RHIC low energy beam loss projections

Abstract

For RHIC low-energy operations, we plan to collide Au beams with energies of E = 2:5-10 GeV/u in RHIC. Beams are injected into collision optics, and RHIC runs as a storage ring with no acceleration. At these low energies, observed beam lifetimes are minutes, with measured beam lifetimes of 3.5 min (fast) and 50 min (slow) at E=4.6 GeV/u in the March 2008 test run. With these lifetimes we can operate RHIC as a storage ring to produce reasonable integrated luminosity. This note estimates beam losses and collimator/dump energy deposition in normal injection modes of low energy operation. The main question is whether a normal injection run is feasible for an FY10 10-15 week operations run from a radiation safety perspective. A peripheral question is whether continuous injection operations is feasible from a radiation safety perspective. In continuous injection mode, we fill both rings, then continuously extract and reinject the oldest bunches that have suffered the most beam loss to increase the overall integrated luminosity. We expect to gain a factor of 2-3 in integrated luminosity from continuous injection at lowest energies if implemented[1]. Continuous injection is feasible by FY11 from an engineering perspective given enough effort, but the requiredmore » extra safety controls and hardware dose risk make it unappealing for the projected luminosity improvement. Low-energy electron cooling will reduce beam losses by at least an order of magnitude vs normal low-energy operations, but low energy cooling is only feasible in the FY13 timescale and therefore beyond the scope of this note. For normal injection low energy estimates we assume the following: (1) RHIC beam total energies are E=2.5-10 GeV/u. (Continuous injection mode is probably unnecessary above total energies of E=7-8 GeV/u.); (2) RHIC operates only as a storage ring, with no acceleration; (3) 110 bunches of about 0.5-1.0 x 10{sup 9} initial bunch intensities (50-100% injection efficiency, likely conservative); (4) 90% collimation efficiency, with 10% of beam losses elsewhere. This is considered mildly conservative [2]; (5) Beam lifetimes range from about 1 minute (E=2.5 GeV/u) to 20 minutes (E=7-8 GeV/u); (6) The primary beam loss mechanism is space charge/IBS blowup of transverse emittance. This implies that most beam is lost at transverse apertures, particularly abort kickers and collimators. Mar 11 2008 test run data also shows some mild chronic losses around the other IR triplets. We were not using collimators during this test run. (See Fig. 1.); and (7) The RHIC ASE states that the 'maximum number of heavy ions in each ring (is) to be less than the equivalent of 2.4 x 10{sup 11} Au ions at 100 GeV/u' [3]. The total stored beam energy of this beam is 9.5 x 10{sup 15} GeV-u.« less