Automatic Sub-Microwave Measurements on a

Beam-Plasma Experiment

Pedro Francisco de Deus Lourenço

Thesis to obtain the Master of Science Degree in

Engineering Physics

Supervisor: Professor Horácio João Matos Fernandes

Examination Committee

Chairperson: Professor Luís Paulo da Mota Capitão Lemos Alves

Supervisor: Professor Horácio João Matos Fernandes

Members of the Committee: Doctor Jorge Manuel Gonçalves Baptista dos Santos

October 2015

Acknowledgements

First of all, I would like to thank Professor Horácio Fernandes for the constant support in the deve-

lopment of this work, which was critical at all levels, from the conception of the work to the resolution of

both theoretical and experimental issues as well as stimulating scientific thinking. The machine’s original

operation records and experimental guides he provided were of the most importance to understand the

physics behind the system and to overcome the challenges presented.

A special thanks to João Fortunato and Rui Dias for their contribution to overtake many of the techni-

calities presented along every stage of this project and, above all, for sharing their personal experiences

and being available even on their personal time. Finally, a remark to Josué Lopes, my colleague and

present member of the e-Lab team. Due to his contribution it became possible to expedite the process

of integrating this experiment into the e-Lab platform.

I am very grateful to Instituto de Plasma e Fusão Nuclear for all the conditions and means made avai-

lable for the execution of the project. This work was financed by the project Incentivo/FIS/LA0010/2014

granted by "Fundação para a Ciência e a Tecnologia", through a research grant for which I would like to

thank.

Resumo

Os aparatos de interação Feixe-Plasma, desenvolvidos entre as décadas de 1960 e 1980, foram

usados no estudo de plasmas de baixa temperatura e propagação de ondas eletrostáticas. Presente-

mente, estes aparatos apresentam condições únicas para a realização de trabalhos experimentais em

física de plasmas. Neste trabalho utilizou-se o aparato existente no IPFN, IST.

Foi realizado um processo de inspeção ao aparato para compreender os mecanismos envolvidos

no seu funcionamento. Este conhecimento permitiu a conceção do sistema CODAC, utilizado com

sucesso na operação do aparato e na aquisição de dados experimentais, tendo garantindo condições

de reprodutibilidade de resultados. O CODAC desenvolvido é compatível com o e-lab, permitindo a sua

futura integração nesta plataforma e consequente operação remota.

As duas técnicas de diagnóstico, cavidade ressonante e interferometria, foram utilizadas para deter-

minar os parâmetros da coluna de plasma. Para um campo de confinamento de 10,8±0,5mT, pressão

de 3,0±0,1×10−2mbar e corrente de feixe de 18,0±0,5mA, foi possível reconstruir a curva de dispersão

do plasma para frequências inferiores à frequência de plasma através da interferometria. Determinou-

se uma frequência de plasma de 200±1MHz e um número de onda transversal de 0,91±0,03cm−1,

tendo portanto uma densidade de 5,0±0,1×108cm−3. Nas mesmas condições, determinou-se uma

densidade de 3,3±0,9×109cm−3 recorrendo à cavidade ressonante. Esta discrepância de uma ordem

de grandeza foi atribuída ao valor considerado para o raio da coluna de plasma na técnica da cavidade

ressonante e ao gradiente de pressão criado no interior do aparato pela bomba de vácuo.

Palavras-chave: Interação Feixe-Plasma, CODAC, e-lab, Cavidade Ressonante, Interferometria,

Eletrónica de Rádio-Frequência.

Abstract

Beam-Plasma interaction apparatuses, developed between the 1960s and the 1980s, were used to

study the underlying mechanisms of low temperature plasmas and wave propagation. In today’s science,

these can provide an unique environment to perform advanced experimental works on plasma physics.

The apparatus used was the Beam-Plasma experimental apparatus from IPFN, IST.

An initial maintenance and inspection process was conducted on the device to understand the phys-

ical mechanisms behind operation. Then, this insight was used to design and implement a CODAC

system which was successfully used to operate the apparatus and acquire the experimental data, guar-

anteeing conditions of reproducibility for the operating conditions and parameters of the experiment. The

CODAC hardware was made compliant with e-lab, allowing for a future integration under this platform

and ultimately leading to the possibility of complete remote operation.

Two diagnostic techniques, resonant cavity and interferometry, were used to determine the parame-

ters of the plasma column. For a confinement field of 10.8±0.5mT, pressure of 3.0±0.1×10−2mbar and

electron beam current of 18.0±0.5mA, it was possible to reconstruct the dispersion relation for frequen-

cies below the plasma frequency using interferometry. The technique allowed the determination of the

plasma frequency at 200±1MHz and the transverse wavenumber of 0.91±0.03 cm−1, thus with a den-

sity of 5.0±0.1×108cm−3. Under the same conditions, the density determined with resonant cavity was

3.3±0.9×109cm−3, thus one magnitude above. This discrepancy was attributed to the plasma column

radius considered inside the cavity and to the pressure gradient created inside the interaction chamber

by the vacuum pump.

Keywords: Beam-Plasma Interaction, CODAC, e-lab, Resonant Cavity, Interferometry, Radio-Frequency

Electronics.

Contents

Contents ii

List of Figures v

List of Tables vi

List of Acronyms vii

1 Introduction 1

1.1 Plasma Physics and Radio Communications . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Fundamental Plasma Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 Wave Propagation in Plasmas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.3.1 Electrostatic and Electromagnetic Waves . . . . . . . . . . . . . . . . . . . . . . . 4

1.3.2 Instabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.3.3 Distribution Function and Landau Damping . . . . . . . . . . . . . . . . . . . . . . 7

1.4 Apparatus Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.4.1 Experimental Set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.4.2 Langmuir Probes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.4.3 Interferometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.4.4 Resonant Cavity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.4.5 Review of Main Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

1.5 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2 Real-Time Control Systems 16

2.1 Control System Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.2 Variable Control Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.2.1 PID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.2.2 Velocity Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.3 Hardware Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.4 Software Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

i

3 The Beam-Plasma Interaction Experiment 24

3.1 General Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.2 Vacuum System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.3 Pressure Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.4 Gas Injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.5 Electron Gun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.6 Electrostatic Collector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.7 Confinement Coils and Quadrupole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.8 Fixed Langmuir Probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

3.9 Movable Langmuir Probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3.10 Resonant Cavity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

4 The CODAC 42

4.1 CODAC Architecture and Integration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.2 Control and Data Acquisition Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.2.1 Control Board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4.2.2 Aquisition Board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

4.3 Microcontroller State Machine and Commands . . . . . . . . . . . . . . . . . . . . . . . . 47

4.4 RF Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5 Experimental Results and Discussion 53

5.1 Resonant Cavity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.2 Interferometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

5.3 Overall Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

6 Conclusions and Future Work 61

References 66

A Appendices 67

A Apparatus and CODAC Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

A.1 Console Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

A.2 Setup Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

B Experimental Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

B.1 Electromagnetic Cavity Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

B.2 Determination of Dispersion Relation via Interferometry . . . . . . . . . . . . . . . 73

C Technical Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

ii

List of Figures

1.1 Dispersion diagram for the waveguide wave and the plasma wave propagation for a filled

waveguide. The variable β stands for the wave propagation constant, k. From reference14. 5

1.2 Front view of the Beam-Plasma experimental apparatus setup. It is possible to observe

the confinement coils placed allong the lenght of the a apparatus as well as the beam

alignment quadrupole. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.3 A Langmuir probe in most simple configuration. From reference24. . . . . . . . . . . . . . 9

1.4 Plasma I-V characteristic. Electron Saturation (A) and Ion Saturation (C). From reference24. 9

1.5 Example of the pattern obtained with the Interferometry technique. From reference26. . . 10

1.6 Plasma dispersion relation. From reference26. . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.7 Fields inside the resonant cavity. From reference26. . . . . . . . . . . . . . . . . . . . . . 13

1.8 Suppression of the electron cyclotron instability with the addition of a second resonant

beam. Time evolution of amplitude measured with probe in fixed positions: (I) in the

absence of the secondary beam, (II) with a second resonant beam and (III) with a non-

resonant second beam. From reference6. . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.1 Simple PID block diagram in frequency domain. The terms Gc and Gp refer to the con-

troller and system gains respectively. From reference31. . . . . . . . . . . . . . . . . . . . 18

2.2 Assembled dsPICnode V3.0 board with a 30F4013 microcontroller. . . . . . . . . . . . . . 20

2.3 Complete e-lab framework architecture. The Beam-Plasma apparatus (Experimental Ap-

paratus) is connected to the ReC through the control and acquisition hardware (Hardware

Controller). Form reference45. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.4 Example of an experiment GUI on e-lab framework, "Ondas estacionárias e velociade do

som experiment". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.1 Cross section schematic of the apparatus. Based on reference6. . . . . . . . . . . . . . . 25

3.2 Vacuum cut and venting valves mounted on a VARIAN DS102 rotary pump with a KF

DN40 T connection joint. The bellow leads to the turbo pump. . . . . . . . . . . . . . . . . 26

3.3 Connection schematic of the valves introduced into the vacuum system. . . . . . . . . . . 27

3.4 Gas injection system. The gas line (blue) from the pressure regulator connects to the

gas cut valve (right), follows to the proportional valve (center) and enters the interaction

chamber through the manual security valve (top). . . . . . . . . . . . . . . . . . . . . . . . 29

iii

3.5 Schematic of the helium gas injection system. . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.6 Plot of pressure as function of the proportional valve aperture for a gas line pressure of

0.3bar over atmospheric pressure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.7 Step response of the MPT100 gauge to MKS248A valve aperture steps: 40-77% and 10-

70% (acquisition intervals of 75ms). It is possible to notice the transition between the dual

gauge measuring elements (Transition). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.8 Linear fit to determine the pressure variation slope as a response to the valve aperture

step as well as the response delay. These were used to calculate the PID parameters.

The data used is the same as in the 10-70% step of the plot on Figure 3.7. . . . . . . . . 32

3.9 Plot of the pressure controlled by the implemented PID algorithm for five different set-points. 33

3.10 View of the components that constitute the electron gun: (a) - Tungsten filament placed at

the holder; (b) - Inner view of the cup; (c) - Side view of the filament cup. The acceleration

plates are electrically isolated from the cup by three ceramic rods; (d) - Electron gun fitted

to the apparatus. The gun is cooled with distillate water in a closed fluid circuit. The

cables connected to the plates and filament are also visible. . . . . . . . . . . . . . . . . . 34

3.11 Model representation of the electrostatic collector: (a) - Collector head detail; (b) - Collec-

tor mounted on the support tube and top end flange. . . . . . . . . . . . . . . . . . . . . . 36

3.12 Rear view of the apparatus. The confinement coils placed form the electron gun to the

end of the interaction chamber in order to provide an homogeneous magnetic field across

the experiment. It is also possible to observe part of the quadrupole . . . . . . . . . . . . 37

3.13 Fixed Langmuir probe positioner: (a) - Manually adjustable positioner with probe; (b) -

Inner view of the inside of the positioner where the below and probe jacket are visible. . . 38

3.14 Movable Langmuir probe: (a) - Inner view of the interaction chamber seen from the elec-

tron gun side. The probe holder slides along the chamber over two parallel rails, fixed on

both ends of the chamber section; (b) - Movable probe mounted on the sliding support

during the process of cable replacement and cable mechanism re-adjustment. . . . . . . 39

3.15 Traction mechanism of the movable probe system: (a) - View of the section which holds

the cable mechanism (inside). The cables are winded on the drive shaft pulleys. Below,

there is a steel tube with a weight that maintains the probes coaxial cable under tension.

The position encoder (blue) connected to the drive shaft and the head of the electrostatic

collector are also visible; (b) - DC motor and reduction gears connected to the drive shaft. 39

3.16 : (a) - Electron gun cup mounted on the section which contains the resonant cavity. On

the side of the section it is possible to observe the port for antenna insertion; (b) - Loop

antenna used in the resonant cavity mounted on the positioner. . . . . . . . . . . . . . . . 40

4.1 CODAC integration on a 3U 19-inch Rack. From left to the right: local-host, control board,

acquisition board, BNC panel and turbo pump driver. . . . . . . . . . . . . . . . . . . . . . 42

4.2 CODAC integration schematic. Both boards can work and communicate separately with

the local-host via RS232 communication protocol. . . . . . . . . . . . . . . . . . . . . . . 43

iv

4.3 The two CODAC boards: Control board on the left and Acquisition board on the right. . . 44

4.4 Bock diagrams for the control and acquisition boards of the CODAC system. . . . . . . . . 46

4.5 ReC State machine50 implemented into the boards microcontrollers (a) and sample of the

acquisition board output (b). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4.6 Schematic of connections made in order to implement the resonant cavity diagnostic tech-

nique. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

4.7 RF equipment: Boonton 42A Microwattmeter (top left), HP 3200B VHF Oscillator (top

right) and HP8620A Sweep Oscillator (bottom). . . . . . . . . . . . . . . . . . . . . . . . . 50

4.8 RF equipment: (a) - dual directional coupler HP 777D with two HP 420A crystal detectors

(on top), HP 423A crystal detector (bottom left) and HP 10514A mixer (bottom right); (b) -

Set of two VHF attenuators, HP 355C and HP 355D and two HP 8447A RF amplifiers; (c)

- Two HP 8447A RF amplifiers connected in series; (d) - RF stub, model GRC 874-D20L. 50

4.9 Schematic of connections made in order to implement the interferometry diagnostic tech-

nique. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5.1 Resonant cavity in vacuum at 2.1×10−5mbar: transmitted, reflected and incident signal. . 53

5.2 Resonant cavity transmitted signal for vacuum (2.1±0.1×10−5mbar) and for different gas

pressures with an electron current of 18mA and confinement field of 10.8±0.5mT (4±0.2A). 54

5.3 Correlation plot between pressure and electron density determined with the resonant cav-

ity technique. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

5.4 Plasma E(x) patterns for three specific frequencies in the sweeped range. . . . . . . . . . 56

5.5 Plasma dispersion relation for an electron beam current of 18mA. . . . . . . . . . . . . . . 57

5.6 Plasma transverse wave number for an electron beam current of 18±0.5mA. . . . . . . . 57

5.7 Experimental determination of phase velocity for an electron beam current of 18±0.5mA. 58

5.8 Experimental determination of group velocity for an electron beam current of 18±0.5mA. 59

5.9 Plasma column inside the cylindrical resonant cavity where a is the radius of the passing

holes and a’ is the actual radius of column. . . . . . . . . . . . . . . . . . . . . . . . . . . 60

A.1 Schematic of the CODAC hardware front BNC connections panel, used to acquire exper-

imental data from the diagnostics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

A.2 Schematic of connections for resonant cavity diagnostic technique implementation. Ob-

serve that the image may mislead in how to correctly alight the loop antennas. The normal

plane of the loop must be aligned along θ, thus with the axis of the loop perpendicular to

z 85. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

A.3 Plots of power as function of frequency for emitted, reflected and transmitted signals. In

this case, the incident power is assumed to be ideally constant. As expected, a maximum

in the transmitted power coincides with a minimum in the reflected power. From reference85. 72

A.4 Schematic of connections for interferometry diagnostic technique implementation.85. . . . 74

A.5 Example of the patterns detected in the interferometry protocol. From reference85. . . . . 75

v

List of Tables

3.1 Resistance and inductance of the gas valves solenoids. . . . . . . . . . . . . . . . . . . . 30

3.2 Derived PID parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.1 CODAC Boards Terminal I/O. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

5.1 Peak Fit results for Figure 5.2. Electron beam current of 18±0.5mA and confinement field

of 10.8±0.5mT (4A). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

5.2 Fit results for plasma dispersion relation from Figure 5.5, calculated parameters and com-

parison with average transverse wave number from Figure 5.6. Electron beam current of

18±0.5mA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

5.3 Comparison between density results determined with resonant cavity and interferometry

techniques under the same experimental conditions: pressure of 3.0±0.1×10−2mbar,

electron beam current of 18±0.5mA and confinement field of 10.8±0.5mT. . . . . . . . . 59

A.1 CODAC BNC Connections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

A.2 CODAC Terminal Commands (Short Version). . . . . . . . . . . . . . . . . . . . . . . . . . 70

A.3 Experimental parameters for the cavity protocol. . . . . . . . . . . . . . . . . . . . . . . . 73

A.4 Experimental parameters for the interferometry protocol. . . . . . . . . . . . . . . . . . . . 75

vi

List of Acronyms

ADC Analog to Digital Converter

ATX Advanced Technology Extended

CA Channel Access

CAN Controller Area Network

CBT Clock-Based Tasks

CODAC Control, Data Access and Communica-

tion

CSS Control System Studio

DDC Direct Digital Control

DSC Digital Signal Controller

DSP Digital Signal Processor

EBT Event-Based Tasks

EEPROM Electrically Erasable Programmable

Read-Only Memory

EPICS Experimental Physics and Industrial Con-

trol System

FCFS First-Come, First-Served

FILO First-In, Last-Out

FPGA Field-Programmable Gate Array

GUI Graphical User Interface

HCI Human-Computer Interface

HDL Hardware Description Language

HF High Frequency

I/O Input/Output

I2C Inter-Integrated Circuit

IOC Input/Output Controller

IS Interactive System

MCU Microcontroller Unit

PV Process Variable

PWM Pulse-Width Modulation

RAM Random-Access memory

RCL Remote Controlled Laboratory

ReC Remote experiment Control

RF Radio Frequency

RTC Real-Time Control

RTCS Real-Time Control System

SPI Serial Peripheral Interface

UART Rniversal Asynchronous Receiver/Trans-

mitter

UHF Ultra High Frequency

VHF Very High Frequency

vii

Chapter 1

Introduction

Plasmas occur on Earth’s atmosphere as a natural phenomenon and they can originate through

two different processes: high-voltage discharges, which result from the accumulation of electrostatic

potential in the atmosphere, cause the ionization of the surrounding particles in the air1; radiation and

energetic particles that hit the atmosphere coming from space, producing particle ionization at the upper

layers of the atmosphere.

This last process and the confinement provided by Earth’s magnetic field creates the atmospheric

layer known as Ionosphere. The principal source of ionization is energetic electromagnetic radiation

from the sun2. Other important sources are energetic charged particles of solar origin and galactic

cosmic rays2. The rate of particle ionization both depends on the intensity of the solar radiation and

on the ionization efficiency of the neutral particles present in the atmosphere2. By passing through the

atmosphere, the radiation of the sun is progressively absorbed. Thus, its ionizing ability depends on the

length of the interaction path2. Due to its importance to fields such as telecommunications, it is crucial

to study the low density plasma that characterizes the Ionosphere.

1.1 Plasma Physics and Radio Communications

Plasma physics is one of the scientific areas of knowledge that has suffered major advances in

the last decades3. It has given a large contribution, along with the scientific development itself, to the

technological achievements in fields such as telecommunications.

In the 1950s, long range telecommunications were still performed on the high frequency band (HF)

and one way to achieve this was to use the ionosphere because of its reflective and blocking properties

to electromagnetic waves of certain frequencies1.

The ionosphere comprises the ionized region of the atmosphere and starts at an altitude around

50km2. The ionization process is the result of interaction between the space radiation, mainly from the

Sun, and the particles that constitute the atmosphere4. The ionosphere is divided in layers2;5 according

to temperature and electronic density, which change with the time of the day, latitude, season and solar

activity4;2, and it has been object of several studies over the past decades4.

1

As a plasma, the medium presents a plasma frequency below which there is no radio wave propa-

gation, named the cutoff frequency. As the density varies with altitude, so does the cutoff frequency of

the medium1. By using this characteristic, it is possible to perform long range communications in the HF

frequency band2. A radio wave transmitted from a given position on Earth propagates until it reaches

an altitude with cutoff frequency equal to the wave’s own frequency forcing the wave to be reflected

back down1. For frequencies higher than the cutoff, reflection does not occur and waves propagate

through the plasma medium. Nevertheless, the ionosphere affects the performance of communication

bands such as VHF, UHF and even higher frequency bands with phenomena such as, wave refraction,

absorption and frequency shift and ultimately leading to the degradation of radio transmissions between

ground stations and satellites4.

Thus, plasma physics plays a fundamental role in telecommunications and it is crucial to under-

stand the wave propagation mechanisms inside the ionosphere. In order to have some insight over the

physical models that govern plasmas, several experiments were conducted on beam-plasma interaction

machines6;7;8 and Q-machines9;10.

The Beam-Plasma experiment is an experimental apparatus that allows to study the propagation of

waves at a low temperature plasma, which is generated by the interaction of a low energetic electron

beam with Helium gas inside a low pressure chamber with cylindrical geometry6;7;8. Therefore, it be-

comes relevant to give a more detailed look into the mentioned experiment to understand its importance

and how it can still present today a relevant part in the study of plasma physics and wave propagation.

1.2 Fundamental Plasma Parameters

A simple definition for a plasma can be "a quasineutral gas of charged and neutral particles which

exhibits collective behaviour"1. This means that an ionized gas cannot be defined as a plasma unless it

presents specific characteristics.

The fundamental characteristic of plasmas is the presence of oscillations resultant from the electric

fields produced between particles of opposite charge: electrons and ions. Electrons have a mass much

smaller than ions and, therefore a much smaller inertia. Considering that ions are fixed in the form of

a background wall relatively to the motion of electrons, the generated electric fields make the electrons

oscillate around their equilibrium positions1. This oscillations are very fast and have a characteristic

frequency, the plasma frequency, which is given by the following expression:

ωp =

(ne2

ε0me

)1/2

, (1.1)

where me stands for the electron mass, e for the electron charge and ε0 for the vacuum permittivity.

Since these are constant parameters, the plasma frequency is proportional to n, the medium electronic

density.

The motion of particles inside the gas does not depend only on local particle collisions. The presence

2

of charged particles leads to the occurrence of electric fields that affect the motion of charged particles in

other regions of the plasma. This is called collective behaviour1;11. Another characteristic is the shielding

that plasma presents to electric potentials, allowing the formation of clouds of charged particles around

a surface of a given electric potential. This cloud, or the sheath, shields the surrounding plasma from

the applied potential and is characterized by a thickness called the Debye length λD:

λD =

(ε0KTene2

)1/2

. (1.2)

The term KTe represents the thermal motion of the electrons, where K is the Boltzmann constant and

Te is the electron temperature. Due to their higher mobility, electrons have a major contribution to the

thickness of the shielding layer1. The number of particles inside the shielding cloud must be high in

order for the shielding to take place. The number of particles inside the Debye shielding shaped as a

sphere can be defined as:

ND = n4

3πλD

3, (1.3)

which must be much larger than one1. For distances larger than the Debye, L λD, occurs an effective

shielding for the applied potentials, allowing for quaisneutrality. This expression refers to the fact that for

a gas under the mentioned conditions, the density of ions, ni, and electrons, ne, is approximately the

same:

ni ≈ ne ≈ n, (1.4)

and therefore, approximately equal to a term n, the plasma density, but still maintaining the presence

of electromagnetic forces. The last criteria is based on the fact that motion on a plasma must be ruled

by electromagnetic forces instead of collisions between charged and neutral particles1. This can be

represented by the condition ωτ > 1, with ω representing the frequency of plasma oscillations and τ the

mean time between collisions of charged particles and neutrals.

