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Beam Chopper Development for Next Generation High Power Proton Drivers. Michael A. Clarke-Gayther. RAL / FETS / HIPPI. Outline. Overview Fast Pulse Generator (FPG) Slow Pulse Generator (SPG) Slow – wave electrode designs Summary. Mike Clarke-Gayther (WP4 Fast Beam Chopper & MEBT).

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Beam Chopper Development forNext GenerationHigh Power Proton Drivers

Michael A. Clarke-Gayther




  • Overview
  • Fast Pulse Generator (FPG)
  • Slow Pulse Generator (SPG)
  • Slow – wave electrode designs
  • Summary

Mike Clarke-Gayther

(WP4 Fast Beam Chopper & MEBT)

Maurizio Vretenar

(WP Coordinator)

Alessandra Lombardi

(WP4 Leader)

Luca Bruno, Fritz Caspers

Frank Gerigk, Tom Kroyer

Mauro Paoluzzi

Edgar Sargsyan, Carlo Rossi

Chris Prior (WP Coordinator)

Ciprian Plostinar

(WP2 & 4 N-C Structures / MEBT)

Christoph Gabor (WP5 / Beam Dynamics)


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)

Aaron Cheng (LPRF)

Simon Jolly (LEBT Diagnostics)

Ajit Kurup (RFQ)

David Lee (Diagnostics)

Jürgen Pozimski (Ion source/ RFQ)

Peter Savage (Mechanical Eng.)

Christoph Gabor

(Laser diagnostics)

Ciprian Plostinar


Javier Bermejo

Pierpaolo Romano

(LEBT / Beam stop)

John Back (LEBT)


A Fast Beam chopper


Next Generation Proton Drivers / Motivation

  • To significantly reduce beam loss at trapping / extraction
    • Enables ‘Hands on’ maintenance (1 Watt / m)
  • To support complex beam delivery schemes
    • Enables low loss ‘switchyards’ and duty cycle control
  • To provide beam diagnostic function
    • Enables low duty cycle (i.e. ‘low risk)’ accelerator tuning

3.0 MeV MEBT Chopper (RAL FETS Scheme A)

4.6 m

Chopper 1 (fast transition)

Beam dump 1

Chopper 2 (slower transition)

Beam dump 2

‘CCL’ type re-buncher cavities


3.0 MeV MEBT Chopper (RAL FETS Scheme A)

2.3 m

Chopper 1 (fast transition)

‘CCL’ type re-buncher cavities

Beam dump 1 (low duty cycle)


3.0 MeV MEBT Chopper (RAL FETS Scheme A)

2.3 m

Chopper 2 (slower transition)

Beam dump 2

(high duty cycle)

‘CCL’ type re-buncher cavities


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


Chop 1: +/- 1.28 kV (20 mm gap)

Chop 2: +/- 1.42 kV (18 mm gap)


0.1 % @ input to CH1, 0.3% on dump 1

0.1% on CH2, 0.3% on dump 2


High peak power loads

Control and interface

Power supply

9 x Pulse generator cards

1.7 m

9 x Pulse generator cards


9 x Pulse generator cards

9 x Pulse generator cards

FPG / Front View


Specified by:

M. Clarke-Gayther

Supplied by:

Kentech Instruments

Wallingford, UK


Specified by:

M. Paoluzzi

Supplied by:

FID Technology

St. Petersburg,



SPG beam line layout and load analysis

Slow chopper



16 close coupled ‘slow’ pulse generator modules


Prototype 8 kV SPG euro-cassette module / Side view

Axial cooling fans

Air duct

High voltage


(output port)

0.26 m

8 kV push-pull MOSFET switch module

Low-inductance HV damping resistors


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

Tr =12.0 ns

Tf =10.8 ns

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

‘E-field chopping / Slow-wave electrode design

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:


Transverse extent of the beam: L2

Beam 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) / 2

Minimum Value for L1 = L2 (V1/ V2)

T(L1) = L1/V1 = T(L2) + T(L3)


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 Planar A2 / Pre-prototype








Helical structure B2 / Prototype

UT-390 semi-rigid

coaxial delay lines


Helical structure B2 / Pre-prototype

Coaxial interface


Extended dielectricconnector (SMA)


‘On-axis field in x, y plane

CERN Planar:

(F. Caspers,

T. Kroyer)

Supplied by:

Kyocera Corp.




    • 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

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.


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


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, 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.

M Clarke-Gayther, ‘Fast-slow beam chopping for next generation high power proton drivers’, Proc. of PAC 2005, Knoxville, Tennessee, USA, 16th – 20th May, 2005, pp. 3637-3639

M Clarke-Gayther, ‘A fast beam chopper for next generation proton drivers’, Proc. of EPAC 2004, Lucerne, Switzerland, 5th – 9th July, 2004, pp. 1449-1451

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

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

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