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LEReC Electron Cooling: Commissioning Summary and Plans

This article provides an overview of the LEReC project, the world's first electron cooler based on RF acceleration of bunched electron beams. It discusses the successful achievement of key elements and experimental demonstrations for this new approach. It also highlights the commissioning of electron cooling in a collider and the beam parameters achieved in the cooling sections.

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LEReC Electron Cooling: Commissioning Summary and Plans

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  1. Low Energy RHIC electron Cooling (LEReC):Commissioning Summary and Plans Alexei Fedotov on behalf of the LEReC team RHIC Retreat 2019 July 31, 2019

  2. LEReC Project Overview LEReC is world first electron cooler based on the RF acceleration of bunched electron beam (all previous coolers used DC beams) All key elements and experimental demonstrations required for this new approach were successfully achieved: • Building and commissioning of new state of the art electron accelerator  • Produce electron bunches with beam quality suitable for cooling • RF acceleration and transport maintaining required beam quality • Achieve required beam parameters in cooling sections • Commissioning of bunched electron beam cooling • Commissioning of electron cooling in a collider  2

  3. LEReC inside RHIC tunnel at Interaction Region @ 2 o’clock (IR2) Transport beamline Injection Section (DC photocathode Gun, SRF Booster cavity) Cooling sections Laser

  4. LEReC electron accelerator (100 meters of beamlines with the DC Gun, high-power fiber laser, 5 RF systems, including one SRF, many magnets and instrumentation) 704 MHz SRF Booster Cavity 704 MHz Cu Deflector Cavity Cathode loading system Transport Beamline RF Diagnostic Beamline Injection beam dump Merger Beamline 2.1 GHz Cu Cavity 704 MHz Cu Cavity 9 MHz Cu Cavity COOLING in Yellow RHIC ring e- e- RHIC TRIPLET DC e- Gun RHIC DX e- COOLING in Blue RHIC ring Quad corrector Trim corrector Corrector 3.8 ID Corrector 6.0 ID BPM 2.4 ID BPM 4.8 ID Bellows Ion pump LF solenoid HF solenoid Transport solenoid ERL solenoid 180° Bending Magnet Extraction beam line High-Power Beam Dump * NOT to scale

  5. LEReC cooling sections

  6. LEReC electron beam parameters Two energies commissioned

  7. LEReC bunched electron beam cooling In order to be accelerated to high energy by the RF cavities electron beam has to be bunched (thus “bunched electron beam cooling”). Bunches are generated by illuminating a photocathode inside the high-voltage Gun with green light laser (high-brightness in 3D: both emittances and energy spread). Electron beam properties resulting from acceleration of bunched beam are fundamentally different from those obtained in standard DC beam coolers. The 704MHz high-power fiberlaser produces required modulations to overlap ion bunches at 9MHz frequency with laser pulse temporal profile shaping using crystal stacking. RF gymnastics (several RF cavities) is employed to accelerate electron beam and to achieve energy spread required for cooling. Electron beams of required quality are delivered to cooling sections. Electron bunches overlap only small portion of ion bunch. All ion amplitudes are cooled as a result of synchrotron oscillations of ions.

  8. LEReC beam structure in cooling section Ions structure: 120 bunches f_rep=120x75.8347 kHz=9.1 MHz N_ion=5e8, I_peak=0.24 A Rms length=3 meters Electrons: f_SRF=704 MHz Q_e=100 pC, I_peak=0.4 A Rms length=3 cm 1.42 nsec 9 MHz bunch structure Electron Macro-bunch 30 electron bunches per ion bunch 110 nsec, f=9 MHz Ion bunches with new 9MHz RF

  9. Attainment of “cold” electron beam suitable for cooling • LEReC is based on the state-of-the-art accelerator physics and technology: • Photocathodes: production and delivery system • High power fiber laser and transport • Laser beam shaping to produce electron bunches of required quality • Operation of DC gun at high voltages (around 400kV) with high charge and high average current • RF gymnastics using several RF cavities and stability control • Energy stability and control • Instrumentation and controls

  10. Transverse phase space measurements of electron beam Bunch charge 75 pC Movable slit and downstream beam profile monitors are installed at the beginning of each cooling section.

