Thermionic Bunched Electron Sources for High-Energy Electron Cooling

Transcription

1 Thermionic Bunched Electron Sources for High-Energy Electron Cooling Vadim Jabotinski 1, Yaroslav Derbenev 2, and Philippe Piot 3 1 Institute for Physics and Technology (Alexandria, VA) 2 Thomas Jefferson National Accelerator Facility (Newport News, VA) 3 Northern Illinois University (DeKalb, IL) MEIC Collaboration Meeting SPRING 2016 Jefferson Lab March 29-31, 2016

2 Outline HEEC schemes (e-sources, beam-beam kicker integration) Beam-beam kicker Required electron sources Emission gating and acceleration schemes Obtaining broad range of bunch repetition rates Summary 2

3 Single Current HEEC with Counter ERL ion bunches (cooled) Energy Recovery Beam (ERB) Solenoid (cooling section) ion bunches (hot) Cooling e-bunches SRF ERL 5 to 50 (10-140) MeV Beams separation Pre-accelerating linac, 2 to 5 MeV Subharmonic bunching Acceleration, MeV Depressed collector Magnetized cooling beam CW injector High average current e-source, 1-2 A 2.1 nc, 1-3 ns FWHM, 952 MHz 4.2 nc, 1-3 ns FWHM, 476 MHz 10 ps rms after compression Y. Derbenev Cooling with Magnetized Electron Beam MEIC Spring 2015 Y. Derbenev Head-on ERL for HEEC JLEIC R&D Meeting, CASA, March 17,

4 Circulating Current HEEC with Counter ERL ion bunches (cooled) Solenoid (cooling section) CCR, x100 ion bunches (hot) Cooling e-bunches FBT kicker-out Reverse FBT ERB kicker-in Reverse FBT FBT cooling beam Reduced power SRF ERL Magnetized cooling beam CW injector Reduced average current and repetition rate by a factor of e-source, A average current 1 nc, 1-3 ns FWHM, MHz 2 nc, 1-3 ns FWHM, MHz 10 ps rms after compression 4

5 Circulating Current HEEC Beam-Beam Kicker with Magnet Dipoles dump Kicking sheet e-bunch Kicking sheet e-bunch Kicking beam e-source From CCR Ejected flat e-bunch Injected flat e-bunch To CCR Kicker-out Flat e-bunch from SRF ERL Kicker-in Kicking beam e-source Bunched sheet beam, ~10x150 mm >2 nc, <1ns FWHM, MHz >1 A average current, MeV 5

6 Bunch Compression and Pre-acceleration Solenoid, T To SRF ERL Bunched e-source 36 th subharmonic 36 MHz 1-3 ns FWHM MeV Ballistic Buncher 1 12 th subharmonic 108 MHz 150 ps FWHM Ballistic Buncher 2 3 rd subharmonic 433 MHz 50 ps FWHM Linac 5 MeV Tapered phase velocity buncher Linac frequency, 9-cells 1296 MHz ps FWHM, 2 MeV The scheme and example values adopted from 1. Yeremian et al. Boeing 120 MeV RF linac injector design and accelerator performance comparison with PARMELA Proc. PAC 89 IEEE (1989) 2. C. H. Kim Electron Injector Studies at LBL LBNL Paper LBL (2010) 3. N. S. Sereno Booster Subharmonic RF Capture Design APS ANL, LS-297 (2002) 6

7 Gating Pulsed [1] Limitations Required Electron Source Emission Gating and Acceleration Limited repetition rate to low subharmonics Jitter errors (gap voltage, current, bunch charge, timing) Poor to no control HOM Limited grid voltage, 200 V (small gap, dense grid, higher emittance) Limited DC floating, 100 kv I gap ~ C gap+c circuit j peak d2 2 3 gap d gap =0.25 mm, j peak =11A/cm 2 U gap = 200 V τ rise C + gap C circuit = 30 pf, t rise = 0.3 ns I gap = 20 A C circuit co-sources jitter. j peak and d gap limit I gap 1. M. J. Browne et al. A multi-channel pulser. for the SLC thermionic electron source PAC 85 SLAC-PUB RF harmonics Repetition rate from the linac frequency to its low subharmonics Advantages No jitter sources DC floating, 500 kv For two and more harmonics, higher grid voltage, >200 V attainable (larger gap, less dense grid, lower emittance) 7

