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Development of an ERL Electron Cooler

Development of an ERL Electron Cooler. Speaker: Stephen Benson, Jefferson Lab Collaborator(s): Slava Derbenev, Fay Hannon, Andrew Hutton, Bob Rimmer, Yves Roblin, Chris Tennant, Haipeng Wang, He Zhang, Yuhong Zhang. JLEIC Collaboration Meeting April 1-3, 2019. Outline.

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Development of an ERL Electron Cooler

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  1. Development of an ERL Electron Cooler Speaker: Stephen Benson, Jefferson Lab Collaborator(s): Slava Derbenev, Fay Hannon, Andrew Hutton, Bob Rimmer, Yves Roblin, Chris Tennant, Haipeng Wang, He Zhang, Yuhong Zhang JLEIC Collaboration Meeting April 1-3, 2019

  2. Outline • Cooler design specifications • Cooling simulation status • CCR design. • Injection/extraction scheme • Injector design • Summary (future work)

  3. JLEIC ion injector chain for 200 GeV • What has changed: • Add a fullsize high energy booster (HEB), also functioning as a stacker • Ramping one ring from 8 GeV proton (and possibly 6.4 GeV p equivalent Pb82+) to 200 GeV equivalent is risky, • Most of the bunch formation process can be done in the HEB, increasing the collider duty factor and time averaged luminosity • The low energy booster (LEB) changed to normal conducting magnets • Circumference almost doubled • Lower the linac energy to ~150 MeV for cost reduction • Double the LEB cycles to 54-56 ~8GeV H+ 2GeV Pb67+ ~200GeV H+ ~78GeV Pb82+ ~12.1GeV H+ ~4.28GeV Pb82+ Collider ring with BB cooler 150 MeV H- 40 MeV Pb67+ Low Energy Booster with DC cooler H- Strip Pb Strip Pb Strip Fullsize high energy booster/ stacker with DC cooler Bucket-to-bucket transfer Multi-turn injection Bucket-to-bucket transfer ion sources SRF Linac

  4. Proton Cooling Scheme • DC magnetized cooling is used in the High Energy Booster to cool the long bunches before bunch formation. • Bunched beam cooling is used in the collider, mostly at intermediate energies. Energy Collider High-energy booster Low-energy booster Bunch splitting/capture Up to 200 GeV Pre-bunch splitting Emittance preservation ε reduction • Stacking, • ε preservation ~20 GeV Bunched Beam ERL cooler Up to 110 MeV 12.1 GeV Injection, accumulation 8 GeV DC cooler 4.3 MeV 150 MeV Time

  5. Lead Ion Cooling Scheme • Heavy ions need cooling in both boosters and the collider. • Space charge is a problem so the stacking and cooling are at different energies in the high energy booster. • BBC is more than for protons so not as much cooling current is needed. Energy Collider High-energy booster Low-energy booster Bunch Splitting/capture Up to 80 GeV/u Pre-bunch splitting Emittance preservation ε reduction ~20 GeV/u Injection, accumulation Stacking, ε preservation Bunched Beam ERL cooler using up to 44 MeV e-beam 4.25 GeV/u DC cooler 21 keV 2 GeV/u DC cooler 2.3 MeV DC cooler 1.1 MeV 41 MeV/u Time

  6. Baseline Design is Cooling Ring Fed by ERL • Same-cell energy recovery in 476.3 MHz SRF cavities with Harmonic linearizer • Uses harmonic kicker to inject and extract from CCR (divide by 11) • Assumes high charge, low rep-rate injector (w/ harmonic linearizer acceleration) • Use magnetization flips to compensate ion spin effects

  7. Strong Cooler Specifications (Electrons) • Energy 20–110 MeV • Charge 1.6 (3.2) nC • CCR pulse frequency 476.3 MHz • Gun frequency 43.3 MHz • Bunch length (tophat) 3 cm (17°) • Thermal (Larmor) emittance <19 mm-mrad • Cathode spot radius 3.1 mm • Cathode field 0.05 T 3 • Normalized hor. drift emittance 36 mm-mrad • rms Energy spread (uncorr.)* 3x10-4 • Energy spread (p-p corr.)* <6x10-4 • Solenoid field 1–2 T • Electron beta in cooler 37.6 cm • Solenoid length 4x15 m • Bunch shape beer can

  8. Cooler Specifications (protons) Case 1 – 44.7 GeV center of mass energy • Energy 100 GeV • Particles/bunch 1.0x1010 • Repetition rate 476.3 MHz • Bunch length (rms) 4.0 cm • Normalized emittance (x/y) 1.0/0.2 mm-mrad • Betatron function in cooler 100 m (at point between solenoids) Case 2 – 80.0 GeV center of mass energy • Energy 160 GeV • Particles/bunch 1.4 x1010 • Repetition rate 238.2 MHz • Bunch length (rms) 4 cm • Normalized emittance (x/y) 2.0/0.4 mm-mrad • Betatron function in cooler 100 m (at point between solenoids) Ion ring lattice may be coupled or dispersed in solenoid. Ion beam may be partially offset from the electron beam.

