1. Electron Cooling for RHIC Dong Wang Collider-Accelerator Department Brookhaven National Laboratory February 26th, 2003 MIT-Bates

2. Feb. 26, 2003, MIT Bates2 Outline RHIC Luminosity Upgrade Electron Cooling Simulations Overall Design Parameters Photo-injector: c.w. RF-gun Superconducting Linac Cavity Transport of Intense, Magnetized Beam Summary

3. Feb. 26, 2003, MIT Bates3 RHIC complex Electron cooling is likely around “4 o’clock”

4. Feb. 26, 2003, MIT Bates4 RHIC: Relativistic Heavy Ion Collider Circumference 3834m Beam Energy (Au ion) 100 GeV/c (proton)250 GeV/c Number of IPs 6 Beta at IP(H/V) 1~2 m Lum. Lifetime~10 hours * N of Bunches60~120 Bunch Length 30~150 cm Emittance(95%) 15~40  mm mrad * present operation phase High luminosity is vital for physics experiments.

5. Feb. 26, 2003, MIT Bates5 Luminosity and Intra-Beam Scattering Ions at RHIC energy have little synchrotron radiation Ions interact each other via Coulomb force(IBS) Overall consequence is emittance growth RHIC Luminosity and beam current Courtesy: W. Fischer

6. Feb. 26, 2003, MIT Bates6 RHIC Luminosity Upgrade Plan RDMRDM+RHIC II Initial emittance(95%) Final emittance(95%) IP beta function Number of bunches Bunch population B-B parameter Peak luminosity Ave. luminosity  m m 10 9 10 27 cm -2 s -1 15 40 2 60 1.0 0.0016 0.8 0.2 15 40 1 120 1.0 0.0016 3.2 0.8 15 <6 1 120 1.0 0.004 8.3 7 RHIC II emittance: Cooling is assumed. RHIC II ave. luminosity: 5 hours luminosity time(instead of 10 hours)

7. Feb. 26, 2003, MIT Bates7 Expected scenario with cooling cooling Beam dimensions need to be reduced: cooling Ion transverse distribution

8. Feb. 26, 2003, MIT Bates8 Electron cooling project at BNL 2000-2001 RHIC e-cooling discussions, inspired by progress in: 1, high current Energy Recovery Linac experiment at Jlab-FEL 2, principle of transport of magnetized beam(Derbenev et al.) initial calculations were done by BINP. 2001 fall Electron Cooling group(2 persons) at Collider-Accelerator Dept., feasibility study, evaluation of cooling, design of e beam facility 2002 fall to (2005?) more support from C-AD and lab in manpower and funding. R&D on cathode, beam dump, gun, lianc cavity, solenoid, etc. some experiments(gun,RF) planned in BLDG 939.

9. Feb. 26, 2003, MIT Bates9 Ions can be cooled by cold electrons Proved technique Good for intense ion beam (compare to stochastic cooling) at low energy so far, up to ~500 keV e- energy.(Fermi Lab: 5 MeV e-, installed). Much more difficult at high energy Ion ring Electron gun Beam dump ‘cool’ electrons mix with ‘warm’ ions ‘Temperature’ of beams: degree of random motion, i.e., emittance, energy spread, etc. Simplest case: 2-component plasma

10. Feb. 26, 2003, MIT Bates10 Electron cooling calculations Basically: high charge strong, high quality solenoid low emittance and energy spread matched beam size Numerical simulations Calculation is complicated with cooler solenoid. No precise analytical approach. Semi-phenomenological model is used(V. Parkhomchuk) New codes being developed. Friction force vs. Solenoid strength, 0 and 1.0T

11. Feb. 26, 2003, MIT Bates11 Electron cooling simulations Very high charge and tiny solenoid errors are required. 5~10 nc/bunch ~8E-6 error level(B_tran/B)

12. Feb. 26, 2003, MIT Bates12 Electron beam parameters RHIC e-cool: electron beam parameters Most challenging issues 1, high average current(record: 5mA in Jlab FEL) 2, transport of high-charge, magnetized beam

13. Feb. 26, 2003, MIT Bates13 E-cooling facility design Photo-cathode RF-gun: produce intense and high quality electron beam Superconducting cavity: for high current beam Energy recovery for main linac: save tremendous power (5 MW) Multi-function arcs: stretch and compress beam, magnetization matching, beam separation and combination. Sketch of e-cooling facility Courtesy: J. Kewisch

14. Feb. 26, 2003, MIT Bates14 Photo-cathode RF-gun Cathode&laser: under study(T. Rao, BI) Gun: 2 ½ -cell, 1.3 GHz to 700 MHz Courtesy: AES 700 MHz gun: ~9 MV/m at cathode Low field at cathode is bad for beam quality

15. Feb. 26, 2003, MIT Bates15 Gun simulations Optimization of beam quality: balanced transverse and longitudinal parameters Gun geometry and field(SF)

16. Feb. 26, 2003, MIT Bates16 Beam combination Low energy beam: 2.5 MeV High energy beam: 55 MeV Avoid a large bending angle for low energy beam (space charge effect makes matching difficult) Septum magnet is chosen. Magnet design is underway. Larger bending angle+achromat compensation, being explored Layout of beam merging scheme