In the presence of an imposed magnetic field ~B, charged particles behave according to the model of

the harmonic oscillator, presenting a cyclotronic frequency:

ωc =|q|Bm

, (1.5)

with q the particle charge, B the magnetic field and m the particle mass. Charged particles also travel

along the ~B field lines, exhibiting circular motion in the plane perpendicular to the field lines1;11. Using

this propriety, it is possible to confine the charged particles in a plasma by controlling the magnetic field

lines. The radius of the orbit in that plane, around the guiding center and with velocity v⊥, is called the

Larmor radius:

rL =v⊥ωc. (1.6)

The circular motion of the charged particles has opposite direction for particles of positive and neg-

ative charge, since the motion produces a field that must contradict the magnetic field of the medium1.

The presence of an imposed electric field ~E in the medium results in a drift of the guiding center of

3

the particle trajectory with a velocity given by:

vE =[ ~E × ~B]

B2. (1.7)

1.3 Wave Propagation in Plasmas

An arbitrary sinusoidal oscillating plane wave can be expressed in the form A(~r, t) = Aej(~k·~r−ωt),

being k the wave propagation constant in the medium. A useful relation is k = 2π/λ. The concepts of

phase and group velocities can be explained as follows: the first represents the velocity at which a point

of constant phase on a wave travels and is given by vϕ = ω/k while the second, given by vg = ∂ω/∂k, is

the velocity of the traveling waves envelope1.

The beam-plasma experiment allows to study the propagation of electrostatic waves. Waves can

have a electromagnetic or electrostatic nature.

1.3.1 Electrostatic and Electromagnetic Waves

Assuming the presence of an electromagnetic field in the form ~B = ~B0 + ~B1, the subscript 0 repre-

sents an homogeneous term, while 1 stands for a small oscillating perturbation1. For an electrostatic

wave, B1 is zero and for an electromagnetic wave it is different from zero. When a wave is denoted as

parallel or perpendicular it refers to the relative position between ~k and ~B01.

The electric field can also be decomposed in ~E = ~E0 + ~E1, similarly. The terms longitudinal and

transverse refer to the direction between ~k and ~E11.

1.3.1.1 Electron Electrostatic Waves

Plasma oscillations can propagate due to the thermal motion of the particles, creating a plasma

wave. Since a plasma is a dispersive medium, i. e. the value of k is not constant for all values of ω in

the medium, it is possible to derive the following expression for the electron plasma12 waves:

ω2 = ωp2 +

3

2k2vth

2, (1.8)

where vth2 = 2KTe/me is the thermal velocity. The experimental confirmation for this dispersion relation

was done by Barrett et al. in 19689. These are parallel waves and exist even if B0 is zero.

These waves can be excited with the use of an electron beam. The interaction between the waves

and the beam make the oscillations grow. Experiments have shown that there were modes associated

with the wave inside the medium13. For a cylindrical plasma column, the values of k are limited to integer

multiples of half the wave length that fit inside the cylinder length, forming standing wave patterns such

as inside a wave guide13.

Another form of electron electrostatic waves is the upper hybrid waves, which are perpendicular

waves. In this situation, the magnetic field is perpendicular to ~k and therefore, apart from the electric

4

(a) Plasma referential. (b) Electron beam referential.

Figure 1.1: Dispersion diagram for the waveguide wave and the plasma wave propagation for a filled waveguide. The variable βstands for the wave propagation constant, k. From reference 14.

field, the Lorentz force is applied, affecting the particles orbits. The higher restitution force that acts on

the particle origins a higher frequency for the wave. Disregarding the thermal velocity, the dispersion

relation for these is given by:

ω2 = ωp2 + ωce

2 = ωh2 (1.9)

with ωce, the cyclotron frequency for electrons and ωh, the upper hybrid frequency.

The experimental observation for this last dispersion relation can be seen on the work from Trivel-

piece and Gould, published in 195914. This paper refers to the study of space charge (electrostatic)

waves inside a cylindrical plasma column. It is mentioned that these can propagate even in the absence

of a drift motion or thermal velocities of the plasma14. They consider a metallic cylindrical container for

the plasma in the cases of partial and complete filling. The container is treated as a waveguide. It is also

presented the dispersion diagram for the waveguide wave and the plasma wave propagation inside the

filled medium, Figure 1.1a. In Figure 1.1b it is shown the same result but on the referential of a electron

beam crossing the medium, which is the result of a coordinate transformation from the plasma to the

beam referential, ω′ = ω + ku0 with u0 the velocity of the drifting electrons in the beam14.

1.3.1.2 Ion Electrostatic Waves

Similarly to sound waves, electrostatic waves can propagate in the plasma through collisions. But

even in the absence of collisions, ions can still transmit vibrations due to their charge1. These are the

so called ion acoustic waves15 or ion plasma waves. For these waves, ~k is parallel to ~B0 and can even

exist if B0 is zero. The dispersion relation is given by:

ω2 = k2cs2, (1.10)

where cs is the speed of sound. For the ion acoustic, cs2 = (γeKTe + γiKTi)/mi, but for higher frequen-

cies, the Debye shielding becomes significant and the speed of sound is given by cs2 = (γiKTi)/mi +

5

((γeKTe)/mi)(1/(1 + γek2λDe

2)) and the waves are classified as ion plasma waves1. The terms γ

represent the adiabatic constant.

In the cases where k is perpendicular to B0, the waves are denominated as lower hybrid waves. In

this case, since electrons flow along the magnetic field lines, there is a break in overall neutrality and the

dispersion relation is:

ω2 = k2cs2 + ω2

l . (1.11)

The term ωl is the lower hybrid frequency, and is given by ωceωci, the product of the cyclotronic frequen-

cies for electrons and ions, respectively.

If the angle between ~k and ~B0 is comprised between 0 and π/2 the overall neutrality can still be kept

and the resulting waves are called ion cyclotronic waves1. Electrons can still keep the Debye shielding.

Like in upper hybrid waves, both electrostatic and Lorentz forces are apllied, thus the dispersion relation

is:

ω2 = k2cs2 + ωci

2. (1.12)

The first experimental work related to the ion cyclotronic waves was published by Motley and D’Angelo

in 196316. Although it was conducted on a Q-Machine, similar work was also performed on Beam-

Plasma experiments, regarding the study of ion electrostatic waves17;8.

The waves that travel on the plasmas can be excited via the interaction with sources, such as,

electron beams or radio frequency pulses. These are called instabilities.

1.3.2 Instabilities

Plasma instabilities are oscillations that grow very fast and result from the presence of available free

energy in the system18. It is important to denote that, in a finite confined plasma, the free energy results

from the fact that the medium is not in complete thermodynamic equilibrium19. The growing rate of

instabilities is denoted by the term γ.

Under certain conditions, the dispersion relation can present complex solutions, thus ω = ωr + jωi

with the subscripts r and i representing the real and imaginary terms of the frequency ω, respectively.

Given the expression for an arbitrary sinusoidal oscillating plane wave and if the term ωi is positive, the

amplitude of the wave becomes proportional to the exponential factor exp(ωit). Otherwise, the wave is

evanescent. The term ωi is then the growing factor of the instability, γ. The simplest case of this situation

is the two-stream instability. Stream instabilities are driven by the drift energy provided by a stream or

beam of particles that travel through the plasma1. Ion acoustic waves can be excited by ion and electron

beams. In a magnetized plasma medium, electrostatic ion cyclotron waves can also be excited either by

streaming electrons or ions19.

Instabilities are classified according with the source of energy that enables the growth of oscillations

and the spacial localization of the oscillations in the medium1. Although there are several categories,

these can be divided into two main groups: absolute and convective instabilities18;19. An instability is

absolute if an initial disturbance produces a response that grows in time at every spatial point18. It is

6

convective if the oscillations move in space while growing in amplitude i.e., the growing amplitude is

convected away19. These are definitions established within a fixed observer frame18. The maximum

amplitude of the instabilities is limited by non-linear processes that intervene when the occurrence of

the instability changes significantly the conditions by introducing perturbations in the medium18. Insta-

bilities can also be divided into microscopic and macroscopic. While microscopic instabilities result in an

increase of turbulence in the plasma, macroscopic instabilities can create significant material displace-

ments in the medium19. These displacements ultimately lead to confinement loss and to the release of

considerable energy.

Given the previous remarks, in order to develop effective suppression mechanisms it is crucial to

understand the underlying processes that drive plasma unstable.

1.3.3 Distribution Function and Landau Damping

Due to the very large number of particles, it is very difficult to describe a plasma collectively through

the individual characterization of the single particles. Statistical physics has provided a solution to this

problem by describing the particles that constitute a gas, the ensemble, via a distribution function20. The

Maxwell-Boltzmann distribution function1 is:

f(v) =

(m

2πKTe

)3/2

exp

(− mv2

2KTe

), (1.13)

and gives the probability of finding a particle in the medium with average velocity v, and consequently,

with a certain energy20. This is valid under equilibrium conditions20.

Landau damping is a characteristic of collisionless plasmas1. This effect is related to the particles

in the distribution function with the velocity in the same order of magnitude of the phase velocity of

an electrostatic wave propagating in the medium. This allows for energy exchange between the wave

and the particles traveling along the wave. If in the velocity distribution there are more slow electrons

rather that fast, there will be an energy transfer from the wave to the electrons causing the damping of

the wave1. The first experimental verification was performed by Malberg and Wharton in 196621. The

power absorption by the plasma is studied by Brownell in his paper published in 197222.

1.4 Apparatus Overview

The beam-plasma experiment allows the study of the interaction between a low energetic electron

beam and a low temperature Helium plasma. In plasmas, the characterization of the medium is achieved

through specific measurement techniques that allow the determination of the plasma parameters. Due to

their importance, diagnostics in plasmas physics are an extensive field which has presented a technolog-

ical and scientific revolution over the past decades. Some of these fundamental diagnostic techniques

are present in the Beam-Plasma apparatus. Thus it is crucial to understand the physical principles

behind their application.

7

1.4.1 Experimental Set-up

The front view of the experimental set-up core can be found on Figure 1.2. The experiment is made

on a cylindrical interaction chamber with a diameter of 8cm and a length of 75cm6. The low pressure

on the system is achieved through two vacuum pumps, a first stage rotary pump and a turbo pump to

achieve background pressures in the order of 10−6mbar. The chamber is filled with Helium until the

final pressure is around 10−4mbar. A vacuum gauge is connected to the chamber and allows to read

pressure values.

Figure 1.2: Front view of the Beam-Plasma experimental apparatus setup. It is possible to observe the confinement coils placedallong the lenght of the a apparatus as well as the beam alignment quadrupole.

The electron beam is produced by an incandescent filament23 in Pierce configuration6, located on

the left side of the machine, while the axial confinement is provided by 10 coils placed along the chamber.

These produce an homogeneous magnetic field (within 1%) with values around 0.01T6.

At the right end of the chamber is placed an electrostatic energy analyzer, aligned with the beam

and machine axis. The energy of the beam is typically in the order of 2keV, 10mA and 4mm diameter6.

The experiment has a resonant electromagnetic cavity for density measurements. There are also one

movable and several fixed Langmuir pin probes along the interaction chamber6.

1.4.2 Langmuir Probes

Langmuir Probes are one of the earliest, most simple and most used diagnostics for the study of low

temperature plasmas24. They allow to do density, electron temperature and potential measurements25.

A thin cylindrical metallic wire, usually made of tungsten and with a diameter bellow 1mm, is immersed

into the plasma medium while a sweeping electrical potential is applied to it. By varying the potential on

the probe it is possible to determine the I-V characteristic of the plasma (Figure 1.4) or, in other words,

the electric current drawn by the plasma versus applied voltage24. In Langmuir probes with a single wire

(Figure 1.3), the electrical potential is measured relatively to the apparatus chamber.

The immersion of the probe into the plasma results on a local perturbation of the medium. For a

floating probe, electrically insulated from the apparatus and in contact with the plasma, the electrons

charge the probe negatively so the total electric current is zero. This means that electron and ion

currents are equal. The potential of the probe is in this case Vf , the floating potential, which is the

8

Figure 1.3: A Langmuir probe in most simple configuration. From reference 24.

potential of the plasma in the presence of the probe. In this case, the plasma potential without the probe

is then denominated as Vp, the plasma potential.

Figure 1.4: Plasma I-V characteristic. Electron Saturation (A) and Ion Saturation (C). From reference 24.

The region (A) (Figure 1.4) is called electron saturation. In this case, the applied potential V is higher

than Vp, and a maximization of the electron current occurs because all electrons arriving to the probe

are being collected while ions are being repulsed24. If V is inferior to Vp, thus negative comparatively

to the plasma, electrons gradually become repelled by the negative potential of the probe (Region B)

until Vf is reached. If V goes below Vf only ions are collected by the probe reaching the ion saturation

(Region C). On a cylindrical probe, the shape of the characteristic curve suffers some changes but it is

still possible to identify the three distinct areas such as on Figure 1.4.

With a detailed analysis of the plot and making adequate assumptions it is possible to determine

the mentioned parameters26. Assuming that electrons follow a Maxwellian distribution function near the

9

floating potential, the electronic temperature Te can be determined with the expression,

ln(−ie) ≈eV

kBTe+ ln(C) (1.14)

were C is a constant. With this, the Debye lenght can be derived using expression (1.2). The ion

saturation current can be directly determined from the plot and knowing that it is approximately given26

by,

i+ ≈ 0.55Aen+0

√kBTemi

(1.15)

were A = 2πrpl is the collecting area of the cylindrical probe, it becomes possible to calculate the

density of the plasma with the expression:

n+0 ≈i+

2π0.55rple√

kBTe

mi

(1.16)

If the radius of the probe rp is much smaller than λD, a correction can be performed to the previous

calculation by replacing the value of rp with the one found for λD 26. The plasma frequency ωpe can

therefore be determined by applying expression 1.1.

1.4.3 Interferometry

The interferometry technique used in this apparatus allows the determination of the dispersion dia-

gram for 0 ≤ ω ≤ ωpe of the plasma column created by the electron beam and Helium gas interaction.

This is achieved by using a RF generator and two Langmuir probes as antennas. The waves injected

via the fixed probe, propagate through the plasma and are detected with the movable probe in different

positions along the plasma column length26. The injected and detected signals are mixed, producing an

interferometry pattern such as in Figure 1.5.

Figure 1.5: Example of the pattern obtained with the Interferometry technique. From reference 26.

In this case, a completely filled cylindrical plasma wave guide aligned with ~z axis is considered and

10

the transverse wave number ~kx is referred as p. For an electric field in the form of:

Ez = E0J0(pr)exp(jmθ)exp (j(ωt− kz)) (1.17)

and considering the fundamental mode (m = 0) and p = 2.405/a, where a is the radius of the plasma

column it is possible to derive the following expression for the plasma dispersion relation:

p2

(1−

ω2pe

ω2 − ω2ce

)+ k2

(1−

ω2pe

ω2

)= 0 (1.18)

for the electrostatic waves26. Transverse propagation is in the form of stationary waves with amplitude

dependence on J0(pr) and phase independent of x26.

Figure 1.6: Plasma dispersion relation. From reference 26.

Since the signal is injected in the center of the interaction chamber, there is propagation along ±z.

Considering propagation in +z, the output signal in the mixer, S(z) is26:

E1 = E0 cos (ωt)

E2 = E2 cos (ωt− kz)

E3 = E3 cos (ωt+ kz)

S(z) = E1E2 = E0E2[cos(2ωt− kz) + cos(kz)]

2.

(1.19)

The mixer output is filtered with a low-pass filter, thus the signal becomes S(z) = E0E2 cos(kz)/2. From

the obtained pattern it is possible to determine the wave lengths to the right and to the left of the injecting

probe, thus the value of the corresponding wave numbers using the relation k = 2π/λ. Nevertheless,

due to the fact that reflection, formation of standing waves and interference between the two waves that

propagate in ±z may occur for lower frequencies, it may be necessary to consider that the wavelength

measured is in fact λ/226.

With this technique it then possible to reconstruct the plasma dispersion relation (Figure 1.6) for

0 ≤ ω ≤ ωpe and determine ωpe by applying a numerical fit of expression (1.18) to the pairs (ω, k).

Phase and group velocities can also be determined and the density of the plasma can be calculated by

11

transforming expression (1.1) to ne = ne(ωpe).

1.4.4 Resonant Cavity

In a cylindrical resonant cavity , excited in TM010 mode, the Maxwell equations can be simplified to:∂Ez∂r

= jωµHθ

∂

∂r(rHθ) = jωεrEz

(1.20)

since the magnetic field only has component in ~θ, the electric field in ~z and both fields change over ~θ.

The solution of the system of equations stands:

∂2Ez∂r2

+1

r

∂Ez∂r

+K2Ez = 0 (1.21)

with K = ω2µε. The equation can be solved with Bessel equation,

∂2R

∂x2+

1

x

∂R

∂x+

(B2 − ν2

x2

)R = 0 (1.22)

leading to the solutions: Ez(r) = E0J0(Kr)

Hθ(r) = jE0

Z0J1(Kr)

(1.23)

where J0 and J1 are Bessel functions and Z0 is the wave impedance in the medium. In the wall of the

cavity the electric field must be zero so Kr ≈ 2.405 and the resonance frequency in vacuum is given by:

f0 ≈c

2π

2.405

R(1.24)

where c is the speed of light in vacuum and R is the radius of the cavity. Even if there is a low pressure

helium gas inside the cavity this expression is still a very good approximation. Nevertheless, the pres-

ence of a plasma, thus charged particles, changes the values of ε and µ inducing a shift in the frequency

of resonance26. Other parameter that can be evaluated in the cavity is the quality factor considering fi-

nite conductivity on the walls and no crossing holes for the plasma beam. It is given by the ratio between

the electromagnetic energy (U ) inside the cavity and the power losses (Wp), Q = ωUWp

. By integration of

the former expression it is possible to arrive to a more intuitive form to calculate the Q factor:

Q ≈ Z

RS

2.405

2

(1 +

R

L

) (1.25)

with RS the surface resistance and L the length of the cavity.

For this propagation mode inside a cylindrical cavity with metallic walls it is possible to derive an

expression that returns the plasma electron density as function of the shift in the resonance frequency,

12

Figure 1.7: Fields inside the resonant cavity. From reference 26.

induced by the presence of a plasma column along the cavity axis (1.26)26.

ne[cm-3] =

(8π2meε0

e2R2

a2J21 (x01)

J20 (x01a/R) + J2

1 (x01a/R)× f0 [MHz] 106

)∆f [MHz] (1.26)

The parameters x01, a and R are with respect, the zero of the J0 function, the radius of the passing holes

for the plasma beam in the cavity tops ends and the radius the cylindrical cavity. For the particles inside

the plasma, the motion equation is given by:

mdv

dt= eE + e

[~v × ~B

]−m~vνc (1.27)

where νc is the collision frequency. This expression can be solved in cylindrical coordinates and expres-

sion ~E = ρ ~J can be used to derive the resistivity tensor ρ. The relative permittivity tensor can be written

as [εr] = [I]− j 1ωε0

[σ] , with the conductivity tensor σ is then given by26:

σ =ne2

2m

2(jω + νc)

(jω + νc)2 + ω2ce

2ωce(jω + νc)2 + ω2

ce

0

− 2ωce(jω + νc)2 + ω2

ce

2(jω + νc)

(jω + νc)2 + ω2ce

0

0 02

jω + νc

=

σrr σrθ 0

σθr σθθ 0

0 0 σzz

(1.28)

Using the fact that vD is zero and the previous tensor, it is possible derivative the previous expression

(1.26) for the plasma density by solving the following expression26:

(1

Q1− 1

Q0

)− 2j

∆ω

ω0=

1

ω0ε0

∫VσE2(r)dV∫

VE2(r)dV

(1.29)

This is done26 considering that the density profile of the plasma column is approximately constant and

that νc/ω0 << 1.

1.4.5 Review of Main Publications

Several works have been published regarding results obtained on beam-plasma experiments.

In 1967, Hopman et al. published work on the deceleration of an electron beam during the electron

13

plasma frequency instability. The interference pattern found along the system axis presented a shrinkage

of the wave length due to the deceleration of the beam by the unstable wave27. In the same year,

Vermeer et al. studied the excitation of ion oscillations as a result of the beam-plasma interaction. They

found that the interaction is excited by the slow cyclotron wave on the beam8.

Later, in 1968, Hopman and Ott studied the saturation of the beam-plasma instability, caused by

a flattening of the beam distribution function7. Clear differences were found in the beam distribution

function and correlated to pulse status7. Hopman et al. also published a paper on the electron cyclotron

instability, regarding its characterization28. For a limited parameter range they were able to compare the

experimental results with the theory28.

Figure 1.8: Suppression of the electron cyclotron instability withthe addition of a second resonant beam. Time evolution of ampli-tude measured with probe in fixed positions: (I) in the absence ofthe secondary beam, (II) with a second resonant beam and (III)with a non-resonant second beam. From reference 6.

Then, in 1972, Wakeren and Hopman stud-

ied the trapping of electrons as a result of the

beam-plasma interaction29. The entrapment is

attributed to the large amplitude of electrostatic

waves that arise from the interaction29.

Cabral and Varandas published a paper on the

suppression of the electron cyclotron instability in

19806. This suppression is attained by the injec-

tion of a secondary parallel electron beam which,

when in resonance, results in cyclotron damp-

ing and causes the reduction of the cyclotron

wave power6. The importance of this suppression

mechanism for controlled fusion machines, due to

the similar behaviour between energetic electron

beams and the cyclotron radiation in Tokamaks,

caused by runaway electrons is also mentioned6.

The relation between cyclotron radiation and run-

away electrons was suggested by Spong et al. in

197430.

Two years later, Silva and Cabral published

on the ion oscillations at low pressure regimes17.

These oscillations propagate in azimuthal direc-

tion inside the plasma column and were found

to be exited due to convection effects associated

with a rotation of the column.

14

1.5 Outline

The scientific work developed by several authors over the years has given considerable insight over

the phenomena that govern low density plasmas. Today, more complex experimental setups are used

to study the behavior of plasmas under other working regimes. Nevertheless, the contribution of the

beam-plasma experiment to today’s science is not over.

