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

† STFC Rutherford Appleton Laboratory, Didcot, Oxfordshire, UK EU contract number RII3-CT-2003-506395 CARE-Note-2007-002-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

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

RAL ‘Fast-Slow’ two stage chopping scheme

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

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

KEY PARAMETERS SCHEME A SCHEME B SCHEME C ION SPECIES H- ENERGY (MeV) 3.0 RF FREQUENCY (MHz) 324 BEAM CURRENT (mA) 40 - 60 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 (%) 30 - 100 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 / 0.3 - 2 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 1.3 MHz / 0.3 – 2 ms / 50 Hz 12 ns / 100 ns – 0.1ms 1.3 MHz / 0.3 -2 ms / 50 Hz 15 ns / 100 ns – 0.1 ms/ 1.3 MHz / 0.3 - 2 ms / SLOW CHOPPER EFFECTIVE LENGTH / GAPS (mm) 450 x 0.85 / 18 450 x 0.85 / 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

Fast Pulse Generator (FPG) development

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

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

Slow Pulse Generator (SPG) development

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

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

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.

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

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 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

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

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

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

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.

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

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

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.)

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.)

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

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.

Slow-wave electrode development

‘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

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

Planar structure A 3D cut-away 300 mm

Helical structure B with L - C trimmers and adjustable delay

Helical structure B1

Helical structure B1 Helical structure B2

Helical structure B1 Helical structure B2

Helical structure B1 Helical structure B2

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

‘On-axis field in x, y plane

Simulation of Helical B structure in the T & F domain

RAL Planar A2 (3.0 MeV design)

RAL Planar A2 (3.0 MeV design)

Selection of coaxial and strip-line dielectric support material AL2O3 AlN Shapal M MACOR BN (HBR) Vespel PEEK Dielectric constant (1 MHz) 9.4-9.9 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) 25 - 30 170 96 1.5 33 - 55 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

Vacuum coaxial support disc / HF Simulation

Semi-rigid to vacuum coaxial transition / HF Simulation

Coaxial to strip-line 90° transition / HF simulation

Planar strip-line stand-off / HF simulation

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

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

Strip-line dimensional tolerance analysis

Strip-line dimensional tolerance analysis

Visualisation and development of 3D models

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) 2.18292e7 2.39032e7 Beam width / 100% (mm) 10 18 Beam aperture (mm) 11 19 Cell periodicity (mm) Cell delay (ns) 0.870394 0.794874 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.

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!