Atomic Energy of Canada Limited

THE HIGH CURRENT TEST FACILITY INJECTOR

by

J.H. ORM&OD, M.D. SNEDDEN and J. UNGRIN

Chalk River Nuclear Laboratories

Chalk River, Ontario

November 1972

AECL-4224

THE HIGH CURRENT TEST FACILITY INJECTOR

by

J.H. Ormrod, M.D. Snedden and J. Ungrin

ABSTRACT

The high current test facility is a proton accelerator consisting of a 750

keV dc injector, beam transport system, buncher and a cw Alvarez structure

to accelerate the beam to 3 MeV. The injector and the first leg of the

transport system are described as is initial runup to VA of the design current.

Chalk River Nuclear Laboratories

Chalk River, Ontario

November, 1972

AECL-4224

L'injecteur du HCTF (High Current Test Facility)

par

J.H. Ormrod, M.D. Snedden et J. Ungrin

Résumé

Le HCTF est un accélérateur de protons qui comprend un injecteur en

courant continu de 750 keV, un système de transport pour le faisceau, un

groupeur de particules et un appareil Alvarez cw destiné â accélérer le

faisceau jusqu'à 3 MeV. On décrit le fonctionnement initial, jusqu'à 1% du

courant prévu, de l'injecteur et de la première section du système de

transport.

L'Energie Atomique du Canada, Limitée

Laboratoires Nucléaires de Chalk River

Chalk River, Ontario

Novembre 1972

AECL-4224

TABLE OF CONTENTS

Page

I INTRODUCTION 1

II BUILDING AND FARADAY CAGE 2

III INJECTOR POWER SUPPLY 4

IV HIGH VOLTAGE DOME 6

IV-1 Dome and Supports 6IV-2 Motor-Generator Assembly 9IV-3 Dome and Pit Cooling 10IV-4 Dome Equipment 11

V ACCELERATING COLUMN i4

V-l Introduction 14V-2 r m Optics 16V-3 Source and Pumping Manifold 19V-4 Accelerating Electrodes and Ceramic Vacuum Vessel 20V-5 SF6 Vessel 21

VI BEAM LINE 23

2325252626

VII DATA ACQUISITION AND CONTROL SYSTEM 28

VIII COMMISSION ING AND INITIAL OPERATION 32

VIII-1 Alignment 32VI1I-2 Voltagj Testing and Accelerator Column Conditioning 32VI1I-3 Low Intensity Beam Operation 35VIII-4 Summary 38

ACKNOWLEDGEMENTS 39

REFERENCES 40

APPENDIX I Buckling and Stability Calculations for Dome 42Legs

APPENDIX II Torque and Resonance-Speed Calculations 44for Motor-Generator Drive Shaft

APPENDIX III Focussing by Reduced Current in a Pierce 46Column

BEAM

VI-1VI-2VI-3VI-4VI-5

LINE

QuadrupolesVacuum SystemProfile MonitorSteering MagnetBeam Stop

LIST OF TABLES

Page

I Electrical and Mechanical Specifications of Epoxy-Resin 8Bonded Fiberglass

II High Voltage Dome Communication Channels 13

III Beam Envelope Parameters at Downstream End of 17Accelerating Column

IV Analogue Parameters Monitored by the Raytheon 30MUX/ADC Unit

V Non-Dome Status Signals 31

VI Control Panel Functions 31

VII Control Relays for Haefely Power Supply 31

VIII High Voltage Dome and Beam Stop Pressures 36

IX Radiation Intensities mR/hr for 750 keV 1 mA Beam 36

THE HIGH CURRENT TEST FACILITY INJECTOR

by

* ??. Ormrod, M.D. Snedden and J. Ungrin

I INTRODUCTION

The high current test facility (HCTF) is a high current proton accelerator

consisting of a 750 keV dc injector, beam transport system, buncher and a

cw Alvarez structure to accelerate the beam to 3 MeV. The current

accelerated to this energy is expected to approach 100 mA. Fig. 1 is an

isometric view of the apparatus.

The main purpose in building the HCTF is to explore the problems

associated with the acceleration of high current continuous proton beams. In

any high current accelerator, the greatest effects from space charge, which

can significantly contribute to diluting the beam's phase-space density, occur

at low energies (up to a few MeV). The HCTF is being built to investigate

this energy interval.

The injector power supply is a 750 kV cascade Cockcroft-Walton

generator capable of currents up to 180 mA dc. The ion source is a von

Ardenne duoplasmatron and the accelerating column is designed for a proton

beam of 120 mA. This portion of die apparatus is housed in the Faraday

cage as shown in Fig. 2. The beam from the column passes through a seriej

of quadrupoles, bending magnets and a buncher to the Alvarez

structured) - a 24 cell 268.33 MHz structure with a field gradient of 2

MV/m. The beam is focussed along the Alvarez linac by magnetic

quadrupoles in the drift tubes which have a maximum gradient of 75T/m

and are oriented ++--.

This report describes the injector, beam transport system to the 750 keV

beam stop and initial runup to 1% of design current. It gives the details on

the different components, the criteria used in their design, notes how

successful they have been to date and proposes modifications to improve

their performance.

- 2 -

II BUILDING AND FARADAY CAGE

Plan views of the facility showing the Faraday cage, control room,

mechanical services room, and beam line and experimental areas are shown

in Fig. 2 and 3. Additional adjacent space has been provided in the building

for the assembly of equipment and for an office.

The Faraday cage is a large light-construction vault which provides a well-

defined ground potential plane and a controlled dust-free environment for

the high voltage equipment. The roof and walls of the cage are covered

internally with 1 mm thick galvanized steel to within 1.5 m of the floor. This

metal surface is electrically isolated from the remainder of the building and

serves also as a shield for radiated fie'ds. The cascade rectifier and high

voltage (HV) dome are housed in the Faraday cage together with the high

voltage transformers and the bouncer rectifier (Fig. 3). The temperature and

humidity of the Faraday cage which is designed to accommodate operation

up to 1 MV are maintained at 22 ± 1°C and 45 ± 5% respectively by a

separate air conditioning unit.

Access to the Faraday cage, which is only possible when the high voltage

supply is disabled, is provided by two doors at the lower level (Fig. 3) and

via a drawbridge which permits entry to the HV dome from Room 203. The

door inter-connecting the cage and Room 103 can only be opened from

within the cage and serves as an emergency exit. Two sections of the cage

roof are removable allowing a 3 ton overhead crane to be used for moving

large units of equipment into and out of the Faraday cage and HV dome.

Standard floor trenching has been used within the vault for power and

water supply lines to the rectifier, transformers, and to the pit below the

high-voltage dome which contains the baseplate for the HV dome legs and

the motor of the motor-generator unit providing ac power within the dome.

Observation of the equipment within the cage during operation is possible

through two lead-glass observation windows at grade level (Fig. 2) and also

from a wire mesh observation corridor which runs along the width of the

cage at its base.

A 1.3 meter square aluminum plate, electrically insulated from the cage

wall, is mounted within the cage at the height of the HV dome (Fig. 2). This

surface acts as a capacitive pickup and can be used to observe changes in the

potential of the HV dome.

- 3 -

The 10 kHz motor-generator set, which supplies the HV transformers,together with a portion of the HV control electronics and the dome coolingsystem are located in the mechanical services room adjacent, to the Faradaycage. The main control panel for the HV supply and a computer based dataacquisition system are located in the control room immediately above themechanical services area (Fig. 2).

The 10 x 20 meter experimental hall outside the Faraday cage will beused for the beam line, Alvarez tank and beam stops as shown in Fig. 1 andfor future diagnostic equipment.

- 4 -

111 INJECTOR POWER SUPPLY*

The injector power supply is a 750 kV, 180 inA cascade Cockcroft-Walton generator. Fig. 4 is a schematic of the apparatus. The frequencyconverter (M-G set) is a 500 hp motor, M, driving a 500 V, 600 A, 10 kHzsingle phase generator, G. The excitation current for the generator issupplied by the variable regulated supply E-2. The two step-up transformershave a maximum output voltage of 125 kV. The output voltage of thestandard cascade Cockcroft-Walton circuit (three 250-kV stages) is connectedto the high voltage dome through the damping resistor Rp. Two dampingresistors are used, one of 5MI2 for column conditioning and one of 100 k£2for normal use.

The power supply can be upgraded to 1 MV by adding another 250 kVstage to the rectifier stack. The rectifier dome, transformers and M-G set aredesigned for the upgraded 1 MV facility.

A compensated (ohmic-capacitive) voltage divider chain from the HVdome provides the signal for the feedback loop and the five digit voltmeterV-l. The three 625 Mf2 resistors in this chain are immersed in oil, the oiltemperature is measured by a thermistor-bridge circuit and V-l is correctedfor temperature on the computer log (see section VII).

The reference voltage E-l determines the operating regulated voltage(100-800 kV). The amplifier A drives the bouncer that provides the fastregulation and sets the dc potential at the bottom of the rectifier stack. Thispotential is monitored by mete.' V-2 and is kept near the middle of thebouncer dynamic range (~ 40 kV) by manually adjusting E-2. The currentdrawn from the power supply is displayed on a 3-digit meter I that must becorrected for the current drawn by the bouncer stabilizing loop ~ 1.3 mA(V-2/30 Mfi).

The peak to peak ripple at 750 kV is less than 500 V. Normal stability ismuch better than 0.1%. A 14 mA current transient (obtained by shorting aportion of a water resistor) produced an 800 volt transient lasting 50 jus.

* The injector power supply which includes the two damping resistors andthe compensated voltage divider chain, was fabricated by the Emile HaefelyCompany, Basel. Switzerland.

— 5 —

An intermittent instability giving several kV fluctuations plagued the

early operation of the supply. This problem has been greatly reduced in

frequency of occurrence and amplitude, but periodically appears as an

instability of several hundred volts.

The attenuation factor of the ohmic-capacitive voltage divider falls off

rapidly beyond 7 kHz. When the HV dome arcs down a large transient (up to

10 kV) appears on the input of V-l and eventually damaged the instrument.

The circuitry installed to protect V-l from this transient is shown in Fig. 5.

- 6 -

IV HIGH VOLTAGE DOME

IV-1 Dome and Supports

The ion source and its ancillary equipment operate at potentials up to

750 kV. This equipment is housed in an aluminum enclosure (the HV dome)

that sits atop four fiberglass legs as shown in F;.£. 6. The dome is a 2.4 m

cubical shell with cylindrical edges and spherical corners, all curves having a

radius of 600 mm. The shell was fabricated from 3.2 mm aluminum sheet

rivetted to an internal aluminum frame. A surface finish of approximately 32

rms was maintained on the exterior dome surface to minimize potential

stress concentrations.

The radius of the curved surfaces was chosen on the basis of a 1 M V dome

potential (upgraded facility) and minimum dome to ground plane separation

of 3 m. (The accelerating column side of the dome is closer than this, but the

potential is graded by the column itself.) Using a sphere to planed)

approximation, the maximum voltage gradient is 1.8 kV/mm, well below the

air corona limit of 3.1 kV/mm.