The beam-plasma provides fundamental insight to the study of plasma physics and slow wave prop-

agation. It allows direct contact with an experimental setup that creates a low-pressure and weakly

ionized plasma in a user controlled environment, equipped with data acquisition systems. The potential

of this machine was limited by the age of the mentioned systems. Moreover, its operation was complex

due to fluctuations in the experimental parameters, which was aggravated by the existing control system,

making it difficult to attain operation regimes in steady-state.

In order to solve this impairment, it became clear that an overall maintenance and upgrade had to

be conducted so the machine becomes compliant with the most recent state of the art control and data

acquisition technologies. Such upgrade now allows achieving reproducibility of the operating conditions

and parameters of the machine and performing advanced experimental works on plasma physics. This

was done by evaluating which options were more suitable to rebuild the machine structure, control and

data acquisition systems, providing a more recent and user friendly interface to replace the previous

one.

15

Chapter 2

Real-Time Control Systems

The process of controlling physical systems via hardware devices and software platforms is a com-

plex task and largely depends on the specific type of control solution appropriate for each specific case.

This is essential in order development an adequate CODAC system for the Beam-Plasma apparatus.

More generally, the data regarding a current state of a given system is collected from sensors positioned

in key locations and this information is used by the control system, through a series of mechanisms,

to adjust the state of the system itself. This can be achieved by a direct decision from the user of the

system or even by a predefined algorithm with user defined parameters. Nevertheless, regardless of the

system, the main goal will always be: actuate in the system variables in order to achieve the desired

behavior for the system.

The term Real-Time Control (RTC) applies when the control system response time, from the sensing

to actuation stages, occurs on a time interval smaller than the time needed for the measured parameters

to change significantly31. Real-Time Control Systems (RTCS) can be classified according to the type of

tasks performed during the control process. Periodic or clock-based tasks (CBT) take place in routines

and are repeated in time intervals adequate to the system characteristics. Each system has its own

characteristic time which can be defined as the time taken by the system to respond to a change in input

or load31. Sampling and performing analog-to-digital conversion (ADC) of an analog output variable of

a sensor or transducer or performing polling on another task or variable status are common examples

for this type of operation. On the other hand, tasks that occur as a response to a specific event rather

than the result of a periodic operation, such as the response of a position switch, are called Event-Based

Tasks (EBT). An Interactive System (IS) appears to be very similar to a combination of the two previously

mentioned tasks. In this case, the characteristic time is not only relative to the process under control but

also to the users response time31.

2.1 Control System Process

The development of a control system for the management of an experimental setup or laboratory

experiment is a common application, especially when dealing with equipment characterized by a wide

16

set of parameters that one needs to control in order to perform a test or a trial.

The interface between the control system and the user is nowadays most commonly performed

through a high level software platform31. But regardless of the application, there are several activities

that can be defined as essential to any control system: data acquisition; analysis and storage; sequence,

loop and supervisory control.

There are also methods that can be implemented into control systems in order to verify and protect

the system against failure and increase overall reliability. System protection can be done both at software

and hardware levels. A possibility is to try to make the systems "fault tolerant"31;32. This means that the

systems are designed in such a fashion that already accommodate mechanisms to suppress faults that

can be predicted to occur - the anticipated faults. Nevertheless, there are faults related to the design of

the system itself - the unanticipated faults32. These design faults are much more difficult to solve.

Given the wide extension of elements, methods and characteristics mentioned, it becomes interesting

to evaluate the adequacy of specific solutions when implementing a control system for a laboratorial

experimental setup.

2.2 Variable Control Algorithms

The most common implementation of the Direct Digital Control principle is done through the PID

algorithm. PID means proportional, integral and derivative and can be implemented either by digital or

by analog methods31. This algorithm is frequently used for variable control because of the conceptual

simplicity it presents. Another alternative is to use the velocity algorithm which is the differential form of

the previous one. Both algorithms are described in detail below as the simplest versions of each one of

them, although, improvement can be done to overcome some of their flaws33;31.

2.2.1 PID

Applying control over a system usually means to set a defined variable to a specific reference value,

named the set-point r(t). The proportional term of the algorithm uses the difference between the ac-

tual value of the output c(t) and the set-point, the error e(t), to generate a control signal m(t). For a

system with fixed parameters this simple approach would be enough, but in reality a controlled variable

presents fluctuations due to the feedback loop. To suppress them, it is possible to add an integral term

to the algorithm. The integration of the error value over a period of time gives a compensation in situ-

ations where a parameter variation is observed. The error that otherwise would achieve a steady-state

situation, can then be reduced to zero. The majority of systems achieve very high performance only with

the described PI algorithm33;31. However, on certain specific situations, the variable under control can

suffer rapid variations and the PI control is no longer able to respond effectively. Measuring the rate at

which the error changes, the derivative term D allows to compensate for these variations by enabling the

controller to respond faster and proportionally to the change rate.

17

The general form of the algorithm in time-domain is given by:

m(t) = Kp

[e(t) +

1

Ti

∫ t

0

e(t)dt+ Tdde(t)

dt

], (2.1)

and the block diagram of the process in frequency-domain (equation 2.2) is given in Figure 2.1. In this

figure, the term Gcis the controller gain and the term Gp is the system gain. The variables Kp, Ti and Td

represent the controller overall gain, the integral action time and the derivative action time, respectively.

These are used to tune the control system to the desired level of performance31.

Gc =M(s)

E(s)= Kp

[1 +

1

Tis+ Tds

](2.2)

With the appropriate Z-transform, the continuous time domain expression for the PID algorithm (2.1)

can be converted into a time discreet expression. By defining a sampling interval for the control system,

Ts, the algorithm can be implemented iteratively through n iterations, each corresponding to a time step

Ts. A variable M can be added and is used to set the operation point for the controller. The term is not

essential but allows a smother change in the value (bumpless transfer)31. The resulting expressions can

be presented as it follows:

m(n) = kpe(n) + kis(n) + kd [e(n)− e(n− 1)] +M,

s(n) = s(n− 1) + e(n),

ki = kpTsTi,

kd = kpTdTs,

(2.3)

where the term s(n) is the integral sum of errors e(n).

Figure 2.1: Simple PID block diagram in frequency domain. The terms Gc and Gp refer to the controller and system gainsrespectively. From reference 31.

2.2.2 Velocity Algorithm

The differential form of the PID is the velocity algorithm. The first is also known as the positional

algorithm since it returns the value of the variable m, the position, while the second gives the variation

rate of the same variable, the velocity31. The expression for the latter case is derived by applying a time

18

derivative to expression (2.1) and then, a discretization in time like previously:

∆m(n) = k1e(n) + k2e(n− 1) + k3e(n− 2),

k1 = kp(1 +TsTi

+TdTs

),

k2 = −kp(1 + 2TdTs

),

k3 = kpTdTs.

(2.4)

One of the main advantages of this method is that it prevents the occurrence of sudden large vari-

ations in m, and consequently, in the actuator. If the tune-up parameters are well adjusted, it allows to

avoid Windup31, and therefore, keeps m within values with physical meaning.

2.3 Hardware Solution

Embedded solutions such as, microcontrollers, are today a powerful choice for the implementation of

control systems, especially when combined with computers that can perform high level tasks. Microcon-

trollers are programmable units that, apart from a CPU and RAM and EEPROM memory storage units,

are equipped with multichannel ADC data acquisition modules, digital Input/Output (I/O) ports, serial

communication controller, interrupt controller, hardware timers, among others features31;34. The devices

usually work on boards that contain the peripheral support, fundamental to implement the desired tasks:

power supply, I/O connections, transducers and actuators, power drivers to control other applications

and communication drivers. It also usually provides a clock source with considerably higher frequency

(MHz) than the internal oscillator of the microcontroller itself (kHz). The boards can be developed as

general purpose "development boards" or designed to maximize the device performance for very specific

solutions. Microcontroller based board units can work alone or communicate as nodes in a computer

network to create a control system in a variety of configurations31.

The dsPICnode V3.0, developed at IPFN-IST/EUROATOM35, was designed as a general purpose

board, or node, being able to accommodate Microchip dsPIC30 family microcontrollers: models 30F4011

and 30F401336. Depending on the model used, the board becomes more suitable for motor control and

power conversion for the first model, whereas the second is more suitable for general purpose applica-

tions. The devices are both 16-bit Digital Signal Controllers (DSC) and therefore, integrate the control

attributes of a Microcontroller (MCU) with the capabilities of a Digital Signal Processor (DSP)36. The

MCUs are equipped with Input Capture (Ic), Output Compare (Oc) and ADC modules, digital I/O pins,

and UART, SPI, I2C and CAN communication modules. The board provides all the necessary support

for the MCU features: power regulation, motor/actuators drivers, MCU programmer’s interface, I/O pin

interface and drivers for RS232, RS485 and Optical communication via the UART bus. The node is

physically compliant with Eurocard form factor with a square footprint layout of 100mm35. The wide con-

nectivity can be used, apart from internode and node-to-computer communication, to control peripherals

as pressures sensors, relay boards, interface panels, and even devices that require sensor/actuator ca-

19

pabilities.

Figure 2.2: Assembled dsPICnode V3.0 board with a 30F4013 microcontroller.

Microchip provides a complete programing environment with code compilers and several manuals

that allow the user to program the MCU with C programing language under certain specific constrains

due to the MUC own characteristics36. C is a well established programming language that can be used

both for computer and microcontrollers software programing. Therefore, there is a certain added value

of using this programing language37.

Xilinx and Altera development boards, based on field-programmable gate array (FPGA) units, are

also a possible solution. These are configured with hardware description language (HDL) and the

boards have a complete set of interface peripherals38;39. Unlike MCUs, FPGAs allow parallel execu-

tion, thus providing a powerful alternative to the first solution. Manufacturers like Siemens provide high

end solutions as the Siemens SIMATIC PLC control system40 that offers top-level performance, flexibility

and functionality for controlling complex industrial environments41. National Instruments provides hard-

ware solutions for sensing and actuation that are compliant with eachother42, allowing for integration in

a control system43.

Nevertheless, the performance and viability comparison between these options must take into ac-

count the cost of each solution and the desired level of performance for the specific case under study.

From the solutions presented above and taking into account all the mentioned characteristics, the dsPIC-

node V3.0 has the most adequate performance-cost ratio for the development of the control system for

the beam-plasma experiment, therefore being the best choice. It provides all the essential characteris-

tics for RTCs implementation and, comparatively, the cost is lower that the other options. Therefore, it

constitutes a cost-effective process unit for the control system.

20

2.4 Software Solution

The Human-Computer Interface (HCI) requires an environment that allows the human user to interact

with the control system. This can be achieved through a computer terminal. A control system can

be designed using several smaller units, or nodes, distributed through the controlled environment, i.e.

distributed control system31;33, performing specific process tasks and connected to a computer that gives

supervisory control to the user. In this case, having a high level graphical interface is a very powerful tool

since it improves significantly the user experience and allows for a clearer monitoring of the variables

and parameters through virtual control panels. In the market there are several tools and platforms for

this specific propose.

The e-lab44 is a framework for control and operation of remote experiments developed at IST45,

falling in the scope of the Remote Controlled Laboratory (RCL) technologies for Interactive Experi-

ments46. It has been in operation since 200147. At the present, it allows remote users to drive 19

experiments through the internet on a "first-come, first-served" principle (FCFS). The user can control

the experiment parameters and retrieve the experimental results via the same framework. Video feed

is also available so the user can have a more authentic contact with the experimental environment and

set-up. Due to these characteristics, e-lab is a suitable platform for operating interactive or real-time

experiments45;47;46.

Figure 2.3: Complete e-lab framework architecture. The Beam-Plasma apparatus (Experimental Apparatus) is connected to theReC through the control and acquisition hardware (Hardware Controller). Form reference 45.

The framework (Figure 2.3) provides a fully integrated software platform, from the interfacing with

the experiment CODAC hardware to the remote user end. e-lab relies mainly on ReC48, the Remote

experiment Control, which is a software framework in a client-server approach47. It allows the integration

of the services and tools provided by the platform and manages the simultaneous multi-client connection

to a multi-experiment laboratory, being based on standards like Java47;49. ReC50 is divided in three

major units: the Hardware Server, the MultiCastController and the Hardware Client. The Hardware

Sever is responsible for the communication between the hardware drivers and the transmission of data

21

to the MultiCastController. The later is responsible for distinct instantiations and expositions of Hardware

Servers and their access in a controlled, secure and coordinated manner with the client side. Finally,

the HardwareClient manages the communication with the MultiCastController, the integration of different

graphic interfaces for configuration and the presentation of data in a common and coherent interface47.

The graphical user interface (GUI) is also developed with Java and can be specifically adapted for each

experimental apparatus47. On the user end, e-lab can run on any desktop platform since the interface

is made through an application based on Java (Figure 2.4). The framework was designed so it can be

integrated easily with any legacy code or receive upgrades. It also can work on any platform: Unix and

Linux, Solaris, Microsoft Windows and Mac OSX47.

Figure 2.4: Example of an experiment GUI on e-lab framework, "Ondas estacionárias e velociade do som experiment".

Besides e-lab, other possible solutions were taken under analysis. The Experimental Physics and

Industrial Control System (EPICS) is a software platform that comprises a wide variety of tools appro-

priate to the control of large and complex distributed systems51. This platform has been successfully

used in large scientific experiments such as, particle accelerators52 and fusion machines53;54;55. The

control platform uses Client/Server and Publishing/Subscribe techniques to communicate through the

control network. The real-world I/O and local control tasks are performed by the Input/Output Con-

trollers (IOCs), while the Channel Access (CA) protocol makes the data available through the network.

The data is stored in variables named Process Variables (PV). This platform is programed using the

C++ programing language. The system architecture was designed to be reliable, maintainable, easily

upgraded and since 2004 it is possible to run the IOCs on Linux, MS Windows and Mac OSX, among

other operative systems51. EPICS can be connected with other platforms that provide a GUI or the

appropriate tools to build one. A possible solution is the Control System Studio (CSS). This software

platform provides a collection of tools to monitor and operate large scale control systems56. It is based

on Eclipse and resulted from the collaboration between different laboratories and universities56;57.

22

Both Siemens SIMATIC PLC40 and National Instruments LabVIEW58 also provide powerful solutions

for system monitoring, GUI and control but again, due to cost-effectiveness reasons, these options

become unsuitable for this specific purpose. Moreover, these software solutions are adjusted to the

already discarded providers proprietary hardware.

Given the considerations previously stated regarding the hardware and the interface needs for a

distributed control system, the e-lab framework presents itself as the most adequate choice. It pro-

vides powerful control and graphic environment tools and the combination allows to achieve the desired

characteristics for the control system with proven results and expansion potential.

23

Chapter 3

The Beam-Plasma Interaction

Experiment

The Beam-Plasma apparatus upgrade was conducted under the scope of making the machine com-

pliant with the most recent control and data acquisition techniques and, at the same time, preserve and

maintain the original integrity of the setup. It provides an unique perspective over the initial constructions

methods of plasma physics apparatus with and on how to apply new technologies into their operation.

3.1 General Scope

The cross-section schematic can be observed on Figure 3.1. The apparatus is centered on a cylin-

drical steel chamber with seven observation hatches, 64.0±0.1cm long and 4.1±0.1cm inner radius.

There, a low pressure helium gas interacts with a low energy electron beam, creating a low temperature

plasma. The chamber is equipped with two Langmuir probes: one fixed probe and one movable probe.

The fixed probe is placed vertically and fixed to the chamber wall. The fixed probe positioner allows

to adjust the penetration depth into the plasma column. On the other hand, the movable probe has

fixed penetration depth but can be moved across the length of the interaction chamber. The holder of

this probe slides on two horizontal rails, allowing to perform measurements in different positions. The

injection of gas is also made near the left side of this section through an injection nozzle.

Globally, the structure can be divided into five fundamental sections, connected in chain but electri-

cally isolated through Bakelite rings and rubber seals.

On the left, according to the presented schematic, is the electron gun that generates the electron

beam. During operation this section is at an electric potential of -2kV and for this reason it is isolated from

the remaining structure of the experiment. It can be divided into two parts: the inner filament holder and

the shielded cylindrical cup where it is inserted. Between the electron gun and the interaction chamber

lies the electromagnetic resonant cavity, placed inside a cylindrical steel section with the same radius

as the interaction chamber but only 21±0.1 cm long. On the right of the interaction chamber lie two

24

sections. The first holds the pulley system of the movable probe cables and the vacuum measurement

gauge while the second on the far right end, contains the electrostatic energy collector and the turbo-

molecular vacuum pump. Additionally, the latter is directly attached to the apparatus steel support frame.

A set of ten coils, equally displaced and aligned, surround the hole interaction path in order to create an

homogeneous6 magnetic field. Alongside, there is a quadrupole visible on Figure 1.2 which allows to

correct any misalignment between beam and interaction chamber. The described parts are connected

together and stand on a steel bed composed by two rails bolted to the holding structure frame.

The present chapter documents the construction and operation details of the setup as well as the

changes implemented during the upgrade.

3.2 Vacuum System

Achieving high vacuum and maintaining a stable low pressure regime was one of the main challenges

presented by the upgrade process conducted on this apparatus. The original system was upgraded in

the late 90’s and the two oil diffusion pumps were then replaced by a Varian Turbo-V 301 Navigator

turbo-molecular pump with the exhaust connected to an EDWARDS E2M-12 rotary pump59 through a

flexible KF DN40 bellow, 1m long. During the present upgrade, several malfunctions were detected in

this configuration, leading to the implementation of crucial modifications to the vacuum pumping line.

In the first attempts to achieve high vacuum inside the interaction chamber, it was observed that

there were leaks in the vacuum line and the time needed to achieve pressures of 0.05mbar exceeded

three hours. For an apparatus with a inner volume around 15dm3 this time was largely exceeding the

manufacturer specifications for the pump. The causes were initially attributed to the deterioration of

the KF DN40 Vitton O-Rings used in the connections. Throughout the inspection, it was verified that

not only the previous assumption was correct, but also a malfunction behavior was found on the pump.

Figure 3.1: Cross section schematic of the apparatus. Based on reference 6.

25

It was leaking oil through the vacuum inlet valve and, at the same time, was unable to keep a low

pressure regime in the bellow after it was turned off. Equalization with atmospheric pressure occurred

after approximately two minutes. The rotary-bellow-turbo configuration presented two major problems.

In case of a sudden power loss during operation, there was no protection valve between the rotary and

the turbo. In a critical situation this could lead to permanent damages on the turbo as a result from fast

raising pressure over 0.1mbar and rotary oil dispersion into the rotating blades. Moreover, it was not

possible to safely inject air into the vacuum line, thus equalizing the pressure in the interaction chamber

with the atmosphere without manually breaking the connections between the line elements. The ability

to perform this is essential to conduct routine inspections.

Figure 3.2: Vacuum cut and venting valves mounted on a VAR-IAN DS102 rotary pump with a KF DN40 T connection joint. Thebellow leads to the turbo pump.

A performance test was conducted on the ro-

tary pump, leading to the conclusion that the inner

gaskets of the rotors were damaged and that only

with specific replacement parts form the manufac-

turer would be possible rehabilitate the pumping

mechanism. This lead to the option of replacing

the rotary pump with a newer VARIAN DS102 ro-

tary dual-stage pump60. The outlet port was fitted

with a PFEIFFER ONF 16 M exhaust filter to pre-

vent the expulsion of vacuum oil particles to the

surrounding atmosphere while the inlet was con-

nected with a KF40 T connection joint. This al-

lowed to connect two cut valves to the pumping

line: an EDWARDS PV25EKS vacuum cut valve

and an MECMAN 533 venting valve. The first was

placed between the T and the bellow while the

second was connected to the remaining connec-

tion of the T. This configuration allows to isolate

the turbo from the rotary and at the same time

creates a practical solution to fill the apparatus with filtered atmospheric air. The vacuum system con-

nections schematic is presented on Figure 3.3.

A thorough inspection on the apparatus itself revealed the presence of remaining oil traces from the

original diffusion pumps. The reachable inner surfaces were cleaned with isopropil alcohol as well as all

the elements in the pumping line.

The system was reassembled (Figure 3.2) and several pumping cycles were made to evaluate the

effect of these modifications. The pressure line was able to maintain low vacuum for longer periods

and no sudden pressure drops were observed after power down. The pressure regimes attained with

only rotary pumping were as low as 1.8±0.1×10−2mbar and the time necessary to reach this order of

magnitude from atmospheric pressure was reduced to approximately thirty minutes. Using the turbo-

molecular pump to reach high vacuum regimes was largely improved by these modifications, since the

26

lower initial pumping pressure for the turbo allows a larger pumping speed (150l/s)61. Pressures in the

order of 10−6mbar can now be reached in approximately thirty minutes after rotary pumping. It means

that the total time needed to set the apparatus in high vacuum conditions was drastically reduced to one

hour. These modifications successfully corrected the system malfunctions and later inspections have

reported no traces of oil leakages form the rotary after power loss. Moreover, during these events the

vacuum cut valve has correctly isolated and protected the turbo pump.

Figure 3.3: Connection schematic of the valves introduced into the vacuum system.

The introduction of the valves provided yet a supplementary capability. During power loss, the cut

valve isolates the turbo pump from the T connection through which the rotary and the venting valve are

connected, maintaining high vacuum regime inside the chamber for longer periods. Without power, the

turbo remains spinning at high speed since no active braking mode can be activated. At the same time,

the pressure inside the T increases faster than inside the chamber and if the cut valve is reopened the

pressure in the line can raise violently. By maintaining the cut valve closed, air can be injected into

the T via the vent valve, equalizing the T with the atmosphere. Then, the vent is closed and the rotary

is restarted, thus dropping the pressure below the turbo upper limit. Tests have shown that the time

needed to perform this operation is less than 5 minutes which can be justified based on the fact that the

volume of the T is considerably smaller when comparing with the pumping performance of the rotary

pump.

The turbo pump is connected to a VARIAN Turbo-V 301-AG Rack Controller which allows to drive

the pump and provides interface with the CODAC62. The driver was configured so that the pump is

operated at the minimum nominal rotation speed possible, 700Hz or 42kRPM, since it allows to reach

ultimate pressures in the order of 10−6mbar. The control of both rotary and cut valve is made by a power

relay board while the vent valve is actuated directly by the CODAC.

27

3.3 Pressure Monitoring

The pressure inside the the apparatus is presently measured with a PFEIFFER MPT100 combined

gauge head comprising two components: thermal conductivity according to Pirani and cold cathode

tube63. The absolute pressure range largely exceeds the necessary range for the experiments26, going

from 1000 to 5.0±0.1×10−9mbar, and was fitted to the apparatus via a DN40CF flange. The gauge

head electronics provide the possibility to perform measuring cycles each 40ms and to establish com-

munication via RS232 or RS485 protocols, becoming a suitable solution for CODAC integration.

