Download
1 / 65
650 likes | 739 Views
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).
E N D
Beam Chopper Development forNext GenerationHigh 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) 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 (MEBT / DTL) John Back (LEBT)
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
The RAL Front-End Test Stand (FETS) Project / Key parameters
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 Voltages: Chop 1: +/- 1.28 kV (20 mm gap) Chop 2: +/- 1.42 kV (18 mm gap) Losses: 0.1 % @ input to CH1, 0.3% on dump 1 0.1% on CH2, 0.3% on dump 2
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)
High peak power loads Control and interface Power supply 9 x Pulse generator cards 1.7 m 9 x Pulse generator cards Combiner 9 x Pulse generator cards 9 x Pulse generator cards FPG / Front View
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 / Summary Measured 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.
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
SPG beam line layout and load analysis Slow chopper electrodes Beam 16 close coupled ‘slow’ pulse generator modules
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
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.
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.
‘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)
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
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.