Fig. 7 shows a sectional view of the HV dome assembly. The interior

framing of the dome is attached to the lower 25 mm thick aluminum

platform 450 mm above the bottom of the dome shell. The platform rests on

four fiberglass legs and supports the dome shell, the accelerating column,

two 1000 fi/sec ion pumps, and the damping resistor. A second 25 mm thick

aluminum platform "floats" inside the dome above the lower platform and is

supported by a second set of cylindrical fiberglass legs nested inside the first

set. The purpose of this "floating" floor is to isolate the accelerating column

from vibrations which may disturb the alignment of the column or put

damaging stresses on the glue joints of the lucite-aluminum SF6 vessel.

Vibrations originate from a 412 Hz generator that supplies the ac power to

the dome equipment and from personnel inside the dome. Both sets of

fiberglass legs are rigidly clamped to a steel platform grouted to the floor of

a 1.2 m deep, 2.4 m wide x 3 m long pit in the Faraday cage floor.

Fig. 7 and 8 show the platform in the pit and the method for clamping

the nested fiberglass legs.

- 7 -

The entire dome assembly has been designed for operation up to 1 MV.The pit below floor level houses the leg support structure and drive motorfor the dome ac generator and is large enough that the entire dome assemblycan be moved back to accommodate a 1 MV accelerating column.

The leg lengths were chosen to match the 0.25 kV/mm voltage gradient ofthe Haefely divider chain. Including allowance for support clamping, theouter legs are 4.85 m long while the inner legs, which project above the outerlegs in the dome to allow the service platform to float above the mainplatform, are 5.13 m long. For operation at 750 kV the pit is covered by agrounded aluminum shield (Fig. 7) 750 mm above the cage floor at the levelof the lower end of the Haefely divider chain. For 1 MV operation this shieldwill be replaced by a plate flush with the Faraday cage floor.

The total weight of equipment supported by the legs is about 2300 kg forthe outer set and 1800 kg for the inner set. Using an iterative designprocedure, the dimensions for the legs were chosen as 300 mm (12") O.D.by 267 mm (lOVi") I.D. for the outer set and 241 mm (9V4") O.D. by 178mm (7") I.D. for the inner set. (See Appendix I for these calculations.)

When nested, the inner and outer legs have a wall clearance of 12.7 mmwhich allows room for installation of a bleeder resistor chain in the gap forgrading the potentials of the legs. The 178 mm diameter bore of the inner legsallows room for fibre-optic communication links from the equipment pit tothe dome. The weights of the individual outer and inner legs are 180 kg and140 kg respectively.

Epoxy-resin bonded fiberglass was chosen as the material for the domelegs because of its mechanical strength and electrical insulation properties.The material is also impervious to moisture. Table I lists the specifications.

The legs were wound* on a polished steel mandrel in one continuousoperation to eliminate delaminations in the fiberglass layers which wouldcreate air pockets and potential corona sources. After curing, the outersurfaces were ground smooth to a 32 mis finish or better and painted.Installation of the legs in the Faraday cage is shown in progress in Fig. 9.

* Manufactured by Canadian General Electric (Montreal)

- 8

TABLE I ELECTRICAL AND MECHANICAL SPECIFICATIONS OFEPOXY-RESIN BONDED FIBERGLASS

Electrical: (1) Surface arcover: 14kV/cm

(2) Breakdown through a section: 500-700 V/milperpendicular to tape layers

Mechanical: (1) Flexural strength: 50-100 x 103 psi

(2) Flexural modulus: 2-4 x 106 psi

(3) Tensile strength: 24-60x l0 3 ps i

(4) Density: 0.056-0.065 lb/in3

(5) Temperature limits: 35O-4OO°F

— 9 —

Two sets of aluminum corona rings are attached to the legs (see Fig. 6) at1 meter intervals between the dome and the pit shield and their potentialsare defined by a 3-section resistor bleeder chain. Each section contains aseries string of 642, 3.9 Mfi ± 5%, 1 watt carbon composition resistorsinserted in 6 mm I.D. teflon tubing and spirally wound on the outer surface ofan inner leg. A constant spacing between the turns of the teflon tubing ismaintained by loops of fiberglass tape epoxied to the surface of thefiberglass leg. The total resistance per section is nominally 2500 Megohmswhich gives a bleeder current of IOOJUA for the rated 250 kV per section.Fig. 9 shows the outer leg being lowered over the inner leg that has thebleeder resistor chain wrapped around it. The junctions of the bleeder chainsections are electrically connected through the wall of the outer leg to anexterior aluminum corona ring. In order to avoid altering the capacity of theHaefely divider chain and the current through it which is used to monitorthe HV dome voltage, no contact is made between the two voltage dividerchains at the 250 and 500 kV levels.

IV-2 Motor-Generator Assembly

Power to the equipment in the dome is supplied by a 412 Hz, 120/208V, 3-phase, 4-wire synchronous generator which delivers 25 kW ratedpower at 3530 rpm. The generator is coupled to a 1:6 speed increaser (Fig.10) in the dome and the assembly is vertically mounted with its drive shaftdown. Mechanical drive for the generator-increaser assembly comes from afiberglass insulating drive shaft coupled to a 6:1 speed reducer and inductionmotor assembly vertically mounted in the equipment pit. The inductionmotor is rated at 40 hp at 3530 rpm with 575 V, 3-phase, 60 Hz voltageinput. A 4.27 m long fiberglass shaft (Fig. 7) spans the high voltage interfacebetween the equipment pit and the dome.

The fiberglass drive shaft was made from the same material as the domelegs. It was wound and cured on a specially machined steel mandrel to ensurestraightness over the full length. Steel journals were embedded and pinned inboth ends of the fiberglass tube and the entire assembly was machined totolerances. The shaft was then dynamically balanced at 600 rpm. Fiberglassweights were inserted through a hole in one journal and attached to the innerwall of the shaft with small brass screws. Seventeen ~ 30 gram weights are

- 1 0 -

located 1.13 m from each end and eight similar weights are located at thecenter portion on the inner surface of the tubing. The final dynamicdisplacement reading for the centre whip of the drive shaft at 600 rpm onthe manufacturer's special balancing machine was 0.13 mm. Appendix IIoutlines the calculations on the shaft dimensions.

Once installed in its vertical orientation in the HCTF, the centre-whip waschecked and measured at 0.23 mm at 600 rpm.

IV-3 Dome and Pit Cooling

The speed reducers used in the 25 kW motor-generator set providing acpower to the HV dome are units originally designed for horizontal operation.In this mode lubrication for the gears is provided by their rotation through asmall reservoir of oil. Operated in the vertical position, it is necessary toimmerse the lower portion of the gear units in an oil bath and to provideadditional cooling. This is particularly true for the reducer attached to themotor where the high speed portion (3530 rpm) of the assembly is theimmersed section. An oil pump and feed-water cooled heat exchangercapable of maintaining the oil temperature at less than 40°C have beeninstalled in the equipment pit. Before this cooling was added, eight hours ofcontinuous operation would raise the oil temperature to 120°C.

Cooling within the HV dome is necessary for the speed increaser, thedamping resistor, and the ion-source and power supplies. This is provided byrunning demineralized water ~ 18 Mfi-cm to the dome through two 9.6 mmI.D. tubes placed along one of the outer HV dome legs. Potential grading ofthe tubes is achieved by passing them through the aluminum corona rings.Thick-wall teflon tubing was used initially for the entire system, but wasfound to be too good an insulator allowing charge build-up along the tubingwhich resulted in severe arcdowns and tubing punctures (Fig. 11). Thisproblem was cured by replacing the 750-500 kV section with Saran tubing.The heat exchanger and on-line water purifier for this cooling system arelocated in the mechanical services room (Fig. 3).

In the dome, the cooling water is run through the ion source and its highcurrent supplies, through a copper cooling plate attached to the speedincreaser, and through the heat exchanger which cools the damping resistoroil. Additional cooling for the dome interior is provided by circulatingapproximately one volume change of air per minute through several largescreen-covered holes in the roof and walls.

11 -

IV-4 Dome Equipment

An overhead view of the dome interior taken with the roof removed isshown in Fig. 12. Housed within this 2.4 meter cube are the ion source andpumping manifold, the damping resistor cooling system, a 60 kV extractorelectrode power supply, and the power generator and supplies required todrive these units and to provide remote monitoring and control of them.Apart from the two 1000 2/sec ion pumps and the ion source and pumpingmanifold, all internal dome equipment is mounted on the upper "floating"floor in order to minimize the transmission of vibrations to the acceleratingcolumn.

A block diagram of the major electrical equipment located within the HVdome is shown in Fig. 13. Power is provided by the 412 Hz generatordescribed above. A sensing circuit provides a remotely monitored indicationof the loss of one or more phases of the generator. A model 8006C3 CTSfrequency converter coupled to a model B129D17C Wells rectifier* is usedto provide 1.5 kVA of 115V, 60 Hz single phase power required foroperating several monitoring and control circuits.

Remote control and monitoring of equipment within the dome istransmitted across the HV interface by ten analogue and twelve digitalfibre-optic light links.'Analogue information is transmitted as a pulse widthmodulated signal, making the accuracy independent of optical transmissionlosses. These twenty-two 16.7 meter long fibre-optic bundles are routed toground potential via the resistor-graded interior dome supporting leg andthen follow the Faraday cage floor trenching from the equipment pit toRoom 103 (Fig. 3) where computer-compatible interfacing is located. Aschematic block diagram of the twenty-six channels of communication (seeTable II) provided between the dome and ground potential is shown in Fig.13 and 14. A more complete description of the control system is provided insection VII and is also published elsewhere^).

* Supplied by CTS Canada Ltd., Streetsville, Ontario

t Supplied by The IPAC Group, Chicago, Illinois

- 12

The ion source power supplies are 412 Hz versions of the Ion Source Test

Stand supplies described elsewhere^). Fibre-optic coupled metering circuits

and S!o-Syn motor drives for the input variacs have been added to provide

remote parameter adjustment. The 2.5 kHz peak-to-peak ripple on the arc,

coil, and filament current supplies at maximum current are < 0.02%, < 0.1%

and 147c respectively. A motor-driven needle-valve assembly controls

hydrogen gas flow rate to the ion source. The source pressure is measured

with a thermocouple gauge.

Several units of equipment not directly associated with the source are

located within the dome. The 60 kV negative polarity extractor electrode

power supply is a Model K160-1 OS Kilovolt*unit which is operated at 412

Hz. A Slo-Syn motor has been added for remote adjustment of the voltage as

well as voltage and current monitoring circuits, and status and reset relays.

The 7.5 kV diode and 5.7 kV triode ion pump supplies are units built within

this laboratory as 412 Hz versions of commercially available supplies.

Circuits have been added to allow for remote readings of the pump current,

for alarm triggering when the pumps are tripped out by overcurrent protects,

and for remote resetting of the pump supplies.

