Prospects for RHIC Low-Energy Operations

1. Prospects for RHIC Low-Energy Operations Todd Satogata(W. Fischer, T. Roser, A. Fedotov, N. Tsoupas, M. Brennan, C. Montag, and others)“Can We Discover the QCD Critical Point at RHIC?” WorkshopMarch 9, 2006 • Scope and History • Initial Machine Projections • Operational Challenges • Recommendations

2. Scope • The workshop is motivated by a growing body of theoretical and experimental evidence that the critical point on the QCD phase diagram, if it exists, should appear on the QGP transition boundary at baryo-chemical potential ~100 - 500 MeV, corresponding to heavy ion collisions with c.m. energy in the range s= 5 - 50 GeV/u. • Beam total energy in RHIC of 2.5-25 GeV/u • RHIC momentum aperture typically 1-3x10-3 • Assume Au as the primary, but not necessarily only, species • 2.5 GeV/u total energy scales very badly as Z/A increases • For reference, normal Au injection total energy is 9.8 GeV/u • Can likely inject up to ~12GeV/u • Provide projections, identify primary challenges • Below injection energy: field quality, IBS, emittance, cooling • Above injection energy: ramping, transition energy T. Satogata - RHIC Low-Energy Operations

3. 2001 9.8 GeV/u Au collisions • 2 days of 9.8 GeV/u collisions • 0.4 mb-1 integrated luminosity • *=3m by necessity • 60-90 minute stores • 56 Au bunches, 0.6x109/bunch • 10-30 Hz ZDC rates • IBS and aperture dominated beam and luminosity lifetime • Another run at this energy may improve this by factor of 2-5 • 1.0+x109/bunch • Raise * to improve lifetime • RHIC is best used as a storage ring collider below beam energies of ~12 GeV/u T. Satogata - RHIC Low-Energy Operations

4. Initial Machine Projections • Scaling laws apply above injection energies • When aperture dominated: • Peak luminosity ag2 • No clear scaling laws apply below injection energies • Injected beam already fills aperture • Magnetic field quality degrades very quickly • Power supply regulation • Strawman model • Peak luminosity ag3-4 T. Satogata - RHIC Low-Energy Operations

5. Initial Machine Projections • Assumes expected luminosity scaling as 3 below 9.8 GeV/u • b*/aperture and integrated luminosity tradeoffs must be studied • Projections do not include potential improvements • Electron and stochastic cooling (peak and integrated luminosity) • Lattice modifications to mitigate IBS (integrated luminosity) • Total bunch intensity from vacuum improvements (peak luminosity) • Small set of specific energies (and species?) should be a workshop deliverable for planning T. Satogata - RHIC Low-Energy Operations

6. Low-Energy Magnetic Field Quality • Magnet currents scale with rigidity B which scale with  • Field quality deteriorates rapidly at very low currents • Currently have no magnet measurements at very low currents, few at low energy • Must extrapolate field behavior for simulations • Low-current magnet measurements are a priority T. Satogata - RHIC Low-Energy Operations

7. Power Supply Regulation Issues • Several power supply issues • Chromaticity sextupoles • Main power supplies • Sextupoles: 0.6-0.7 A -> 0.15-0.2 A • CMOS regulation, works to 0.01 A • Study option of using only some sextupoles with higher current • Aperture and lifetime concerns • Correction of large main dipole b2 • Main dipoles: 430 A -> 110 A • Requires testing to check regulation • Will test during Run6 maintenance • Pulsed injection/extraction kickers • May have low-voltage limitations T. Satogata - RHIC Low-Energy Operations

8. Other Issues for Beam Energies 2.5-10 GeV/u • Injector issues (also covered by Nick Tsoupas) • Au at 2.5 GeV/u is above AGS injection energy (1.0 GeV/u) • AGS Au transition energy is 8 GeV/u • AGS/ATR extraction aperture dominated by G10 kicker • IBS growth rates (from Alexei Fedotov) • Consistent with 30-minute stores at 5 GeV/u • Lower energies likely require cooling or lattice modifications T. Satogata - RHIC Low-Energy Operations

9. Beam Studies for Low-Energy Injection • ~1 day of studies required in run before low-energy operations • Initial studies • Trivially scale nominal injection to lower energies • Provides reality check of power supplies, optics • Test injection, establish circulating beam, optimize lifetime • Initial global optics measurements, field quality, tune scan, energy resolution/momentum aperture • IBS growth time study require 3-6 hours extra time • All but IBS growth evaluation can be done with Run6 p • Later studies • IBS modification lattice development • Field quality and detailed optics measurement/correction T. Satogata - RHIC Low-Energy Operations

10. Issues for Beam Energies 10-25 GeV/u • Raising RHIC injection energy • Limited by injection kicker performance, ~20% increase feasible with 55 bunches • Higher energies than ~12 GeV/u require RHIC ramping • Squeeze b* with acceleration to maintain constant aperture • 2-3 days of setup per energy is probably sufficient • Optimize operations for length of stores/ramping time • Nominal RHIC transition energy: gT=21.3 for Au • Operation at g near gT is infeasible • Lattice modifications using gT-jump power supplies • Successful in Run 6, lowered gT by ~1 unit T. Satogata - RHIC Low-Energy Operations

11. Transition Energy Modification • For Run 6, transition energy was successfully modified • Lowering polarized proton injection energy for spin matching • Nominal gT=23.5, modified gT=22.8 • Primarily changes horizontal dispersion, momentum aperture • No effort to tune dispersion matching, limit triplet dispersion Modified Nominal T. Satogata - RHIC Low-Energy Operations

12. Cooling at Low Energies • Stochastic cooling is feasible but requires development • Different mixing regime than high-energy stochastic cooling • Cannot use filter cooling, as Schottky bands overlap • Higher cooling rates at lower energies, but requires different method • Cooling rate estimates under development • Palmer cooling • Under active development at C-AD as frontier of cooling • Will test cutting chord from pickup to kicker in Run 7 • Requires new cold pickup in arc (high dispersion, low beta) • Concerns about 10 Hz coherenct signal rejection • Electron cooling discussed in Alexei’s talk T. Satogata - RHIC Low-Energy Operations

13. Summary • No apparent show-stoppers for RHIC collisions at s=5-50 GeV/u • Lattice modifications to suppress IBS should be studied • For beam energies 2.5-10 GeV/u, luminosity scaling is uncertain • Field quality at very low currents should be measured/modeled • b*=10m likely required for reasonable lifetime/aperture • IBS drives very short store times (<30 min) below 4-5 GeV/u • A ~1 day study period in preceding run will be very beneficial • Identify power supply, lifetime, tuning issues/limitations • Above 10-12 GeV/u, luminosity scales as g2 with constant aperture • Study transition energy changes • Required to avoid ~3 GeV transition “hole” around 20-23 GeV/u • Very similar to normal heavy ion operations, 2-3 days setup/energy • Low-energy cooling • Stochastic Palmer cooling under development, unknown cooling time • Electron cooling is quite promising (see A. Fedotov’s talk) T. Satogata - RHIC Low-Energy Operations