The MPT100 gauge head replaced the previous EDWARDS AIM-X gauge head and controller. This

gauge needed a manufacturer unique controller to perform the conversion and communication interface.

Besides, the pressure range was comprised between 10−2 and 10−9mbar and the controller only allowed

serial communication protocol. These were key factors in the decision of replacing the gauge with a more

recent and compact solution.

Apart from this main gauge there is a supplementary PFEIFFER TPR265 compact Pirani gauge

head64 with KF DN16 fitting, connected to a RVC300 controller of the same manufacturer65. The mea-

suring range goes from 1000 to 5×10−4mbar. Although it is not connected to CODAC, it was funda-

mental to initially corroborate the measurements made during the implementation of the communication

protocol between the MPT100 head and the CODAC. This probe was also used as a reference to deter-

mine the working status of the previous gauge. The implementation of the communication protocol via

RS485 is discussed ahead.

Although the gauge head has a permanent magnetic unit for cold cathode operation, due to the

proximity (15cm) between the fitting port of the gauge and the confinement coils it was crucial to de-

termine if the induced magnetic field had any influence over the pressure readings. For pressures

of 2±0.1×10−2 and 1,5±0.1×10−5mbar the current on the coils was varied from 1±0.2 to 12±0.2A

(2,7±0.5 to 32,4±0.5mT in the center of the coils) and no significant changes on the readings were

detected. It was determined that, for this distance and field intensities, the concentration of field lines in

the gauge region is not strong enough to induce faulty readings.

Considerations regarding the gauge response to a pressure step and the calibration factors for the

helium gas are discussed along with the characterization of the gas injection system.

3.4 Gas Injection

The gas injection system presented a barrier to the utilization of the apparatus. It was composed by

a cut valve and a needle valve both manually operated, connected from the pressure regulator to the

entrance of chamber with 4/6mm inner/outer diameter gas tubes. Operation of this setup revealed that

attaining constant pressure inside the interaction chamber by manually actuating the needle valve was

a complex task.

28

Figure 3.4: Gas injection system. The gas line (blue) from thepressure regulator connects to the gas cut valve (right), followsto the proportional valve (center) and enters the interaction cham-ber through the manual security valve (top).

Although the valve was reliable and still in

good condition, frequent corrections needed to

be made to maintain the equilibrium between the

turbo pumping and the amount of gas being in-

jected. The turbo pump has a large pumping

speed when compared to the volume of the ap-

paratus and the needle valve is subject to a large

pressure gradient: 0.3±0.1bar over the atmo-

sphere on the inlet port and 10−6mbar in the out-

let. Also, the length of the tubes used was consid-

erably long making difficult to purge the gas line

of air.

Nevertheless, the configuration of the gas line

components was found to be adequate for the

needs of the experiment. With the upgrade pro-

cess, all elements were replaced with exception

of the manual security valve, fitted to the cham-

ber inlet port (Figure 3.4). Both gas bottle and

pressure regulator used are from AIR LIQUIDE.

According to the supplier, the gas was helium with a purity grade N55, fed to the gas line through a

GA2 pressure regulator. The gas line was implemented with TECALEMIT C98 flexible tube, inner/outer

diameter of 4/6mm and 3.2m long, and the connections with conic cooper fittings. The manual valves

were replaced by two electrically actuated valves: an EDWARDS TAV5 as cut valve, fitted with a FESTO

MSFG-12 12V DC solenoid, followed by an MKS248A proportional valve66, since both allow direct inte-

gration into the CODAC. The proportional valve output was then connected to the manual security valve.

These connections can be observed on the schematic present on Figure 3.5.

Figure 3.5: Schematic of the helium gas injection system.

The choice of the proportional valve was based on its specifications: this model is actuated with a

0 to 5V DC tension, presents good linearity relation between the applied tension and the aperture of

29

the valve and the current on the solenoid does not exceeds 60mA when fully opened. Again, a series

of tests were conducted to verify the correct operation of the valves as well as characterize the overall

of the gas injection process. The impedance of the solenoids was measured with a Digimess 200 RLC

meter and the results are displayed on Table 3.1. These parameters were used in the design of the

respective electronic control modules integrated into the CODAC.

Figure 3.6: Plot of pressure as function of the proportional valve aperture for a gas line pressure of 0.3bar over atmosphericpressure.

In order to calibrate the proportional valve, the gas line was filled with gas and purged using the

rotary pump to remove any air present. Then, the valve was closed, the gas line refilled with a pressure

of 0.3bar over the atmospheric pressure and the rotary was left pumping. The MKS248A presents

a tuneup screw that allows to regulate the leakage of the valve pinhole. By using this mechanism it

became possible to adjust the valve linear working range, thus minimizing the gas leakage and achieving

lower pumping pressures. After the tuneup, the final pressure achieved was of 1.8±0.1×10−2 mbar with

no pressure loss on the gas line. Finally, the turbo pump was started and the screw was re-tuned,

registering final pressures of 8±0.1×10−6 mbar. Notice that the calibration process was done with the

valve placed at its final position and location on the apparatus structure. This was done to ensure that

the process was as precise as possible. In Figure 3.6, the correlation between the aperture of the

proportional valve and the pressure inside the interaction chamber indicates that the pressure range of

10−4mbar is attained for valve apertures between 65% and 70%.

Table 3.1: Resistance and inductance of the gas valves solenoids.

Solenoid MKS248A MSFG-12R (Ω) 90.6±0.1 324±1

L (mH) 445±1 386±1

Integrating a PID control algorithm to manage and maintain a desired pressure level inside apparatus

would be a considerable improvement. To evaluate the possibility of doing so, the vacuum gauge was

submitted to pressure step tests in order do determine lag between actuation on the valve and the

30

sensing of the measuring head. The two most significant cases are present on Figure 3.7 in intervals of

75ms between each acquisition. It can be observed that in the first step with 40 to 70% of valve aperture,

the pressure goes form 2.6±0.1×10−5 to 1.6±0.1×10−2 mbar, crossing the 4.4±0.1×10−3mbar area

marked on the plot as Transition. These series of measurements with same pressure were also observed

in this region on other step tests in similar conditions. This plateau presents itself again when the

turbo pump is started and the pressure drops, crossing this region at similar speed. The cause for

this unexpected phenomena was attributed to the fact that the MPT100 is a combined gauge head

comprising two components, corresponding this plateau to the transition between the Pirani and the

cold cathode, automatically managed by the gauge electronics. On the second step with 10 to 70% of

valve aperture, the pressure goes form 9.3±0.1×10−6 to 2.6±0.1×10−3 mbar and the plateau region is

no longer found. This is supported by both the previous hypothesis and other step tests with the same

aperture limit.

Although it would be possible to account for this plateau in the PID algorithm, after a thorough dis-

cussion it was decided that the implementation would only cover pressures bellow this limit. The desired

pressure range for the experiments is comprised in the 10−4mbar range26 which implies the use of

the turbo pump. Maintaining pressures above 10−3mbar with the turbo pump pumping requires steady

rate injection of large quantities of gas, thus largely increasing the operating costs of the apparatus.

The hypothesis of implementing a secondary algorithm to work with the rotary pump at pressures from

2±0.1×10−2 to 6±0.1×10−2mbar was also considered. The tests carried out have shown that, since

these values are very close to the lower pumping limit of the rotary, it is easier to actuate over the valve

through the CODAC and adjust it to achieve the desired pressure.

Figure 3.7: Step response of the MPT100 gauge to MKS248A valve aperture steps: 40-77% and 10-70% (acquisition intervals of75ms). It is possible to notice the transition between the dual gauge measuring elements (Transition).

By analyzing the 10-70% step response it was possible to determine the delay L between actuation

over the valve and the gauge sensing, as well as the slope R of the raising pressure, used to determine

the PID parameters. The cold cathode element presents a logarithmic relation between the pressure

31

and the voltage measured on the element67: V∝log10p. Since the pressure range for the PID falls into

the working range of this element it is fundamental to take the previous relation into account. Therefore,

the R and L parameters were determined by linear fit with the pressure axis in logarithmic scale. The

plot fitting is present of Figure 3.8 and the derived PID parameters on Table 3.2.

Figure 3.8: Linear fit to determine the pressure variation slope as a response to the valve aperture step as well as the responsedelay. These were used to calculate the PID parameters. The data used is the same as in the 10-70% step of the plot on Figure3.7.

The PID parameters were derived based on the standard Ziegler-Nichols relations31 for a step re-

sponse and consequently, the k1, k2 and k3 parameters of the velocity algorithm (Eq. (2.4)). These are

also present on table 3.2. The units of R were converted to mPa/s in order to simplify the implementation

of the algorithm at hardware level. However, it implies no change from the user point of view.

Table 3.2: Derived PID parameters.

R (mPa/ms) L (ms) Ts (ms)250 375 2000Kp Ti Td

0.0189 750.0 187.5k1 k2 k3

0.0710 -0.0224 0.0018

For this pressure range it was observed that in order to obtain consistent pressure readings was bet-

ter to let the pressure become stable after changing the valve aperture. This settling time was between

one and two seconds, thus justifying the reason for the long Ts time. A different alternative was also

superficially tested. It consisted on using the average of the pressures measured between PID cycles

in order to attain a more reliable pressure value. Preliminary results showed that the pressure oscillated

around the defined set-points without becoming stable. Nevertheless, it would be interesting in the fu-

ture to test the performance of this last technique but with a weighted FILO (First In, Last Out) average

instead. The experimental results obtained with the implementation of the velocity algorithm and using

32

the derived parameters can be observed on Figure 3.9. The curves correspond to the time evolution of

the pressure to the specified set-points as a result of the PID control.

Figure 3.9: Plot of the pressure controlled by the implemented PID algorithm for five different set-points.

The PID was able set the pressure inside the apparatus to the desired set-points. It can be observed

that the final steady-state errors fall within the ±25% band of measurement uncertainty of the gauge

head63. Furthermore, since the time-scales of the apparatus are in the order of minutes, a PID with

settling times in that same order can be considered satisfactory. Additionally, it can be observed that the

settling time increases as the pressure set-points decrease. For higher set-points and considering the

same start pressure, the initial error entering the PID algorithm is larger than for lower set-points, which

given the pressure-valve aperture characteristic (Figure 3.6) accounts for the different settling times.

To set the pressure to values outside the PID operation range, it is still possible to manually operate

the proportional valve through the CODAC interface. Since the signal that actuates over the valve has

a resolution of 0.04±0.001%, the operation can be carried out with very satisfactory results. Specific

aspects regarding the integration of the previous systems into the CODAC are further explained ahead.

3.5 Electron Gun

The electron gun used in this apparatus consists on a hot filament made of tungsten wire with a

diameter of 0.98±0.01mm. According to the manufacturer, the wire is 99.95% tungsten (W) with traces

of molybdenum (Mo). The components that constitute the electron gun are displayed on Figure 3.10.

The tungsten wire is bent with a shape similar to a parabola and the top area with major curvature

is pruned until the width is half the initial. It is done so that the emission of electrons is concentrated

in this area. The filament is mounted on an holder that acts both for support and positioning. The two

33

rods where the filament is bolted are fixed to the base through sliding vacuum seals and a screwed

ring. Rotating the screw makes the filament to go forward and reverse along the rods guides. The

mechanism allows to adjust the position of the filament even when the apparatus is working and also

provides connections for the cooling of the electron gun with distilled water. It is fitted onto the filament

cup (Figure 3.10(c)) that provides shielding to the filament and acceleration to the electrons. The inner

side of the cup is cylindrical with a graphite cone placed at the top end (Figure 3.10(b)). Since the cup

is at the same potential as the filament and holder, the shielding created protects the filament against

bending caused by the confinement coils magnetic field. It is fundamental to guarantee that the filament

tip is properly aligned with the 4±0.1mm diameter pinhole. The cup is flowed by three metallic discs

with openings aligned with the pinhole. The first and second are called grids one and two and the

remaining as plate. These acceleration disks are electrically isolated for the cup with three, axially

displayed ceramic rods and spacers. They have individual connections to the outside of the cup (Figure

3.10(c) and (d)) to allow the application of acceleration potentials. When in operation these are usually

connected to the apparatus ground potential while the filament and cup are at -2kV. Again, since the

walls of the cup are at the same potential, the electrons are channeled through the pinhole, following the

path to the region where the potential is higher. The stream of expelled electrons, accelerated by the

potential and aligned by the disks create the beam of electrons.

Figure 3.10: View of the components that constitute the electron gun: (a) - Tungsten filament placed at the holder; (b) - Inner viewof the cup; (c) - Side view of the filament cup. The acceleration plates are electrically isolated from the cup by three ceramic rods;(d) - Electron gun fitted to the apparatus. The gun is cooled with distillate water in a closed fluid circuit. The cables connected tothe plates and filament are also visible.

The electron gun uses two external power supplies: a FLUKE415B68 which provides the -2kV electric

34

potential at a maximum current of 30mA and a current source that supplies the heating current for the

filament and is capable of providing currents over 50A. The heating current goes through the filament

wire, in vacuum, making it incandescent and capable of emitting electrons. The source is manually

operated and the current values used lie between 25 and 40A. These values, measured on the source

ammeter, were crosschecked with a AC/DC Clamp Meter ST-337. Moreover, the high voltage supply

is connected to the filament through the circuit of the current source. Since the ejected electrons are

at -2kV, the electron current of the beam can be determined by measuring the current that flows from

the high voltage supply to the filament. This is done with a insulated YOKOGAWA YEW 2011 ammeter

placed on the connection between sources.

During the inspection to the electron gun, it was detected that the filament was in need of being

replaced due to oxidation and to the long time of operation is had been submitted to. A new filament

was made replicating the characteristics of the previous one. It was placed in position and consequently

both focus and alignment were re-adjusted. For safety reasons, the electrical connections of the cup

were also rewired and the electrical insulation between disks and cup verified. A through inspection

was conducted on the heating current source and although being more than forty ears old is remains

in perfect condition. The high voltage source was also inspected and only the BNC cable that feds

HV to the filament through the high current source was replaced. At the same time, the electron gun

ground and filament cables were maintained since no malfunction was detected. Besides, no cooling

system was present on the electron gun, therefore a solution based on a plastic water container of 5l,

connected to an EHEIM 1048A submersible pump was implemented. The water circuit is closed so that

the pumped water that goes to the electron gun returns to the container. A Parker DFC flowmeter69,

connected to the CODAC, was introduced between the container and the pump to monitor the flow and

an air filter was fitted to the container for venting purposes. The container was filled with 4±0.1l of

distilled water (<30µS/cm) and the 3/8" plastic flexible tube, 2.2±0.1m total length, was purged. No

leakage or air formation inside the tubes was detected. It is fundamental to monitor the coolant flow in

the electron gun since overheating can permanently damage the filament, the vacuum seals or even the

positioning mechanism. Finally, the cooling system was carefully isolated from the work bench, since

during operation the filament is at a potential of -2kV. Operation tests showed that the cooling system

was working properly with no over heating of neither water container or gun. Also, no sparks were

registered on the gun inner connections and both heating and electron currents were in the expected

range.

Neither of the power sources are at the present controlled by the CODAC, except through the power

relay board, since a future replacement of these units is being taken under account. Although no mal-

functions were detected, neither of the sources allow direct integration on a more advanced operation

system. Moreover, these are sensitive equipments and account for many years of operation. Although

it would be possible to control the sources with the introduction of actuators and sensors, the amount of

effort implied could be in future directed to the design of new units, compliant with the present CODAC

architecture.

35

3.6 Electrostatic Collector

At the right end of the interaction chamber, there is an electrostatic collector aligned with the appara-

tus axis and therefore with the pinhole of the electron gun. The electron beam interacts with the helium

gas across the apparatus length terminating its path at the surface of the collector. On Figure 3.11 is

displayed the head of the collector (a) and the structure that holds the collector in position (b). In the

detail, it is possible to observe that the sensor is constituted by two elements: a central circular disk and

a secondary ring. These two detection areas produce an output signal proportional to the amount of

charge deposited by the beam electrons.

Figure 3.11: Model representation of the electrostatic collector: (a) - Collector head detail; (b) - Collector mounted on the supporttube and top end flange.

Although the collector presents two detection areas these were found to be connected to the same

output BNC connector. During the upgrade process it was not possible to change these connections

since the apparatus did not had another available output to pass the signal, and modifying the existing

support flange could permanently corrupt the ability to achieve high vacuum. Nevertheless the signal is

connected to the CODAC using the single BNC connection. Another problem was detected during the

inspection. The tube that holds the collector in position was excessively long, thus the collector head

was placed into the motion range of the movable probe. This situation was corrected by reducing the

length of the holding tube to 29±0.1cm. The detection surface is now located over the gears of the

movable probe mechanism, preventing the possibility of collision or interfering with the probe motion.

This detector was also used during the experimental process to detect and correct the miss alignment

of the beam.

3.7 Confinement Coils and Quadrupole

The apparatus is equipped with a set of ten coils that provide magnetic confinement to the plasma,

connected in series and equally placed along the apparatus to provide an homogeneous magnetic

field, from the electron gun to the right end of the interaction chamber (Figure 3.12). The coils are

180/350±1mm inner/outer diameter and 70±1mm wide. The intensity of the field created by those is

2,7mT/A at the axis26. An HP 6274B DC power supply, operated as a current source, provides up to

60V and 15A according to manufacturer specifications70. Although the current used does not exceed

6A under normal operation conditions, the power supply was tested up to 12A without any malfunctions

36

being detected. The connections between coils and from the coils to the supply were rewired. The set

has three rods, parallel to the axis and equally spaced, which enable to adjust the spacing between coils

and maintain the alignment with the interaction chamber. The resistance measured on the set of coils

was 5.7±0.1Ω and the inductance 45.6±0.1mH.

Figure 3.12: Rear view of the apparatus. The confinement coils placed form the electron gun to the end of the interaction chamberin order to provide an homogeneous magnetic field across the experiment. It is also possible to observe part of the quadrupole .

The quadrupole consists on two rectangular shaped coils aligned with the beam axis but perpendicu-

lar to the confinement coils (Figure 1.2 and partially on 3.12). They are operated with a LEYBOLD 52155

power supply as current source. The two coils make an angle of approximately 80±5o between them

and are held into position by the metallic frame of the apparatus. During operation, it was determined

that the current applied on the quadrupole to properly align the beam with the chamber axis is 6A, using

only the vertical coil of the quadrupole. This coil presented 2.9±0.1Ω and 6.0±0.1mH.

None of these power supplies allow integration into the CODAC. At the present, both currents have

to be manually adjusted by the user, similarly to those used with the electron gun. For similar reasons,

these units were not replaced during the present upgrade.

3.8 Fixed Langmuir Probe

As previously mentioned, the apparatus is equipped with a vertical fixed Langmuir probe (Figure

3.13) on the top of the interaction chamber. The tungsten wire is 15±0.1mm long and 0.3±0.1mm

diameter. It is attached to a mechanical positioner which allows to regulate the insertion depth. This is

achieved through a vacuum bellow attached to a hollow screw spindle. The probe is connected to the

output BNC through a shielded copper conductor with a configuration similar to a coaxial cable. This

tube is fitted with epoxy seals inside a steel cylindrical jacket to improve rigidity and guarantee optimal

alignment between the probe and the positioner. The jacket, placed inside the bellow (Figure 3.13 (b)),

is vacuum sealed on the bellow-spindle junction with a conic rubber gasket. Since the nut is axially

fixed to the frame, rotating it makes the screw spindle move up or down and consequently changing the

position of the probe. Also, the base of the positioner has a pivot which allows for the change the angle

of the probe while two screw bolts on the sides maintain the desired level inclination. Moreover, the base

37

of the positioner seals the bellow to the port of the chamber with a Viton O-Ring.

Figure 3.13: Fixed Langmuir probe positioner: (a) - Manually adjustable positioner with probe; (b) - Inner view of the inside of thepositioner where the below and probe jacket are visible.

During the inspection process on this device it was found that the mechanism of the positioner was

in excellent working conditions. The probe wire was realigned and all the sealants were checked. It was

verified that the electric contact between the probe shielding and the positioner frame was not adequate

due to the conic gasket and the gap between the jacket and the hollow screw spindle. A copper wire

was added connecting these two elements to ensure that the interaction chamber and probe shield

are at the same potential. Moreover, an extra conductor was introduced to connect these to ground of

the apparatus frame. No leaks were detected when operating the positioner with the chamber in high

vacuum regime.

3.9 Movable Langmuir Probe

Apart from the fixed probe there is also a movable probe with the same dimensions but with no depth

variation. Instead, this probe can be moved across the length of the interaction chamber. On Figure

3.14 (a) it is possible to observe the inner side of the chamber. The probe holder slides on two parallel

rails, held in position at both ends of the chamber. It is fixed to a steel cable under tension which allows

for the movement of the the probe back and forward on the rails. Both ends of the cable are winded on

two separate pulleys, fixed to the motion drive shaft, but on opposite directions. At the left end, there

is also a return sheave placed horizontally. The cable winded to one pulley, goes through the tension

mechanism, across the chamber and is fixed to the probe holder. Then, the cable goes around the return

sheave, crosses back the chamber and is rewind into the other pulley. This means that when the drive

shaft is rotated, one end of the cable is winded while the other is re-winded, thus maintaining the length

of unwinded cable. Rotating the shaft clockwise makes the probe move to the right and vice versa. The

connection of the probe to the outside is assured with a coaxial cable. This is kept under tension by

using a weight which slides inside a steel tube placed vertically. This tube is located under the pulleys of

the traction system. The coaxial goes from the probe directly to the traction mechanism where instead

of being winded, passes over the pulleys and goes down the vertical steel tube. It makes one turn on

the weights sheave and follows back up the tube where the BNC connector is located.

Although this mechanism constitutes one of the most delicate and difficult to access in the hole

apparatus, it was decided that a thorough inspection needed to be conducted which resulted in the

38

Figure 3.14: Movable Langmuir probe: (a) - Inner view of the interaction chamber seen from the electron gun side. The probeholder slides along the chamber over two parallel rails, fixed on both ends of the chamber section; (b) - Movable probe mountedon the sliding support during the process of cable replacement and cable mechanism re-adjustment.

introduction of several modifications. The steel cable was replaced since it presented visible signs of

corrosion and fractures. LINEA EFFE coated steel cable, 0.5±0.1mm diameter and 2±0.01m long,

was employed, followed by the recalibration and alignment of the entire traction system. Alongside, the

coaxial cable was verified to ensure perfect connection between the probe wire and the output BNC plug.