Supplied by Kilovolt Corporation, Hackensack, New Jersey

- 1 3 -

TABLE II HIGH VOLTAGE DOME COMMUNICATION CHANNELS

ANALOGUE LEVEL TRANSMISSION

1) Extraction electrode voltage2) Extraction electrode current3) Arc current4) Arc voltage5) Coil current6) Filament current7) Source pressure8) Diode ion pump current9) Triode ion pump current

10) Analogue calibration voltage

STEPPING MOTOR ADJUSTMENTS

1) Extraction electrode voltage2) Arc current3) Coil current4) Filament current5) H2 gas flow

RELA Y RESET ACTIONS

1) Extraction electrode supply2) Diode ion pump3) Triode ion pump4) Arc start circuit5) Arc trip-out circuit

STA TUS MONITORING

1) Generator phase voltages2) Extraction electrode supply3) Diode ion pump supply4) Triode ion pump supply5) Arc variac at limit6) Coil variac at limit

- 14

V ACCELERATING COLUMN

V-l Introduction

The accelerating column is located between the high voltage dome and

the wall of the Faraday cage. To obtain the transition from an acceptable

voltage gradient in air to the high vacuum gradients required, Curtis'(->)

geometry was used with the accelerating gaps re-entrant in a vacuum vessel

which in turn is re-entrant in an SF6 vessel. This system permits easy

access to the source which is open to the air, i.e. not immersed in SF6 .

A cross section of the column is shown in the insert of Fig. 1. The

vacuum vessel and accelerating electrodes are shown in more detail in

Fig. 15.

The column is designed for the space charge equivalent of a 120 mA beam

of protons assuming zero emittance. This current is determined by the

desired accelerated current in the Alvarez structure (65 mA), the anticipated

bunching efficiency^") (~ 80%) and the expected proton percentage in the

beam (~ 15%). The value of 65 mA was the ING study(^) design current; by

using it, much of the ING design work is applicable to the HCTF.

The design of the corona rings both in air and within the SF6 vessel was

governed by the criterion that the maximum electric fields should have a

safety factor of approximately two over the corona limit.

To compensate for space charge divergence of the beam during

acceleration, the Pierce gradient^") is usually used, i.e.

v=/_JL_ / ^ j ' z™ (v-i)

where

V is the applied voltage in volts

eo is the dielectric constant of free space (8.85 x 1CT1 2 farads/m)

m is the mass of the particles in kilograms

e is the charge of the particles in coulombs

j is the current density in amps/m2-

Z is the distance from the emitting surface in meters

- 1 5 -

For a given current, the maximum vacuum gradient is set by the totalvoltage and the emitting area. The voltage of 750 kV was a compromisebetween high voltage reliability in air and difficulties in the first cells of theAlvarez linac. The source diameter of 14 mm was chosen as a realisticmaximum from the experiments of Sluyters et al(9). Substituting thesevalues into V-l yields a maximum vacuum gradient of 4.7 MV/m which wasconsidered high for reliable dc operational^). However, space charge effectsdecrease as the beam energy increases. To reduce the maximum vacuumgradient we have used the Pierce potential distribution up to 200 keV* and auniform gradient of 3.1 MV/m (the average gradient in the last Pierce gap)for the rest of the column. The transition at 200 keV was somewhatarbitrary, but considered a reasonable compromise between the deleteriouseffects on the beam over the non-Pierce section and the maximum electricfield.

The number of accelerating electrodes in the column was chosen to givethe least complicated system that would satisfy two requirements. Thevoltage that can be held across two electrodes varies approximately as thesquare root of the separation^ 1), i.e. it is better to hold a given field in aseries of gaps than a single gap. The choice of 3.1 MV/m and 100 kV gapssatisfies the criterion of no breakdowns(^), although some conditioning isrequired to achieve reliable operation. The second factor in the choice ofnumber of electrodes is the ability of the column to withstand micro-discharges During a microdischarge between two electrodes, the potentialdifference collapses and the other seven gaps in the column have to assume agreater voltage until the transient decays. The distribution of this voltagetransient among the other gaps depends on the inter-electrode capacities andwhere the microdischarge occurs. The gaps have been designed to withstanda 13% voltage transient which assumes a uniform distribution of the extrapotential.

In accelerating a dc beam, beam current intercepted by the acceleratingelectrodes is a much more serious problem than in pulsed operation. Forpulsed beams, the interelectrode capacity is sufficient to absorb quite aserious spill. For example, consider a 100 ixs pulse in our column.Interelectrode capacities are ~ 100 pf and for 1 mA of intercepted current

In describing the accelerating column, the electrodes are defined bypotential (kV) when referenced from ground, and by particle energy.(keV)when referenced from the dome, e.g. the 200 keV electrode is the same asthe 550 kV electrode.

- 1 6 -

IAT icr3 x icr4

AV fj— ~ 1000 V, i.e. a 1% change.

In dc operation, the important parameter is not the interelectrodecapacitance, but the current iri the accelerating column voltage divider chain.This resistor chain sets the potentials on the accelerating electrodes. Thecurrent intercepted by each electrode distorts the field distribution byincreasing the potential drop across all downstream gaps at the expense ofthe potential drop in the upstream gaps. Field distortion in the 100 keVPierce section was considered the most serious problem. To reduce fielddistortion in this section, the first accelerating electrode is powered from aseparate supply (0 -60 kV) in the dome. The electrode acts as a grid and needonly supply the current that it intercepts. It has a beam aperture of 16 mmdiameter and the rest of the column has 20 mm apertures; hence it acts as ashield for at least the adjacent electrode.

The intercepted current is only important if it is significant compared tothe normal current drain in the voltage divider network. The resistor chaindescribed below draws 0.5 mA; if operating experience indicates this is notsufficient, we can use a water resistor and adjust the water conductivity togive an adequate drain.

Backstreaming electron* in the column piv,duce unwanted X-radiation.The electrons are produced from ionization of the gas in the column by thebeam particles, secondaries from the electrodes struck by the beam particlesand electrons generated by these processes beyond the column. To eliminatethis last source, the penultimate electrode is held at a voltage negativerelative to the beam line, which acts as a potential barrier to electronsproduced beyond the column. The potential for this suppressor electrodecomes from a separate power supply which is normally operated at -10 kV.

V-2 Ion Optics

The ion optics of the accelerating column does not lend itself to anelegant solution. The problem is complicated by the finife emittance of thebeam, space charge forces, a mixture of mass components and the initial ionoptics determined by the plasma-beam interface which is notorious for notfulfilling the initial conditions prescribed in the mathematical treatment ofthe problem. It probably is possible to treat the system numerically in aself-consistent manner, but more probably not worth the effort becauseempirical adjustments will inevitably determine the final parameters.

17

Let us first consider space-charge defocussing. We will assume a 120 mAbeam consisting only of protons uniformly distributed across the 14 mmaperture, having zero emittance and extracted from a plane plasma-beaminterface. The first 200 keV of the accelerating column is designed for such abeam and will accelerate it without expansion, the graded field providingfocussing just sufficient to offset the space-charge forces. The beam willexpand in the uniform gradient of the next 500 keV of acceleration. Thebehaviour of the envelope through this section can be solved by a simplecomputer code using Gauss' theorem to determine the radial acceleration ofa peripheral particle

f =I

2mV(V-2)

For the initial conditions of r = 7 mm and r = — = 0, the final values are r =dz

7.6 mm and r = 5.2 mrad.

The dominating influence in the last two gaps of the column is thefocussing action of the varying electric field. We treat the last threeelectrodes as thin lenses with focal lengths given W

E 2 - E ,4V

where eV is the ion's energy, Et and E2 are the axial fields upstream anddownstream of the electrode. Table III lists the lens' parameters, beam radiusand divergence at each electrode.

TABLE III Beam Envelope Parameters at Downstream End of Accelerating

Column

Electrode Ion Upstream Downstream Focal Beam r' r'Number Energy Field Field Length Radius in out

keV x*\//_ »*»//_ „, m m mrad mradMV/m MV/m m mm

89

10

700760750

3.11.9

-0.3

1.9-0.3

0

-2.3-1.410

7.67.98.3

5.28.7

12.8

8.712.812.0

- 1 8 -

Consider next how the actual initial conditions affect the above analysis.A typical measured mass distribution is 70% H,+ , 25% H 2

+ and 5% H3+. Tocorrect for this, m in eq'n V-l must be replaced byO^) meff = (fi + f2>/ 2 +f3 \/~3)2 • In practice, one only has to reduce the current so that I (extracted)

A % x I (design). For the distribution cited above the extracted currentmust be reduced to 105 mA. With this modification, the Pierce conditionsare satisfied.

The beam diameter increases throughout the column because of its finiteemittance. At the plasma-beam interface, assumed plane, the two dimen-sional phase space can be represented by a rectangle in x -vx space with thebase of the rectangle equal to the diameter of the beam and the half-altitudevxmax = fp where

e is the emittance invariant;

c is the velocity of light;

r is the radius of the source.

Ignoring other radial effects, after traversing the column, the rectangleshown in Fig. 16-a has sheared into the parallelogram shown in Fig. 16-bwhere Ax = vxmax.T, and r is the column transit time = 55 nsec. For anemittance of .32 ir cm mrad(^), Ax ~ 5 mm !! If the satellites in theemittance pattern^-*) c a n be eliminated, the emittance is reducedapproximately 10-fold and Ax is reduced proportionately.

Again, a reduction in current can compensate for this divergence in thePierce section of the column (see Appendix III). The net focussing for an ionon the beam edge produced by a reduction of 10 mA produces an inwarddirected velocity equal to v x m a x in Fig. 16 for an emittance of .32 n cmmrad. The current reduction required is proportional to the beam emittance.Because there is no focussing through the uniform gradient section of thecolumn, reducing the current will not compensate; the beam expansion isagain proportional to the transit time = 20 ns, i.e. ~ 2 mm for an emittanceof .32 7r cm mrad.

Perhaps the most serious departure from the initial conditions specified inthe space charge analysis is the assumption of a plane plasma-beamboundary. If the plasma density at the expansion cup is not uniform, theboundary will be curved; the initial particle trajectories are normal to this

- 19

surface and the beam will be converging, diverging or a combination of both.The easiest solution is to start with as near a plane boundary as possible. Inour present source, the plasma density is lower near the edge of the sourceexpansion cup than over the central region; work is progressing to improvethe uniformity of the plasma density.

V-3 Source and Pumping Manifold

The ion source and extraction electrode assembly are shown in Fig. 17.This source arrangement produces a 100 mA dc beam with an emittanceinvariant of .32 ir cm mrad, a proton component in excess of 70% and alifetime of a few hundred hours. Development to improve the emittancecontinues. For the initial experiments reported in section VIII, the 14 mmplasma aperture has been reduced to 4 mm which gives an upper limit to thecurrent of 9.8

The extraction electrode is powered from a separate 0-60 kV supply inthe high voltage dome, thus permitting a greater degree of control on itspotential. The ion optics are especially critical at the beginning of thecolumn and, as mentioned above, even a small intercepted current can upsetthe potential distribution. The first electrode has a smaller aperture than therest of the column and acts as a shield for the electrodes immediatelydownstream. Because it has an independent voltage source it can interceptbeam, and this intercepted current is all that the power source need supply.

The voltage reliability of the extraction electrode system is not as good asthe main column. This problem will be discussed in section VIII.

