1. Beam Chopper Development for Next Generation High 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

4. † STFC Rutherford Appleton Laboratory, Didcot, Oxfordshire, UK
EU contract number RII3-CT
CARE-Note HIPPI
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

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

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

7. 3.0 MeV MEBT Chopper (RAL FETS Scheme C)
‘CCL’ type
re-buncher
cavities
Chopper 1 (fast transition)
Chopper 2 (slower transition)

8. FETS Scheme A / Beam-line layout and GPT trajectory plots
Voltages:
Chop 1: +/ kV (20 mm gap)
Chop 2: +/ kV (18 mm gap)
Losses:
0.1 input to CH1, 0.3% on dump 1
0.1% on CH2, 0.3% on dump 2

9. KEY PARAMETERS
SCHEME A
SCHEME B
SCHEME C
ION SPECIES H-
ENERGY (MeV)
3.0
RF FREQUENCY (MHz)
324
BEAM CURRENT (mA)
NORMALISED RMS INPUT EMITTANCE IN X / Y / Z PLANES
( π.mm.mr & π.deg.MeV)
0.25 / 0.25 / 0.18
RMS EMITTANCE GROWTH IN X / Y / Z PLANES (%)
6 / 13 / 2
4 / 8 / 0
5 / 8 / 0
CHOPPING FACTOR (%)
CHOPPING EFFICIENCY (%)
99.9
FAST CHOPPER PULSE: TRANSITION TIME / DURATION / PRF/ BURST DURATION / BRF
2 ns / 12 ns / 2.6 MHz / 0.3 – 2 ms / 50 Hz
2 ns / 15 ns / 2.6 MHz /
ms / 50 Hz
FAST CHOPPER ELECTRODE EFFECTIVE LENGTH / GAPS (mm)
450 x 0.82 = 369 / 20
FAST CHOPPER POTENTIAL(kV)
± 1.3
± 1.2
± 1.4
SLOW CHOPPER PULSE: TRANSITION TIME / DURATION /
PRF/ BURST DURATION /
BRF
12 ns / 100 ns – 0.1 ms MHz / 0.3 – 2 ms / 50 Hz
12 ns / 100 ns – 0.1ms MHz / ms /
50 Hz
15 ns / 100 ns – 0.1 ms/ 1.3 MHz / ms /
SLOW CHOPPER EFFECTIVE LENGTH / GAPS (mm)
450 x 0.85 / 18
450 x / 18
450 x 0.85 / 14
SLOW CHOPPER POTENTIAL (kV)
± 1.5
± 2.0
± 5.0
POWER ON FAST / SLOW BEAM DUMPS (W)
150 / 850
OPTICAL DESIGN CODE(S)
GPT

10. Fast Pulse Generator (FPG) development

11. 9 x Pulse generator cards
SPG / Front View
9 x Pulse generator cards
High peak power loads
Control and interface
Combiner
Power supply
1.7 m

12. SPG waveform measurement / HTS 81-06-GSM-CF-HFB

13. Slow Pulse Generator (SPG) development

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

15. 8 kV SPG pre-prototype Test Set-up
- 8 kV ~ 5 μF LF cap. bank
HV damping
resistor
8 kV push-pull MOSFET switch
+ 8 kV ~ 5 μF LF cap. bank
+ 8 kV ~ 3 nF HF cap. bank
- 8 kV ~ 3 nF HF cap. bank
Two turn load inductance
~ 50 nH
Load capacitance
~ 30 pf
6 kV, 400 MHz ÷ 1000 probe
Trigger input
Auxiliary power supplies
Cooling fan

16. SPG waveform measurement /HTS 81-06-GSM HFB
Tr =11.9 ns
Tr =15.5 ns
Tf =11.1 ns
Tf =19.7 ns
SPG waveforms at ± 6 kV peak & 50 ns / div.
SPG waveforms at ± 6 kV peak & 50 ns / div.
SPG waveforms at ± 6 kV peak & 2.0 μs / div.
SPG waveforms at ± 6 kV peak & 50 μs / div.

17. Measured performance parameters / HTS 81-06-GSM HFB 8kV SPG
Pulse Parameter
ESS Requirement
Measured
Compliancy
Comment
Amplitude (kV into 50 Ohms)
± 6.0
± 4.0
Yes
± 4 kV rated
Transition time (ns)
~ 12.0
Trise ~ 13, Tfall ~ 12
Limited
First ~ 10 pulses
Duration (μs)
0.2 – 100
FWHM
Droop (%)
DC coupled
Repetition frequency (MHz)
1.2
Burst duration @ 1.2 MHz
1.5 ms
Burst limitation
Burst repetition frequency (Hz) 50
Duty cycle ~ 0.27 %
Post pulse aberration (%)
± 2
≤ ± 2
Pulse width stability (ns)
± 0.1
≤ ± 0.1
+ve shifting, -ve OK
Timing stability (ns over 1 hour)
± 0.5
± 0.4
Peak to Peak
Burst amplitude stability (%)
+ 10, - 5
< + 10, -5
@ 0.1 MHz PRF

18. 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

19. SPG Development Plan / October 2006
Bench test 4 kV rated switch
Compare results with existing 8 kV rated switch
Re-formulate specification for SPG
Based on new optical design for FETS
Obtain quotes for a custom designed switch
Based on re-formulated specification for FETS

20. BEHLKE HTS 41-06-GSM-CF-HSB (4kV) & 81-06-GSM-CF-HSB (8kV)

21. 4kV MOSFET switch (BEHLKE HTS 41-06-GSM-CF-HSB) / Test Set-Up

22. 4kV MOSFET switch (BEHLKE HTS 41-06-GSM-CF-HSB) / Test Set-Up

23. SPG waveform measurement / HTS 41-06-GSM-CF-HFB (4 kV)
Tr =12.0 ns
Tr =11.2 ns
Tf =10.8 ns
Tf =10.8 ns
SPG waveforms at ± 4 kV peak & 50 ns / div.
SPG waveforms at ± 4 kV peak & 50 ns / div.
SPG waveforms at ± 4 kV peak & 2.0 μs / div.
SPG waveforms at ± 4 kV peak & 50 μs / div.

