Upgrading Rhic for Higher Luminosity*

While RHIC has only just started running for its heavy ion physics program, in the first run last summer, we achieved 10% of the design luminosity. In this paper we discuss plans for increasing the luminosity by a factor of 35 beyond the nominal design. A factor of 4 should be straightforward by doubling the number of bunches per ring and squeezing the β* from 2 to 1 m at selected interaction points. An additional factor of 8 to 10 could be possible by using electron cooling to counteract intrabeam scattering and reduce emittances of the beams. 1 GOLD-GOLD LUMINOSITY UPGRADE The RHIC lattice allows for simultaneous operation at six different interaction regions, each with a design luminosity of 2 ×10 cm s for gold beams. It is expected that this design luminosity will be reached during the FY2001 heavy ion run. The machine parameters are shown in Table 1 in column “RDM” (RHIC Design Manual). Scheme Units RDM RDM+ RHIC II ε initial πμm 15 15 15 ε final πμm 40 40 <6 β* m 2 1 1 NB 60 120 120 N 10 1 1 1 ξ 0.0016 0.0016 0.004 σ’* μrad 108 153 95 σ* μm 216 150 95 L0 10 cms 0.8 3.2 8.3 10cms 0.2 0.8 7 Table 1: The luminosity performance of RHIC in scenarios of Au+Au collisions at 100 GeV/nucleon. The luminosity averages given for “RDM” and “RDM+” are averaged over a 10 hour store. For the “RHIC II” scenario luminosity is averaged over 5 hours due to the beam-beam burn-off from actual collisions. In table 1 (and in the following), ε is the normalized 95% emittance, β* is the IP beta function, ξ is the beambeam parameter per IP, NB the number of stored bunches, N the number of ions per bunch, σ’* angular beam size at IP, σ* rms beam size at the IP, L0 the initial luminosity and the average luminosity. A first upgrade of the * Under contract with the United States Department of Energy, Contract Number DE-AC02-98CH10886 [email protected] luminosity by about a factor of four consists of increasing the number of bunches from about 60 to about 120 and decreasing * from 2 m to 1 m. This will not require any substantial new hardware. However, due to the larger beam size in the interaction triplets the non-linear local correction elements will have to be carefully optimized. It is expected that this level of performance can be reached during the FY2003 running period. The machine parameters for this enhanced luminosity are shown in column “RDM+”. A further increase of the number of bunches or decrease of * is possible and has been studied. However, it would require substantial upgrades or modifications of the collider detectors. The former will reduce the time interval between collisions to less than 100 ns and the latter would require additional triplets close to the collision point. Alternatively the luminosity can be enhanced by increasing the number of ions per bunch or by de-creasing the transverse emittance of the beam. However, already at the present bunch intensity and beam emittance the luminosity is expected to decrease very rapidly during a store due to intrabeam scattering (IBS). This is the reason for the large difference between peak and average luminosity in Table 1. To overcome this limitation we are proposing to counteract intrabeam scattering by electron cooling the gold beams at storage energy. Cooling the gold beams at 100 GeV/nucleon requires electron beam energy of about 50 MeV and an average beam current of about 10 mA. A detailed discussion of the electron cooling of RHIC can be found in a companion paper [1]. With electron cooling the beam emittance can be reduced and maintained throughout the store and the luminosity increased until non-linear effects of the two colliding beams on each other limit any further increase (beam-beam limit). With the parameters shown in Table 1 in column “RHIC II”, a luminosity increase by 35-fold over RHIC design luminosity could eventually be achieved. The RHIC electron cooler could be completed by FY2006. Upgrading the heavy ion beam from gold to uranium ions at similar bunch intensities will require the Electron Beam Ion Source (EBIS), which is presently in development [2]. 0-7803-7191-7/01/$10.00 ©2001 IEEE. 3126 Proceedings of the 2001 Particle Accelerator Conference, Chicago