Beam Chopper Development for Next Generation High Power Proton Drivers

M. A. Clarke-Gayther RAL/FETS/HIPPI CARE-07 October 30th 2007

Beam Chopper Development for

Next GenerationHigh Power Proton Drivers

Michael A. Clarke-GaytherRAL / FETS / HIPPI


Overview

Fast Pulse Generator (FPG)

Slow Pulse Generator (SPG)

Slow – wave electrode designs

Summary

Outline

Overview


Maurizio Vretenar(WP Coordinator)Alessandra Lombardi(WP4 Leader)Luca Bruno, Fritz CaspersFrank Gerigk, Tom KroyerMauro PaoluzziEdgar Sargsyan, Carlo Rossi

Mike Clarke-Gayther (WP4 Fast Beam Chopper & MEBT)

Chris Prior (WP Coordinator) Ciprian Plostinar (WP2 & 4 N-C Structures / MEBT)Christoph Gabor (WP5 / Beam Dynamics

Overview


John Back (LEBT)

Aaron Cheng (LPRF)Simon Jolly (LEBT Diagnostics)Ajit Kurup (RFQ)David Lee (Diagnostics) Jürgen Pozimski (Ion source/ RFQ)Peter Savage (Mechanical Eng.)

Mike Clarke-Gayther (Chopper / MEBT)Adeline Daly (HPRF sourcing & R8)Dan Faircloth (Ion source)Alan Letchford (RFQ / (Leader)Jürgen Pozimski (Ion source / RFQ) Chris Thomas (Laser diagnostics)

Christoph Gabor (Laser diagnostics)Ciprian Plostinar (MEBT / DTL)

Overview


Project History and Plan

Overview


A Fast Beam chopper for

Next Generation Proton Drivers / Motivation

To reduce beam loss at trapping and extraction• Enable ‘Hands on’ maintenance (1 Watt / m)

To support complex beam delivery schemes• Enable low loss ‘switchyards’ and duty cycle control

To provide beam diagnostic function• Enable ‘low risk’ accelerator development

Overview


Design Project Position Type Chopping Status

RAL ESS & FETS MEBT

Slow-wave& Array

Uni-directional Prototype

CERN SPL MEBT Slow-waveUni-

directionalAdvanced prototype

LANL/LBNL SNSMEBT

& LEBTSlow-wave& Discrete

Uni & quadInstalled& tested

JAERI JPARCMEBT

& LEBTCavity &Solenoid

Bi &Longitudinal

Installed& tested?

FNAL ‘X’ MEBT Slow-wave Uni Prototype

Fast beam chopper schemes

Overview


The RAL Front-End Test Stand (FETS) Project / Key parameters

Overview


RAL ‘Fast-Slow’ two stage chopping scheme

Overview


3.0 MeV MEBT Chopper (RAL FETS Scheme A)

Chopper 1 (fast transition)

Chopper 2 (slower transition)

‘CCL’ type re-buncher cavities

4.6 m

Beam dump 1

Beam dump 2

Overview





2.3 m

Beam dump 1 (low duty cycle)

Overview





2.3 m

Beam dump 2(high duty cycle)

Overview


FETS Scheme A / Beam-line layout and GPT trajectory plots

Losses:0.1 % @ input to CH1, 0.3% on dump 10.1% on CH2, 0.3% on dump 2

Voltages:Chop 1: +/- 1.28 kV (20 mm gap)Chop 2: +/- 1.42 kV (18 mm gap)

Overview


Open animated GIF in Internet Explorer

Overview


KEY PARAMETERS SCHEME A

ION SPECIES H-

ENERGY (MeV) 3.0

RF FREQUENCY (MHz) 324

BEAM CURRENT (mA) 40 - 60

NORMALISED RMS INPUT EMITTANCE IN X / Y / Z PLANES( π.mm.mr & π.deg.MeV)