  9. He Zhang Cooling Rate is Not the Same in All Dimensions • Proton beam (CM energy 44.7 GeV): • Energy: 100 GeV • Proton number: 1.0x1010 • Normalized emit. (rms): 1.0/0.20 μm • Bunch length (rms): 4.0 cm • No transverse coupling • No dispersion at the cooler • =100 m at the cooler • Electron beam: • Current: 3.2 nC/bunch • Beer can shape • Radius: 0.528 mm • Full bunch length: 3.0 cm • 0.246 eV, eV • Cooler length: 30 m 2

  10. Consequence of Mismatch Electron beam 3.2 nC • Proton beam (CM energy 44.7 GeV): • Energy: 100 GeV • Proton number: 0.804x1010 (82%) • Normalized emit. (rms): 0.50/0.15μm • Beta function in cooler: 60/200 m Longitudinal overcooling reduces the bunch length, which increases the charge density and thus the IBS rate. Transverse equilibrium is broken. Will try to decrease RF to keep bunch long.

  11. Maintain the emittance w. 3.2 nC/bunch e- beam • Momentum spread: 8x10-4 • Bunch length (rms): 4 cm • Dispersion at cooler: 2.0/0.8 m • Transverse coupling: 50% • Proton beam (CM energy 44.7 GeV): • Energy: 100 GeV • Proton number: 0.59x1010 (60%) • Normalized emit. (rms): 1.0/0.2μm • Electron beam: • Current: 3.2 nC/bunch • Beer can shape • Radius: 0.528 mm • Full bunch length: 3.0 cm • 0.246 eV, eV • Cooler length: 30 m 2

  12. Circulating Cooler Ring Specifications The proposed design is to use a Circulating Cooling Ring (CCR) to provide high current in the cooler (~1 A) without requiring such high current in the electron source. • The CCR has the following requirements: • Isochronous. • Achromatic • Need RF compensation to counter SC and CSR • High periodicity with rational tune • Moderate size • Local axial symmetry • Local isochronicitysmall compaction oscillations (for µBI) • Local dispersion suppression • No tune resonances except for coupling resonance • We would also like the ring to use conventional magnet and vacuum chamber technology as far as possible. Should take advantage of CSR shielding.

  13. Simple Arc Layout • design by D. Douglas dipole quadrupole sextupole

  14. Lattice Functions

  15. Microbunching Gain for Simple bend • mBI gain is ≤ unity • needs to be less than unity for multiple passes (gain grows exponentially)

  16. Longitudinal Phase Space Comparison: After 10-Turns elegant – Stupakov + RF correction elegant – without csrdrifts Bmad – with shielding

  17. Performance of CCR at 1.6 nC

  18. Exchange Region Layout • CCR back leg • ERL to CCR

  19. Harmonic Kicker (G. Park) Harmonic Beam Kicker. A first 952.6 MHz copper cavity has been prototyped, bench measured, and satisfies beam dynamic requirements for a Circular Cooler Ring design for the bunched electron cooler.

  20. Injector Design (Fay Hannon) • Currently uses DC photocathode gun with room temperature buncher. • The SRF booster will have three single cell cavities followed by a 3rd harmonic linearizer. • Optimization in progress

  21. Preliminary results

  22. Issues and Potential Solutions • Injector bunch is not a top hat • Lower frequency and add harmonic RF • CSR and space charge accelerate the bunch ends • Go to longer bunch • Simple arc does not preserve magnetization • New arc design?

  23. Voltage with 3rd Harmonic and phase and amplitude offsets If we want to accelerate a very long bunch and then stretch it out even more we can use 3rd harmonic cavities in the linac. Before going into the CCR, take out the slope using a 952.6 MHz de-chirper. We can also put in a quartic correction if necessary by changing the amplitude

  24. ERL Linac Four 3-cell 476.3 MHz Cavities Two 5-cell 1428.9 MHz Cavities

  25. Summary: Where are We, and Where Do We Go? • ERL Design • Add doglegs and update injector design. • Calculate collective effects (BBU, ion trapping, halo formation) • Beam exchange design • Linac design • Optimize HOM damping. • Lower frequency and add 3rd harmonic cavities • Cooling Insertion • Balance cooling partition • Explore offset beams • CCR Design • Micro-bunching gain is low. • Optimize vs. tune and explore new designs • Calculate collective effects (ion trapping, wakes, resonances) • Injector design • Magnetization is preserved up to end of booster • Lower frequency helps. Add harmonic RF • Merger Design • Assume RF kicker to date. Need to explore other options.

  26. BACKUPS

  27. Possible Merger Option Recirculated Beam Injected Beam • Use an RF Separator to separate the injected beam from the recirculating beam • Immerse the RF Separator in a DC magnetic field • Arrange timing and relative amplitudes so that the injected beam is not deflected • Bunches are at maximum of RF deflection – bunch center has zero slope • This means that the kick seen by the recirculated beam is doubled • Needs to be sufficient to provide adequate separation at the septum RF Separator with Superposed Magnetic Field Injector Cryomodule Septum Magnet ERL Cryomodule

  28. Waveforms Recirculated Bunch Injected Bunch

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