  11. Longitudinal phase space measurement of electron beam 1 macro-bunch of electrons (total charge 3nC) 704MHz RF vertically deflecting cavity 6 macro-bunches, 3 nC each. • First dogleg merger dipole is off • Beam goes to RF diagnostic line • 20 degree dipole produces dispersion • Deflecting cavity produces time dependent vertical kick In pulsed mode, subsequent electron macro-bunches have lower energy due to beam loading in RF cavities.

  12. LEReC: First observation of electron cooling using bunched electron beam, April 5, 2019 Ion bunch #4 which is not being cooled Bunch length time evolution for two Au ion bunches in RHIC Ion bunch #2 is being cooled Longitudinal profiles of six ion bunches Energy of electrons and ions matched

  13. First cooling observation: April 5, 2019 BNL, CAD Main Control Room

  14. Simultaneous cooling in Yellow and Blue rings (76kHz mode, 6 ion bunches: bunch #1 is being cooled; bunch #6 does not see electrons) Longitudinal bunch profiles Blue bunch length cooled cooled Yellow cooled bunch intensity cooled and non-interacting bunches Blue Yellow Cooled, heated and non-interacting bunches Blue loss rate Yellow

  15. Cooling of 111x111 ion bunches at 3.85GeV (1.6MeV 9MHz CW electrons) Cooled vs uncooled store (+/-0.7m trigger) Electron cooling in a collider: Cooling of hadron beams in collisions.

  16. Cooling using 2MeV electron beam (ions at 4.6GeV) Transverse cooling: vertical beam size No cooling OFF Cooling on Cooling on Longitudinal cooling: ion bunch length

  17. Cooling of 111x111 ion bunches at 4.6GeV (2MeV 9MHz CW electrons) 25 min into the store rate became essentially constant, which should allow for longer stores with cooling and save time spent on refills.

  18. Potential benefits from cooling • Cooled bunch is kept shorter, more useful events within trigger window • Minimize ion beam de-bunching and losses from the RF bucket • Peak current significantly higher for cooled bunch • Transverse cooling can help with reduction of beta* lifetime BLUE bunch length Non-interacting Cooled Ion bunch profiles Ion bunch profiles lifetime YELLOW Cooled bunch Cooled bunch Non-interacting bunch length Ion lifetime Ion lifetime Cooled

  19. Observed issues and limitations 1. Electron beam optics has to account for additional focusing from ions. Optics matching between two rings needs to take this into account for both cooling sections, and allow high-current CW running of e-beam without ions at the same time. 2. Over focusing of electrons by ions. Different focusing on electrons distributed at different longitudinal slices of ion bunch. The higher ion bunch intensity the stronger are over focusing effects. 3. Observed strong “heating” effects of electrons on ions (which are coherent space-charge beam-beam kicks from electron bunches on ions). 4. Ion lifetime limitations at low energy: physical aperture, dynamics aperture, beam-beam, space charge and IBS.

  20. Blue cooling section optimization Bunch length uncooled cooled Ions lifetime • e-beam optics with negative alpha_x,y (diverging e-beam without ions) to account for focusing from ions in both cooling sections.

  21. Over focusing from ion beam • To reduce observed strong “heating” effects of electrons on ions (which are space-charge beam-beam kicks from e-beam on ions), we moved to smaller ion beta-functions in cooling sections. This increased over focusing effects from ions. • To reduce observed “heating” effects, we have to operate with lower than design electron bunch charge. This increased over focusing effects from ions. • Ion bunch intensity is higher than expected, which increased over focusing effects from ions.

