Towards Achieving the Design Number of Bunches in PEP-II

PEP-II, the asymmetric B-Factory at SLAC, has delivered a luminosity of 2.20 {center_dot} 10{sup 33} cm{sup {minus}2} s{sup {minus}1}. This was achieved with 553 bunches in each ring, about a third of the design bunch number of 1,658. Different problems arise when the authors tried to increase the number of bunches and keep the single bunch current constant: increased loading of the RF, heating of the vacuum chamber with synchrotron radiation, and more background in the detector. Lowering the current can reduce these effects, while also reducing the luminosity. The lower beam currents allow them to study the effects of parasitic crossings, different higher order modes, rate dependent effects of the luminosity monitor and other equipment like feedbacks, intensity monitors, etc. Instead of going directly from 829 to 1,658 bunches, they tried to increase the number of bunches in steps by adding additional bunches between the 829-bunch pattern or by increasing the bunch number by 33% leaving every third bucket position empty (2,4,2,4 bunch spacing). The first scenario gives two parasitic crossings for the added bunches, while the +33% case gives half of the bunches a left and the other half a right parasitic crossing. The vertical tune shifts duemore » to the parasitic crossing (all together four: left, right for both beams in HER and LER) were measured to be about 0.01. A 30% difference of the left and right side in LER indicates a possible asymmetric beta function set up near the interaction point.« less