July LEReC Review July 2014 Low Energy RHIC electron Cooling (LEReC) Alexei Fedotov LEReC overview: project goal and cooling approach

July Outline Low Energy RHIC Physics Program Luminosity improvement with electron cooling LEReC scope Cooling parameters Cooling approach Challenges Project timeline Summary 2

July LEReC reviews Accelerator Physics Review, August : Report and talks are available. Most reviewed accelerator topics stayed the same. Changed from new 100 MHz SRF gun to already existing 704MHz SRF gun which is presently under commissioning. Technical, Cost, Schedule and Management Review, July 9-11, 2014: 3

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6 Search for QCD phase transition Critical Point Beam Energy Scan I, center of mass energies:  s NN = 5, 6.3, 7.7, 8.8, 11.5, 14.6, 19.6, 27 GeV ( 2010 & 2011 & 2014 RHIC runs ) Low Energy RHIC Physics program

July

Wolfram Fischer

July E ke (MeV) Luminosity is especially low at lowest energies Higher energies in 2019

July

July Low-energy RHIC operation Electron cooling (a well known method of increasing phase-space density of hadron beams): - “cold” electron beam is merged with ion beam which is cooled through Coulomb interactions - electron beam is renewed and velocity spread of ion beam is reduced in all three planes requires co-propagating electron beam with the same average velocity as velocity of hadron beam. Energy scan of interest:  s NN = 5, 7.7, 9, 11.5, 14.6, 19.6 GeV To cover all energies of interest need electron accelerator: E e,kinetic = MeV At low energies in RHIC luminosity has a very fast drop with energy (from  3 to  6 ). As a result, achievable luminosity becomes extremely low for lowest energy points of interest. However, significant luminosity improvement can be provided with electron cooling applied directly in RHIC at low energies. LEReC (2018): MeV LEReC energy upgrade (2019): 2-5 MeV

July Luminosity with cooling and new RF system 12 L avg in BES-I Min projection Max projection LEReC 2018 LEReC energy upgrade 2019 BES-II request (2018)BES-II request (2019)

July LEReC cooling sections LEReC SRF gun

July Low-Energy RHIC electron Cooler (LEReC), 2018 LEReC: Existing 704 MHz SRF gun and 5-cell cavity accelerator (presently under commissioning in Bldg. 912) -Needed beam parameters from the SRF gun should be demonstrated in the nearest future ( ) -Many relevant questions related to the SRF gun performance for LEReC will be tested in Bldg. 912 before moving hardware to RHIC. -Will need to add electron beam transport lines with magnets, 2.1 GHz warm RF cavity and two cooling sections with short solenoids. Cooling sections Beam transport lines Beam dump 704 MHz cavity 704 MHz SRF gun e-e- e-e- e-e- e-e- e-e- IP2 DX 2.1 GHz cavity

July Existing ERL loop layout in Bldg MeV 20 MeV 2-3 MeV 704 MHz SRF gun 5-Cell SRF cavity Beam dump R&D ERL accelerator is now a baseline accelerator for LEReC Most of existing hardware (Gun, 5-cell cavity, quadrupoles, diagnostics, powers supplies, beam dump) will be reused for LEReC.

July LEReC scope 704 MHz SRF gun with maximum energy of operation 2 MeV. 704 MHz SRF 5-cell cavity for energy chirp correction. 2.1 GHz (3 rd harmonic of the SRF frequency) copper cavity for energy spread correction. Electron beam transport from IP2 region to cooling sections Cooling section in Yellow RHIC ring – about 20 m long. Short Correction solenoids are located every 2 m. Free space between the solenoids is covered by several layers of the mu metal to shield magnetic field to a required level. U-turn dipole magnets between cooling section in Yellow and Blue RHIC Rings. Cooling section in Blue RHIC Ring. Return electron beam transport line. Electron beam dump. 16

July Luminosity limits for RHIC operation at low energies On top of dynamic aperture limitation at lowest energies in RHIC some fundamental limitations come from: Intra-beam Scattering (IBS): Strong IBS growth at lowest energies- can be counteracted by electron cooling Beam-beam: Becomes dominant limitation for RHIC parameters at  > 20 Space-charge: At lowest energies, ultimate limitation on achievable ion beam peak current is expected to be given by space-charge effects (note: not the same as typical space-charge limit in low-energy machines or space-charge dominated beams)

July For present 28 MHz RHIC RF at lowest energies we are limited both by space charge and RF bucket acceptance (significant beam losses), which strongly limits luminosity improvement with cooling. Better gain in luminosity is possible if one can tolerated operation with longer bunches for lowest energies: If bunch length is relaxed, we can now cool transverse emittance which in turn allows to reduce  *. Losses on transverse acceptance will be minimized as well. Luminosity gains from electron cooling will be maximized by using new 9 MHz RHIC RF system which is being built to improve beam lifetime at low energies. 18 Luminosity limits for RHIC operation at low energies

July LEReC (2018) cooler requirements Ion beam parameters Gamma4.1 RMS bunch length3.2 m N au 0.5e9 I_peak0.24 A Frequency9.1 MHz Beta cooling section30 m RMS bunch cooling section4.3 mm RMS angular spread140 urad Electron beam parameters Cooling sections2x20 m Charge per ion bunch3 nC (30x100pC) RMS norm. emittance< 2.5 um Average current30 mA RMS energy spread<5e-4 RMS angular spread<150 urad

