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RHIC Run-19 Performance and Plans for 2020

Overview of RHIC Run-19, challenges faced, and performance details. Plans for Run-20. Beam energy scan issues. LEReC cooling and space charge challenges. Improvements and notable events.

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RHIC Run-19 Performance and Plans for 2020

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  1. RHIC Run-19 Performance and Plans for 2020 Chuyu Liu 2019 RHIC Retreat Physics building, BNL 07/31/2019

  2. Outline • Overview of RHIC Run-19 • Challenges for Beam Energy Scan II (BES-II) • Run-19 performance in colliding mode • Run-19 fixed target experiments • Plans for Run-20 • Summary

  3. Overview of Run-19 • LEReC electron beam commissioning before RHIC started on Feb. 11th. • Interleave of physics program and LEReC commissioning. • Physics at 9.8 GeV/nucleon went smoothly. STAR was flexible on LEReC dedicated time allocation. • Physics at 7.3 GeV/nucleon went smoothly as well with physics running during weekends. • Fixed target experiments at various beam energies were conducted with large beta star, with rates being controlled by BBQ kicker, orbit control or other beam parameters. • Short physics running at 3.85 and 4.59 GeV/nucleon gave a guidance for the operations in coming years.

  4. Challenge #1: Intra-beam scattering • At BES-I/II beam energies, which are below the transition energy (∼24 GeV/nucleon for Au beam), both longitudinal and transverse beam emittance grow rapidly due to intra-beam scattering (IBS).

  5. LEReC cooling • The Low Energy RHIC electron Cooling (LEReC), using a linear electron accelerator, is designed to combat the IBS effect by cooling RHIC ion beams, and therefore to improve luminosities at the three lowest beam energies.

  6. Challenges #2: Space charge • At the BES-I/II beam energies, the ions in the beam experience a strong direct space charge force. The space charge force introduces incoherent and coherent tune shifts. • The space charge effects are significantly reduced by using the new 9 MHz cavities due to reduced peak beam intensity. • The 9 MHz cavities provide a larger bucket area than the 28 MHz cavities.

  7. Challenges #3: beam-beam Much improved lifetime with collision Significant beam loss Tune: 0.09 Tune: 0.23

  8. Challenge #4: persistent current in superconducting magnet • Persistent currents in superconducting magnets introduce significant field errors. • The decay of magnetic field errors causes the drifts of beam orbits, tunes and chromaticities. Demagnetization cycle Regular cycle

  9. Reduction of persistent current by demagnetization cycle More stable tunes More stable orbit More stable chromaticity

  10. Performance at 9.8 GeV/nucleon

  11. Performance at 7.3 GeV/nucleon 7.3 GeV

  12. What’s new compared to Run-11 • For physics at 9.8 and 7.3 GeV/nucleon: • Wiggle (demagnetization) ramp for RHIC superconducting magnets to reduce persistent current effects. • New betatron working point at 0.09/0.08. • Beta squeeze at IP6 for higher luminosity. • Upgrade of corrector control from 12 to 16 bit. • ATR re-alignment, additional BPM at WD4. • For LEReC cooling commissioning: • New 9 MHz cavities for 3.85 GeV/nucleon, much improved lifetime. • Improved lifetime due to wiggle (demagnetization) ramp. • Flat orbit in LEReC cooling section with g1-tvx separation bump.

  13. Mode-switching and wiggles • Benefits: quickly switch between energies, zero wait time for persistent currents to settle down; Orbit, tune and chromaticity stability much improved; automatically restore correct settings for systems. 9.8GeV 7.3GeV 9.8GeV 9.8GeV 3.85GeV 7.3GeV

  14. Less beam loss at 9.8 GeV with the new working point and other factors 2011 Now One collision point, 2 m beta star at IP6, wiggle hysteresis Two collision points, 2.5/3 m beta star at IP6&8, regular hysteresis, gap cleaning

  15. Comparison of bunched beam lifetime

  16. Operational challenges for physics at 9.8 and 7.3GeV • Beam-gas interaction induced background, beam scrubbing was not quite effective at 9.8 GeV. • Limited bucket acceptance at 7.3 GeV: tight tolerance on injection matching, more de-bunched beam. • Background control: injection kicker kicking de-bunched beam; beam scraping in the triplets with squeezed beta star. • Run gap cleaning at the end of stores to avoid generating background and hurting bunched beam.