The original motor and position monitoring board were obsolete and could not be efficiently integrated

into the CODAC system. The AC motor was also overpowered for the needs of the apparatus. Moreover,

its gearbox frame was designed for this motor, making it difficult to use with a different unit. Taking into

account the needs of the apparatus, a new motor and gearbox system was designed and implemented

(Figure 3.15 (b)). This new gearbox has a ratio of 400:1 and the motor is operated at 12VDC. Tests have

shown that the implemented solution provides optimal speed for the motion of the probe and keeps the

current on the motor below 200mA. In order to track the position of the probe two sensing devices were

implemented. An HP HEDS 5700 rotating optical encoder71 was attached to the drive shaft (Figure 3.15

(a)), providing a resolution of 512 positions per revolution. After calibration, the relation between probe

displacement and rotation was found to be 0.192±0.001mm per encoder unit. Consequently, the probe

moves inside the chamber at a maximum speed of approximately 4.5mm/s.

Figure 3.15: Traction mechanism of the movable probe system: (a) - View of the section which holds the cable mechanism (inside).The cables are winded on the drive shaft pulleys. Below, there is a steel tube with a weight that maintains the probes coaxial cableunder tension. The position encoder (blue) connected to the drive shaft and the head of the electrostatic collector are also visible;(b) - DC motor and reduction gears connected to the drive shaft.

39

Besides the encoder, a reed relay was placed outside the steel tube where the coaxial cable weight

slides. A small permanent magnet was fitted to the weight so the reed becomes active when in proximity

with the magnet. Reed relays present hysteresis and this characteristic was taken into account during

the calibration process. Since the reed was used to mark the alignment of the two probes, these were

set in position while the reed was placed on the side of the tube and aligned with it. The reed was

moved upwards along the length of the tube until it became activated due to the proximity with the

magnet. Under this configuration, when the movable probe slides to the right, the reed is activated when

the probes become aligned. The alignment of the probes is also used to mark the reference zero of

the interaction chamber. The combination reed-encoder proved to be a safe and accurate method for

determining the position of the movable probe inside the chamber. Besides, it was also determined that

the optimal range for probe sweeping is comprised between positions -800 and +800 on encoder units,

thus 307.4±0.2mm of total range. This interval is justified based on the fact that sending the movable

probe near extreme positions creates unnecessary risks such as misalignment of the cables or even

collision. Moreover, this interval comprises all the relevant physical characteristics which are studied

with the movable probe.

All of the implemented changes were tested under high vacuum and no leaks were detected during

operation. Motor, encoder and reed relay were consequently integrated into the CODAC. These choices

have proven to be reliable and allow to correctly operate the movable probe without any direct contact

with the apparatus.

3.10 Resonant Cavity

The cylindrical resonant cavity is located in the section between the electron gun and the interaction

chamber (Figure 3.16(a)). It is made with a copper hollow cylinder with inner and outer radius of 32 and

40mm respectively. The inner length of the cavity is 50mm and is closed at both ends with two circular

disks of the same material26. Each disk presents a circular hole with 5mm radius so that the beam can

pass through the cavity26.

Figure 3.16: : (a) - Electron gun cup mounted on the section which contains the resonant cavity. On the side of the section it ispossible to observe the port for antenna insertion; (b) - Loop antenna used in the resonant cavity mounted on the positioner.

40

On the sides there are two circular ports aligned with the center of the cavity through where the

antennas are introduced. These single loop antennas are used to stimulate the cavity with a RF signal

on one side and pick the signal that propagates along the wall of the cavity on the opposite. In Figure

3.16(b) it is possible to observe one of the antennas mounted at the respective positioner. Besides

providing support it also has two screw bolts which permit to adjust the insertion depth of the antenna

inside the cavity. A Viton O-Ring is fitted on the positioner, thus creating a vacuum seal on the port-

positioner interface. The positioners have BNC plugs to connect the antennas to the RF electronic

equipment.

The inspection conducted on this section revealed that the resonant cavity was in perfect condition.

Moreover, the vacuum seals were also verified and only the alignment and insertion depth of the loop

antennas needed to be adjusted. This was achieved using the RF equipment described in the next

chapter, resulting in a maximization of the transmitted power at resonance frequency.

41

Chapter 4

The CODAC

4.1 CODAC Architecture and Integration

The CODAC system implemented during the upgrade process can be divided into three major units:

the local-host, the control and the acquisition boards. The system was designed under the scope that

both boards can work and communicate separately with the host (Figure 4.2). Both are integrated into

a 3U 19-inch rack, presented on Figure 4.1 where it is possible to observe the complete set frontal

connections.

Figure 4.1: CODAC integration on a 3U 19-inch Rack. From left to the right: local-host, control board, acquisition board, BNCpanel and turbo pump driver.

The local-host is based on a MSI MS-9832 ITX motherboard72, with a SanDisk 60GB SDD disk, 2GB

DDR2 RAM and a 250W ATX power supply. It runs the CentOS 6.5-x386 Linux operative system73 and

constitutes the central unit of the CODAC. This motherboard provides a compact solution for hosting the

system with all the necessary interfaces. The serial ports allow to establish serial communication with

the two boards via RS232 protocol. Moreover, the Ethernet port enables to make the CODAC available

through the network. This means that the apparatus can be operated either locally, using the host as

a standalone desktop computer, or through the internet. All the usual desktop connections are made

available through the front frame shield of the local-host (left panel). The ATX power supply is also used

42

to supply both control and acquisition boards and consequently all actuators and sensors. The rotating

key switch is used to turn on the power supply. The boards can be independently turned on or off via the

three toggle switches present of the front shield. Thus, starting the local-host does not imply the start of

the boards.

The front panel is similar for both boards: a power indicator LED (red), two press switches (black for

function and red for reset), four indication LEDs (green), one DB9 connector for serial communication

and a RJ11 connector for microcontroller firmware upgrades. These are vertically mounted on the Rack

slots using 3U 6HP aluminum panels. All the connections, with exception of the RF diagnostics, are

connected to the back panel of the boards. Instead, the acquisition inputs for the RF diagnostics are

located on the front panel via a series of eight BNC connectors, linked to the acquisition board back

plane. Finally, at the right, is located the driver of the turbo pump. Bellow this driver, is the auxiliary

vacuum gauge controller. As previously explained, this vacuum gauge is not connected to the CODAC.

Figure 4.2: CODAC integration schematic. Both boards can work and communicate separately with the local-host via RS232communication protocol.

Each board has a specific set of assigned functions and responds directly to the host. In this appara-

tus, the control board performs the following tasks: pressure measurement, control of gas injection and

valves, vacuum pumps, movable probe positioning, electron gun coolant flow and power relay board. It

already has control I/Os for the future implementation of DC-DC converters for confinement coils and

filament. The power relay board is used for both rotary and turbo pumps, water pump, vacuum cut valve,

high voltage and current power supplies. On the other hand, the acquisition board is oriented to acquire

the signals related to the apparatus diagnostics: Resonant cavity transmitted, reflected and incident

signals, interferometry mixed signal and electrostatic collector signal. Besides, it also has a control line

for a secondary relay board to power on/of the RF and diagnostic equipments. The block diagrams for

each board are presented in Figure 4.4.

4.2 Control and Data Acquisition Electronics

Both control and acquisition boards, illustrated on Figure 4.3, are based on dsPICnode V3.0 develop-

ment boards with Microchip dsPIC30F4011 and dsPIC30F4013 microcontrollers, respectively. For each

board it was developed an EUROCARD expansion shield, 100mm wide and 130mm long, containing

the specific control and acquisition electronics . The connection between dsPICnode and shield is made

43

through a DIN96 connector and the power supply via MOLEX connectors located at the shields back

panel. With exception of the fiber optic connectors, all the back panel connections are made with RJ45

connectors.

Figure 4.3: The two CODAC boards: Control board on the left and Acquisition board on the right.

Although both microcontroller models share many similar features, the reason for using different mi-

crocontrollers for control and acquisition is justified based on their specific characteristics. For instance,

the 30F4011 has the Quadrature Encoder module, useful to interface with the movable probe position

encoder. On the other hand, the 30F4013 has 12bit-ADC resolution instead of the 30F4011 10bit-ADC,

becoming a more suitable choice for the acquisition board. Both microcontrollers work at 30MIPS, using

the UART2 to establish serial communication (RS232 driver) with the host at 115200 bit/s. In the at-

tachments it is possible to observe the electric schematics of the dsPICnode V3.0 as well as the shields

developed during the upgrade process.

4.2.1 Control Board

The control board can be described according to the following characteristics:

• Power Supply - Three pin MOLEX connector with Ground, 5VDC and 12VDC;

• Pressure Monitoring - 24VDC power supply (shield DC-DC converter) and serial communication

for the MPT100 vacuum gauge (dsPIC UART1 and dsPICnode RS485 driver);

• Warning Devices - Buzzer to provide warning beeps;

• Power Control - Connection to drive one external power relay constituted by six relays (dsPICnode

ULN2803);

• Flow Monitoring - Connection with the DCF Flowmeter (dsPIC Input Capture) to monitor the elec-

tron gun coolant flow;

• Movable Probe - Control and supply of the motor (two relays), connection for the position encoder

(dsPIC Quadrature Encoder) and reference reed-relay;

44

• Turbo Pump - Interface connection with the turbo pump driver with start/stop signal and rotation

speed measurement (dsPIC ADC);

• Valves - Connection for gas cut/off and venting valves (dsPICnode ULN2803) and PWM signal to

control the opening of the proportional gas valve (dsPIC Output Compare);

• Confinement Field - Connection with PWM signal output (dsPIC Output Compare) and analog

input (dsPIC ADC) for future implementation of the confinement field current power supply;

• Filament Current - Connection with analog input (dsPIC ADC) and PWM optical fiber output (dsPIC

Output Compare and shield HFBR) for future implementation of the filament current power supply;

• Trigger Signal - Optical fiber output (shield HFBR).

Since no filament current power supply was yet implemented and the remaining I/Os of the dsPIC30F4011

have other assigned functions, the optical fiber output (shield HFBR) was used to send the trigger signal

to the acquisition board. The signal pulse is sent each time there is a change in the position encoder

and only during the interaction chamber sweep.

4.2.2 Aquisition Board

The acquisition board can be described according to the following characteristics:

• Power Supply - Four pin MOLEX connector with Ground, -12VDC, 5VDC and 12VDC;

• Warning Devices - Buzzer to provide warning beeps and external 12VDC Bulb connection (dsPIC-

node ULN2803);

• Power Control - Connection to drive one external power relay constituted by six relays (dsPICnode

ULN2803);

• Filament Current - Optical fiber input (dsPIC Input Capture and shield HFBR) for future implemen-

tation of the filament current power supply;

• CH1 - Connection for resonant cavity sweep oscillator out signal (gain 0.91V/V);

• CH2 - Connection for resonant cavity transmitted signal crystal detector output signal (gain -

194.4V/V);

• CH3 - Connection for resonant cavity reflected signal crystal detector output signal (gain -2555.6V/V);

• CH4 - Connection for resonant cavity incident signal crystal detector output signal (gain -100.0V/V);

• CH5 - Connection for interferometry mixer output signal (gain 3.2V/V);

• CH6 - Connection for electrostatic collector output signal (gain 4.7V/V);

Due to the previous considerations regarding the optical fiber output of the control board, the optical

fiber input of the acquisition board is used as the acquisition trigger signal input.

45

Figure 4.4: Bock diagrams for the control and acquisition boards of the CODAC system.

46

4.3 Microcontroller State Machine and Commands

During the present upgrade process, the communication protocol implemented in the control and

acquisition boards was based on the e-lab ReC Generic Diver50. This means that the boards are

compliant with the e-lab middleware, making it possible to further integrate this apparatus into the e-

lab platform. The high level implementation into the platform was not part of this upgrade process.

Nevertheless, it was possible to correctly operate the apparatus using the driver specific commands and

state machine. Moreover, by doing so it has been proven that it is possible to integrate the Beam-Plasma

with the e-lab mainframe installed on the local host of the CODAC.

e-lab works under the scope of pre-configurable experiments. It means that the user configures the

apparatus for a given experimental protocol and the CODAC executes the experiment autonomously.

Due to the high complexity of the apparatus, this topic was taken under serious consideration. In or-

der to provide the maximum flexibility during operation, several protocols are implemented for the same

diagnostic technique. For the specific case of the interferometry, the user can start to calibrate the align-

ment position between probes, then send the probe to the power calibration position and finally sweep

the preset range of the interaction chamber with the movable probe. These operations are performed

autonomously by the microcontroller but their execution is dependent on the user configuration and start

command. Another feature is that machine parameters can be preserved between protocols i.e. rotary

vacuum pump keeps working during interferometry protocol changes. There is a specific protocol (pro-

tocol 0) to actuate over the fundamental apparatus parameters such as gas pressure, vacuum pumps

or power relays. This implementation method aims to preserve operation flexibility and at the same

time ensure compatibility with the e-lab frame work. In Figure 4.5, it is possible to observe the state

machine implemented into the microcontrollers (a) and a communication sample between the user and

the acquisition board (b).

Figure 4.5: ReC State machine 50 implemented into the boards microcontrollers (a) and sample of the acquisition board output(b).

The state machine consists in five fundamental states and eight possible transitions between states.

The default machine state is Stopped. If an Unknown state is reached, the machine will enter the

Stopped (1) or Reseted (C) states. For a given state, the hardware sends the string "IDS \t HWID \t

47

Table 4.1: CODAC Boards Terminal I/O.

General I/OInput Output Observations

stp\n STP\n Stop command recognized;STPOK\n STOPPED state entered.

rst\n RST\n Reset command recognized;RSTOK\n RESETED state entered.

Acquisition Firmware Rev. 2.0.4 - HWID "ELAB_BPLASMA_HW02_V2.0.4"cur\n CUR\t[protocol]\t[Relay1]...

...\t[Relay6]\t[BUZZER]\t[BULB]\nReturns the current status

cfg\t[protocol]\t[Relay1]......\t[Relay6]\t[BUZZER]\t[BULB]\n

CFG\t[protocol]\t[Relay1]......\t[Relay6]\t[BUZZER]\t[BULB]\n

Configure command and param-eters recognized

CFGOK\n CONFIGURED state enteredstr\n STR\n Start command recognized

STROK\n STARTED state entered;DAT\n Data set is beginning;[CH1]\t ...[CH6]\t[pos]\t[vel]\n Data output;... Repeats while trigger signal is

received and timeout or maxi-mum number of acquisitions arenot reached;

END\n Data set has ended.Protocols

0 Change of parameters and no acquisition mode activated.1 Triggered acquisition mode activated with timeout of 180s.

Control Firmware Rev. 5.0.1 - HWID "ELAB_BPLASMA_HW01_V5.0.1"cur\n CUR\t[protocol]\t[OC1RS]\t[VG]\t

[pid]\t[pidstp]\t[turbo]\t[OC2RS]\t[OC3RS]\t[BUZZER]\t[Relay1]...\t[Relay6]\n

Returns the current status.

cfg\t[protocol]\t[OC1RS]\t[VG]\t[pid]\t[pidstp]\t[turbo]\t[OC2RS]\t[OC3RS]\t[BUZZER]\t[Relay1]...\t[Relay6]\n

CFG\t[protocol]\t[OC1RS]\t[VG]\t[pid]\t[pidstp]\t[turbo]\t[OC2RS]\t[OC3RS]\t[BUZZER]\t[Relay1]...\t[Relay6]\n

Configure command and param-eters recognized;

CFGOK\n CONFIGURED state entered.str\n STR\n Start command recognized;

STROK\n STARTED state entered;DAT\n Data set is beginning;[disp]\t[press]\t[OC1RS]\t[OC2RS]\t[OC3RS]\t[adc0]\t[adc1]\t[adc2]\t[flow]\t[POSCNT]\n

Data output;

... Repeats each second whilemaximum number of acquisi-tions is not reached;

END\n Data set has ended.Protocols

0 Change of parameters and no routine activated.1 Reset movable probe to position zero.2 Calibrate movable probe zero position.3 Send movable probe to power calibration position.4 Sweep probe in the preset range and send trigger signal to the

acquisition board. Return to position zero.

48

STATE \n", where HWID is the ID of the hardware unit and STATE refers to the state machine. This

is done in intervals of one second so the user knows the actual state of the machine except when the

experimental data is being sent. The command interfacing can be seen on Figure 4.5 (b). The user sent

commands correspond to the small caption strings, followed by the microcontroller output response. In

case the command is not recognized, no action is taken. The "cfg" command string is followed by the

protocol and experiment parameters: "cfg \t [protocol number] \t [parameter 0] ... \t [parameter n] \n". It

is also possible so send the command "cur" which returns the current status of the hardware. Table 4.1,

shows the commands, parameters and actions for each board. Further details on parameters can be

found in Apendix A.

4.4 RF Equipment

During the upgrade process, two major diagnostic techniques were rehabilitated: resonant cavity and

interferometry. These techniques depend on sensitive RF equipments such as generators and crystal

detectors. One of the major features of this CODAC project is the possibility to acquire signals and op-

erate both diagnostics simultaneously, improving the capability to compare experimental results. Since

all the original RF equipment was in perfect working condition, all the diagnostics were implemented

according to the original specifications.

For the resonant cavity (Figure 4.6) a HP 8620A sweep oscillator74 with 8621B RF section, a HP

777D75 dual directional coupler with two HP 420A crystal detectors76, a HP 423A crystal detector77 and

a GRC 874-D20L stub78 were used.

Figure 4.6: Schematic of connections made in order to implement the resonant cavity diagnostic technique.

The HP 8620A sweep oscillator and 8621B RF section (Figure 4.7) provide a frequency range com-

prised between 1.8 and 4.2GHz making it suitable to cover the 3.6GHz resonance frequency of the

cavity in TM010 mode. Besides providing fine tuning of the central frequency, the generator also allows

to set a frequency window that can be automatically seeped in a preset time base. The generator gives

an output DC signal comprised between 0 and 10.2V, proportional to frequency output into the sweep

range. Thus, by knowing the frequency windows range, it becomes possible to determine the frequency

output of the oscillator at a given time. The 8621B RF section provides the ALC mode which was used

49

to flatten and control the output power level. The output power was set to 4/6 of the scale with ALC INT

mode on. The power was determined with the Boonton 42A Microwattmeter (Figure 4.7) as oscillating

around 1±0.1mW.

Figure 4.7: RF equipment: Boonton 42A Microwattmeter (top left), HP 3200B VHF Oscillator (top right) and HP8620A SweepOscillator (bottom).

The output signal of the sweep generator was connected to the HP 777D dual directional coupler

(Figure 4.8). This device allows to sample at -19.9 dB the signal traveling from the generator to the cavity

and vice-versa. Since the working range is comprised between 1.9 and 4.0GHz, it allows to sample both

incident and reflected waves at the working frequency of the cavity. Moreover, the sampling outputs are

connected to two HP 420A crystal detectors. These provide a DC signal output and consequently allow

to measure the RF power for a given frequency.

Figure 4.8: RF equipment: (a) - dual directional coupler HP 777D with two HP 420A crystal detectors (on top), HP 423A crystaldetector (bottom left) and HP 10514A mixer (bottom right); (b) - Set of two VHF attenuators, HP 355C and HP 355D and two HP8447A RF amplifiers; (c) - Two HP 8447A RF amplifiers connected in series; (d) - RF stub, model GRC 874-D20L.

Between the dual-coupler output and the input of the cavity, a GRC 874-D20L stub (Figure 4.8) was

introduced. This aimed to match the impedance of the load, antenna and cable, to the output of the

dual-directional coupler. It was observed that after tuning the stub properly (length of 10.5±0.5cm), the

50

power delivered to the load was maximized.

In order to pick the output signal of the cavity a HP 423A crystal detector (Figure 4.8) is employed. An

attempt to introduce a second GRC 874-D20L stub into this path was made but no significant advantages

were observed. Moreover, it as observed that the signal would be less intense. It was tested in the

position of the first stub and it was concluded that the second stub was not working properly causing

high insertion losses.

For the interferometry technique (Figure 4.9) a HP 3200B VHF oscillator79 (Figure 4.7), a 50Ω T-

junction, two VHF attenuators, one RF mixer and two VHF amplifiers were used. The VHF generator

allows to manually tune the output frequency from 10MHz to a maximum of 500MHz. This is adequate for

the technique since the frequency range used is comprised between 50 and 250MHz. The signal output

connector in the back of the device also allows to regulate the power level via a piston-type attenuator.

The output signal is divided using the T-junction, connecting one output to the attenuators and the other

to the fixed Langmuir probe. Both fixed and movable Langmuir probes are used as antennas.

Figure 4.9: Schematic of connections made in order to implement the interferometry diagnostic technique.

The signal injected into the plasma column is picked by the movable probe. Due to the low signal

strength, two HP 8447A RF amplifiers80 (Figure 4.7) were connected in series to provide a total gain

of 40dB so the signal power would became in the order of hundreds of micro Watt. These provide a

working range comprised between DC and 400MHz80.

Finally, the HP 10514A mixer81 (Figure 4.8) was used to mix the signal picked by the movable probe

with the one injected into the plasma. Since the two signals must have power intensities as equal

as possible, the signal from the generator must be attenuated. This was done through two variable

attenuators connected in series (Figure 4.7), the first scaled in steps of 10dB (HP355D VHF Attenuator,

0-120dB) and the second in steps of 1dB (HP355C VHF Attenuator, 0-12dB). The power of the signals

entering the mixer must never exceed the limit of 5mW or the device will become permanently damaged.

As the phase variations induced by the propagation along the cables and the RF equipment are constant,

the mixing of the two signals allows to determine the phase shift induced by the plasma. The movable

probe signal was connected to the R input of the mixer while the attenuated signal was connected to

the L input. The output was then connected to the CODAC. It is critical to adjust the attenuation each

time the frequency of the generator is changed during the execution of the interferometry technique.

51

Moreover, the power calibration must not be done with the two probes aligned. Tests have demonstrated

that the most suitable position is 7±0.2mm off alignment to the left of the fixed probe.

Besides these equipments a HP 85688 1.5GHz Spectrum Analyzer82, a HP54100A/D 1GHz Digi-

tizing Oscilloscope83 and a Tektronix TDS 2024C 200MHz Oscilloscope were also used. Prior to any

measurements, all the RF equipments were turned on for at least one hour. This was done to ensure

that the output was as stable as possible. The connections between RF equipment, apparatus and CO-

DAC are established using 50Ω RG-58 coaxial cables. These are further explained in the Attachments

section.

52

Chapter 5

Experimental Results and Discussion

This section refers to the experimental results attained with the resonant cavity and interferometry

diagnostic techniques. The most representative results for both techniques are addressed separately,

followed by a final discussion regarding both techniques. Moreover, the ability to use the CODAC to

perform the experiments is also addressed.