0.25 / 0.25 / 0.18

RMS EMITTANCE GROWTH IN X / Y / Z PLANES (%) 6 / 13 / 2

CHOPPING FACTOR (%) 30 - 100

CHOPPING EFFICIENCY (%) 99.9

FAST CHOPPER PULSE: TRANSITION TIME / DURATION / PRF/ BURST DURATION / BRF

2 ns / 12 ns / 2.6 MHz / 0.3 – 2 ms / 50 Hz

FAST CHOPPER ELECTRODE EFFECTIVE LENGTH / GAPS (mm) 450 x 0.82 = 369 / 20

FAST CHOPPER POTENTIAL(kV) ± 1.3

SLOW CHOPPER PULSE: TRANSITION TIME / DURATION /PRF/ BURST DURATION /BRF

12 ns / 250 ns – 0.1 ms 1.3 MHz / 0.3 – 2 ms /

50 Hz

SLOW CHOPPER EFFECTIVE LENGTH / GAPS (mm) 450 x 0.85 / 18

SLOW CHOPPER POTENTIAL (kV) ± 1.5

POWER ON FAST / SLOW BEAM DUMPS (W) 150 / 850

OPTICAL DESIGN CODE(S) IMPACT / TRACEWIN/ GPT


Fast Pulse Generator (FPG) development





2.3 m

Beam dump 1 (low duty cycle)

FPG development


9 x Pulse generator cards

High peak power loads

Control and interface

Combiner


Power supply



1.7 m

FPG / Front View

FPG development


Pulse Parameter FETS Requirement Measured Compliancy Comment Amplitude (kV into 50 Ohms) ± 1.4 ± 1.5 Yes Scalable Transition time (ns) ≤ 2.0 Trise = 1.8, Tfall = 1.2 Yes 10 – 90 % Duration (ns) 10 - 15 10 - 15 Yes FWHM Droop (%) 2.0 in 10 ns 1.9 in 10 ns Yes F3dB ~ 300 kHz Repetition frequency (MHz) 2.4 2.4 Yes Burst duration (ms) 0.3-1.5 1.5 Yes Burst repetition frequency (Hz) 50 50 Yes Duty cycle ~ 0.27 % Post pulse aberration (%) ± 2 ± 5 No Reducible Timing stability (ps over 1 hour) ± 100 ± 50 Yes Peak to Peak Burst amplitude stability (%) + 10, - 5 + 5, - 3 Yes

FPG waveform measurement

FPG development


FPG duty cycle induced baseline shift compensation

FPG baseline shift for five bunch chopping at 324 MHz Circuit schematic: Duty cycle droop compensation

Timing schematic: Compensation ‘off’ @ 1 μs & 0.5 kV/div Timing schematic: Compensation ‘on’ @ 1 μs & 0.5 kV/div


FPG / SummaryMeasured performance parameters, for the FPG indicate that the design is generally compliant with the FETS specification. Passive techniques to reduce post-pulse aberration can be implemented when the precise configuration of the load circuit is determined.

A scheme to compensate for the duty cycle induced baseline shift, for the case of a fixed or slowly varying duty cycle, has been described, and indicates that the resulting residual baseline shift due to LF cut-off can be balanced around the zero volt level, giving values of ± 1.5 % for five bunch chopping in the FETS MEBT. For the case of a rapidly varying duty cycle, duty cycle induced baseline shift can be eliminated, by utilising an FPG with a bipolar output pulse, resulting in alternate beam bunches, or sets of beam bunches, being deflected, in opposite directions.


Slow Pulse Generator (SPG) development





2.3 m

Beam dump 2(high duty cycle)

M. A. Clarke-Gayther RAL/FETS/HIPPI

SPG development

CARE-07 October 30th 2007

16 close coupled ‘slow’ pulse generator modules

Slow chopperelectrodes

Beam

SPG beam line layout and load analysis


SPG development


Prototype 8 kV SPG euro-cassette module / Side view

Low-inductance HV damping resistors

8 kV push-pull MOSFET switch module

High voltagefeed-through(output port)

Axial cooling fans

Air duct

0.26 m


SPG development


SPG waveforms at ± 4 kV peak & 50 ns / div.

