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 (HIPPI WP coordinator) Alessandra Lombardi (WP4 Coordinator) 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) Dan Faircloth, Scott Lawrie (Ion source) Alan Letchford (RFQ / FETS coordinator) Mike Perkins (Ion source power supplies) Jürgen Pozimski (Ion source / RFQ) Pierpaolo Romano (Beam stop) Philip Wise (Mechanical Eng.) Saad Alsari (RF) Simon Jolly, Ajit Kurup (RFQ) David Lee (Laser Diagnostics) Jaroslav Pasternack (UK-NF) Jürgen Pozimski (Ion source/ RFQ) Peter Savage (Mechanical Eng.) John Back (LEBT) Christoph Gabor (Diagnostics) Ciprian Plostinar (MEBT / DTL) Jesus Alonso (ESS) Rafael Enparantza (ESS) Javier Bermejo (ESS)

5. Project History and Plan

6. A Fast Beam chopper for Next Generation Proton Drivers (NGPDs) / Motivation • Key enabling componentfor all NG synchrotron and accumulator ring based proton drivers • Beam loss during trapping is a ‘show stopper’ • Order of magnitude reduction in loss required to support • operating regime of ‘hands on maintenance’ (1W/m) • All existing NGPDs have suboptimal chopper designs

7. A Fast Beam chopper for Next Generation Proton Drivers (NGPDs) / Motivation • FETS will test a unique,UK designed, fast beam chopper with the potential to be the first to demonstrate efficient operation on ring based NGPDs for spallation neutron sources and neutrino factories

8. A Fast Beam chopper for 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

9. Fast beam chopper schemes

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

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

12. 3.0 MeV MEBT Chopper (RAL FETS Scheme A) 4.8 m Chopper 1 (fast transition) Beam dump 1 Chopper 2 (slower transition) Beam dump 2 ‘CCL’ type re-buncher cavities

13. 3.0 MeV MEBT Chopper (RAL FETS Scheme A) 2.4 m Chopper 1 (fast transition) ‘CCL’ type re-buncher cavities Beam dump 1 (low duty cycle)

14. 3.0 MeV MEBT Chopper (RAL FETS Scheme A) 2.4 m Chopper 2 (slower transition) Beam dump 2 (high duty cycle) ‘CCL’ type re-buncher cavities

15. 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

17. Open animated GIF in Internet Explorer

18. Fast Pulse Generator (FPG) development

19. 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

20. FPG waveform measurement

21. Slow Pulse Generator (SPG) development

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

23. 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

24. SPG waveform measurement / HTS 41-06-GSM-CF-HFB (4 kV) Tr =11.3 ns Tf =11.3 ns • SPG waveforms at ± 4 kV peak & 0.2 ms / div. • SPG waveforms at ± 4 kV peak & 50 ns / div.

25. Slow-wave electrode development

26. ‘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)

27. ‘On-axis field in x, y plane

28. Preliminary test assemblies • Coaxial • Helical • Planar

29. Preliminary test assemblies • The manufacture and test of these preliminary assemblies will provide important information on the following: • Construction techniques. • NC machining and tolerances. • Selection of machine-able ceramics and of suitable copper and aluminium alloys. • Electroplating and electro-polishing. • Accuracy of the 3D high frequency design code.

30. Coaxial test assembly

31. Coaxial test assembly / Shapal-M version

32. Helical test assembly

33. Helical B2 / Short length prototype UT-390 semi-rigid coaxial delay lines

34. Helical B2 / CAD view

35. Planar test assembly

36. RAL Planar / Short length prototype

37. RAL Planar / Short length prototype

38. FPG • Meets key specifications • SPG • 4 kV version looks promising • Slow-wave electrode designs • Measurements on coaxial test assembly have: • Verified accuracy of high frequency modelling code • Tested effect of mechanical tolerances • Tested machining properties of selected ceramic material • Measurements on helical test assembly have: • Tested effect of strip-line tolerances and electro-polishing • Probed limitations of NC machining practice

39. Slow-wave electrode designs (continued): • Planar test assembly – design in progress – to test: • Machining properties of ceramic support pillars • Strip-line clamping and positioning tolerances • The design and manufacture of the subsequent planar and helical ‘short length’ prototype structures, will build on the experience gained from the preliminary test assemblies, and should facilitate the choice of a candidate design for the full scale structure.

40. HIPPI WP4: The RAL† Fast Beam Chopper Development Programme Progress Report for the period: January 2007 – June 2008 M. A. Clarke-Gayther STFC Rutherford Appleton Laboratory, Didcot, Oxfordshire, UK

41. M Clarke-Gayther, ‘The development of a fast beam chopper for next generation high power proton drivers’, Proc. of EPAC 2008, Genoa, Italy, 23rd – 27th June, 2008, pp. 3584-3586. 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.