The column is pumped at the dome end by two 1000 2/s (nominal air)ion pumps, one a diode and the other a triode. This combination was chosento include the superior argon pumping speed of the triode; the diode pumpwith reinforced cathodes has a longer hydrogen-pumping life. The capacityof these pumps was chosen on the basis of the ion source gas efficiencywhich is ~ 50%, i.e. a gas load of ~ .02 torr C/s. Ion pumps can accom-modate prolonged hydrogen consumption provided p ~ 10~6 torr; hencea speed of ~ 104 C/s is required. The combined pumping speed forhydrogen is 5000 C/s. The gas is pumped through the annulus between thesource and vacuum cylinders (see Fig. 1) that has a conductance of 4400 fi/s.At normal operating conditions, the pressure at the pump throats should be~ 4 x 1CT6 torr and near the source ~ 8 x 10"6 torr.

- 2 0

V-4 Accelerating Electrodes and Ceramic Vacuum Vessel

The assembled vacuum vessel is shown in Fig. 15. Fig. 18 is a photographof the column and the individual titanium alloy accelerating electrodes. Theseventeen ceramic rings are interleaved with flat rings of Ti6A14V andbonded together with polyvinyl acetate. The 96% A12O3 (AD96) ceramicrings are right cylinders 534 mm O.D. x 417 mm I.D. and 32 mm high with a6 mm square cutaway on both internal edges. This cutaway shields the triplejunction of metal, solid and vacuum dielectric. The Ti6A14V rings are 564mm O.D. x 422 mm I.D. and alternately 1.5 mm and 3.0 mm thick. Thethick rings are used to support the accelerating electrodes; the thin ringssupport stress-relieving tori that do not require as rigid support.

One of the prime considerations in the choice of the accelerating columnbonding agent was that it have a low vapour pressure to minimizecontamination of the accelerating electrodes and insulators. (This concernabout contamination predicated the choice of ion pumps rather thandiffusion pumps.) Brazing was considered, but discarded because it appearedbeyond the state of the art for reliable ceramic-metal joints of this diameter.Because of its low vapour pressure, the polycarbonate LEXAN was to beused as the bonding agentO^). Tests on small samples gave average shearstrengths of 4900 psi. A partial column of three ceramic rings and one eachof the titanium alloy discs was assembled, vacuum tested and subjected to ashearing force of 4 500 kg without destruction.

The column was bonded with the polycarbonate but one of the jointsfailed under the tension of the weight of the column. The polycarbonate atthe failed joint had a crystalline appearance. The column was disassembled,thoroughly cleaned and again bonded with the polycarbonate but cooledmore rapidly than the previous attempt. Again, a joint failed under tension.Polyvinyl acetate has been used successfully as a bonding agent on columnsof this diameter(S), but it has a greater vapour pressure. Because of theuncertainty in the reason for failure of the polycarbonate and the effortrequired in cleanup after an unsuccessful bond, polyvinyl acetate was used inthe final assembly.

- 2 1 -

All of the electrodes were fabricated from Ti6A14V, chosen because of itsexcellent voltage holding capacity in vacuum^6). The individual electrodeswere machined to tight tolerances and polished to a 5 rms finish*. Theelectrodes are shaped so that the beam can nowhere see the ceramic ringsand the first three are contoured^) so that V « Z^/3. The column was boredafter bonding to align the I.D.'s of the 3 mm thick Ti6A14V rings with theaxis. This boring, along with the tight tolerances on the machining of theindividual electrodes was the sole means of axial alignment. Fig. 19 showsthe axial offsets of the electrodes after assembly. Small adjustments to thelongitudinal placements of the electrodes were made during assembly bymachining the offsets in the semi-circular clamping rings that are screwed toboth the accelerating electrodes and the thick titanium alloy rings (see Fig.15). This method of fine positioning of the electrodes has the advantage ofno voltage stress points. All of the electrodes were positioned longitudinallyto better than ± 0.3 mm. This is relatively more accurate than the electrodepotential differences which are within ± 1%.

Aluminum corona rings fit over the titanium alloy discs on the outside(SF6 side) of the column. These have a minor radius of 7.9 mm and centerto center spacing of 34.2 mm. The maximum voltage stress between adjacentrings for 1 psig of SF6 is less than 1.4 kV/mm air equivalent, satisfying thecriterion of a safety factor of two over the corona limit.

V-5 SF6 Vessel

The insert in Fig. 1 shows the SF6 vessel mounted in its position betweenthe Faraday cage wall and the high voltage dome. It is made of seventeen1.02 m I.D. lucite cylinders of 12.5 mm wall thickness bonded to thealuminum corona rings with Hysol 4144 epoxy and has a total length of 1.57m. The vessel was designed to operate at gauge pressures up to 0.6atmospheres. Conventional 3 mm thick "Fairprene" gasketing is used toform the seals at the vessel ends.

Fabricated by Stemac Co. Ltd., Montreal, Quebec

Voltage tests were performed in air on a three-section, full-scale model ofthe assembly during design studies. No detectable corona occurred forvoltages up to 20% above the design value of 50 kV between adjacent coronarings, and the leakage current along the 86 mm wide lucite surface was <1/uA. At voltages above 80 kV per section flashovers began to occur mostlyalong the lucite surface in the region of the butt joint of the twosemi-cylinders used to form the sections.

Four parallel strings of 400 MS2 ± 1% Victoreen metal-oxide film resistorsare used to define the potential distribution along the accelerating column.These resistors are mounted between the corona rings outside the SF6 vessel;at a dome potential of 750 kV they define a potential difference of 50 kVbetween adjacent rings and draw a total current of 0.5 mA (50 kV/100 MJ2)requiring a power dissipation of ~ 6 watts per resistor.

Several types of potential-grading resistors were tested on the three-section model. Although rated for only 25 kV operation, the Victoreen typeMOX-F metal-oxide film resistors withstood steady-state voltages up to 90kV with no degradation of resistance film and no surface flashovers. Theresistors are 100 mm long by 14.2 mm diameter. Operating experience onthe accelerating column has been very good; only two resistors out of a totalof 68 have failed (open-circuit) in six months of operation.

The resistors are protected from damaging overvoltages by spark-gapsmounted between adjacent corona rings in parallel with the resistors. Highlypolished 50 mm diameter aluminum hemispheres are screwed onto threadedresistor end-caps, as shown in Fig. 20. The spark gap between two adjacenthemispheres can be adjusted from 5 to 30 mm, ± 0.05 mm, to cover therange of voltages from 30 to 85 kV. Fig. 21 shows the resistors mountedalong the accelerating column.

Shaped, spring-loaded copper conductors connect the corona rings of theSF6 vessel to the accelerating column electrodes (Fig. 22). These conductorswere installed after both the accelerating column and SF6 vessel werepositioned. Adjacent conductors are rotated 90° (Fig. 23); in-line conductorsare therefore 200 kV apart.

A cross-sectional view at the column showing the conductor positions isshown in Fig. 24a. The shapes were chosen to follow as closely as possiblethe calculated( 17) equipotential lines between the ceramic column and SF6

vessel corona rings. Fig. 24b shows the results of a full-scale two-dimensionalconducting paper study of the equipotential lines instigated by voltagebreakdowns during initial testing of the column (section VIII-2).

- 2 3 -

Vi BEAM LINE

VI-1 Quadrupoles

Fig. 25 is a plan view of the beam line from the last electrode of theaccelerating column to the beam stop. A 45° bending magnet will be locatedbetween the bellows and the fifth quadrupole (Q-5) when the beam is to bedirected to the Alvarez tank.

Two doublets are used in the focussing system. A four element focussingsystem was chosen in preference to a triplet because of the reducedellipticity possible in the beam envelope, and with four variables at ourdisposal, a greater degree of freedom is available in the ion optics. Thequadrupoles each have an effective length of 140 mm, an aperture of 82.5mm and for 2.6 amps < I < 20 amps, the gradient G in Tesla/meter is givenby

G = 0.20+ 0.301

"Electron sweepers" produce a transverse electric field inside the vacuumchamber throughout the quadrupole section of the beam line. In the firstdoublet, the electrodes are curved plates as shown in Fig. 26; in the seconddoublet the electric field is produced by pairs of water-cooled copper tubes.

To keep the beam free of electrons, we require that no potential wellsexist within the beam and the sweeping time be short compared to theneutralization time. The latter condition is easily fulfilled and the firstcondition is satisfied if the applied transverse field is equal to the maximumspace charge field which occurs at the edge of a cylindrical beam,

E = - i -m a x 27reorv

where

E m a x is the field in volts/meter

I is the current in amperes

eQ is the permittivity of free space (8.85 x 1CT12 farads/meter)

r is the beam radius in meters

v is the beam velocity in meters/second.

For a 120 mA beam with a radius of 10 mm, this field is 18 kV/m. This

requires a potential difference of 1150 V for the sweeper in Fig. 26. Over the

534 mm length £, this field, E, will deflect the beam

E£d = — = 6.4 mrad.

2V

- 24

The second sweeper is 600 mm long, has the electric field rotated IT/2 andwill produce a deflection of 7.2 mrad. These deflections correspond to dis-placements approaching 10 mm at the steering magnet (where the beam canbe redirected). However, much more modest transverse fields will probablyclear the beam because most electrons are ionized with an initial kineticenergy of several eV. If the deflection produced by the sweepers becomes aproblem, they can be replaced by four sections, each rotated 90° to itsneighbours so the net deflection is zero with the beam having a smalldisplacement. The focussing effect from the sweepers is negligible.

Two computer programs, ENVELOPE and QUADSEARCH have beenused in the ion optics calculations. ENVELOPE uses the Kapchinskii-Vladimirskii(18) equations to follow the trajectories of the particles on theedge of the beam through the transport system assuming the charge densityis uniformly distributed and takes both emittance and space charge forcesinto account. The program is written for a monokinetic beam passingthrough drift regions and magnetic quadrupoles.

The program QUADSEARCH is used to determine what gradients arerequired to give a double waist at the bending magnet. Using ENVELOPE asa subroutine and varying the four quadrupoles independently the beamdependence on the gradients is determined, the matrix is inverted and thegradients are adjusted for the required output values of rx, ry, rx and r ' .(Primes denote differentiation with respect to z, the beam direction.) Threepasses are usually adequate for convergence.

Fig. 27 shows the beam envelope for protons from the last electrode ofthe accelerating column to the face of the 45° bending magnet. The inputbeam conditions are I = 120 mA, e = 0.32TT cm mrad, rx o = ry o = 9.2 mmand rXQ = ryg = 18 mrad. These latter values are greater than the columnoutput values listed in Table III because of the increase expected from finiteemittance.

This, and other ENVELOPE calculations, can only be considered as closeexamples of the expected trajectories because the input conditions cannot berigidly specified and the space charge forces decrease as the beam driftsdownstream. The focal lengths of the quadrupoles are different for the threemass components and the H2

+ and H3+ components are gradually removed

from the beam as they are scraped off on the periphery. As an example, Fig.

28 shows the beam envelope for H2+ for the same conditions as Fig. 27. Fig.

29 shows the envelope through modified gradients for a 1.2 mAbeam - what might be expected if the beam were space charge neutralized.