17. Feb. 26, 2003, MIT Bates17 Superconducting Cavity Initial choice: TESLA 9-cell 1.3 GHz cavity Recently we decide to develop a new cavity with fewer cells lower frequency TESLA 9-cell L-band sc cavity Major Issues high current operations: high average current means huge HOM power high bunch charge makes situation even worse Multibunch effects driven by high-Q sc cavities Single bunch effects

18. Feb. 26, 2003, MIT Bates18 New sc cavity: fewer cells G240  R/Q710  Q bcs 4.910 E p /E a 2.1 H p /E a 5.94mT/MV/m there are fewer trapped modes in a structure with fewer cells. fewer cells per structure makes coupling of HOMs easier BCS resistance vs. temp. Courtesy: I. Ben-Zvi

19. Feb. 26, 2003, MIT Bates19 New sc cavity: lower frequency Lower frequency features: large aperture(19 cm radius), low loss factor Cavity (single)TESLA 1.3 GHz0.7 GHz K l (V/pC)7.81.2 Power (kW)39.66.6 Energy spread30x10 -4 5x10 -4

20. Feb. 26, 2003, MIT Bates20 Damping HOMs with Ferrite Absorber Ferrite absorber in B-Factory One of the worst higher modes Waveguide coupler: J. Setukowicz

21. Feb. 26, 2003, MIT Bates21 HOMs with absorbers ModeFrequency(Re) (MHz) Frequency(Im) (MHz) Q 1672.81.2E-95.61E11 2680.44.9E-91.39E11 3690.01.07E-86.4E10 4698.21.66E-84.2E10 5701.49.59E-97.3E10 6110134.132 7110134.232 8123166.219 9127515.1384 1012760.3843323 Material: TT2, Ferrite-50 N of absorbers: 2/cavity Monopole modes with ferrite Local fields around an absorber, the worst mode

22. Feb. 26, 2003, MIT Bates22 Beam Break-Up(BBU) Multi-bunch instability Double-pass in ERL case Beam energy: 2.5 ~ 55 MeV Cures: Reduce Q(HOM) High injection energy(expensive) Low R/Q Proper optics Simplest case

23. Feb. 26, 2003, MIT Bates23 Beam Break-Up simulations TDBBU code Threshold: > 500 mA ~1A with some frequency Spread(0.001(f_hom-f_o)) L-band: ~ 120mA ERL circulating length: 107.42 m Distance between cavities: ~ 2.0 m

24. Feb. 26, 2003, MIT Bates24 Transport of Magnetized Beam magnetizedangular momentum dominated’beam ‘magnetized’ or‘ angular momentum dominated’ beam Electrons get angular momentum while they experience the radial field. Troubles in cooler: coherent motions. Cause: cooler solenoid Busch’s theorem: Other e cooling facilities: continual solenoid, no such trouble. RHIC e cooling: discrete elements. certain optical matching is a must. Linear theory: Burov, Derbenev, et al. PRST, 2001. 1, beam must be magnetized at cathode, 2, global matching is needed

25. Feb. 26, 2003, MIT Bates25 Simulating magnetized beam Description of the magnetized beam: Angular momentum is the fundamental thing. Beam: E = 55 MeV, emit = 30 mm mrad, beta = 5 m Angular momentum of an e- bunch after experiencing end-field of 1T solenoid at different positions.

26. Feb. 26, 2003, MIT Bates26 Compare different phase spaces Angular speed vs. r A good measure (linear correlation) New PARMELA (x,y’) or (y, x’) Non-Invariant, but maybe useful in some cases

27. Feb. 26, 2003, MIT Bates27 ARC: stretcher and compressor Function of arcs: Stretch(compress) e- bunch by a factor of 10~30(M 56 =30) 2 cavities are used to manipulate longitudinal phase space Lattice functions of arc with MAD.

28. Feb. 26, 2003, MIT Bates28 Particle tracking: envelope PRAMELA: tracking along beam line, cathode to cooler

29. Feb. 26, 2003, MIT Bates29 Particle tracking (2) PARMELA, Evolution of beam emittance and energy spread

30. Feb. 26, 2003, MIT Bates30 CAM preservation Preservation of angular momentums is seen though not perfect It is feasible. Improving matching. Simulations with errors, etc. At exit of linac At end of the first arc

31. Feb. 26, 2003, MIT Bates31 Cooler solenoid (s.c.) Main field: 1.0T Total length: ~30 m N of sections: TBD Field error: <8e-6 (trans. field/main field)Challenging! Correctors: h/v M. Harrison, A. Jain of Magnet Division Courtesy: Magnet Division

32. Feb. 26, 2003, MIT Bates32 Summary Feasibility of electron cooling in RHIC has been explored. Electron cooling simulation shows that a high performance cw e- beam facility is needed. Beam quality in RF-gun is good but somewhat limited by power issue. 700 MHz linac cavity is new choice to address HOM issues. Ferrite absorbers are effective. Magnetized beam simulations are exploited. Start-to- end tracking shows that transport line works properly. CAM can be mostly preserved with matching. Still a lot of work, solenoid, cathode, error effects, etc.