Increasing the Number of Bunches in PEP-II

The PEP-II B-factory at SLAC has delivered a luminosity of 4.6 x 10{sup 33} cm{sup -2}s{sup -1}. The bunch pattern now used in PEP-II has colliding bunches spaced every 4th rf bucket in what we call the ''by4'' pattern. During the past year we have increased the number of bunches per train in the by4 pattern from 21 to 23, out of a possible 24 bunches per train, for a total of 728 to 796 bunches respectively. The bunch trains were initially necessary to allow the electron cloud around the positron beam, which induces a transverse size beam blow-up, to dissipate. By adding solenoid windings around the low energy ring straight and arc section vacuum chambers the beam size blow-up has been reduced which has allowed us to run without gaps between trains. Presently, we use a straight by4 pattern with 836 bunches. At this point the increase in machine luminosity requires an introduction of a new bunch pattern that permits a large number of bunches since the bunch charge is already limited by the beam-beam effects. Increasing the number of bunches in the PEP-II ring should linearly raise the luminosity when the charge per bunch is kept constant. Sincemore » we reached the maximum number of bunches possible in the by4 pattern, we have been exploring different patterns for higher luminosity operations. Present limitations for higher beam current are, higher order mode heating, electron cloud effects, differential tune shift along the bunch train, availability of rf power, and phase margin in the longitudinal feedback system. All of these limitations are directly or indirectly related to the bunch pattern.« less