SPG waveform measurement / HTS 41-06-GSM-CF-HFB (4 kV)

SPG waveforms at ± 4 kV peak & 50 μs / div.

Tr =12.0 ns

Tf =10.8 ns

Pulse Parameter FETS Requirement Measured Compliancy CommentAmplitude (kV into 50 Ohms) ± 1.5 ± 4.0 Yes ± 4 kV ratedTransition time (ns) ~ 12.0 Trise ~ 12, Tfall ~ 11 Yes 500 pulsesDuration (μs) 0.23 – 100 0.17 – 100 Yes FWHMDroop (%) 0 0 Yes DC coupledRepetition frequency (MHz) 1.3 1.3 YesBurst duration @ 1.3 MHz 0.3 – 1.5 ms 1 ms Close Limited by coolingBurst repetition frequency (Hz) 50 25 Close Limited by coolingPost pulse aberration (%) ± 5 ≤ ± 5 Yes Damping dependentPulse width stability (ns) ± 0.1 8.2 ns (n=1 to 2) Limited Can be correctedTiming stability (ns over 1 hour) ± 0.5 ± 0.3 Yes Over temperature Burst amplitude stability (%) + 10, - 5 < + 10, -5 Yes Limited by power reg.


SPG development


SPG / Summary

Measured performance parameters indicate that the design is generally compliant with the RAL specification at a burst repetition frequency (BRF) of 25 Hz. Further upgrades to power supplies and cooling should allow testing at the full BRF of 50 Hz.

Measurements show that for positive polarity pulses, there is a step change in the trigger to output pulse delay time between the first pulse in the burst and subsequent pulses, and that the magnitude of the change in delay time between the second pulse in the burst and the subsequent 500 pulses is then less than ~ 1 ns. Although these shifts in delay time are not compliant with the required specification, they can, in principle, be corrected by a programmable compensation technique.


Slow-wave electrode development



Where:

Transverse extent of the beam: L2Beam transit time for distance L1: T(L1) Pulse transit time in vacuum for distance L2: T(L2) Pulse transit time in dielectric for distance L3: T(L3) Electrode width: L4

For the generalised slow wave structure:Maximum value for L1 = V1 (T3 - T1) / 2Minimum Value for L1 = L2 (V1/ V2)T(L1) = L1/V1 = T(L2) + T(L3)

The relationships for field (E), and transverse displacement (x), where q is the electronic charge, is the beam velocity, m0 is the rest mass, z is the effective electrode length, is the required deflection angle, V is the deflecting potential, and d is the electrode gap, are:

zqmE

2

0tan dVE 2

0

2

2

mzEq

x

‘E-field chopping / Slow-wave electrode design



Strategy for the development of RAL slow–wave structures

Modify ESS 2.5 MeV helical and planar designs • Reduce delay to enable 3 MeV operation• Increase beam aperture to ~ 20 mm• Maximise field coverage and homogeneity• Simplify design - minimise number of parts• Investigate effects of dimensional tolerances• Ensure compatibility with NC machining practise• Identify optimum materials

Modify helical design for CERN MEBT• Shrink to fit in 95 mm ID vacuum vessel



RAL Helical B1 & B2 structures



Preliminary test assemblies

Effort during the current reporting period has been directed towards the design, manufacture, and test of three preliminary assemblies that are viewed as an essential first step on the path to the realisation of the full scale planar and helical slow-wave structures. The manufacture and test of these assemblies is expected to provide important information on the following: Construction techniques. NC machining and tolerances. Selection of machine-able ceramics and of copper and aluminium alloys. Electroplating and electro-polishing. Accuracy of the 3D high frequency design code.