- 2 5 -

VI-2 Vacuum System

The beam line is pumped by a 220 C/s triode ion pump immediatelydownstream of the quadrupoles and two 500 8/s diode ion pumps (withheavy gauge cathodes for hydrogen pumping) at the beam dump. Thevacuum vessel is mainly 300 series stainless steel with copper gaskets. Thegate valve is an exception, aluminum with Viton-A O-rings. Through thequadrupoles, the vacuum pipe is 75 mm I.D.; downstream the minimum pipeI.D. is 105 mm. Access ports for diagnostics are available at the 220 E/spump manifold and along the drift length between the ac dipole and thebeam stop pipes.

The main load on the two 500 fi/s pumps is from the stopped protonsevolving from the beam stop. For a 90 mA beam at equilibrium, this is theequivalent of .016 torr fi/s. The hydrogen pumping speed is 2500 C/s, which,at full beam current should maintain a pressure ~ 7 x 10~6 torr.

The 200 £/s pump must exhaust a portion of the (higher masscomponents) spilled beam. The pump size was selected to roughly match thepipe conductances (~ 100 fi/s each).

VI-3 Profile Monitor

Fig. 30 and 31 show the schematic and mechanical assembly of thenon-destructive ionization profile monitorO^) The design of the monitor isbased on a crossed-field technique used at CERN(20) A set of Helmholtzcoils and parallel electrodes establish orthogonal magnetic and electric fields,B and E, with directions parallel to and perpendicular to the proton beamrespectively. Electrons produced by protons ionizing the residual gas driftperpendicular to both E and B to a collector.

For a given proton energy, the electron current density is proportional tothe proton current density and the residual gas pressure. By placing anelectron detector at the zero equipotential plane and displacing theelectric-field equipotentials transversely so that the zero equipotential movesacross the beam it is possible to obtain a one-dimensional proton densitydistribution for the beam. Total electron currents of 60 nA per mm beamlength are expected for a 750 keV, 100 mA proton beam and a residual gaspressure of 10~6 torr. Applied fields of 100 kV/m and .035 Tesla aresufficient to minimize distortion from the beam space-charge. Spatialresolution from a single detector can be varied down to ~ 1 mm by a simplevoltage adjustment. The dc beam allows slow scanning rates of ~ 1 Hz.

- 26 -

VI-4 Steering Magnet

The steering magnet is located immediately downstream of the ionizationprofile monitor and is centered 1.95 m from the last column electrode. It is asquare box frame of 12.7 mm thick mild steel plate, 127 mm long and withinside dimensions of 159 mm. A 1300 turn coil of #16 copper wire is woundaround each of the four sides of the frame; each coil has a resistance of ~6J2. Opposite coils are powered in series and 1 amp excites a peak field of0.01 Tesla with a transverse field uniformity of ± 0.5% over the central 30mm. The effective length is 210 mm and for 750 keV protons, 1 ampproduces a calculated deflection of 17 mrad.

VI-5 Beam Stop

The beam radius is ~ 10 mm, and 90 mA at 750 keV corresponds to apower density greater than 20 kW/cm2. To reduce the power density, thebeam is defocused in the horizontal plane by the defocussing quadrupole Q-5and swept up and down by the ac dipole. The expanded beam is thenintercepted by the beam stop, 23 vertical 12.7 mm O.D. copper pipesoverlapping to form a V with the vertex downstream. The beamstop presentsto the beam a normal area 165 mm wide and 490 mm high which, for a 90mA 750 keV beam corresponds to an average power density of 84 W/cm2.The sinusoidal field variation in the ac dipole does not distribute the beamuniformly in the vertical direction and the peak power density near theextremes of the sweep is approximately twice the average.

Each copper pipe is half concealed by the adjacent upstream pipe andonly one quarter of the circumference intercepts beam. The I.D. of the pipesis 6.4 mm and the average power density at the copper-water interface is 54W/cm2 on the pessimistic assumption that only half the circumference iseffective in heat transfer. For a total flow in the beamstop of 3.3 C/s, thetemperature rise would be 4.9°C; the linear velocity in the pipes of ~ 4.5m/sec gives a modest film drop(-l) of ~ 40°C.

The outlet temperature of each of the 23 pipes is individually monitoredby a thermocouple and displayed by a 24 pen recorder. An adjustableovertemperature alarm rings if any pipe overheats. The inlet and outletheader temperatures are measured with resistance-temperature-detectors(R.T.D.) and the total water flow measured by a turbine flowmeter. TheR.T.D.'s and turbine flowmeter are read directly into the computer forcalorimetric power measurements, which, with the HV dome voltage givesthe current.

- 27 -

The defocussing quadrupole is centered 3.60 m downstream from the lastaccelerating electrode. This is 1.08 m downstream from the double waist atthe entrance to the proposed bending magnet, i.e. at r = 18 mm and r' = 13mrad. These numbers assume no space charge neutralization which may notbe valid. If the envelope continues to diverge at 13 mrad, at the beam stopwhich is 3.2 m further downstream, the beam width will be 100 mmcompared to the 165 mm width of the array of pipes. A small defocussingforce is needed to illuminate the entire width of the beamstop.

The defocussing quadrupole Q-5 has an aperture of 127 mm and a length of180 mm, and a 1 ampere current excites a gradient of ~ 0.05 Tesla/meter.The power supply is capable of delivering 50 amps to the 0.33 £2 ceils, butonly a small fraction of this should be required in practice.

The ac dipole is centered 3.96 m from the last column electrode,immediately downstream of the defocussing quadrupole. The c-shaped yokeis fabricated from old transformer laminations. The gap is 75 mm and thepole face is 123 mm x 115 mm; the effective length is 160 mm. Twoeighteen-turn coils of 6.4 mm square hollow copper conductor are woundaround the yoke. An ac current of 1 amp produces a peak field of 9 x 10~4

Tesla and a maximum deflection of half-angle 1 mrad. Ignoring possiblespace charge neutralization, the beam enters the ac dipole with r = 20 mm andr' i 15 mrad; hence a current of approximately 60 amps should sweep thebeam over the entire height of the beam stop.

The vacuum chamber that fits in the magnet gap is made of series 300stainless steel plate, 3.2 mm thick. The walls of the vacuum chamber areheated by the eddy currents generated by the 60 Hz magnetic field and reachtemperatures ~ 75°C for a current of 50 amps.

- 2 8 -

VII DATA ACQUISITION AND CONTROL SYSTEM

The data acquisition and control system is centered around a SUCCESS Icomputer system which uses a DEC PDP-8/I computer with 8k corememory(3). jfa SyStem will gather and log data from equipment in the highvoltage dome and the accelerator beam line, control stepping motors andcontact closures within the HV dome to adjust dome parameters, and sensestatus contacts throughout the equipment and initiate appropriate action inresponse to contact states. Fig. 32 is a simplified block diagram of thesystem.

Table IV lists the parameters which are sampled by the 32 channel highspeed Raytheon multiplexer and A/D converter. All 27 parameters aresampled every second at a scan rate of 60,000 channels per second and arelogged on the teleprinter at preselected intervals (between one minute andone hour) or on demand.

A variety of status events (see Tables II and V), as represented by contactclosures, are monitored by the computer through the Digital Multiplex Unit(DMU) and the SUCCESS I General Purpose Interface (GPI). A variety ofunits are connected to the DMU for both manual and automatic control ofthe system. Among these is the Task Generator which provides the facilityfor initiating, either manually from pushbutton switches or automaticallyfrom external signals, the control operations listed in Table VI. Outputsfrom the DMU include signals for driving the control relays listed in TableVII for adjusting the High Tension Supply Voltage. Also provided are twoaudible alarms to indicate abnormal conditions in the system. The DomeAlarm is a bell which rings continuously if one or more of the Dome StatusSignals in Table II becomes true. The Process Alarm is a buzzer whichsignifies that one or more of the analogue parameters in Table IV havewandered outside permitted limits, as determined by the computer program.

One of the most important parameters in the system is the dome voltage.It is measured from the Haefely potential divider chain by a 5'/2 digitintegrating digital voltmeter (HP-3450A) which is interfaced to theSUCCESS System GPI through a special DVM Interface Unit. This parameteris logged with the other analogue parameters under program control.

Adjustment of the dome parameters listed in Table II is accomplishedwith Slo-Syn stepping motors and relay contact closures. A Decoder in thedome selects the motor and relays in response to coded pulses received from

- 2 9 -

the Motor Control Unit in the control room via the fibre-optic data links.The control pulses may be generated either manually from the Manual MotorBurst Generator or automatically under program control from the Timingand Control Unit in the SUCCESS I System.

The computer system will be programmed to operate in three basicmodes:

(1) Filament Conditioning Mode:

Each time a new filament is installed in the ion source it will beconditioned automatically by the computer system.

(2) Column Conditioning Mode:

The accelerating column will be voltage-conditioned automatically bythe computer system after any maintenance which requires exposure ofthe electrodes to the atmosphere.

(3) Operating Mode:

This will be the general control mode for obtaining and maintaining theproper voltage and current conditions for a given experiment.

30

TABLE IV ANALOGUE PARAMETERS MONITORED BY THE RAYTHEONMUX/ADC UNIT

Source Channel

01234

from dome 56789

10111213141516

from beamline & 17Haefely power supply 18

192021222324252627

Parameter

Extractor Plate CurrentExtractor Plate VoltageArc CurrentArc VoltageFilament CurrentCoil CurrentSource PressureDiode Ion Pump CurrentTriode Ion Pump CurrentDome Calibration Voltage

High Tension Supply CurrentMeasuring Resistor Oil TemperatureSuppressor CurrentSuppressor VoltageBeam Line Ion Pump CurrentNo. 1 Quadrupole Magnet CurrentNo. 2 Quadrupole Magnet CurrentNo. 3 Quadrupole Magnet CurrentNo. 4 Quadrupole Magnet Current

Code Word

EXPCEXPVARCCARCVFILCCOLCSORPDIPCTIPCDCAV

HTSCMROTSUPCSUPVBIPCQMC1QMC2QMC3QMC4

Defocussing Quadrupole Magnet Current DFQCAC Dipole Magnet CurrentFaraday Cup CurrentFaraday Cup VoltageFaraday Cup FlowFaraday Cup TemperatureRadiationSpan Calibration VoltageZero Check

ACMCFACCFACVFACFFACTRADNSCAVZERO

- 3 1 -

TABLE V NON-DOME STATUS SIGNALS

1. Arcdown2. Fine HI Limit3. Fine LO Limit4. Rapid Arcdowns

TABLE VI CONTROL PANEL FUNCTIONS

1. Reset Extractor Power Supply2. Start Arc3. Reset Diode Pump Power Supply4. Reset Triode Pump Power Supply5. Trip Arc Power Supply6. Increase HTSV7. Normal Log Most Important Parameters8. Normal Log All Parameters9. Column Conditioning Log

10. Filament Conditioning Log

TABLE VII CONTROL RELAYS FOR HAEFELY POWER SUPPLY

1. Coarse UP2. Coarse DOWN3. Fine UP4. Fine DOWN5. Trip OFF High Tension Supply Voltage

- 3 2 -

VIII COMMISSIONING AN D INITIAL OPERATION

VIII-1 Alignment

The alignment axis of the accelerator is defined by the centre-line of theceramic accelerating column. Optical targets were mounted in the aperturesof the 0 and 700 kV electrodes at the ends of the ceramic column duringassembly, and the ion source assembly and beam transport system werealigned along the axis defined by these two points using a Taylor-Hobsontelescope. After completion of the beam line assembly, access to the lowpotential electrodes is restricted by the location of the quadrupole magnets(Fig. 25). To provide a reference line for future realignment an adjustabletarget holder remountable on the vacuum manifold to + 0.05 mm of itsinitial position was also aligned along this axis. This target together with the700 kV electrode aperture centre which can be exposed by removal of theion source are now used to obtain the beam reference line.