5.1 Resonant Cavity

In order to perform the resonant cavity technique, the sweep generator central frequency was tuned

for the resonance frequency 3.588GHz. This was done with the apparatus in high vacuum, thus with no

electron beam or gas injection. Then, a sweep window of 40±2MHz was set for the oscillator to sweep

around the central frequency. The power was set to 1±0.1mW with the generator ALC mode activated.

Figure 5.1: Resonant cavity in vacuum at 2.1×10−5mbar: transmitted, reflected and incident signal.

Figure 5.1 presents the intensity of the transmitted, reflected and incident signals, under the previ-

53

ous experimental conditions. It is possible to observe a transmission maximum peak centered in the

resonance frequency, coincident with a minimum in the reflected signal. This is according to what was

expected since for a resonant cavity, a minimum in the reflected signal at the cavity input port corre-

sponds to a maximum in the transmission across the cavity, detected in the output port. Looking in detail

at both signals, it can be noticed that they are not perfectly symmetrical around the central frequency as

it could be ideally expected. The reason for this deformation is justified mainly by the variations in the

oscillator output power, thus in the intensity of the cavity incident signal. For instance, for frequencies

above 30MHz, the power drop in incident signal causes a clear drop in both transmitted and reflected

signals. To obtain the expected symmetry, the output power of the generator should be as constant as

possible. Several attempts to achieve a flat power output were made but with no significant improve-

ments on the results. Nevertheless, these results allow the determination of the central frequency (f0)

as well as the width (W) of the peak. These values, determined without plasma and with the cavity in

high vacuum, were used as reference to determine the frequency shift induced by the presence of the

column of plasma in the center of the cavity.

Figure 5.2: Resonant cavity transmitted signal for vacuum (2.1±0.1×10−5mbar) and for different gas pressures with an electroncurrent of 18mA and confinement field of 10.8±0.5mT (4±0.2A).

The current of the electron beam was set to 18±0.5mA with a confinement field of 10.8±0.5mT, thus

4±0.2A in the coils, and the helium pressure was varied from 0.01 to 0.05mbar. During this experimental

process, the configuration of the sweep oscillator and stub were not changed. The attained transmission

peaks for these conditions are present on figure 5.2, including the reference peak. The peaks were fitted

(Lorentz) and the determined values are presented on table 5.1. The table also contains the frequency

shifts between peaks and reference (∆f) and the calculated electron densities (ne). Electron density was

calculated using expression (1.26).

The values derived for the electron density are in the order of 109cm−3 as expected26. It is also

observable that maintaining the remaining parameters constant, an increase in pressure corresponds to

an increase in plasma density. The correlation between pressure and plasma density can be observed

54

Table 5.1: Peak Fit results for Figure 5.2. Electron beam current of 18±0.5mA and confinement field of 10.8±0.5mT (4A).

p (mbar) f0 (MHz) W (MHz) ∆f (MHz) ne (cm−3) Q2.1±0.1×10−5 18.831±0.004 3.22±0.01 — — 11141.0±0.1×10−2 18.906±0.004 3.22±0.01 0.075±0.008 7.6±0.8×107 11142.0±0.1×10−2 20.443±0.004 3.21±0.01 1.612±0.008 1.6±0.8×109 11183.0±0.1×10−2 22.072±0.005 3.19±0.01 3.241±0.009 3.3±0.9×109 11264.0±0.1×10−2 23.566±0.005 3.14±0.01 4.735±0.009 4.8±0.9×109 11445.0±0.1×10−2 25.753±0.005 3.12±0.02 6.922±0.009 7.0±0.9×109 1152

in Figure 5.3. Linear correlation can be observed for pressure values above 0.01mbar. On the other

hand, the pressure magnitude at which the experiment was conducted is two orders of magnitude above

expected, 10−2mbar instead of 10−4mbar26;6. This discrepancy in pressure values is addressed ahead.

Figure 5.3: Correlation plot between pressure and electron density determined with the resonant cavity technique.

The quality factor of the cavity in vacuum was determined as the ratio between the resonance fre-

quency and the peak half height width: Q=f0/W0=1114. Theoretically and according to expression (1.25),

the quality factor of the resonant cavity is approximately 17900 for a surface resistance RS of 15.63mΩ26.

The difference can be justified based on the fact that the theoretical formula assumes a cavity without

losses, which cause broadening of the resonance peak, resulting in a reduction on the quality factor26.

The two holes is the cavity tops also cause a reduction in the quality factor in 5.6%26, due to the for-

mation of fringing fields84. In the apparatus, the cavity is not isolated but instead integrated into a more

complex system such as in a wave guide - cavity in transmission. The connections between the waveg-

uide elements - interaction chamber, electron gun section, loop antennas - create load for the cavity and

consequently induce a drop in the quality factor (quality factor under load)84;26. Also, the connection

between the cooper cylinder and the tops creates additional Joule losses.

Nevertheless, the expected value for the experimental quality factor should be approximately 200026,

thus in the same order of magnitude of the determined value. Moreover, it can be also observed that

the quality factor increases with the increase in electron density. This is also as expected due to the fact

that the central part of the cavity was filled with a plasma column, thus with ε < ε026.

55

5.2 Interferometry

To perform the interferometry diagnostic technique, several configurations of confinement field and

electron beam current were tested. For a confinement field of 10.8±0.5mT, the threshold beam current

was found to lie between 10 and 12mA for pressures in the order of 10−2mbar. The lower limit pressure

was of 1.0±0.1×10−2mbar. The pressure range should be 10−4mbar26, thus two orders below. At this

pressure, no conclusive results were found and the experiment was conducted under the previous condi-

tions. The most expressive results here represented were attained for a pressure of 3.0±0.1×10−2mbar,

electron beam current of 18±0.5mA and confinement field of 10.8±0.5mT (4A).

Figure 5.4: Plasma E(x) patterns for three specific frequencies in the sweeped range.

In this technique, the wave number k is determined by imposing fixed values of frequency to the

system. The frequency was varied between 70 and 200MHz and the E(x) patterns were registered

using the CODAC (Figure 5.4). For each measurement, the power level of the signals entering the mixer

was calibrated using the microwattmeter and attenuators. The wavelengths were determined to the left

and to the right side of the fixed probe by measuring the distance between points with the same phase.

For frequencies above 190MHz, it was observed that the E(x) pattern did not exhibited any measurable

wavelengths. This is due to the fact that the frequency imposed by the VHF oscillator is close to the

plasma frequency, thus in the limit where wave propagation passes from real to evanescent26. Moreover,

it was also observed that as the imposed frequency increases, the wavelength decreases until no phase

variation is observed and evanescent propagation is established (amplitude variation26). The difficulty

in determining fpe by changing the imposed frequency results from the presence of the electron beam

which affects the plasma dispersion diagram26. The presence of the electron beam was disregarded in

this analysis but it was still possible to conclude that the plasma frequency should lie between 200 and

220MHz. For frequencies bellow 70MHz the wavelength is larger that the sweep range of the movable

probe making it impossible to perform any usable measurements.

The most adequate method for determining the plasma frequency consists on fitting the pairs (f,k)

with the expression for the dispersion relation (1.18), thus reconstruct the diagram for dispersion relation

of the plasma for frequencies below fpe. This was done for values determined to the left (superscript

56

Figure 5.5: Plasma dispersion relation for an electron beam current of 18mA.

-) and to the right side (superscript +) of the fixed probe, separately. By using expression (1.1) to

calculate the cyclotron frequency (302±14MHz), it was possible to retrieve the plasma frequency and

the transverse wave number p for both situations. These results are displayed on Table 5.2, along with

the calculated plasma density and plasma column radius. The density was calculated using expression

(1.1) and the radius of the plasma column with a=2.405/p.

Figure 5.6: Plasma transverse wave number for an electron beam current of 18±0.5mA.

On figure 5.6 is presented the variation of the parameter p with the frequency imposed to the plasma,

attained by replacing the experimental pairs into the dispersion relation and using the empirical value of

plasma frequency (200MHz). These values were averaged and presented on the last column of Table

5.2. It is possible to observe that the values from both methods are in the same order of magnitude,

with differences covered by the errors. The radius of the plasma column in also in the expected order

of magnitude, corroborating the hypothesis that the interaction chamber is not completely filled with the

plasma column - partially filled cylindrical waveguide. During the experimental trial it was difficult to

visually determine the actual radius of the plasma column. Nevertheless, this value was estimated in

approximately two centimeters, thus comparable with those from the dispersion equation fit. Moreover,

57

it was also observed that the visible radius of the plasma column was not constant along the plasma

column. This was attributed to the fact that the confinement field not being completely homogeneous

across the full length of the interaction chamber.

Table 5.2: Fit results for plasma dispersion relation from Figure 5.5, calculated parameters and comparison with average transversewave number from Figure 5.6. Electron beam current of 18±0.5mA.

k(f) Fit Calculated Parameters p(f)fpe (MHz) p (cm−1) a (mm) ne (cm−3) p (cm−1)

k− (left) 202±4 0.90±0.05 27±1 5.1±0.2×108 0.9±0.6k (average) 200±1 0.91±0.03 26.4±0.9 4.96±0.05×108 0.9±0.5k+ (right) 194±2 0.84±0.04 29±1 4.7±0.1×108 0.9±0.5

Using the (f,k) pairs it was also possible to build the plots for phase and group velocity presented

on Figures 5.7 and 5.8, respectively. As expected, both phase and group velocity decrease when the

imposed frequency approaches the plasma frequency. Disregarding frequencies below 120MHz due

to measurement uncertainty, it can be verified that the phase velocity is larger that the group velocity.

Globally the relation vg < vph < c could be verified, where c in the speed of light in vacuum.

Figure 5.7: Experimental determination of phase velocity for an electron beam current of 18±0.5mA.

Looking at the results displayed on Table 5.2, it is possible to observe that the plasma frequency

determined to the left of the fixed probe is larger than to the right. These results are critical since they

corroborate the hypothesis that the density of the plasma decreases along the plasma column, thus

along the interaction path of the electron beam with the low pressure helium gas26. Moreover, the

plasma order of magnitude for the plasma density is within the expected range of 108cm−3.

It should be also noticed that for frequencies close to the plasma frequency, small variations on

the density induce large variations on k due to the low group velocity at this frequency range26. This

phenomena may justify the discrepancy found between the plasma frequency from the fits and the

expected value. Although the plasma frequency retrieved from the fits was coincident with the empirical

limit of 200MHz, the difference between the actual plasma frequency and this limit was expected to be

higher.

58

Figure 5.8: Experimental determination of group velocity for an electron beam current of 18±0.5mA.

5.3 Overall Assessment

Comparing the density results obtained by both techniques (Table 5.3) for the same experimental

conditions, it is possible to observe that the density of the plasma decreased along the plasma column,

from the electron gun to the electrostatic collector. The plasma is generated by the interaction between

the low density gas and the electron beam, thus it can be expected that the density of the plasma

decreases along the interaction path.

Table 5.3: Comparison between density results determined with resonant cavity and interferometry techniques under thesame experimental conditions: pressure of 3.0±0.1×10−2mbar, electron beam current of 18±0.5mA and confinement field of10.8±0.5mT.

Resonant Cavity Interferometry3.3±0.9×109cm−3 5.0±0.1×108cm−3

Nevertheless, the density values differ in one order of magnitude and the respective errors do not

cover the difference between values. This can be explained based on two main reasons.

In Expression 1.26, used to determine the density via the resonant cavity diagnostic technique, it

was assumed that the diameter of the plasma column inside the cavity (a) was equal to the diameter

of the cavity passing holes. Actually, the plasma column created inside the cavity by the electron beam

and helium gas interaction is not confined to the size of the holes. Instead, the plasma diffuses inside

the cavity, presenting a radius larger than the one considered initially14 (Figure 5.9). The previous

assumption is a reasonable initial approximation since it is not possible to know in advance the radius of

the plasma column. The actual value of the plasma column radius (a’) can be estimated by considering

the density determined by the interferometry technique, the measured frequency shift in the cavity under

the same conditions and the inner radius of the cavity (R). These values were used to numerically solve

Expression (1.26) which returned a plasma column radius a’ of 14.7mm, approximately half of R as it

could be expected14. It can also be observed that a’ is approximately three times the value considered

in the first approximation. Given these considerations, the actual value for the radius must be lower than

14.7mm but significantly higher than 5mm. This correction would lead to cavity density values larger

than in the interferometry but yet in the same order of magnitude.

59

Figure 5.9: Plasma column inside the cylindrical resonant cavity where a is the radius of the passing holes and a’ is the actualradius of column.

The discrepancy can also be explained based on the existence of a pressure gradient inside the

apparatus caused by the vacuum pump. If the helium pressure decreases along the interaction path so

will the density of the plasma leading to the observed results. The same mechanism can be used to

explain the absence of plasma at pressures of 10−4mbar as well as the first ionization of the helium gas.

In order to achieve lower pressures, it is necessary to resort to the turbo-molecular pump. The pump

will largely increase the pressure gradient inside the interaction chamber, making it difficult to create the

plasma.

The results published regarding this apparatus26;6 revealed a different configuration which may par-

tially corroborate the previous discrepancies. The right end section of the apparatus where the turbo-

molecular pump is presently connected was longer, providing a larger inner volume between the pumps

and the interaction chamber. This extra volume could largely decrease the formation of a significant

pressure gradient along the interaction path. Moreover, the turbo-molecular pump, which replaced the

oil diffusion pumps, was designed for a volume of 150L whereas the inner volume of the apparatus is ap-

proximately 15L. Furthermore, under the previous configuration, the vacuum gauge was located directly

above the diffusion pumps which may have induced pressure readings below the actual pressure inside

the interaction chamber. Based on these assumptions, it can be concluded that the pressure gradient

formation is a critical matter to the operation of the apparatus and must be addressed in the future.

Another fundamental issue is to study plasma formation for pressures of 10−3mbar. At lower pressures,

no plasma was detected and for higher pressures the helium gas was clearly in the second ionization

regime. Since the best regime for performing experiments in this apparatus is the first ionization of the

helium26, it is critical to find under which specific conditions this regime appears.

The implementation of the CODAC system in the Beam-Plasma experiment has successfully al-

lowed the rehabilitation of the resonant cavity and interferometry diagnostic techniques. Moreover, the

experimental protocols to study the plasma parameters can now be implemented systematically and

within conditions of reproducibility. Additionally, the experimental results have shown that the apparatus

is working with the expected performance and it can now be used to perform advanced experimental

works on plasma physics.

60

Chapter 6

Conclusions and Future Work

The main goal of the present project was to rehabilitate and upgrade the Beam-Plasma apparatus in

order to make it compliant with the most recent solutions for control and data acquisition so it could be

used for advanced experimental works on plasma physics. This constituted a significant challenge since

the original integrity of the set-up should be preserved. The upgrade conducted on the set-up allowed

to understand and document the mechanisms and techniques used in its construction. Moreover, it pro-

vided the necessary insight to correctly integrate both sensors and actuators, thus to guarantee that the

CODAC would become integral part of the apparatus. The CODAC was designed to allow future expan-

sions and modifications but at the same time making the control and acquisition hardware compliant with

the e-lab platform. Experimental trials conducted on the apparatus showed that both interferometry and

resonant cavity diagnostic techniques are working properly and can be used to determine fundamental

plasma parameters. Also, it became possible to gain insight over the physical mechanisms behind the

techniques and consequently on how the different apparatus parameters affect the plasma.

Despite being operational, the process of completely upgrading this apparatus is not yet concluded.

It is still necessary to develop power supplies for the electron gun filament, confinement coils and

quadrupole that can be integrated into the CODAC. Moreover, it could be interesting to fully automate

the radio frequency equipments in order to make them accessible through the CODAC. Although it was

possible to show that the apparatus can actually be integrated into the e-lab platform, no GUI was de-

veloped at this stage. Equipping the CODAC with this feature would be a considerable improvement to

its usability. An interesting alternative for e-lab integration would be to have one of the CODAC boards

directly connected to the e-lab hardware server while the other is connected through the first one. This

configuration requires only one serial communication port at the hardware server for the hole CODAC

system and, at the same time, preserves all the available features. The gas pressure PID implementa-

tion still requires optimization in order to reduce the set-point settling time. Finally, reducing the pressure

gradient inside the apparatus is critical to conduct experiments at lower pressures, which can possi-

bly be achieved by throttling the connection between the turbo-pump and the apparatus. These future

improvements would certainly extend the experimental scope of the apparatus, giving continuity to the

present upgrade work.

61

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66

Appendices A

Appendices

A Apparatus and CODAC Operation

The CODAC system is designed so it gives the user the capability to control and monitor all the

relevant parameters in the experimental apparatus: pressure, gas injection, turbo-molecular vacuum

pump, movable Langmuir probe and electrostatic collector, among others. The DC-DC hardware units

that produce the filament high-voltage and current as well as the current in the confinement coils and

quadrupole are currently under development and testing, thus these operations are still made manually

with the original equipment. Nonetheless, both control and acquisition boards already have specific I/Os

assigned for the future integration of these modules.

Figure A.1: Schematic of the CODAC hardware front BNC connections panel, used to acquire experimental data from the diag-nostics.

The CODAC hardware front panel is presented in Figure A.1. It is composed by a local-host, a

control board and an acquisition board. The hardware modules are assembled on an 3U 19-inch Rack

crate. The local-host is based on an industrial form factor motherboard running Linux that performs the

interface between the user and the control and data acquisition boards. The front panel also presents a

series of BNC inputs connected to the acquisition board. These allow the connection of RF equipment

and signals from the resonant cavity and interferometry. The BNC connections are described, both on

Figure A.1 and on Table A.1. The origin of each input signal is further explained on the protocols.

67

Table A.1: CODAC BNC Connections.

Channel Connection/Function Gain (V/V)CH1 HP8620A sweep oscillator reference output 0.91CH2 HP423A crystal detector output (cavity transmitted wave) -194.4CH3 HP420A crystal detector output (cavity reflected wave) -2555.6CH4 HP420A crystal detector output (cavity incident wave) -100.0CH5 HP10514 RF mixer output (interferometry) 3.2CH6 Electrostatic collector output 4.7CH7 Not connected (N.C. - Future Upgrade) —CH8 Not connected (N.C. - Future Upgrade) —

Obs: Channel resolution of 1.2mV (12-bit ADCs)

A.1 Console Interface

The user interface is made through two serial text consoles present on the local-host, one for each

board. A reduced list of commands necessary to perform the protocols is presented on Table A.2.

Command application is later explained in detail and related with each stage of setup operation. When

the user sends a command to either one of the consoles, it returns CMD OK or CMD ERROR to inform

ether the command was recognized or not.

A.1.1 Control Console

The control board interface console returns a string, with the parameters enumerated below, at a

constant rate of 250ms. The variable pressure corresponds to the pressure measured inside the inter-

action chamber in mbar units. The remaining variables are integers. The opening of the needle valve

is set by the variable pr. It has a factor of ~0.04%/unit and the usage is explained in detail ahead. VG

and VV correspond to the status of the gas cut and air vent valves, respectively. In this case, zero

means the valve is closed while one means it is open. The variable turbo returns the rotation speed of

the turbo pump (~54.7rpm/unit) while h2oflow returns the coolant flow rate (~0.0798lmin-1/unit). Finally,

position refers to the position of the movable probe inside the interaction chamber. When the movable

and fixed probes are aligned, the variable is zero. If the moveable probe moves in the direction of the

filament, thus to the left of the fixed probe, the variable will have a negative value. Otherwise, if it moves

to the right, thus in the direction of the electrostatic collector it will assume a positive value. The sweep

range goes from -800 to +800 in counter units, relatively to the fixed probe, with a conversion factor of

0.192mm/unit.

//Rev. 4.0.4 (simplified console output)

"[pressure ]\t[pr]\t[VG]\t[VV]\t[turbo]\t[h2oflow ]\t[position ]\n"

A.1.2 Acquisition Console

The acquisition board is usually operated in trigger mode (mode 2). This means that it sweeps all the

channels and prints their respective values each time a trigger signal is sent by the control board. The

output is sent as a string with the following indentation:

68

//Rev. 1.5.0

"[CH1]\t[CH2]\t[CH3]\t[CH4]\t[CH5]\t[CH6]\t[counter ]\t[rate]\t[mode]\n"

All the variables are integers. Since the acquisition in made with a 12-bit ADC with 0 to 5V input

range, the CHx variables can assume values from 0 to 4095. When this mode is activated, the board

sends a sample string with the previous format and the acquisition counter is set to zero. At the end

of each sweep of the movable probe it is necessary to reset the counter so the trigger mode must be

turned off and back on. Notice that each increment on the counter corresponds to a step increment on

the encoder of the movable probe. Again, the conversion factor is 0.192mm per counter unit. This mode

allows for the establishment of a precise relation between each acquisition and the position at which it

was taken.

A.2 Setup Operation

The following sequence of instructions resume the operating procedure of the apparatus:

• Primary vacuum - Turn the rotary vacuum pump on. Depending on the initial pressure of the cham-

ber, the time to achieve pressures of ~2×10-2mbar may vary. If the pump cannot drop the pressure

below ~1mbar, turn the pump off, open the vent valve and wait until the chamber equalizes with

the atmospheric pressure. Close the vent valve and start the pump again. It may be necessary to

repeat this extra step more than once.

• High vacuum - Verify if the pressure in the helium gas line is 0.3bar over the atmospheric pressure

and open the gas cut valve. Leave the needle valve fully closed. When the pressure is below

0.01mbar, start the turbo pump and wait until the pressure is in the order of 10-5mbar.

• Electron gun - With the chamber in high vacuum, turn the water pump on. Verify if the coolant is

flowing and turn the filament current source on. Gradually adjust the current to 25A. The pressure

inside the chamber will rapidly increase due to the outgasing induced by the warmup of the tung-

sten filament. Once the pressure has reached ~10−3mbar, it will gradually decrease again until

the previous high vacuum regime is recovered. During this process, turn the high-voltage power

supply on. Wait for the stand-by pilot light to turn on and only then apply the high voltage. It must

be set to −2kV . Do not change any parameter in the power supply when it is running. The power-

off procedure is done in the reverse way. Turn the apply switch off, wait until the standby light is

on and then turn the supply off. Notice that the current of ejected electrons is directly measured

on the ammeter connected in series with the high-voltage supply and proportional to the heating

current. Do not exceed 35A in the heating current or 20mA in the high-voltage supply.

• Confinement coils - Turn the power supply for the confinement coils on and verify that the current

is set to zero. Adjust the voltage rotary control to half the scale and only then increase the current

to the desired value (operation as current source). Do not provoke sudden changes in the current

or the power source will enter in short circuit. If this happens it is necessary to restart the power

supply.

