Beam Chopper Development for Next Generation High Power Proton Drivers

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Beam Chopper Development forNext
GenerationHigh Power Proton Drivers
Michael A. Clarke-Gayther
RAL / FETS / HIPPI
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Outline

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

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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
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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 (MEBT / DTL)
John Back (LEBT)
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Project History and Plan
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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

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Fast beam chopper schemes
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The RAL Front-End Test Stand (FETS) Project / Key
parameters
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RAL Fast-Slow two stage chopping scheme
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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
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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)
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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
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FETS Scheme A / Beam-line layout and GPT
trajectory plots
Losses 0.1 _at_ input to CH1, 0.3 on dump 1 0.1
on CH2, 0.3 on dump 2
Voltages Chop 1 /- 1.28 kV (20 mm gap) Chop
2 /- 1.42 kV (18 mm gap)
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Fast Pulse Generator (FPG) development
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FPG / Front View
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FPG waveform measurement
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Slow Pulse Generator (SPG) development
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SPG beam line layout and load analysis
Slow chopper electrodes
Beam
16 close coupled slow pulse generator modules
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Prototype 8 kV SPG euro-cassette module / Side
view
Axial cooling fans
Air duct
High voltage feed-through (output port)
0.26 m
8 kV push-pull MOSFET switch module
Low-inductance HV damping resistors
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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.
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Slow-wave electrode development
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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
Where 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)
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• Strategy for the development of RAL slowwave
  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

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RAL Planar A2 / Prototype
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RAL Planar A2 / Prototype
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RAL Planar A2 / Pre-prototype
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RAL Planar A2 / Pre-prototype
Coaxial interface adapter
Extended dielectricconnector (SMA)
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Helical structure B2 / Prototype
UT-390 semi-rigid coaxial delay lines
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Helical structure B2 / Prototype
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Helical structure B2 / Pre-prototype
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Helical structure B2 / Pre-prototype
Coaxial interface adapter
Extended dielectricconnector (SMA)
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On-axis field in x, y plane
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Simulation of Helical B structure in the T F
domain
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• 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

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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.
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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
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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.