8 Acceleration DC RF TM 010, l/4 Required Electron Source Emission Gating and Acceleration 10 MV/m (up to 30 MV/m with Mo) possible, CW (advantage) Applicable to long bunches, no RF curvature (advantage) HV DC insulation, Floating cathode (limitation) Limited to 0.5 MeV (limitation) no HV DC insulation (advantage) Higher energies > 0.5 MeV in Linacs attainable (advantage) Limited accelerating gradient, <7 MV/m CW (limitation) Due to larger TM 010 cavity, bunch duration to be <0.3 ns FWHM (limit.) l/4 structures can work with longer bunches, <100 ns FWHM (advan.) Cooling beam CW e-source Bunched magnetized beam, ~3 mm radius A average current 2.1 nc, 1-3 ns FWHM, MHz 4.2 nc, 1-3 ns FWHM, MHz 10 ps rms after compression Kicking beam CW e-source Bunched sheet beam, ~10x150 mm >1 A average current, MeV >2 nc, <1ns FWHM, MHz 8

9 Bunched electron sources. Gating and acceleration Single frequency gating of thermionic emission Gridded Cathode DC (IOTs, TRIUMF) or RF (TM 010 or l/4) Acceleration Drawback: long bunch duration (slide 12) Acceleration Single-frequency RF source Voltage break Bidirectional coupler Coaxial transmission line Slug tuner Thermionic cathode Grid 9

10 Dual-Frequency Gating of Thermionic Emission 1 st and (2n+1)l/4-modes 3 rd -harmonic 3 rd harmonic RF source Bidirectional coupler Beam magnetizing solenoid < 30 mm bore radius RF (TM 010 or l/4) or DC acceleration 1 st harmonic RF source Low/high pass rejection filter Slug tuner 3 rd harmonics tuning Voltage break Coaxial transmission lines V. Jabotinski et al. A Dual-Frequency Approach to a High Cathode region Average Current Thermionic Source MEIC Fall

11 Electric field (MV/m) Electric field (MV/m) Dual-Frequency Gating of Thermionic Emission Shortening Bunch Duration 7 injection phase Cavity mode (reference) 7 injection phase Cavity mode (reference) 0 Bias emission duration Emission gating Phase/p emission duration Emission gating 1 st +3 rd harmonics Phase/p Gating the emission with the 1 st harmonic (IOT, TRIUMF) Gating the emission with the 1 st and 3 rd harmonics. 11

12 Electric field (MV/m) Electric field (MV/m) Bias emission duration RF Gating of the Emission Effect of higher harmonics Emission gating 1 st +3 rd harmonics Phase/p 1 st harmonic 1 st vs. 1 st + 3 rd Amplitude x7.9 (RF power x62.4) Bias x τ > 4 sin 1 j peak d gap E peak (rad) t>92.6 o at 20 A/cm 2, 0.5 mm, 5 MV/m emission duration Phase/p 1 st 1 st + 3 rd 1 st + 3 rd + 5 th 1 st + 3 rd + 8 th Harmonics Emission duration (deg.) 1 st st + 3 rd st + 3 rd + 5 th st + 3 rd + 8 th

13 Kicking Sheet Beam E-source Low subharmonic repetition rate 3 rd harmonic 30 MHz 1 st harmonic 10 MHz MeV RF or DC acceleration 10 MHz sweep Sheet beam slit aperture Sheet beam gridless thermionic cathode, e.g. 0.5 x 10 mm 4-11 ns FWHM sheet beam e-bunch Kicking sheet beam e-bunch < 1 ns FWHM, 10 MHz Sweeping is not needed for >40 MHz repetition rates or can be avoided with 3-harmonics gating for the lower frequencies, 10 MHz. 13

14 E (V/m) RF Gated Thermionic Electron Source Low subharmonic repetition rates l/4-mode MHz 3l/4-mode MHz x (mm) 14

15 f l/4 (MHz) x/(l/4) f 3l/4 / f l/ RF Gated Thermionic Electron Source Low subharmonic repetition rates x < l/ f l/4 (MHz) Two off-axis ports for 3-harmonics gating f l/4 (MHz) x (mm) 15