69

• Quadrupole - The power-supply for the quadrupole is operated in current mode source, similarly

to the confinement coils.

• Gas injection - The injection of Helium gas is done through the needle valve. Refer to Table A.2

for commands. The opening of this valve is proportional to the value of pr, introduced on the

command line. This variable is initially set at 0 meaning that the valve is fully closed. The valve

is fully opened when pr is 2460, although the maximum value allowed for the user to set is 1800,

corresponding to an opening of approximately 73%. This limitation is introduced to avoid flooding

the chamber and consequently damage the turbo pump in case it is working. For the protocols,

the pressure range is achieved as it follows: with the chamber in high vacuum, the filament hot

and the high-voltage applied, open the needle valve to approximately 30% and the pressure will

increase superficially; send the stop turbo-pump command and the pump driver will enter in active

breaking mode; the pressure will rise until the desired pressure regime is attained; proceed to fine

tuning of the pressure value, acting over pr.

• Experiment Standby - To stop the experiment, start by closing both needle and gas cut valves. Turn

off the high-voltage supply as previously explained. Gradually reduce the current on the filament

to zero and then turn the supply off. Proceed in the same way for the confinement coils and

quadrupole power supplies. Turn off any of the peripheral RF equipment and leave both coolant

pump and primary vacuum pumps running.

Table A.2: CODAC Terminal Commands (Short Version).

Control TerminalFunction Command

Air vent valve open: "vnt op←"; close: "vnt cl←"Gas cut valve open: "gas op←"; close: "gas cl←"

Gas needle valve first "pr←", then set the value "XXX←"Turbo pump start: "trb st←"; stop: "gas sp←"

Auto sweep probe "5←"Set probe to power calibration position "6←"

Reset probe to zero "8←"Calibrate probe zero "9←"Stop probe motion ".←"

Acquisition TerminalFunction Command

Trigger mode on: "trg on←"; off: "trg of←"

B Experimental Protocols

In the present configuration of the Beam-Plasma experimental apparatus, the CODAC system and

adjacent RF equipment allows to perform two experimental protocols simultaneously: Resonant Cavity

and Interferometry. This feature largely improves the ability to compare the results attained with both

70

techniques, since it guarantees that the experimental conditions are the same. Due to the optimization of

the experimental process and reproducibility introduced by the CODAC and the upgrade on the machine

specific hardware, from where arises the possibility to maximize the number of conditions tested in each

experimental session. This is a significant enhancement to the potential of the apparatus in the scope

of teaching the underling physical mechanisms of the plasma and the beam-plasma interaction.

B.1 Electromagnetic Cavity Protocol

B.1.1 Objectives

The objective of this work is to determine the shift in the resonance frequency of an electromagnetic

resonant cavity with cylindrical geometry. This cavity works on the TM010 mode, resultant form the

presence of the plasma column along the symmetry axis of the cavity.

B.1.2 Experimental Setup

The resonant cavity, located between the electron gun and the interaction chamber, has an interior

radius R of 32mm and a length L of 50mm. On each top there is a passing hole for the beam with radius

equal to 5mm.

The resonance frequency of the cavity in vacuum is given by the following expression:

fres =c

2π

2.405

R.

For the used cavity dimensions, fres is about 3.6GHz and a sweep oscillator (HP8620A with HP8621B

RF Section) that covers this band must be employed in order to properly stimulate the cavity and sweep

the band around the central frequency. The bandwidth shall be in the order of 10MHz. Using a sweep

oscillator brings the advantage of automatically sweeping the frequency band on a time base defined by

the user. The device provides a reference signal proportional to the output frequency. The later is con-

nected to the CH1 input of the CODAC system, as it allows for the syncing of the power measurements

explained below with the frequency produced by the generator at a given time instant.

Figure A.2: Schematic of connections for resonant cavity diagnostic technique implementation. Observe that the image maymislead in how to correctly alight the loop antennas. The normal plane of the loop must be aligned along θ, thus with the axis ofthe loop perpendicular to z 85.

71

The output of the sweep oscillator is connected to a dual directional coupler (HP777D) that works in

the desired frequency band. This device allows to sample at -20dB the signal traveling from the generator

to the cavity and the reflected wave coming towards the generator. These signals are detected with two

crystal detectors (HP420A) directly attached to the coupler sample outputs. The crystal detectors allow

to monitor and measure the power of both reflected and emitted waves with the variation of the frequency

applied to the cavity. The signals are connected to the CH3 and CH4 inputs, respectively.

A stub (HP874-D20L) is placed between the output of the coupler and the input of the cavity due to the

length of the cables. Devices called "stubs" are terminal short-circuited lines with variable length. These

allow to maximize the power delivered from the generator to the load, in this case the cavity, through

the compensation of the reactive part of the total equivalent impedance of the line. The connections

schematic can be observed on Figure A.2

The TM010 mode is the fundamental mode of this cavity, thus with lower frequency, and it also pro-

vides the possibility to excite the cavity without perturbing the plasma column. Given these characteris-

tics, the cavity is stimulated with a single loop antenna placed near the wall while on the axially opposite

side there is a receiving antenna with the same geometry. The loop produces a magnetic field orthogo-

nal to the loop plane. It propagates parallel to the cavity cylindrical wall surface along θ and the signal

transmitted is picked by the receiving antenna. The power of this signal is detected, as before, with a

crystal detector (HP423A). Again, due to the length of the cable, it may be necessary to introduce a

second stub in the reception transmission line. The output of the detector is connected to the CH2 input.

Figure A.3: Plots of power as function of frequency for emitted, reflected and transmitted signals. In this case, the incident poweris assumed to be ideally constant. As expected, a maximum in the transmitted power coincides with a minimum in the reflectedpower. From reference 85.

B.1.3 Experimental Parameters

In case the two experimental protocols are performed separately, it is crucial to be able to compare

the results obtained with this method and those from interferometry. Therefore, the same parameters

72

must be used for both the protocols. The experimental conditions shall be the following:

Table A.3: Experimental parameters for the cavity protocol.

Seep Generator Power fixed, 1mW (4/6 Scale), ALC INT (Balanced)

Helium Pressure fixed, value between 2 and 4×10-2mbarBeam Potential fixed, -2kVMagnetic Field variable, 8.1, 10.8 and 16.2mT (4, 5, and 6A)

Quadrupole Current fixed, 6A

Use the CODAC system as previously explained to retrieve the experimental pairs PTransmitted(f),

PReflected(f) and PEmitted(f). Notice that the power is proportional to the tension measured in the

crystals, PdB = 10logV0. Consult the manufacturer data-sheets for the parameters. Prior to the first

measurement, the sweep generator must be set to the desired conditions and run for at least 15 minutes

in order to warmup and guarantee that the output is as stable as possible. Remember to retrieve curves

for the cavity in vacuum without plasma to set the resonance peak reference.

B.1.4 Results and Discussion

With the data retrieved with the CODAC system, determine the following parameters:

(i) - Resonance frequency of the cavity without plasma (ω0) and the corresponding quality factor (Q0);

(ii) - The shifts in the resonance frequency and corresponding quality factors, resultant from the

variation of if and IB ;

(iii) - Using the following expression, derive the density of the plasma for each situation:

ne(cm-3) =

(8π2meε0

e2R2

a2J21 (2.405)

J20 (0.375) + J2

1 (0.375)× f0 [MHz] 106

)∆f [MHz] .

(iv) - Plot the curves ne(if ) and ne(B) and comment on the evolution of the parameters according to

the experimental conditions;

(v) - Compare the values of electron density (and plasma frequency) attained with this method with

those form the interferometry technique.

B.2 Determination of Dispersion Relation via Interferometry

B.2.1 Objectives

The objective of this work is to determine the dispersion diagram of the magnetically confined plasma

in the band 0 < ω < ωpe. The diagram is constructed with the determination of several (ω,k) pairs where

ω is imposed to the medium.

The plasma frequency ωpe is determined experimentally in the limit where the propagation of waves in

the medium becomes evanescent and through the fitting of the dispersion relation to the pairs. Ultimately,

the method allows to determine the density of the plasma.

73

B.2.2 Experimental Setup

The experimental determination of a (ω, k) pair is done via injection of a RF signal with a specific

frequency inside the plasma through the fixed Langmuir probe. The signal propagates inside the plasma

and using the movable Langmuir probe it is possible to measure the electrical field along the interaction

length. Since the plasma is this apparatus has a typical density in the order of 1014m-1, the RF generator

used must cover the 0 to 500MHz frequency band (HP3200B VHF Oscillator). The generator has an

output power of about 10mW to guarantee that the signal can properly propagate inside the medium.

The connection diagram can be observed on Figure A.4. The output signal from the VHF Oscilator is

sent to the fixed Langmuir probe through the "pick-off". This device behaves as a directional coupler,

retrieving a sample at -20dB of the traveling signal. This sample is used to provide a reference signal for

the determination of the variation in phase of the signal injected in the plasma.

Figure A.4: Schematic of connections for interferometry diagnostic technique implementation. 85.

Since the fixed probe is immerse into the plasma column, the waves travel in both directions of the

interaction chamber until the formation of a pattern resultant from the multiple reflections at the chamber

ends occurs. Using the movable probe as a receiving antenna it is possible to determine the intensity of

the pattern along the interaction chamber.

From a radiation point of view, the coupling between the plasma and the antennas is small so it is

fundamental to amplify the signal picked by the probe. This is done with two amplifiers in chain with

a 20dB gain each and band of 400MHz (HP8447A). Even with a gain of 40dB, it is still necessary to

attenuate the signal from the input pick-off so the two signals have the similar intensity when entering

the mixer (HP10514A, 500MHz BW). This is done through two variable attenuators connected in series,

the first scaled in steps of 1dB (HP355C VHF Attenuator, 0-12dB) and the second in steps of 10dB

(HP355D VHF Attenuator, 0-120dB). The power of the signals entering the mixer must never exceed the

limit of 5mW or the device will become permanently damaged.

Since the phase variations induced by the propagation along the cables and the RF equipment are

constant, the mixing of the two signals allows to determine the phase variations induced by the plasma.

The output signal of the mixer is the product of the R and L inputs, thus having the form:

S(z) = E[cos(2ωt− kz) + cos(kz)]/2,

74

Figure A.5: Example of the patterns detected in the interferometry protocol. From reference 85.

and is connected to the CH5 input of the acquisition board. The movable probe is operated by the control

board that produces a reference signal used by the acquisition board as an index to sync the position

of the probe with the measured value. It allows to construct the plot of S(z), such as in Figure A.5. It is

possible that for relatively low frequencies, the wave length measured on the pattern is not λ but λ/2, as

a result from the formation of standing waves inside the plasma column.

B.2.3 Experimental Parameters

In order to further compare the results obtained with this method with those from the resonant cavity,

it is crucial to use the same parameters for both the protocols. The experimental conditions shall be the

following:

Table A.4: Experimental parameters for the interferometry protocol.

VHF Generator Frequency variable, start at 50MHz and increment in 10MHz steps

Helium Pressure fixed, value between 2 and 4×10-2mbarBeam Potential fixed, -2kVMagnetic Field variable, 8.1, 10.8 and 16.2mT (4, 5, and 6A)

Quadrupole Current fixed, 6A

Use the CODAC system as previously explained to retrieve the experimental pair VInterferometry(z)

for each sweep of the movable probe inside the interaction chamber. Prior to the first measurement, the

VHF generator must be set to the desired conditions and run for at least 15 minutes in order to warmup

and guarantee that the output is as stable as possible. Remember that for each frequency it is necessary

to tune the attenuation in order that the signals entering in the (R) and (L) inputs of the RF mixer have

the same power magnitude and do not exceed 5mW in any situation. It is not adequate to perform the

calibration when the probes are overlapping on the standby position - use the power calibration position.

B.2.4 Results and Discussion

With the data retrieved with the CODAC system, determine the following parameters:

(i) - For each curve determine the wavelength to the left and to the right of the injection plan;

75

(ii) - For each experimental condition construct the plots k = k(ω) and determine the plasma fre-

quency ωpe, the electron density ne;

(iii) - Determine both phase (vph = ω/k) and group (vg = ∂ω/∂k) velocities and verify if the relation

vg < vph < vf stands, where vf is the velocity of the beam electrons;

(iv) - Using the dispersion relation for the plasma electronic waves,

p2

(1−

ω2pe

ω2 − ω2ce

)+ k2

(1−

ω2pe

ω2

)= 0

calculate the value of p for each (ω, k) pair. Evaluate the variation p(ω) and notice that p = 2, 405/a,

where a is the radius of the plasma column;

(v) - For the most reliable value of p calculated in (iv) and using ωpe from (ii), fit the theoretical curve

to the pairs (ω, k). If necessary, vary these two parameters and tune the curve fitting to the experimental

pairs. Perform another theoretical curve fit to the experimental pairs but without imposing the parameters

p and ωpe. It might be useful to establish plausible boundaries for them in order to accelerate the fitting

process. Compare and comment on the results.

(vi) - Derive the dispersion relation when ω << ωpe << ωce. Fit the resulting expression,

ω

k∼=ωpep

(1 +

ω2pe

ω2ce

)−1/2

to the experimental pairs with lower values of ω. Comment and compare with the previous case;

(vii) - Compare the values of electron density (and plasma frequency) attained with this method with

the results from the resonant cavity technique.