  22. LEReC – electron and ion beam parameters (2019) Ions structure: f_rep=9.1 MHz N_ion=6e8, I_peak=0.3 A Rms length=3.2 m Electron bunches: f_SRF=703.5 MHz Q_e=50 pC, I_peak=0.13 A FWHM length =400 psec With small ion beam size in cooling sections and higher ion peak current, stronger over focusing of electron beam by ions 9 MHz bunch structure Electron Macro-bunch 30 electron bunches per ion bunch 110 nsec, f=9 MHz Long ion bunches with 9MHz RF

  23. Effects of ions on e-beam (low intensity of ions, N_ions=3e8, Q_electrons=55pC) with ions e-beam envelopes for groups of electrons at ion bunch center, 1 and 2 longitudinal sigma of ion bunch. Cooled ion bunch length Peak current e-beam angles Ions lifetime cooled non-interacting

  24. Effects of ions on e-beam (high intensity of ions, N_ions=6e8) Without ions e-beam envelope e-beam envelopes for groups of electrons at ion bunch center, 1 and 2 sigma. e-beam angles 15 middle bunches within electron Macro-Bunch (30 bunches) are over focused and are not effective for cooling.

  25. Effects of electrons on ions: “heating” (March 2019) Electrons and ions energies are NOT matched • Even before cooling was established we observed strong “heating” effects of electrons on ions (which are space-charge beam-beam kicks from e-beam on ions). • These “heating” effects were reduced by going to smaller ion beta-function in cooling section and by finding better working point in tune space.

  26. Heating studies: loss rate of ions (April 2019) beta@cooler=30m beta@cooler=50m beta@cooler=30m cooled bunch better working point Non-interacting bunch w.p. scan

  27. LEReC project timeline May 2015: LEReC project approved by DOE for construction December 2016: DC gun successfully conditioned in RHIC IR2 February 2017: Gun test beamline installed in RHIC April-Aug., 2017: Gun tests with beam July-Dec., 2017: Installation of full LEReC accelerator Jan.-Feb., 2018: Systems commissioning (RF, SRF, Cryogenics, Instrumentation, Controls, etc.) March-Sept. 2018: Commissioning of full LEReC accelerator with e-beam Sept 2018 : All project Key Performance Parameters achieved Oct.-Dec., 2018: Scheduled upgrades and modifications December, 2018: Gun Conditioning and HVPS tests without e-beam Jan.-Feb., 2019: Restart operation with electron beam. Achieved e-beam quality suitable for cooling March 2019: Start commissioning with Au ion beams April 2019: First cooling demonstration. Cooling in both RHIC rings using e-bunches at 76kHz frequency. May 2019: Simultaneous cooling of many ion bunches using 9MHz CW e-beam June 2019: Cooling optimization at 1.6MeV, cooling of beams in collisions (3.85GeV ions) July 2019: Cooling commissioned at higher electron energy of 2MeV (4.6GeV ions)

  28. Summary • All LEReC cooling commissioning milestones were successfully achieved. • First in the world electron cooling of hadron beams based on RF acceleration of electron bunches was demonstrated. Such cooling approach is new (all previous coolers used DC beam) and opens the possibility of using this technique to high beam energies. • First electron cooling using electron beam without any magnetization on the cathode or cooling section, “non-magnetized” cooling, was demonstrated (all previous coolers used magnetization on the cathode). • First electron cooling of ion bunches in two different hadron rings simultaneously using the same electron beam. • First electron cooling in a collider (cooling of ion beams in collisions with various effects impacting beam lifetime). • Cooling was commissioned at electron energy of 1.6MeV (ion energy 3.85GeV) • Cooling was commissioned at one more energy of 2MeV (ion energy of 4.6GeV) • Cooling of full RHIC stores with 111 ion bunches in both RHIC rings was demonstrated, both at 3.85 and 4.6 GeV ion beam energies. • The next step will be to maximize collision rates with cooling in next year’s RHIC low-energy collisions.

  29. Acknowledgement Thank you! LEReC project greatly benefits from help and expertise of many people from various groups of the Collider-Accelerator and other Departments of the BNL. As well as FNAL, ANL, JLAB and Cornell University.

  30. More pictures and a story (June 5, 2019): https://www.bnl.gov/newsroom/news.php?a=215585

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