July LEReC beam structure in cooling section Example for  = 4.1 (E ke =1.6 MeV) 110 nsec, f=9.1 MHz Ions structure: 120 bunches f_rep=120x kHz=9.1 MHz N_ion=5e8, I_peak=0.24 A Rms length=3.2 m 1.42 nsec 30 electron bunches Electrons: f_SRF= MHz Rms length=3 cm, I_peak=0.4 A Q_e=100 pC 9 MHz RHIC RF

July Un-magnetized cooling: very strong dependence on relative angles between electrons and ions. Requires strict control of transverse angular spread of electrons in the cooling section. Need to keep total contribution (including from emittance, space charge, remnant magnetic fields) to about150  rad (for  =4.1). LEReC: un-magnetized electron cooling Requirement on electron angles: For  =4.1:  p =5e-4;  <150  rad For LEReC based on RF electron beam continuous magnetic field (in cooling section) is not required. asymptotic for Un-magnetized friction force:

July Example (  =4.1): Emittance of 2.5  m rms: 145  rad angles (  =30m in cooling section). We have to keep all other contributions to a minimum. Space-charge defocusing in cooling section is controlled by correction solenoids, for example. If rms angular spread is 200  rad -> factor of 2 reduction from optimum cooling with 150  rad spread; results in reduction of maximum projected luminosity improvement. We have about factor of 1.7 safety margin in current and factor of 1.5 from cooling section length, so that we could accommodate up to a factor of 2 worse electron beam parameters than optimum (requirements). Control of electron angular spread

July Cooling section The cooling section is the region where the electron beam overlaps and co- propagates with the ion beam to produce cooling. The electron beam first cools ions in Yeloow RHIC ring then it is turned around (U-turn) and cools ions in Blue RHIC ring and then goes to the dump. The electron beam must maintain its good quality all the way through the second cooling section in Blue ring. The Blue and Yellow ring cooling sections are about 20 meters each. No recombination suppression is planned. Some space is taken up by matching solenoids, space-charge correction solenoids, steering dipoles and beam position monitors used to keep the electron beam and ion beam in close relative alignment. Short (10cm) correction solenoids will be place every 2 m of the cooling section. Distance covered by magnetic field from solenoids ( G) will be lost from cooling. Expect about cm to be lost from cooling from each solenoid, every 2m of cooling section. 23 Cooling region 2m Yellow Blue e-e- e-e-

July Requirement on magnetic field in the cooling section  =4.1: B_residual=0.2mG ->angles: 35  rad after L=12m. Passive (mu-metal shielding) to suppress B_residual below required level (<0.2 mG) in free space between the solenoids. FNAL at z=10.7 cm Residual magnetic field from solenoids in cooling region: W. Meng Distance covered by magnetic field from solenoids ( G) will be lost from cooling. Expect about cm to be lost from cooling from each solenoid, every 2 m of cooling section Solenoid design shielding

July Bunched beam cooling is a natural approach for high energies. For intermediate/low energies, like in LEReC, there are some challenges which has to be carefully addressed: - Beam transport of electron bunches without significant degradation of beam emittance and energy spread at low energies. - Keeping low transverse angular spread for the electron beam in the cooling section with a proper engineering design. - Electron beam with small emittance and energy spread should be provided for several energies of interest. - Quality of the beam should be preserved through the entire beam transport and both cooling sections. 25 Electron beam transport

July May - Aug 2014: SRF Gun commissioning w/beam (in 912 blockhouse). Sep-Dec 2014: SRF Gun to dump commissioning w/beam. Jan - June 2015: Shutdown for gun modifications and LEReC test preparation (in 912 blockhouse). Jul Jun 2016: High-current commissioning CW mode (LEReC tests in 912) July 2016 – March 2017: Move and install Gun, 5 cell cavity, beam dump, cryo, PS, magnets in RHIC April-June 2017: Systems commissioning (RF, cryogenics, etc.) July - Sept 2017:Cooldown SRF gun w/RHIC refrigerator; LEReC commissioning with beam in RHIC tunnel October 2017:Run 18, Cooling of Au ion beams LEReC commissioning timeline Project start in FY14 is required to meet deadline of Physics in 2018.

July Mike Blaskiewicz LEReC Energy Upgrade with added ERL loop (2019) SRF gun Cooling sections Energy upgrade uses 5-cell SRF cavity for acceleration and adds ERL loop (2019). Beam dump Beam transport lines 5-cell SRF cavity e-e- Gun-only mode return line ERL mode return line

July Electron cooling can provide significant luminosity improvement for low-energy RHIC operation, which is required by planned physics program (BES-II). 2.Present electron cooler design is based on the SRF gun which is presently under commissioning with beam in Bldg Many relevant questions related to the SRF gun and its performance will be tested in Bldg. 912 before moving hardware to RHIC. 4.After commissioning LEReC accelerator in Bldg. 912 it will be moved and installed at RHIC 2:00 o’clock region. Thank you. Summary

July Back-up slides 29

July

July Projection (maximum possible or “ideal”) for luminosity improvement low-frequency RF (9MHz) and long bunches 31 with cooling without cooling Up to about factor of 10 gain in total luminosity (*assuming that beam lifetime due to other limitations is mitigated). L~6L~6 with cooling without cooling vertex cut  =4.1 C-A/AP/449 (for details) without vertex cut with vertex cut Example for 9 MHz RF

July Luminosity with cooling and new RF system 32 L avg in BES-I Min projection Max projection LEReC 2018 LEReC energy upgrade 2019 BES-II request (2018)BES-II request (2019)