The magnetic centres and relative radial orientations of the quadrupoledoublets were found using the Cotton-Mouton effect. The doublets werealigned to within ~ 5 mrad of each other and to within ± 0.05 mm of thebeam axis using the technique of Cobb and Muray(22)

VIH-2 Voltage Testing and Accelerating Column Conditioning

The initial high voltage testing of the accelerating column in situ wasperformed without SFn in the Incite vessel surrounding the ceramic column.Operation in this mode up to ~ 375 kV was expected to be possible beforehigh-voltage breakdowns between the accelerating column corona ringswould occur. Initial flash-overs, however, occurred at a voltage of 326 ± 1kV. These breakdowns occurred between the contoured conductors joiningthe 700 and 500 kV corona rings of the ceramic column and SF6 vessel (Fig.24). Fig. 33 shows an "open-shutter" photograph of one of these arc-overstaken through one of the openings in the accelerating column supportmanifold (end plate in Fig. 23); the main flash-over occurs from the highstress area of the bend in the 700 kV conductor to the centre of the 500 kVconductor (lower portion of Fig. 33). After modifications to the 700 kVinter-column conductor to have it follow more closely the predictedcquipotential lines (Fig. 24), it was possible to operate the acceleratingstructure in air at a voltage of 350 kV without voltage breakdowns. Voltagetesting of the entire column was halted at this point in order to condition

- 3 3 -

the individual voltage gaps to a higher voltage. The relative electric strengthof SF6 (air = 1.0) at S.T.P. is ~ 2.5(23>; a safety margin of > 125 kV istherefore expected for the accelerating column operated at design voltage atan SF6 absolute pressure of one atmosphere.

Individual accelerating gaps of the accelerating column were high voltageconditioned in situ to a voltage > 100 kV. To do this a single string of 800MS2 resistors was used along the column and voltage was applied only to theaccelerating gap under study by shorting out the remaining resistors. Thelarger resistance was used to improve the microdischarge-to-drain currentratio. The resistor chain was connected to ground potential through amicroammeter and the current was observed as the applied voltage wasincreased. Slight deviations (typically 1-2%) from ohms law charactorized bysharp rapid spiking (1-5 pps) of the current occurred during voltage increasesabove 50 kV per gap. These current spikes produced by microdischargesbetween the electrodes decreased in amplitude and frequency after severalminutes, and the voltages would then again be increased in ~ 1 kV steps. Thegaps were considered to be "conditioned" when observed microdischargerates were <~ 10/hr. The typical conditioning time per gap to a voltage >100 kV was ~ 3 hrs.

Following the conditioning of the individual gaps, the 400 Mfi gradingresistors were reinstalled, SF6 to a pressure of 0.1 atmosphere gauge wasintroduced to the vessel, and voltage was again applied to the entire column.Two diagnostic observation points, namely the capacitive pick-up mountedin the Faraday cage and the current of the 220 £/s beam line ion pump,were used to monitor the inierodischarge rates during the testing period. Asample of a typical pressure record is shown in Fig. 34. Discharges within theaccelerating column which produce bursts of outgassing are seen as sharpshort period increases in the pump current record. It can be seen from thisrecord that some reconditioning of the electrodes at a decreased voltage isrequired following the initial breakdown at 740 kV.

The output of the 1.3 meter square capacitive pick-up (Fig. 2) can bedisplayed directly on an oscflloscope screen and was continuously monitoredduring the conditioning of the entire column to design voltage. Fig. 35 showsseveral oscilloscope traces taken at a slow (0.5 s/cm) sweep rate. The traceenvelope corresponds to the 10 kHz ripple (~ 300 V at 750 kV) on the high

- 3 4 -

voltage supply. A slight 60 Hz modulation of this band can also be seen. Thenarrow negative spikes on the traces correspond to microdischarges in theaccelerating column. The upper trace of Fig. 35a was taken immediatelyafter an increase in voltage from 745 to 750 kV; the lower trace was taken 5minutes later. The decrease of microdischarge rate as a function of time isclearly seen. The dependence of microdischarge rate with applied voltage canbe seen in Fig. 35b where the three traces going down the figure correspondto operating voltages of 725, 750 and 760 kV. These three traces were takenabout two minutes apart and voltage changes were, made before themicrodischarge rates had appreciably decreased at the previous voltage. Thelarge spike in the lowest trace corresponds to a flash-over across the sparkgaps protecting the column.

Several days of operating were required to condition the acceleratingcolumn from 350 kV to 797 kV which is the upper limit of regulation of thehigh voltage supply. During this period both microdischarge and machinearcdown rates were found to decrease steadily in frequency. After the highvoltage had been shut down for several days, some deterioration in theaccelerating column's ability to withstand the previously applied voltageswas found, but the maximum voltage could be reached with only ~ 1-2 hrsconditioning. After ~ one month of accelerator operation with 1-2 mAbeams in the range of 600-750 kV, it is possible to expose the column to airfor periods of ~ 1 day and after pumpdown to a pressure ~ 10~7 torr to raisethe applied voltage from 0 to 800 kV in less than 15 minutes withoutnoticeable microdischarge rates. Throughout this entire period the 5 MSIdamping resistor (Rp of Fig. 4) has been used.

The ion source and extraction electrode assembly (Fig. 17) was installedafter the accelerating column had been conditioned to full voltage. Highvoltage conditioning of this assembly was performed using the 60 kV supplyin the HV dome which was held at ground potential during this procedure.Occasional flash-overs occurred during this conditioning mainly from theextraction electrode to the source container. Modifications in the shapes andmaterials used for the insulators between the extraction electrode andground potential were successful in reducing i^e breakdown rates, butreliable operation at the design voltage of 50 kV acn. ̂ s this gap has not yetbeen achieved. Further studies of this problem are being carried out on aduplicate source assembly mounted in the Ion Source Test Stand.

- 3 5

VIII-3 Low Intensity Beam Operation

Initial extracted beams from the accelerating column were limited to 1-2mA in intensity while the performance of the beam handling and HV domecontrol systems were checked and while measurements of the X-radiationintensities were made. Experiments aimed at attempting to correlate domearcdown rates with such factors as extractor gap voltage and radiation fieldintensities were also carried out.

Operation of the ion source and vacuum systems at low beam levelsproved satisfactory. For the initial experiments an ion source having aplasma aperture of only 4 mm was used. Normal operation requires a gaspressure (before arc ignition) of ~ 0.30 torr within the source and in orderto determine the relationship between X-radiation intensity and the columngas pressure, two anode assemblies with anode apertures of 1.00 and 0.50mm were used. The air-equivalent system pressures measuied with the HVdome and beam stop ion pumps for no-beam and 1 mA operation are shownin Table VIII. Also shown are the corresponding pressures for no hydrogeninput.

X-radiation produced by electrons accelerated upstream within theaccelerating column and striking the source assembly constitute the mainsource of radiation from the HCTF injector. The backstreaming electrons areproduced by the ionization of gas within the accelerating column or by theinterception of the beam by the column structure and, depending on thelocation of their production, can have energies up to 750 keV when theystrike the source assembly.

Intensity and energy distribution measurements of the radiation fieldsnear the accelerating column were made with portable radiation surveymeters and with a 38 mm thick x 50 mm diameter Nal scintillation detector.Fig. 36 shows the radiation spectra taken at two locations with thescintillation counter for a 1.2 mA 750 keV beam. The upper spectrum wastaken with the detector located in the screened observation corridor in theFaraday cage (Fig. 3) and pointed at the accelerating column while the lowerspectrum was taken with the detector against the upraised drawbridge (Fig.2) in line with the column. The low energy cutoffs of the spectra areinstrumental.

3 6 -

TABLE VIII HIGH VOLTAGE DOME AND BEAM STOP PRESSURES

Dome Diode Pump Beam Stop(air equivalent pressures in mm Hg)

No Gas Feed

Large Anode Aperture

No Beam1mA Beam

Reduced Anode Aperture

No Beam1mA Beam

3 x 1(T8

4 x 10"6

4 x 1CT6

9 x 10~7

9 x 10~7

2x 1CT8

8 x 1(T8

2x 10"7

4 x 10"8

1.5 x icr7

TABLE IX RADIATION INTENSITIES mR/hr FOR 750 keV 1 mA BEAM

Location 1 2 3 4 5(see Fig. 2)

'High11

'Low"PressurePressure

9.54.0

104.5

0.60.35

21.4.2

3.3

1.6

- 3 7 -

Bremsstrahlung radiation produced by the electrons should extend inenergy to the full energy of the electrons and is characterized by a forwardangle peaking of the intensity distribution. The lower spectrum of Fig. 35shows a maximum near ~ 500 keV and extends to ~ 700 keV in energy.Attenuation of the spectrum by absorption by the source and columnstructure distort the spectrum from the normal bremsstrahlung shape. Theupper spectrum of Fig. 36 showing mainly radiation scattered by theassembly is typical of the distributions found at various locations about theexperimental area. The relatively high end-point of the energy distributionindicates that an appreciable portion of the backstreaming electrons isproduced near the ground potential end of the column. Examination of theelectrodes with the source removed after several days of 1 mA operationindicated some impingement of the proton beam on the suppressor electrodeand the one upstream from it. Currents of the order of 2-4 /iA have beenobserved from the suppressor power supply at 1 mA operation of thecolumn.

An intensity scan (E > ~ 30 keV) of the accelerating column was madeusing the Nal counter collimated with a lead tube. The detector was locatedin the screened observation corridor in the Faraday cage and moved parallelto the column. The intensity profile (Fig. 37) shows a maximum in thevicinity of the source, but a smearing of the spectrum by scattering from thecolumn structure makes more accurate measurements difficult.

Absolute radiation field intensity measurements were made with radiationsurvey meters. The initial beam was extracted using a 1.00 mm anodeaperture diameter (see section VIII-2) and the radiation fields were measuredat a number of locations. These locations are numbered 1 to 5 in Fig. 2. Thecorresponding intensities are shown in Table IX. In an attempt to determinewhat fraction of the backstreaming current is produced by gas ionization,the anode aperture diameter was reduced to 0.50 mm. The correspondingvalues are also shown in Table IX. The reduction by a factor of four in theconductance of the anode aperture and in the column pressure produces areduction in the radiation intensity by a factor of about 2.5. No noticeablechange in the general spectral distribution was observable.

During initial operation the suppressor electrode (which is powered by aseparate voltage supply) was operated at -10 kV and the electron sweepers at+ 40 V. Radiation field intensities increase by a factor of 2-3 when theseunits are held at ground potential for 1 mA currents. No appreciable changein the radiation intensities is seen for increases in the above operatingvoltages.

