Beam Chopper Development for Next Generation High Power Proton Drivers

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

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

3. 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

4. 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)

5. Project History and Plan

6. 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

7. Fast beam chopper schemes

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

9. RAL ‘Fast-Slow’ two stage chopping scheme

10. 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

11. 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)

12. 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

13. 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

14. Open animated GIF in Internet Explorer

16. Fast Pulse Generator (FPG) development

17. 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

18. FPG waveform measurement

19. Slow Pulse Generator (SPG) development

20. SPG beam line layout and load analysis Slow chopper electrodes Beam 16 close coupled ‘slow’ pulse generator modules

21. 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

22. 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.

23. Slow-wave electrode development

24. ‘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)

25. 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

26. RAL Planar A2 / Prototype

27. RAL Planar A2 / Prototype

28. RAL Planar A2 / Pre-prototype

29. RAL Planar A2 / Pre-prototype Coaxial interface adapter Extended dielectricconnector (SMA)

30. Helical structure B2 / Prototype UT-390 semi-rigid coaxial delay lines

31. Helical structure B2 / Prototype

32. Helical structure B2 / Pre-prototype

33. Helical structure B2 / Pre-prototype Coaxial interface adapter Extended dielectricconnector (SMA)

34. ‘On-axis field in x, y plane

35. Simulation of Helical B structure in the T & F domain

36. 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

37. 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.

38. 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

39. 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.