1. New capabilities of the Coherent electron Cooling system at RHIC 1 Vladimir N. Litvinenko Stony Brook University, Stony Brook, NY, USA C-AD, Brookhaven National Laboratory, Upton, NY, USA AAC 2016,

2. Outline  What is CeC PoP?  Where we are?  What are new possibilities?  Conclusions

4. CeC Proof-of-Principle Experiment

5. Panoramic views

6. SRF gun

7. CsK2Sb Cathode system

8. Main Beam Parameters Parameter Value Status Electron energy21.95 MeV < 10 MeV Charge per e-bunch0.5-5 nC ✔ (> 3.5 nC) Rep-rate78.17 kHz 5 kHz* e-beam current0.39 mA Few μΑ Electron beam power8.6 kW < 10 W * We did not operated 5 kHz with 4.6 nC per bunch at the same time ** Numbers listed in blue do not require modification of equipment

9. Record performance of 112 MHz SRF photo-electron gun 3.7 nC ε n < 0.5 mm mrad at 0.5 nC SRF gun at 1.15 MV Beam loading

10. Bunching RF cavities Low energy transport beam-line with 5 solenoids Dog-leg: 3 dipoles 6 quads 20 MeV SRF linac Low power Beam dump SRF photo-gun and cathode manipulation system CeC modulator 4 quads Common section with RHIC CeC “kicker” 4 quads High power beam dump 2 dipoles 4 quads The CeC system commissioning CeC FEL amplifier 3 helical wigglers Beam was generated, compressed, accelerated and propagated through the entire system to the high power beam dump Compressed 8.7 MeV beam in the dogleg BPM signal in the common sction Beam at the end of the system

11. Electron bunch trans with flexible time structure for DWA and PWFA at kHz rep-rates and, potentially, high transformer ratio Use low frequency SRF gun to generate train of pulses with ~ 10 driving bunches and, when necessary, a witness bunch Charge per micro-bunch of 0.25 – 0.5 nC, up to 5 nC per bunch train Train rep-rate – up to 78 Hz Compress this train to a desirable period to match the resonant plasma/DWA frequency In PWFA, distance between bunches will differ from period of the plasma wave to keep the same deceleration For PWFA, use energy chirp and chromaticity of the focusing beam line for transverse matching of individual bunches to the transverse focusing

12. Compressed 8.7 MeV beam in the dogleg t E … t E … Ballistic compression of bunch train (up to 10-100 fold) Chirp in energy between bunches to match into variable parts of the buble Current: Ballistic compression of 500 ps bunch to ~ 2 psec

13. The wake-field is excited in an uniform cylindrical dielectric wake-field (DWA) accelerator by a train of four bunches with 250 pC change. The beam propagates from left to right. (a) the DWA structure (blue) and the train of four bunches (red dots); (b) the longitudinal electric field on the axis (blue) and the electron bunch train profile (red), X. Wei, D. Stratakis, G. Andonian, J. B. Rosenzweig, in Proceedings of 2011 Particle Accelerator Conference, New York, NY, USA, 2011, p. 124

14. High transformer ratio Transformer ratio enhancement using a ramped bunch train in a collinear wakefield accelerator, J.G. Power, W. Gai, X. Sun, A. Kanareykin, In Proc. of Particle Accelerator Conference, PAC 2001, 2001, p. 114 P. Muggli et al., Phys. Rev. Lett. 101, 054801 (2008) G. Loisch, Proceedings of IPAC2016, Busan, Korea Pre-Release Snapshot 13-May-2016 09:00 WEPMY006

15. Possible up-grade We propose to establish research program of generating trains of high- brightness bunches with tunable time structure from an existing 20 MeV CW SRF accelerator by adding a new femtosecond (fsec) drive- laser. The uniqueness of our proposal is that we can generate such trains with controllable number, charge and bunch-by-bunch spacing of bunches and high charge and repetition rate. Such bunch trains can serve as an excellent driver for resonant excitation in high-transformer ratio PWFA and DWA accelerators. Furthermore, application of bunch trains extends to variety of THz and X-ray sources. DWA/ PWFA

16. Nested interferometer design to produce eight equally spaced laser pulses High power ultrafast lasers, S.Backus, C.G. Durfee III, M.M. Murnane and H.C. Kapteyn, REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 69, 1998, p. 1207

17. Laser & photocathode parameters Range of parameters for Ti: Sapphire laser and photocathode Wavelength, nm800 Optical pluses at 2 nd harmonic, nm400 Energy per pulse (400nm, one per train), mJ0.1-1 Pulse duration, fsec, rms 50 Rep-rate, kHz 10-100 Number of pulses in train2-10 Photocathode material CsK 2 Sb QE at 400 nm, %4-20 Charge per bunch train1-5 Charge in sub-bunch (μ-bunch), pC1-500 Electron beam current, mA0.01-0.5

18. Conclusions Existing CW CeC accelerator with low frequency SRF photo- injector and system for ballistic compressions provides an unique possibility of generating trains of driving and test bunches with nearly arbitrary structure, high charge and high repetition rate Being CW, this system will offer very stable and repeatable electron beam trains This system can operate with high rep-rate of 10s of kHz and potentially provide high transformer ration – two of the important milestones in PWFA roadmap The system would need just an additional driver laser driver to become an excellent test-bed for testing advanced accelerators with high-transformer ratio

19. Back-up slides

20. Record Beam Generated by SRF Gun Operating in CW Mode Bunch charge exceeds 4 nC Beam energy 1.6-1.7 MeV (CW), >2 MeV (pulse) We have demonstrated electrical field at time of emission exceeding 21 MV/m FZDHZDNPSWiscons in CeC Charge, pC 3006781004600 E, MV/m55-76.51221 Frequenc y 1.3 GHz 500 MHz200 MHz113 MHz CathodeCs 2 TePbNbCuCsK 2 Sb

21. Diagnostics for Low Energy Beam Integrating current transformer (1.25 nV s/nC) Two beam profile monitors with 1.3 megapixel cameras Pepper-pot in front of the second profile monitor Two BPMs Low power beam dump with Faraday cup Profile monitor Gun solenoid ICT Pepper-pot BPM Solenoids

22. Modified Cathode Launch System Garage Manipulator

23. Problems Encountered Multipacting in the FPC area – long conditioning cycle with molybdenum puck Excessive dark current – helium discharge cleaning Photocathodes found dead prior insertion into the gun – added port for QE monitoring inside the garage Substantial spikes in the residual pressure during insertion into the gun – added NEG getters Multipacting inside the cathode stalk – used mask for the cathode deposition system, developing start procedure Continuous vacuum problem with cathode launch system – re-build Photocathode end assembly

24. Cathode QE Evolution 24 Vacuum at 1.6×10 -10 torr Initial QE is 8-10%, the evolution after transfer is shown below.

25. Measuring Quantum Efficiency The photocathode was excited with 1 nsec laser pulse with varied optical energy. Gun voltage was 1.2 MV and laser spot size 1 mm r.m.s.

26. Compressing triangular bunches Laser pulses with 100s psec duration can be “trimmed” into semi-rectangular, semi-triangular, etc. Ballistic compression

27. Plans for CY 2016 shutdown Disassemble Repair and clean cavity Clean FPC Assemble and re-install Repair/re-build tuner Suppress microphonics 113 MHz SRF gun 704 MHz SRF Accelerator Replace gun power amplifier Improve coupling control Replace FPC drive Align the gun (need to verify) Improve cathode garages Laser transport/pulse shape Test multialkaline cathodes