- 3 8 -

VHI-4 Summary

The HCTF injector accelerating column was successfully conditioned to

797 kV and low intensity beams up to 750 keV in energy were extracted

from the column and transported to the beam stop. The problem of initial

HV conditioning proved simpler than anticipated. Following the initial

conditioning the column seemed unaffected by exposures to the atmosphere

for periods up to 48 hr.

Proton beams up to ~ 1 mA were transported along the beam handling

system to the beam stop. Measurements of the beam width at the 220 2/s ion

pump (Fig. 24) using a wire scanned 24) showed the beam to be well

centred and to have'a radial profile width at half-maximum of ~ 7 mm. The

temperature profile of the beam stop cooling pipes showed the beam to be

horizontally centred at that location. Because of the small ionization

currents produced in the profile monitor at these beam intensities no

detailed information was obtained on the shape of the beam or on the

performance of the quudrupolc magnets and electron sweepers.

Radiation measurements showed the X-radiation field intensities to be

fairly modest and no major problems were indicated in going to larger beam

currents. Improved source efficiency and column focussing at currents

approaching the design values are expected and field intensities are therefore

not expected to increase linearly with the beam current. The application of

-10 kV to the electron suppressor electrode produced reductions by a factor

of 2-3 in the field intensities.

The data acquisition system operated successfully during low beam

operation and was fully commissioned. A number of problems involving the

1DVM (section 111) and electrical noise generated relay trips in the HV dome

were encountered and solved^). The fibre-optic light link system withstood

many arcdowns during HV conditioning and low beam operation with no

indication of damage to the light links or deterioration of transmission.

Problems were encounteied in the operation of the extraction electrode

at the design voltage of -50 kV due to insulator breakdowns and changes in

design and materials are required.

Some of the lucite-aluminum bonds in the SF6 insulating vessel proved to

be faulty and should operation at pressure > 0.2 atmosphere gauge be

required for improved reliability, changes in the vessel assembly will be

required.

Further details of the accelerator hardware performance and of

experience gained in operation up to beam intensities of ~ 10 mA will be

presented in a future publication.

- 3 9 -

ACKNOWLEDGEMENTS

A great many people contributed to building this apparatus, among whom

we would like to mention J.B. Barks, E.C. Carlick, A. Harvey, G.L. Mead,

W.L. Michel, R.E. lVMks, I.O. Mottram, D. Warren and A.E. Weeden.

- 4 0 -

REFERENCES

1) B.G. Chidley, Atomic Energy of Canada Limited, Unpublished Internal

Report FSD/ING-90 (1967).

2) F.W. Peek, Dielectric Phenomena in High Voltage Engineering,

McGraw-Hill, New York (1929).

3) L.M. Watkins et al. - to be published.

4) J.H. Ormrod and A.E. Weeden - to appear as an Atomic Energy of

Canada Limited publication.

5) CD. Curtis and G.M. Lee — Midwestern Universities Research Associa-

tion, Report MURA-707 (1965).

6) J.H. Ormrod, Atomic Energy of Canada Limited, Unpublished Internal

Report FSD/ING-119 (1968).

7) T.G. Church (editor), Atomic Energy of Canada Limited, Report

AECL-2750(1967).

8) J.R. Pierce, Theory and Design of Electron Beams, Van Nostrand,

Princeton (1954).

9) Th. J.M. Sluyters, Los Alamos Scientific Laboratory, Report LA-3609

p. 383(1966).

10) W.D. Kilpatrick. Rev. Sci. Inst. 28: 824 (1957).

ID L. Cranberg, J. Appl. Phys. 23: 518(1952).

12) J. Fasolo, CD. Curtis, G.M. Lee, Los Alamos Scientific Laboratory,

Report LA-3609 p. 371 (1966).

13) J.H. Ormrod, Brookhaven National Laboratory, Report BNL-50310 p.

151 (1971).

14) O.B. Morgan, G.G. Kelley and R.C. Davis, Rev. Sci. Inst. 38: 467

(1967).

15) S.E. Newfield, and E.P. Ehart, Los Alamos Scientific Laboratory,

Report LA-3890( 1968).

16) J. Huguenin and R. Dubois, CERN Report 65-23 (1965).

17) J. Hayes - private communication.

18) l.M. Kapchinskii, V.V. Vladimirskii, Int. Conf. on High Energy

Accelerators. CERN( 1959) p. 274 (1959).

- 4 1 -

19) M.D. Snedden, Atomic Energy of Canada Limited, Unpublished

Internal Report CRNL-606 (1971).

20) CD. Johnson and L. Thorndahl, IEEE NS 16: 909 (1969).

21) H.T. Stoever, Applied Heat Transmission, McGraw-Hill, New York

(1941).

22) J.K. Cobb and J.J. Muray, Nucl. Inst. Meth. 46: 99 (1967).

23) L.L. Alston, High Voltage Technology, Oxford Press, London (1968).

24) J.H. Ormrod, Rev. Sci. Inst. 40: 1247 (1969).

25) J. Marin, J.A. Sauer, Strength of Materials, 2nd edition, Macmillan,

New York (1960).

26) Mechanical Engineer's Handbook, 6th edition, McGraw-Hill, New York

(1958).

7) G. Gautherin et al., Plasma Physics II: 397 (1969).

- 4 2 -

APPENDIX I

BUCKLING AND STABILITY CALCULATIONS FOR DOME LEGS

The cross-sectional dimensions of the tubular dome legs were influenced

by a number of factors which included minimum buckling loads, vibrational

stability, minimum wall clearances, etc. The following calculations show the

safety margins against buckling and the rigidity of the dome-end of the legs.

The leg dimensions are as follows:

Parameter Inner Leg Outer Leg

inner radius, r̂

outer radius, rQ

length, L

3.50 in.

4.75 in.

16.8 ft.

5.25 in.

6.00 in.

16.0 ft.

BUCKLING LOADS

The buckling load for a tubular column with one end fixed and oneend free is given by Euler's formula(25);

p _ ^ E Ac 4(L/r)2

where P c = buckling load

A = column cross-sectional area

r = radius of gyration = Vi (rQ + x\ ~)^2

L = column length

E = modules of elasticity

= 2 x 106 psi for epoxy-resin fiberglass

For A = 7r(ro - x\) and r = Vi (rQ + t\ ) i / z , the above expression expands to

c 1 6 L 2 ' 0 l)

Substituting values for E, L, rQ and r} from the above table gives thefollowing results:

- 4 3 -

Inner Leg Outer Leg

Buckling Load, Pc

Actual LoadFactor of Safety

34,1001b1,000 lb

34

56,4001b1,2501b

45

VffiRATIONAL STABILITY

The deflection 5 of the free end of a column with one end clamped isgiven by(25);

3EI

where P = deflecting force perpendicular to column centre-line

E = modulus of elasticity

L = column length

I = moment of inertia of column

For a cylindrical column, I = (ro — r-1) ir/4

For design purposes, the ratio P/5 = K, the "rigidity modulus", gives anindication of the column stability when subjected to a perpendiculardeflecting force. Based on experience from a smaller 150 kV dome using thesame leg material, a rigidity modulus of 200 lb/in was considered adequatefor good column stability. Thus, for an outer leg,

3 ElK = — =358 lb/in

Li

This is well above the reference value of 200 lb/in. For an inner leg, K = 206lb/in which is just above the reference value. This was the criterion whichdetermined the wall thickness of the inner legs.

— 4 4 —

APPENDIX II

TORQUE AND RESONANCE-SPEED CALCULATIONS

FOR MOTOR-GENERATOR DRIVE SHAFT

TORQUE

The required full-load torque for the drive shaft at the operating speed of

600 rpm and 40 hp is

e^oooxhpF L 12 x N

The design torque for a cylindrical hollow shaft is given by

ro

where T = torque

S = design shear stress

J = polar moment of inertia = (rQ — if) JT/2

rQ = outer radius

q = inner radius

For the epoxy-resin fiberglass drive shaft, rQ = 2.5 in and r.j = 2.0 in.

If the design shear stress is convervatively chosen as 1000 psi (about 40

times less than the tensile strength of the fiberglass material), then

10001b/in2 x 36.2 in4

T = _ 1 . = 1200 ft ib2.5 in x 12 in/ft

Thus the design torque for the shaft is 3.4 times higher than the required

full-load torque.

RESONANCE SPEED

The first resonance speed for the epoxy-resin fiberlgass shaft can becalculated from^6)

- 4 5

where C ] = a constant dependent on boundary conditions

L = shaft length = 14.5 feet

W = weight of shaft per unit length = 5.1 !b/ft

E = modulus of elasticity = 2 x 106 psi

I = moment of inertia = (ro — if ) JT/4

g = gravity acceleration constant = 32.2 ft/sec2

For a shaft with both ends clamped, C1 = 3.58, and the above formula givesN( = 15QQ rprn. Thus, the first resonance speed is well above the 600 rpmoperating speed.

- 4 6 -

AFPENDIX IIIFOCUSSING BY REDUCED CURRENT IN A PIERCE COLUMN

Consider a particle on the periphery of a circular beam being accelerated

in a Pierce gradient. The particle is subjected to an outward radial force from

space charge and an equal but inward directed force from the field gradient.

If the current is veduced slightly, say by AI, then the net force on the

particle is inward directed, and the magnitude is given by the radial force

exerted by a current of AI. If AI is small enough that the radius of the beam

can be considered constant, then one can solve for the radial velocity of the

particle.

The radial electric field is found from Gauss' theorem

ER (Z) =27reoRv(z)

and

dpi _ dpi dt_ eAI 1

dz dt dz 2?reoRv v

mAIdzdpi =

47reoRV(z)

. -Vi -VAFrom V-l, dz = - \ - — + \ - - l - \ V dV

4 \^f2eo \ e 7rR2j

Substitute this in the above equation and integrate

( a s ) <«°1) a i l V i ~Vf IWe encounter here the familiar infinity inherent in the derivation of the

Child-Langmuir equation. The ions in the source have a directed velocity

corresponding to ~ 50 eV(27). Substituting this for Vj and 200 kV for Vf.

Avx= 1.06 x 107 AI

CROSS SECTIONOF ALVAREZ TANK

CROSS SECTION OFACCELERATING COLUMN

ACCELERATINGCOLUMNALVAREZ TANK

BEAM STOP

COCKCROFT-WALTON UNIT

MOTOR-GENERATOR

HIGH CURRENTTEST FACILITY

HIGH VOLTAGEDOME

Figure 1 - View of the high current test facility.

208

1S METERS

FLOOR TRENCH

EXPERIMENTAL AREA

AND

ALVAREZ TANK

B E t U STOP

HATCH

I 1 /MR CONDITIONER

201

CONTROL ROM

©

OH.

i lR

C0NDIT10NIHS

UNIT 208

Figure 2 — Plan of main floor.

Figure 3 - Plan of basement.

.015uf =f=

015uf=fc

.03uf=fc

4=3000pf

Figure 4 — Schematic of 750 kV injector power supply.