  17. Fixed target runs in 2019 • Larger beta-function (10 m) at IP6 is very helpful for controlling the rates. • Background was well under controlled. • BBQ and orbit control are the two primary ways to control STAR event trigger rate. Beam size at the fixed target location was too small at beam energy 31.2 GeV and the diffusion rate was too low. Yellow horizontal chromaticity was reduced to blow up emittance to get reasonable rate. • For future fixed target runs at high energy, the following options can be explored: large beta function, ARTUS kicker, adjustment of chroms and tunes.

  18. Notable events, lessons, facts • RHIC has been operated at 11 different modes in 2019. • With earlier cooling of the Blue ring, the heat load from the Yellow ring prevented us getting Blue triplets cold enough for beam operation. • Beam scrubbing was performed at 9.8 GeV to improve vacuum, however, not very effective due to limited peak current. Quad-pumping in AGS can’t provide sustainable high peak current in RHIC due to longitudinal decoherence. • Blue injection kicker was found to be charging too early which affected the injection efficiency. • Loss started to show up in the injection area when the beam emittance was blow up to 1.5 um at 3.85GeV. • Injection kicker resistor was switched to 40 ohm for fast rise time and flatter top when RHIC started operation at 7.3GeV. • Nick Kling discovered that injection background was mostly due to debunched beam being kicked by the injection kickers. • Injection damper was helpful for injection efficiency. The selection of BPMs are critical, which need to consider the beam direction and whether the kicker is upstream or downstream of the BPMs. • Abort gap aligned at IP6 improves luminosity by ~10%. Flip-flop injection scheme (half Yellow, full Blue, then full Yellow), for higher rates during injection, has been demonstrated and implemented for days. • 3-1 and 2-1 merge in AGS were implemented to explore the benefit of more intensity at low energy. It was helpful at 4.59 GeV but not at 3.85GeV. • Luminosity (collision rates) in +/-2 m vertex cut is 2.5 times that in +/-0.7 m vertex cut with 9 MHz cavity.

  19. Plans for Run-20 • Plan to have LEReC cooling operational for 4.55 GeV/nucleon, but not for 5.75 GeV/nucleon because electron beam energy is limited by RF system. • The improvement of luminosity at 5.75 GeV is expected from more bunch intensity, better lifetime, more stable machine condition. • Smooth running for all fixed target energies are expected. • 2 weeks of LEReC commissioning each at 4.59 and 3.85GeV/nucleon.

  20. 5.75 GeV • Operation with 28 MHz cavity at 5.75 GeV is mandatory. LEReC commissioning use 9 MHz cavity. There is no way to switch quickly 9 MHz cavity frequency between the frequency required for LEReC and physics at 5.75 GeV. • The bucket acceptance with 28 MHz cavity is limited to accept bunch produced by 3-1 merge. There was proposal use both RF systems together for larger acceptance, which is not practical for the same reason as stated above.

  21. 4.59 GeV • Is 2 weeks enough for LEReC to be operational at this energy? • LEReC imposes constraints on what we can do in RHIC, like pushing up intensity.

  22. Summary • RHIC physics running and LEReC commissioning coexisted near perfectly during 2019. • RHIC performance at 9.8 and 7.3 GeV were substantially improved with new developments compared to previous running. • LEReC team achieved their tremendous goals during cooling commissioning in 2019, and demonstrated for the first time cooling of ion bunches with RF accelerated electron bunches. • Fixed target experiments running conditions were improved with larger beta star. Future FXTs will benefit as well. • We are planning for a challenging RHIC running with operational LEReC next year.

  23. Backup

  24. Run-20 outline MIN goals only • 0.5 weeks cool-down • 1 week setup • 6 weeks 5.75 x 5.75 GeV/nucleon (MIN goal only) • 1.5 week fixed target (up to 6 energies, interleaved) • 2 weeks LEReC setup for 4.59 x 4.59 GeV/nucleon (interleaved) • 10.5 weeks 4.59 x 4.59 GeV/nucleon (MIN goal only) • 2 weeks LEReC setup for 3.85 x 3.85 GeV/nucleon (interleaved) • 0.5 weeks warm-up • 24 weeks total (4K cryo operation) 24

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