24. Measured performance parameters / HTS 41-06-GSM-CF-HSB (4kV)

25. Measured performance parameters / HTS 41-06-GSM-CF-HSB (4kV)

26. Fast-Slow Chopper / FPG & SPG synchronisation / ESS Timing
FPG (0.2 ms/div.)
FPG (4.0 μs/div.)
SPG (0.2 ms/div.)
SPG (4.0 μs/div.)

27. Fast-Slow Chopper / FPG & SPG synchronisation / ESS Timing
FPG (0.2 ms/div.)
FPG (4.0 μs/div.)
SPG (0.2 ms/div.)
SPG (4.0 μs/div.)

28. Measured performance parameters / HTS 41-06-GSM-CF-HSB (4kV) SPG
Pulse Parameter
FETS Requirement
Measured
Compliancy
Comment
Amplitude (kV into 50 Ohms)
± 1.5
± 4.0
Yes
± 4 kV rated
Transition time (ns)
~ 12.0
Trise ~ 12, Tfall ~ 11
500 pulses
Duration (μs)
0.23 – 100
0.17 – 100
FWHM
Droop (%)
DC coupled
Repetition frequency (MHz)
1.3
Burst duration @ 1.2 MHz
0.3 – 1.5 ms
0.4 ms
Limited
Scalable
Burst repetition frequency (Hz) 50 20
Post pulse aberration (%)
± 5
≤ ± 5
Adjustable
Pulse width stability (ns)
± 0.1
8.2 ns (n=1 to 2)
Can be corrected
Timing stability (ns over 1 hour)
± 0.5 -
Not yet measured
Burst amplitude stability (%)
+ 10, - 5
< + 10, -5
0.4 ms burst

29. Summary / 4 kV SPG development
Transition time and transition time stability are now compliant (just) with four bunch chopping at 324 MHz.
Maximum burst duration at 50 Hz BRF will be tested with an upgraded auxiliary power supply and improved cooling.
Timing stability (jitter) will be tested when the auxiliary power supply and cooling have been upgraded.
The 4kV SPG results are encouraging – particularly the improved transition time and pulse duration stability.

30. Slow-wave electrode development

31. ‘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)

32. 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

33. ESS planar and Helical slow-wave electrode designs
Helical B
Helical C

34. Planar structure A
3D cut-away
300 mm

35. Helical structure B with L - C trimmers and adjustable delay

36. Helical structure B1

37. Helical structure B1
Helical structure B2

38. Helical structure B1
Helical structure B2

39. Helical structure B1
Helical structure B2

40. RAL helical B / Field in x - y plane/ line integrals along z
8.0 mm radius
inscribed circle

41. ‘On-axis field in x, y plane

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

43. RAL Planar A2 (3.0 MeV design)

44. RAL Planar A2 (3.0 MeV design)

45. Selection of coaxial and strip-line dielectric support material
AL2O3
AlN
Shapal M
MACOR
BN (HBR)
Vespel
PEEK
Dielectric constant
(1 MHz)
8.7
6.9
5.9
4.1
3.55
3.3
Loss Tangent (1 MHz)
< 1 e-3
5 e-4
1 e-3
5 e-3
< 5 e-4
3 e-3
Thermal conductivity
(W/m oC)
170 96
1.5
0.35
0.25
Flexural Strength MPa
~ 400
~ 250
~ 100
~ 50
~ 150
Service temperature
(In vacuum)
1500
1000
800
350
300
Metallise-able Y Y* ? N
Machine-able
Diamond

46. Vacuum coaxial support disc / HF Simulation

47. Semi-rigid to vacuum coaxial transition / HF Simulation

48. Coaxial to strip-line 90° transition / HF simulation

49. Planar strip-line stand-off / HF simulation

50. Planar strip-line components / HF simulation
Beam aperture
180 degree bend

51. Strip-line dimensional tolerance analysis
Plot variation in Z0 with:
Strip width
Displacement in x & y planes
Strip edge radius
Strip thickness

52. Strip-line dimensional tolerance analysis

53. Strip-line dimensional tolerance analysis

54. Visualisation and development of 3D models

55. RAL slow-wave electrode structures / Key parameters
Design parameter
Planar A1 & A2
Helical B1
Helical B2
Helical C
ESS
FETS
H‾ beam energy (Mev)
2.5
3.0
Beam velocity (m/s) e7 e7
Beam width / 100% (mm) 10 18
Beam aperture (mm) 11 19
Cell periodicity (mm)
Cell delay (ns)
Coverage factor:
Centre / Edge (%)
80 / 77
82 / 81
80 / 75
81 / 79
Characteristic impedance (Ω)
50 ± 0.5
External dimensions (mm)
45 x 300
x 400
45 x 280
x 450
< 75 rad.
< 48 rad.
< 70 rad.

56. As usual – the Devil is in the details!
FPG
Meets most 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
As usual – the Devil is in the details!