Beam-Beam Performance Of The SLAC B-Factory

The beam-beam performance of PEP-II has been monitored by parasitic measurements recorded during routine physics running, and by a few dedicated accelerator physics experiments. These measurements indicate that in some cases, the beam-beam interaction and the electron-cloud-induced blowup of the low-energy positron beam are somehow coupled and enhance each other. Tailoring the bunch pattern to carefully balance these effects has proven very effective in maximizing the integrated luminosity. The comparison of recent simulation results with experimental data is encouraging. * Work supported in part by the Department of Energy under Contract No. DE-AC02-76SF00515. INTRODUCTION The main parameters of the SLAC B-Factory [1] are listed in Table 1. In contrast to what is naturally enforced in single-ring colliders, the emittances and IP β -functions can be quite different in the two rings. The best performance has so far been achieved with rather different e and e beam-beam parameters. High luminosity also reproducibly favours an e/e current ratio of 1.5 to 2.0, where one would naively expect 2.9 from the simplified energy-transparency condition [2]: − − + + = E I E I Steady luminosity and background improvements have relied on maintaining a delicate empirical balance between the currents, tunes, beam-beam parameters, and ECI-induced blowup as these quantities vary along each bunch train. Spot-size, beam-current and luminosity diagnostics (both bunch-by-bunch and averaging over the entire train) have proven essential to unravel these coupled phenomena. TABLE 1. PEP-II Collision Parameters. The symbols "LER" and "HER" refer to the low-energy (e) and highenergy (e) ring, respectively. IP Parameter Design Recent peak performance Units Beam energy E (LER/HER) 3.1 / 9.0 3.1 / 9.0 GeV Crossing angle 0 < 1 mrad Luminosity 3.00 6.57 10 cms Number of bunches 1658 1034 Beam current I (LER/HER) 2146 / 75