16 RF Gated Thermionic Electron Source Quarter-wave bunching structure x=1 m 74.4 MHz 16

17 RF Gated Thermionic Electron Source Quarter-wave bunching structure with ERB drift tube Radius 50 mm 663 MHz 17

18 Summary We have considered HEEC schemes, identified critical components, their integration, and requirements including the needed electron sources and beam-beam kicker. Beam-beam kicker scheme using magnet dipoles is proposed Thermionic emission is inherently suitable for attaining high average current electron beams that are imperative for HEEC Methods for the emission gating and acceleration have been preliminary explored and e-sources for the cooling and the kicking beams are presented. Techniques aimed at low subharmonic repetition rates along with the linac frequency from the thermionic e-sources are discussed. Preliminary studies outlined the most critical approaches important to developing highly efficient HEEC and the electron sources. 18

19 Back up slides 19

20 Counter ERL. In-Solenoid Beams Separation ion bunches (cooled) ERB Solenoid (cooling section) ion bunches (hot) Cooling e-bunches Side SRF ERL Linac Staggered solenoid with movable pole pieces. Beams separation Acceleration Bunching Depressed collector Magnetized cooling beam CW injector e-source, 1-2 A average current 2.1 nc, 1-3 ns FWHM, 952 MHz 4.2 nc, 1-3 ns FWHM, 476 MHz 10 ps rms after compression Bent solenoid drift: Y. Derbenev Head-on ERL for HEEC JLEIC R&D Meeting, CASA, March 17,

21 Beams Separation using Bent Solenoid Drift To/from Linac Orbits are separated perpendicular to the viewing (top) plane Buncher Acceleration Bent Solenoid [1] Side e-source Spent ERB Twisted Solenoid Staggered solenoid Staggered solenoid movable coil sections movable pole pieces Y. Derbenev Head-on ERL for HEEC JLEIC R&D Meeting, CASA, March 17, 2016

22 Single Current HEEC Concurrent SRF ERL. Counter Injector Linac Solenoid (cooling section) ion bunches (cooled) ion bunches (hot) FBT Cooling e-bunches Reverse FBT ERB kicker-out (~10 MeV) or Dipole (>>5 MeV) 5 MeV SRF ERL ERB kicker-in (~10 MeV) or Dipole (>>5 MeV) Flat ERB, MeV 5 MeV Reverse FBT FBT Magnetized cooling beam CW injector e-source, 1-2 A average current 2.1 nc, 1-3 ns FWHM, 952 MHz 4.2 nc, 1-3 ns FWHM, 476 MHz 10 ps rms after compression 22

23 Circulating Current HEEC Concurrent SRF ERL. Counter Injector Linac Solenoid (cooling section) ion bunches (cooled) ion bunches (hot) FBT kicker-out ERB kicker-out (~10 MeV) or Dipole (>>5 MeV) Reduced power SRF ERL ERB kicker-in (~10 MeV) or Dipole (>>5 MeV) CCR, x100 Flat ERB, MeV 5 MeV kicker-in 5 MeV Reverse FBT FBT Reverse FBT Cooling e-bunches Magnetized cooling beam CW injector Reduced average current and repetition rate by a factor of e-source, A average current 1 nc, 1-3 ns FWHM, MHz 2 nc, 1-3 ns FWHM, MHz 10 ps rms after compression 23

24 RF Gated Thermionic Electron Source Two off axis ports for three-harmonics gating f/3-120 mm f=451 MHz f/3-120 mm ~ 3f 24

25 10 mm RF Gated Thermionic Electron Source Quarter-wave bunching structure, E-field plots 74.4 MHz 25

26 RF Gated Thermionic Electron Source Quarter-wave bunching structure with ERB drift channel ERB drift ERB drift (35 mm off axis) cooling beam cooling beam 663 MHz 26

27 Dual-Frequency Thermionic Electron Source Modeling and Simulations Dual-frequency emission gating RF structure Grid region Thermionic cathode 3mm beam radius Cathode-grid gap mm RF accelerating cavity half-cell, 1 st harmonic 27