E-2

LOW INPUT <•

HI INPUT

R5

R1-WAr

18K

D1

D5

FD3OO

1N3024BC1

0.01

A AR3

-VVSr10K

R4-vw10K

D2

1

D3

R2

5D4

FD300

FD300

FD300 C2

D6 0.01

18K1N3024B

B OUTPUT

A OUTPUT

TODVM

27V 27V GND

Figure 5 — Protection circuit on input of injector power supply digitalvoltmeter.

Figure 6 - View inside Faraday cage.

ION CUMP SUPPORT STAND

HIGH VOLTAGE DOME

GENERATOR - INCREASER ASS'Y

UPPER FLOOR

LOVER FLOOR

HAEFELV POTENTIALDIVIDER COLUMN

SUPPORT FOR HAEFEL*POTENTIAL DIVIDER COLUMN

INNER LEG CLAMPS I-BEAM FRAMEOUTER LEG CLAMPS

Figure 7 — Sectional views of high voltage dome assembly.

MOTOR-REDUCER ASS'V

INNER LEG

OUTER LEG

BASE PLATE

Figure 8 - Clamping arrangement for nested dorae legs.

Figure 9 - Installing outer leg over inner leg. Note the bleeder resistors wound around theinner leg.

Figure 10 — Motor-generator power train.

Figure 11 - High-voltage arcdown along teflon water lines.

Figure 12 - Overhead view of equipment in HV dome.

25kVA, 208V30, 412HzGENERATOR

GENERATORSENS NG CIRCUIT "

COILSUPPLY

5A,270Vdc

-ION SOURCE

ARCCURRENT

35A,155Vdc

SUPPL ES

FILAMENTCURRENT35A,4Vdc

4

7.5kVDIODE IONPUMPSUPPLY

5.7kVTRIODE

ION PUMPSUPPLY

EXTRACTORELECTRODEVOLTAGE60kV

DAMPINGRESISTOR

COOLING

1.5kVA115V, fcOHz

FREQUENCYCONVERTER

COIL

REGULATOR

ARC

REGULATOR

H2 GASFLOW

SOURCEGAS

PRESSURE

1H?

ARCSTARTCIRCUIT

CONTROL

SYSTEM

Figure 13 - Block diagram of equipment in HV dome.

FIBRE-OPTICUNITS

CODINGUNIT

RECEIVERS

TRANSMITTERS

1

L

r

-^J LI

i . I

SLO-SYN MOTORTRANSLATORS

RELAY RESETUNITS

STATUS MONITORINGUNITS

ANALOGUE SIGNALS

Figure 14 — Communication channels across H V interface.

COIL CURRENTARC CURRENTFILAMENT CURRENTH2 GAS FLOWEXTRACTOR VOLTAGE

TRIODE PUMPDIODE PUMPEXTRACTOR VOLTAGEARC OFF-ON CONTROL

COIL VARtAC LIMITARC VARIAC LIMITTRIODE PUMPDIODE PUMPEXTRACTOR VOLTAGEGENERATOR

COIL CURRENTARC CURRENT, VOLTAGEFILAMENT CURRENTSOURCE GAS PRESSUREDIODE PUMP CURRENTTRIODE PUMP CURRENTEXTRACTOR CURRENT,

VOLTAGECALIBRATION VOLTAGE

-ID

600

700

\o

Figure 15 - Assembly of accelerating column vacuum vessel. Diagonal broken hatchingdenotes ceramic; all else is titanium except the outside rings which are aluminum. Theelectrode unattached in the figure is supported from the source assembly (not shown) andits potential is derived from the extractor power supply. The normal operating potentials inkV are listed on the right.

a)

b)

Figure 16 — a) Phase-space diagram of beam at plasma aperture, b) Phase-space diagram ofbeam at some downstream position.

Figure 17 - Assembly of ion source and extraction electrode. A detailed description of the

ion source can be found in reference 4.

\

Figure 18 — Accelerating column vacuum vessel and exploded view of acceleratingelectrodes #2 to #10. Uppermost electrode is #10.

0.2

Figure 19 — Position of the electrode centers relative to the column centre-line. Numbers inparentheses refer to electrode numbers. Units are millimeters.

AIR

LUCITE VESSELWALL

ADJUSTABLESPARK GAP

HEMISPHERES

SF,

1 cm

Figure 20 — Resistor and spark-gap assembly on SF6 vessel corona rings.

Figure 21 — The accelerating column mounted between the wall of the Faraday cage on the left and the HV

dome on the right showing the staggered resistor spark-gap assemblies on the SF6 vessel.

Figure 22 — Electrical connector for joining 700 kV SF6 corona ring to corresponding electrode on

accelerating column vacuum vessel.

Figure 23 — Partially assembled accelerating column looking through wall of Faraday cagetowards HV dome. Note intercolumn connectors. Plate on ceramic column end is atemporary valve protector.

/i

rX)KV

t3 f-

KV 6C

z; s

OKV 55OKV 50C

(

KV

c

\

4SOKV

J

\

V

• \

40QKV

\

\

35OJ<V •!9SCy 35OKV ? 2 ? ^ JJ^

\

r*t

r (

r 1

!

T

/

»*,

/

*

-*.

- •

£*•

_ ,

^ >

\

7

5Ci^ '

/t-

W -IQKV

K4

1J

Figure 24 — a) Equipotential lines between interior corona rings of SF6 vessel and corona rings on ceramicvacuum vessel; b) Cross-sectional view of intercolumn connectors. In actual assembly, each connector isrotated 90° from its neighbour.

WALL OFFARADAY CAGE

BEAMSTOP

#10ELECTRODE SCALE LENGTH 32/1

SCALE BREADTH 16/1

Figure 25 — Plan view of beam line from the last electrode of the accelerating column to the beam stop.

B = bellows, Q = quadrupole, I .P. = ion pump, P.M. = profile monitor, S.M. = steering magnet, G.V. = gate

valve, A.C.D. = ac dipoie.

o I O

Figure 26 - Cross-sectional view of electron sweepers. The sweeper electrodes are the solid curved plates atthe side of the beam pipe.

ZCMETERS)Figure 27 — Beam envelope for proton component of full current beam coming to a double 9 mm waist at45° bending masnet. Z = 0 at last electrode of accelerating column. A = r.,, + = rv. Field gradients:q-1 = 1.31 T/m, Q-2 = 2.99 T/m, Q-3 = 3.33 T/m, Q-4 = 1.90 T/m.

TJ 1 Z( METERS

Figure 28 - Beam envelope for H2+ component of full current beam. Same conditions as Fig. 27.

o 1 ZCMETERS)Figure 29 — Beam envelope for proton component of 99% neutralized beam coming to a double 9 mmwaist at the 45° bending magnet. Field gradients: Q-l =2.65 T/m, Q-2 = 3.31 T/m, Q-3 = -1.37 T/m,0-4 = 0.16 T/m.

• D E T E C T O R

D.C. PROGRAMMABLE

POWER SUPPLY

<V*-*V*-l-lV*Xv*l^X+*4-'*i-V>-

D.C. FLOATING

POWER SUPPLY

STRIP ELECTRODES

SIGNAL

PROTONBEAM

ELECTRON SDRIFT | |

• s

COLLIMATORELECTRODE

aoo

COLLECTORELECTRODE

ELECTRON DRIFTCYCLOIDAL TRAJECTORY

Figure 30 — Schematic of crossed-field profile monitor.

SIGNAL

COLLIMATORELECTRODE

MAINELECTRODE

STRIPELECTRODE

ROTATABLEFLftNGE

HELMHOLTZ'COIL

ELECTRICAL FEED-THROUGH CONNECTOR TEFLON

SPACERS

MAINELECTRODE

COLLECTORELECTRODE

BEAM t

B-WIREELECTRICALFEEDTHROUGHCONNECTOR

ELECTRODE-ASSEMBLYSUPPORT

Figure 31 — Mechanical assembly of profile monitor.

DETECTORASSEMBLY

STRIPELECTRODE

D TO +10 VOLTi 0 TO +IOV

TYPICAL OF 5 DOMEADJUSTMENTS USINGSTEPPING MOTORS

TYPICAL OF 4 DOMEADJUSTMENTS USINGCONTACT CLOSURES

TYPICAL OF G DOMESTATUS EVENTS ASREPRESENTED BYCONTACT CLOSURES

32 CHANNELRAYTHEON AD

CONVERTER + MULTIPLEXER

•••-L..LREDCOR INTERFACE

SUCCESS I

COMPUTER SYSTEM

CONSISTING OF

( I iDEC PDP8 I COMPUTER WITH6K CORE

(2(GENERAL PURPOSE INTER-

FACE (GP1)

(3IPERIPHERAL INTERFACE

UNITS A + B

(410SCILL0SC0PE D I S P L A Y

(5(TELEPRINTER TELETYPE

UNIT

(6)FAST PUNCH

WiFAST READER

(B)REAL TIME CLOCK

(9)TIMING + CONTROL UN IT

(T0 iPOWER ENTRY UNIT

< I I (DISTRIBUTION PANEL

GPI

D TO I0MV

OTO+IOV A l (

NTDATA

AMPLIFIER

TYPICAL NON-DOME

ANALOG INPUTS

DVM

INTER-

FACE

H-P 3450A

DVMDOME VOLTAGEFROU HAEFELVBLEEDER CHAIM

I OF BINPUTS

nCONDITIONINGMODE STATUSWORD UNIT

TO & FROM GPI

DIGITAL MULTIPLEXERUNIT (DMU)

(GENERAL PURPOSE INPUT-OUTPUT UNIT FOR CONTACTCLOSURES & VOLTAGE LEVELS)

CONDITIONINGMODE CONTROL

CONTACTCLOSURES

TO DOME HIGHVOLTAGE REGULATOR

DOME ALARMLIGHTS & BELL

75OK V GROUND POTENTIAL

Figure 32 — Block diagram of SUCCESS computer data acquisition and control system.

Figure 33 - High-voltage breakdown between inter-column conductors which join the SF6 insulating vesseland accelerating column.

RELATIVE PUMP CURRENT

Ifen CJl o

1 CJl

too

o

II

-a

mo

en

o

3D.

CO

o

too roCJl

Figure 35 — Oscilloscope traces of capacitive pickup output:

a) HV dome at 750 kV with lower trace taken 5 min after upper trace

b) Comparison of microdischarge rates at 725, 750 and 760 kV respectively down the figure.

o

I I -

100 200ENERGY (keV)

400ENERGY (keV)

300 400

600

Figure 36 - Energy spectra of X-radiation observed with scintillation counter:

a) spectrum taken from screened observation corridor in Faraday cage with detector at rightangle to the column

b) spectrum taken directly in line with accelerating column from location 4 in Fig. 2.

COLLIMATEDN a i (T I )COUNTER

OO

10

8

6

4

DETECTOR POSITION

Figure 37 — Schematic view of scintillation detector radiation scan of accelerating columnand corresponding intensity profile. Sections of the column observed at each position areindicated by error bars.

A d d i i i o n a l conies of this documen tmay br. ob ta ined f rom

Scienti f ic Doc jmen t D is t r i bu t i on O f f i ceAtomic Energy of Canada L imi ted

Chalk River, On ta r i o , CanadaKOJ 1J0

Pr ice - $ 2 . 0 0 per c o p y

2914-72