C Technical Details

76

5 5

4 4

3 3

2 2

1 1

DD

CC

BB

AA

Only use with

I2C and only if

needed

MC

LRAN

0/R

B0AN

1/R

B1

UAR

T2_T

X

UAR

T2_R

X

MC

LR

UAR

T2_T

XU

ART2

_RX

RX2

TX2

PGC

/SD

I/SD

A/R

F2PG

D/S

DO

/SC

L/R

F3

MC

LR

CAN

_H

CAN

_L

CAN

_RX

CAN

_TX

CAN

_RX

CAN

_TX

CAN

_RX

CAN

_TX

U1A

RX

INT0

U1A

TX

U1A

RX

U1A

TX

AN2/

RB2

AN3/

RB3

AN4/

RB4

AN5/

RB5

AN6/

RB6

AN7/

RB7

AN8/

RB8

INT1

/OC

2/IC

2/R

D1

OC

4/R

D3

PWM

1L/R

E0PW

M1H

/RE1

PWM

2L/R

E2PW

M2H

/RE3

PWM

3L/R

E4PW

M3H

/RE5

SEN

SE

PWM

1L/R

E0

PWM

1H/R

E1

PWM

2H/R

E3

PWM

3L/R

E4

PWM

3H/R

E5

PWM

2L/R

E2

EXT_

Vcc

OC

3/R

D2

OC

1/IC

1/IN

T1/R

D0

SCK1

/RF6

AN7/

RB7

AN8/

RB8

CAN

_RX

CAN

_TX

OC

2

OU

T_1

OU

T_2

OU

T_3

AN0/

RB0

AN1/

RB1

AN2/

RB2

AN3/

RB3

AN4/

RB4

AN5/

RB5

AN6/

RB6

AN7/

RB7

AN8/

RB8

EXT_

Vcc

EXT_

Vcc

OU

T_1

OU

T_2

OU

T_3

CAN

_LC

AN_H

EXT_

Vcc

OPE

N_1

OPE

N_2

OPE

N_3

OPE

N_4

OPE

N_5

OPE

N_6

OPE

N_7

OPE

N_8

OPE

N_1

OPE

N_2

OPE

N_3

OPE

N_4

OPE

N_5

OPE

N_6

OPE

N_7

OPE

N_8

PGC

/SD

I/SD

A/R

F2PG

D/S

DO

/SC

L/R

F3SC

K1/R

F6

U1A

RX

U1A

TXR

S485

_A

CAN

_RX

CAN

_TX

RS4

85_A

RS4

85_B

U1A

RX

U1A

TX

RS4

85_B

UAR

T2_T

X

UAR

T2_R

X

EXT_

Vcc

INT1

/OC

2/IC

2/R

D1

OC

4/R

D3

OC

1/IC

1/IN

T1/R

D0

OC

3/R

D2

OC

4

OC

1O

C3

OC

2

OC

1O

C3

OC

4

PGC

/SD

I/SD

A/R

F2PG

C/S

DI/S

DA/

RF2

PGD

/SD

O/S

CL/

RF3

PGD

/SD

O/S

CL/

RF3

OC

4

OC

3O

C1

OC

2

+5V

+5V

+5V

+5V

+5V

+5V

VCC

_5

+5V

+5V

V_U

nreg

V_U

nreg

4096

mV

4096

mV

+5V

+5V

+5V

+5V

+5V

+5V

+5V_

PWR

V_U

nreg

+5V

+5V+5

V

V_U

nreg

V_U

nreg

+5V

+5V

V_U

nreg

4096

mV

+5V_

PWR

+5V_

PWR

+5V

4096

mV

+5V_

PWR

Title

Size

Doc

umen

t Num

ber

Rev

Dat

e:Sh

eet

of

1 Sc

hem

atic

v3.0

dsPI

Cno

de

C

11

Sund

ay, A

ugus

t 26,

201

2

Title

Size

Doc

umen

t Num

ber

Rev

Dat

e:Sh

eet

of

1 Sc

hem

atic

v3.0

dsPI

Cno

de

C

11

Sund

ay, A

ugus

t 26,

201

2

Title

Size

Doc

umen

t Num

ber

Rev

Dat

e:Sh

eet

of

1 Sc

hem

atic

v3.0

dsPI

Cno

de

C

11

Sund

ay, A

ugus

t 26,

201

2

U7

HFB

R-1

412

U7

HFB

R-1

412

AN

OD

E2

6

CA

THO

DE

3

AN

OD

E1

2

NC

34

NC

25

NC

41

AN

OD

E3

7

NC

18

R4

0R4

0

C17

470n

CAS

E B

C17

470n

CAS

E B

J18

CO

N7

J18

CO

N7

1 2 3 4 5 6 7

R38

120

R38

120

C23

220n

FC

2322

0nF

R32

120

R32

120

C31

100n

C31

100n

R13

470

R13

470

C10

10n

C10

10n

U5

MAX

3227

U5

MAX

3227

RE

AD

Y1

C1+

2

V+

3

C1-

4

C2+

5

C2-

6

V-

7

R1I

N8

R1O

UT

9

_IN

VA

LID

10

T1IN

11

FOR

CE

ON

12

T1O

UT

13

GND14

VCC15

_FO

RC

EO

FF16C

1410

0nC

1410

0n

R44

120

R44

120

R9

0RR9

0R

J13B

DIN

96

J13B

DIN

96

3333

3434

3535

3636

3737

3838

3939

4040

4141

4242

4343

4444

4545

4646

4747

4848

4949

5050

5151

5252

5353

5454

5555

5656

5757

5858

5959

6060

6161

6262

6363

6464

D5

LED

D5

LED

C22

1uF

C22

1uF

C1

10n

C1

10n

J21

CO

N2

J21

CO

N2

12

R47

120

R47

120

TP4

TEST

PO

INT

TP4

TEST

PO

INT

1

R35

120

R35

120

U8

HFB

R-2

412

U8

HFB

R-2

412O

UT

6

GN

D2

3V

CC

2

NC

24

NC

35

NC

11

GN

D1

7N

C4

8

R39

120

R39

120

C11

100n

C11

100n

R33

120

R33

120

C8

100n

C8

100n

P11

DB9

Fem

ale

P11

DB9

Fem

ale

594837261

R21

120

R21

120

R10

120

R10

120

R46

4k7

R46

4k7

R16

100

R16

100

R25

120

R25

120

C19

220p

FC

1922

0pF

R5

0R5

0

R19

160

R19

160

U4 MC

P255

1

U4 MC

P255

1

TXD

1

VSS2

VDD3

RX

D4

Vre

f5

RS

8C

AN

H7

CA

NL

6

SW5

SW D

IP-8

/SM

SW5

SW D

IP-8

/SM

J14

CO

N2

J14

CO

N2

1 2

C24

10nF

C24

10nF

J13A

DIN

96

J13A

DIN

96

11

22

33

44

55

66

77

88

99

1010

1111

1212

1313

1414

1515

1616

1717

1818

1919

2020

2121

2222

2323

2424

2525

2626

2727

2828

2929

3030

3131

3232

9797

9898

C15

100n

C15

100n

J12

RJ1

1

J12

RJ1

1

123456

R40

120

R40

120

TP5

TEST

PO

INT

TP5

TEST

PO

INT

1

U18 LT66

56

U18 LT66

56

GN

D 1

1

GN

D 2

2

NC

_13

VIN

4N

C_2

5V

OU

T6

C2

10n

C2

10n

TP1

TEST

PO

INT

TP1

TEST

PO

INT

1

R1

33k

R1

33k

R28

120

R28

120

J15

CO

N2

J15

CO

N2

1 2

D7

D1N

4148

D7

D1N

4148

Y1 ZTA

Y1 ZTA

C12

100n

C12

100n

R22

120

R22

120

R11

120

R11

120

R15

100

R15

100

C18

47u

CAS

E B

C18

47u

CAS

E B

R41

120

R41

120

SW2

Prog

Sw

itch

SW2

Prog

Sw

itch

C20

220p

FC

2022

0pF

R26

120

R26

120

R6

2k2

R6

2k2

J20

CO

N3

J20

CO

N3

123

R29

120

R29

120

TP6

TEST

PO

INT

TP6

TEST

PO

INT

1

C3

470n

CAS

E B

C3

470n

CAS

E B

D4

LED

D4

LED

D1

DIO

DE

D1

DIO

DE

D8

D1N

4148

D8

D1N

4148

U16 ULN

2803

U16 ULN

2803

IN_1

1

IN_2

2

IN_3

3

IN_4

4

IN_5

5

IN_6

6

IN_7

7

IN_8

8

GN

D9

OP

EN

_118

OP

EN

_217

OP

EN

_811

OP

EN

_712

OP

EN

_613

OP

EN

_514

OP

EN

_415

OP

EN

_316

CO

M10

D2

LED

D2

LED

C16

10n

C16

10n

U19

LM29

40/T

O26

3U

19LM

2940

/TO

263

IN1

OU

T3

GND2GND_PAD4

J13C

DIN

96

J13C

DIN

96

6565

6666

6767

6868

6969

7070

7171

7272

7373

7474

7575

7676

7777

7878

7979

8080

8181

8282

8383

8484

8585

8686

8787

8888

8989

9090

9191

9292

9393

9494

9595

9696

D6

LED

D6

LED

R42

120

R42

120

D3

LED

D3

LED

J16

CO

N2

J16

CO

N2

12

TP2

TEST

PO

INT

TP2

TEST

PO

INT

1

J1PWR

Jac

k

J1PWR

Jac

k

2 31

R2

470

R2

470

U15

SN75

452

U15

SN75

452

1A1

1B2

2A6

2B7

1Y3

2Y5

VC

C8

GND4

C62

100n

C62

100n

SW3

RES

ET

SW3

RES

ET

C5

18p

C5

18p

R36

120

R36

120

C21 100n

C21 100n

R30

120

R30

120

R27

0R25

1%

5W

R27

0R25

1%

5W

R18

160

R18

160

U2

MAX

485

U2

MAX

485

RO

1

RE

2

DE

3

DI

4

GND5

A6

B7

VCC8

R23

120

R23

120

R3

330

R3

330

R7

160

R7

160

R48

0RR48

0R

R12

33k

R12

33k

TP7

TEST

PO

INT

TP7

TEST

PO

INT

1

U10 L623

4

U10 L623

4

OU

T11

IN1

2

EN

13

VS14

GND15

GND26VS2

7E

N3

8IN

39

OU

T310

VR

EF

11

VC

P12

VB

OO

T13

SE

NS

E2

14

GND315

GND416

SE

NS

E1

17

EN

218

IN2

19

OU

T220

C9

10n

C9

10n

J19

CO

N3

J19

CO

N3

123

R20

160

R20

160

C13

100n

C13

100n

R43

120

R43

120

C25

100n

FC

2510

0nF

R17

160

R17

160

R8

0RR8

0R

C26

100u

FC

2610

0uF

C64

100n

C64

100n

R37

120

R37

120

R14

160

R14

160

U1

dsPI

C30

F401

1U

1ds

PIC

30F4

011

MC

LR1

EM

UD

3/A

N0/

VR

EF+

/CN

2/R

B0

2

EM

UC

3/A

N1/

VR

EF-

/CN

3/R

B1

3

AN

2/S

S1/

CN

4/R

B2

4

AN

3/IN

DX

/CN

5/R

B3

5

AN

4/Q

EA

/IC7/

CN

6/R

B4

6

AN

5/Q

EB

/IC8/

CN

7/R

B5

7

AN

6/O

CFA

/RB

68

AN

7/R

B7

9

AN

8/R

B8

10

VD

D11

VS

S12

OS

C1/

CLK

I13

OS

C2/

CLK

O/R

C15

14

EM

UD

1/S

OS

CI/T

2CK

/U1A

TX/C

N1/

RC

1315

EM

UC

1/S

OS

CO

/T1C

K/U

1AR

X/C

N0/

RC

1416

FLTA

/INT0

/RE

817

EM

UD

2/O

C2/

IC2/

INT2

/RD

118

OC

4/R

D3

19

VS

S_1

20V

DD

_121

OC

3/R

D2

22E

MU

C2/

OC

1/IC

1/IN

T1/R

D0

23S

CK

1/R

F624

PG

D/E

MU

D/U

1TX

/SD

O1/

SC

L/R

F325

PG

C/E

MU

C/U

1RX

/SD

I1/S

DA

/RF2

26U

2TX

/CN

18/R

F527

U2R

X/C

N17

/RF4

28C

1TX

/RF1

29C

1RX

/RF0

30V

SS

_231

VD

D_2

32P

WM

3H/R

E5

33P

WM

3L/R

E4

34P

WM

2H/R

E3

35P

WM

2L/R

E2

36P

WM

1H/R

E1

37P

WM

1L/R

E0

38A

VS

S39

AV

DD

40

R31

120

R31

120

L2 CH

OKE

L2 CH

OKE

R34

120

R34

120

SW4

SWIT

CH

SW4

SWIT

CH

TP3

TEST

PO

INT

TP3

TEST

PO

INT

1

J17

CO

N2

J17

CO

N2

12

U17

LM29

40/T

O26

3U

17LM

2940

/TO

263

IN1

OU

T3

GND2GND_PAD4

C4

47u

CAS

E B

C4

47u

CAS

E B

D9

DIO

DE

D9

DIO

DE

C7

4.7u

CAS

E B

C7

4.7u

CAS

E B

C6

18p

C6

18p

R45

4k7

R45

4k7

J22

CO

N8B

J22

CO

N8B

1 2 3 4

5 6 7 8

C63

100n

C63

100n

R24

120

R24

120

1N41

48

2N3904100 1000uF 25V

10k

10k

1k

1k

470n

HFBR-1522

330

44520-0001 44520-0001

44520-0001 44520-0001

44520-0001 44520-0001

44520-0001 44520-0001

F/C

M04

P

1k

MSTBA3

560560

1N41

481N

4148

100u

50V

1N4148

1N4148

GND

DG20-7011-35-1012

DG20-7011-35-1012

GND

2N3904

2N3904

2N3904

2N3904

68

FABC96R FABC96R FABC96R

3302N3904

3302N3904

3302N3904

D1

T1

R3

C1

R2

R1

R5

R4

C2

XT1

2

1

R9

X1A1A1-1A1-2A1-3A1-4A1-5A1-6A1-7A1-8

X1A2A2-1A2-2A2-3A2-4A2-5A2-6A2-7A2-8

X1B1B1-1B1-2B1-3B1-4B1-5B1-6B1-7B1-8

X1B2B2-1B2-2B2-3B2-4B2-5B2-6B2-7B2-8

X1C1C1-1C1-2C1-3C1-4C1-5C1-6C1-7C1-8

X1C2C2-1C2-2C2-3C2-4C2-5C2-6C2-7C2-8

X1D1D1-1D1-2D1-3D1-4D1-5D1-6D1-7D1-8

X1D2D2-1D2-2D2-3D2-4D2-5D2-6D2-7D2-8

SG

1

-+

R11

X2-1 1

X2-2 2

X2-3 3

R12

R13D

3D

2

C3

D4

D5

U$1

U$1

U$2

U$2

T3

T2

T8

T7

R10

X3-A1X3-A2X3-A3X3-A4X3-A5X3-A6X3-A7X3-A8X3-A9

X3-A10X3-A11X3-A12X3-A13X3-A14X3-A15X3-A16X3-A17X3-A18X3-A19X3-A20X3-A21X3-A22X3-A23X3-A24X3-A25X3-A26X3-A27X3-A28X3-A29X3-A30X3-A31X3-A32

X3-B1X3-B2X3-B3X3-B4X3-B5X3-B6X3-B7X3-B8X3-B9

X3-B10X3-B11X3-B12X3-B13X3-B14X3-B15X3-B16X3-B17X3-B18X3-B19X3-B20X3-B21X3-B22X3-B23X3-B24X3-B25X3-B26X3-B27X3-B28X3-B29X3-B30X3-B31X3-B32

X3-C1X3-C2X3-C3X3-C4X3-C5X3-C6X3-C7X3-C8X3-C9

X3-C10X3-C11X3-C12X3-C13X3-C14X3-C15X3-C16X3-C17X3-C18X3-C19X3-C20X3-C21X3-C22X3-C23X3-C24X3-C25X3-C26X3-C27X3-C28X3-C29X3-C30X3-C31X3-C32

R8

T6

R7

T5

R6

T4

JP1123

JP2123

JP3123

GNDGNDGNDGNDGNDGNDGNDGNDGNDGND

GNDGNDGNDGND

GND

GNDGND

GNDGNDGNDGNDGND

GNDGND

GNDGND

GND

GNDGND

GND

GND

GND

GND

GND

GND

GND

GND

GND

GND

GND

GND

GND

GND

GND

GND

GND

GND

GND

GND

GND GND

GND GND

GND

GN

D

GND

GND

+12V+12V

+12V

+12V

+12V +12V

+12V

+12V +12V

+12V +12V

+12V

+12V

+12V

+12V+12V

+12V +12V +12V

+5V

+5V

+5V

+5V

+5V

+5V

+5V

+5V +5V

+5V

+5V

+5V

+5V

+5V

VG

VG

VG

VV

VV

VV

+24V

+24V

+24V

MT_A

MT_A

MT_A

MT_B

MT_B

MT_B

MT_CW

MT_CW

MT_ACW

MT_ACW

BUCKI_PWM

BUCKI_PWM

BUZZER

BUZZER

VP

VP

VP_PWM

VP_PWM

ADC_RPM

ADC_RPM

MPT_A

MPT_A

MPT_A

MPT_B

MPT_B

MPT_B

BUCKQ_PWM

BUCKQ_PWM

BUCKB_PWM

BUCKB_PWM

R1

R1

R2

R2

R3

R3

R4

R4

R5

R5

R6

R6 ADC_BUCKB

ADC_BUCKB

ADC_BUCKQ

ADC_BUCKQ

FLOW_H2O

FLOW_H2O

ENCODER_AENCODER_A ENCODER_BENCODER_B

TURBO_SW

TURBO_SW

REED_SW

REED_SW

TRB

TRB

RPM

RPM

BUCKB_VCC

BUCKB_VCC

BUCKQ_VCC

BUCKQ_VCC

BUCKQ

BUCKQ

BUCKB

BUCKB

+

LL

Tx

+

exp1

Sheet: 1/1

A

B

C

D

E

F

1 2 3 4

A

B

C

D

E

F

1 2 3 4

Beam-Plasma Control Board Eurocard Expansion Rev1.0 Pedro Lourenço, IPFN, MAY 2015

Pin

Functio

nsUsed

ICIN

OUT

Conn

ector

Row

Pin

Functio

n

1MCLR

2EM

UD3

/AN0/VR

EF+/CN

2/RB

0AN

0J13

C65

ADCRP

M

3EM

UC3

/AN1/VR

EF/CN3/RB

1AN

1J13

C66

ADCBU

CKB

4AN

2/SS1/CN

4/RB

2AN

2J13

C67

ADCBU

CKI

5AN

3/INDX

/CN5/RB

3CN

5J13

C68

Flow

H20

6AN

4/QEA

/IC7

/CN6/RB

4QEA

J13

C69

ENCO

DER_

A

7AN

5/QEB

/IC8

/CN7/RB

5QEB

J13

C70

ENCO

DER_

B

8AN

6/OCFA/RB

6RB

6TurboSW

9AN

7/RB

7RB

7Re

lay3

10AN

8/RB

8RB

8ULN

2803

IN_6

OPEN_6

J13

C88

Relay4

11VD

D

12VS

S

13OSC1/CLKIN

14OSC2/CLKO

/RC1

5

15EM

UD1

/SOSCI/T2CK

/U1A

TX/CN1/RC

13U1A

TRX

MAX

485

DI///

J13

C93

RS48

5_A

16EM

UC1

/SOSCO/T1C

K/U1A

RX/CN0/RC

14U1A

RXMAX

485

RO///

J13

C94

RS48

5_B

17FLTA

/INT0/RE8

RE8

Reed

18EM

UD2

/OC2

/IC2

/INT2/RD1

OC2

ULN

2803

IN_7

OPEN_7

J13

C89

PWM

Buck

B

19OC4

/RD3

RD3

ULN

2803

IN_8

OPEN_8

J13

C90

Relay6

20VS

S

21VD

D

22OC3

/RD2

OC3

ULN

2803

IN_4

OPEN_4

J13

C86

PWM

Buck

I

23EM

UC2

/OC1

/IC1

/INT1/RD0

OC1

ULN

2803

IN_3

OPEN_3

J13

C85

PWM

VP

24SCK1

/RF6

RF6

J13

C82

Relay2

25PG

D/EM

UD/U1TX/SD

O1/SCL/RF3

RF3

J13

C81

Buzzer

26PG

C/EM

UC/U1R

X/SD

I1/SDA

/RF2

RF2

J13

C80

40A_

EN

27U2TX/CN

18/RF5

28U2R

X/CN

17/RF4

29C1

TX/RF1

C1TX

MAX

485

DE///

J13

C93

RS48

5_A

30C1

RX/RF0

C1RX

MAX

485

RE///

J13

C94

RS48

5_B

31VS

S

32VD

D

33PW

M3H

/RE5

RE5

J18

7R e

lay5

34PW

M3L/RE4

RE4

J18

6Re

lay1

35PW

M2H

/RE3

RE3

J18

5VV

36PW

M2L/RE2

RE2

J18

4VG

37PW

M1H

/RE1

RE1

J18

3Motor

ACW

38PW

M1L/RE0

RE0

J18

2Motor

CW

39AV

SS

40AV

DD

dsPIC3

0F40

11dsPICn

odeV3

.0

1H2

OFlow

+5V

1MPT10

0+24V

2H2

OFlow

Sig

2MPT10

0GN

D

3H2

OFlow

GND

3RS48

5A

4N.C.

4RS48

5B

5N.C.

5EN

CCH

A

6N.C.

6EN

CCH

B

7N.C.

7EN

C+5V

8N.C.

8EN

CGN

D

1VP

+1

RB+12V

N.A

.

2VP

GND

2RB

+12V

N.A

.

3N.C.

3R1

N.A

.

4N.C.

4R2

N.A

.

5GA

S+

5R3

N.A

.

6GA

SGN

D6

R4N.A

.

7VN

T+

7R5

N.A

.

8VN

TGN

D8

R6N.A

.

1TR

BSW

1+

2T R

BSW

1GN

D

3TR

BSW

2+

4TR

BSW

2GN

D

5RP

M+

6RP

MGN

D

7PW

R+

8PW

RGN

D

1Re

edGN

D

2Re

edSIG

3N.C.

4N.C.

1GN

D

5MTA

2+5V

6MTB

3+12V

7MTA

8MTB

1BU

CKB+12V

N.A

.N.C.

Not

Conn

ected/used

2BU

CKBGN

DN.A

.N.A

.Co

lour

Not

assig

nedyet

3N.C.

N.A

.

4BU

CKBPW

M+

N.A

.

5BU

CKBPW

MN.A

.

6N.C.

N.A

.

7BU

CKBAD

C+

N.A

.

8BU

CKBAD

CN.A

.

1BU

CKI+5V

N.A

.

2BU

CKIG

ND

N.A

.

3N.C.

N.A

.

4N.C.

N.A

.

5BU

CKI_EN

N.A

.

6N.C.

N.A

.

7BU

CKIA

DC+

N.A

.

8BU

CKIA

DCN.A

.

RJ45

RJ45

PWR

1 64 532

6(X1B

2)7(X1C

2)8(X1D

2)

1(X1A

1)2(X1B

1)3(X1C

1)4(X1D

1)

8

5(X1A

2)

7

Con

trol

Boa

rd C

onne

ctio

ns R

ev. 1

.0

FABC96R FABC96R FABC96R

RJ45-6L-B

RJ45-6L-B

RJ45-6L-B

MSTBA4

LM358N

LM358N

LM358N

LM358N

LM358N

LM358N

10k

9.1k

560

560k

10k

100k33k

470n

1k

470k

1k

100k

10k

120k

22k

10k

220n

100k

470k

100n

HFBR-2521

100n120

F/TM

B120

120

X1-A1X1-A2X1-A3X1-A4X1-A5X1-A6X1-A7X1-A8X1-A9

X1-A10X1-A11X1-A12X1-A13X1-A14X1-A15X1-A16X1-A17X1-A18X1-A19X1-A20X1-A21X1-A22X1-A23X1-A24X1-A25X1-A26X1-A27X1-A28X1-A29X1-A30X1-A31X1-A32

X1-B1X1-B2X1-B3X1-B4X1-B5X1-B6X1-B7X1-B8X1-B9

X1-B10X1-B11X1-B12X1-B13X1-B14X1-B15X1-B16X1-B17X1-B18X1-B19X1-B20X1-B21X1-B22X1-B23X1-B24X1-B25X1-B26X1-B27X1-B28X1-B29X1-B30X1-B31X1-B32

X1-C1X1-C2X1-C3X1-C4X1-C5X1-C6X1-C7X1-C8X1-C9

X1-C10X1-C11X1-C12X1-C13X1-C14X1-C15X1-C16X1-C17X1-C18X1-C19X1-C20X1-C21X1-C22X1-C23X1-C24X1-C25X1-C26X1-C27X1-C28X1-C29X1-C30X1-C31X1-C32

X212345678

X312345678

X412345678

X5-1 1

X5-2 2

X5-3 3

X5-4 4

IC1A

2

31 IC1B

6

57

IC2A

2

31

IC2B

6

57

IC3A

2

31

IC4A

2

31

R1

R2

R3

R4

R5

R6R7C

1

R8

R9

R10

R11

R12

R13

R14

R15

C2

R16

R17

C3

XR1VCC3

RL4

VO1

GND2C4

R18

SG

1

-+ JP11

2

R19

R20

GNDGNDGNDGNDGNDGNDGNDGNDGNDGND

GNDGNDGNDGND

GND

GNDGND

GNDGNDGNDGNDGND

GNDGND

GNDGND

GND

GNDGND

GND

GND

GND

GND

GND

GND

GND

GND

GND

GND

GND

GND

GND

GND

GND

GND

GND

GND GND

GND

GND

GND

+12V+12V

+12V +12V +12V

+12V

+12V+12V

+12V

+12V

BUZZER

BUZZER

BULB

BULB

ADC1

ADC1

R1

R1

R2

R2

R3R3R4R4

R5

R5

R6

R6

ADC2

ADC2

ADC3

ADC3

ADC4

ADC4

ADC5

ADC5

ADC6

ADC6

CH1

CH1

CH2

CH2

CH3

CH3

CH4

CH4

CH5

CH5

CH6

CH6

+5V

+5V

+5V

-12V

-12V

BUCKI

BUCKI

exp2

Sheet: 1/1

A

B

C

D

E

F

1 2 3 4

A

B

C

D

E

F

1 2 3 4

LL

Rx

Beam-Plasma Acquisition Board Eurocard Expansion Rev1.0 Pedro Lourenço, IPFN, MAY 2015

Pin

Functio

nsUsed

ICIN

OUT

Conn

ector

Row

Pin

Functio

n

1MCLR

2AN

0/VR

EF+/CN

2/RB

0AN

0J13

C65

ADCVS

W

3AN

1/VR

EF/CN3/RB

1AN

1J13

C66

ADCCA

V

4AN

2/SS1/LVDIN/CN4/RB

2AN

2J13

C67

ADCRF1

5AN

3/CN

5/RB

3AN

3J13

C68

ADCRF2

6AN

4/IC7/CN

6/RB

4AN

4J13

C69

ADCINT

7AN

5/IC8/CN

7/RB

5AN

5J13

C70

ADCCO

L

8PG

C/EM

UC/AN

6/OCFA/RB

6J13

C71

EXTR

A_1

9PG

D/EM

UD/AN

7/RB

7J13

C72

EXTR

A_2

10AN

8/RB

8AN

8ULN

2803

IN_6

OPEN_6

J13

C88

EXTR

A_3

11VD

D

12VS

S

13OSC1/CLKI

14OSC2/CLKO

/RC1

5

15EM

UD1

/SOSCI/T2CK

/U1A

TX/CN1/RC

13

16EM

UC1

/SOSCO/T1C

K/U1A

RX/CN0/RC

14J13

C92

FILCu

rren

t

17INT0/RA1

1

18IC2/INT2/RD9

RD9

ULN

2803

IN_7

OPEN_7

J13

C89

Relay5

19OC4

/RD3

RD3

ULN

2803

IN_8

OPEN_8

J13

C90

Relay6

20VS

S

21VD

D

22OC3

/RD2

RD2

ULN

2803

IN_4

OPEN_4

J13

C86

Relay2

23IC1/INT1/RD8

RD8

ULN

2803

IN_3

OPEN_3

J13

C85

Relay1

24EM

UC3

/SCK

1/RF6

J13

C82

25EM

UD3

/U1TX/SD

O1/SCL/RF3

J13

C81

26U1R

X/SD

I1/SDA

/RF2

J13

C80

27U2TX/CN

18/RF5

28U2R

X/CN

17/RF4

29C1

TX/RF1

30C1

RX/RF0

31VS

S

32VD

D

33EM

UD2

/OC2

/RD1

J18

7

34EM

UC2

/OC1

/RD0

J18

6

35AN

12/COFS/RB1

2RB

12J18

5Re

lay4

36AN

11/CSD

O/RB1

1RB

11J18

4Re

lay3

37AN

10/CSD

I/RB

10RB

10J18

3BU

LB

38AN

9/CSCK

/RB9

RB9

J18

2BU

ZZ

39AV

SS

40AV

DD

dsPIC3

0F40

13dsPICn

odeV3

.0

1AD

C02

GND

3AD

C14

GND

5AD

C26

GND

7AD

C38

GND

1AD

C42

GND

3N.C.

4GN

D5

N.C.

6GN

D7

N.C.

8GN

D1

RB+12V

N.A

.2

RB+12V

N.A

.3

R1N.A

.4

R2N.A

.5

R3N.A

.6

R4N.A

.7

R5N.A

.8

R6N.A

.

1GN

D2

12V

3+5V

4+12V

RJ45

1 2 3

PWR

Acq

uisit

ion

Boar

d C

onne

ctio

ns R

ev. 1

.0

Parameter Accepted Range Observations

Relay(n) 0 - 1 Relay(n) on-off

Buzzer 0 - 1 Buzzer on-off

Bulb 0 - 1 Bulb on-off

CH(n) 0 - 4095 12-bit, 5V ADC channel(n)

pos 0 - 1601 Aquisition index referent to probe position

vel 0 - 30 Average number of samples per second

Parameter Accepted Range Observations

VG 0 - 1 Gas cut valve open-close

pid 0 - 1 Gas PID start-stop

pidstp 0 - 200 Gas PID setpoin in mPa

trubo 0 - 1 Turbo pump start-stop

OC1RS 0 - 2460 Gas proportional valve openning 0-100% @ 11.98KHz

OC2RS 0 - 2460 Confinement Field Supply PWM duty cycle 0-100% @ 11.98KHz

OC3RS 0 - 2460 Optical Fiber PWM duty cycle 0-100% @ 11.98KHz

Relay(n) 0 - 1 Relay(n) on-off

Buzzer 0 - 1 Buzzer on-off

disp 1 - 1000 Data index display

press X.XEXX Pressure in mbar

adc(n) 0 - 1023 10-bit, 5V ADC chanel(n)

flow 0 - 20 Flow meter pulses per second

POSCNT -800 - +800 Encoder absolute porsiton referent to probe alignment zero

Relay 1

Relay 2

Relay 3

Relay 4

Relay 5

Relay 6

Relay 1

Relay 2

Relay 3

Relay 4

Relay 5

Relay 6

Spectrum Analyzer

Bontoon Microwattmeter

VHF Oscilator

Sweep Oscilator

Board 2

High-Voltage Power Supply

Filament Current Power Supply and Coolant Pump

Rotary Vacuum Pump

Vacuum Cut Valve

Trubo Vacuum Pump

Confinement Field Power Supply

N. A. - Future imeplementation of Langmuir Probe I-V sweeper

Digitizing Osciloscope

Comunication Parameters SpecificationsAquisition Firmware 2.0.4

Control Firmware 5.0.1

Power Relay Boards ConnectionsBoard 1