Coaxial test assemblies / High frequency models and measurements

The RAL planar and helical electrode designs make use of machine-able ceramic pillars and discs to support and align the transmission line structures. The characteristic impedance of the transmission line at the position of these supports must be carefully controlled using compensating techniques if reflections are to be minimised.

Two candidate ceramic materials have been identified, ‘Shapal-M’, and BN (HBR), and an interchangeable set of coaxial test assemblies has been designed, manufactured, and tested during this reporting period. These assemblies are viewed as an essential first step on the path to the realisation of the full scale planar and helical slow-wave structures.



RAL Planar A2 / Prototype



Coaxial test assembly / Shapal-M version

































Coaxial test assembly / Measurements in the F-domain



Coaxial test assembly / Measurements in the T-domain



Coaxial test assembly / Measurements in the T-domain



Helical B2 / High frequency model



Helical B2 / High frequency model



Helical B2 / CAD view



Helical structure B2 / Short length prototype

UT-390 semi-rigidcoaxial delay lines









RAL Planar A2 / Pre-prototype



RAL Planar A2 / Pre-prototype

Coaxialinterfaceadapter

Extendeddielectricconnector(SMA)



Helical structure B2 / Prototype



Helical structure B2 / Pre-prototype



Coaxial interfaceadapter

Extended dielectricconnector (SMA)

Helical structure B2 / Pre-prototype



‘On-axis field in x, y plane



Simulation of Helical B structure in the T & F domain

Summary


FPG• Meets key specifications

SPG• 4 kV version looks promising

Slow-wave electrode designs• Planar and Helical designs now scaled to 3.0 MeV• Beam aperture increased to 19.0 mm• HF models of components with trim function• Analysis of coverage factor• Analysis of effect of dimensional tolerances• Identification of optimum materials / metallisation• Identification of coaxial components and semi-rigid cable• Designs compatible with NC machining practice

Summary


Some final comments and the next steps

The development of FETS optical scheme A has lowered the working voltage requirement for the FPG and SPG. The existing FPG is now compliant, and the results of recent tests on a 4 kV SPG switch module are promising. Modification of the existing 8 kV euro-cassette design will enable the 4 kV switch to be tested at the specified duty cycle.

The RAL slow wave electrode designs are mechanically more complex than the CERN design, but simulations indicate that E-field coverage factor and transverse uniformity should be superior. The design of planar and helical pre-prototype modules is nearing completion, and results of HF tests should be available by the year end.

Summary


HIPPI WP4: The RAL† Fast Beam Chopper Development Programme Progress Report for the period: July 2005 – December 2006

M. A. Clarke-Gayther †

† STFC Rutherford Appleton Laboratory, Didcot, Oxfordshire, UK

EU contract number RII3-CT-2003-506395 CARE-Note-2007-002-HIPPI

References


M Clarke-Gayther, ‘Slow-wave chopper structures for Next Generation High Power Proton Drivers’, Proc of PAC 2007, Albuquerque, New Mexico, USA, 25th – 29th June, 2007, pp.1637-1639

M Clarke-Gayther, ‘Slow-wave electrode structures for the ESS 2.5 MeV fast chopper’, Proc. of PAC 2003, Portland, Oregon, USA, 12th - 16th May, 2003, pp. 1473-1475

M Clarke-Gayther, G Bellodi, F Gerigk, ‘A fast beam chopper for the RAL Front-End Test Stand’, Proc. of EPAC 2006, Edinburgh, Scotland, UK, 26th - 30th June, 2006, pp. 300-302.

F Caspers, A Mostacci, S Kurennoy, ‘Fast Chopper Structure for the CERN SPL’, Proc. of EPAC 2002, Paris, France, 3-7 June, 2002, pp. 873-875.

F Caspers, ‘Review of Fast Beam Chopping’, Proc. of LINAC 2004, Lubeck, Germany, 16-20 August, 2004, pp. 294-296.

Documents

Beam Chopper Development for Next Generation High Power Proton Drivers