Lattice with smaller momentum compaction factor for PEP-II high energy ring

At present, the PEP-II bunch length and vertical beta function β_y*, at the Interaction Point (IP) are about of the same size. To increase luminosity, it is planned to gradually reduce β_y*. For the maximum effect, bunch length has to be also reduced along with β_y* to minimize luminosity loss caused by the hourglass effect at IP. One of the methods to achieve a smaller bunch length is to reduce momentum compaction factor. This paper discusses a lattice option for the High Energy Ring,where the nominal 60° cells in four arcs are replaced by 90° cells to reduce momentum compaction factor by 30% and bunch length by 16%. The increased focusing in 90° cells results in 40% stronger,arc quadrupoles and 150% stronger arc sextupoles due to reduced dispersion and larger chromaticity. Tracking simulations predict that dynamic aperture for this lattice will be ≥ 10 times the rms size of a fully coupled beam for a horizontal emittance of 30 nm and,β_y* = 1 cm. The lattice modification and results of simulations are presented.