  At present, the PEP-II bunch length and vertical beta function at the
Interaction Point (IP) are about of the same size. To increase luminosity, it
is planned to gradually reduce the IP beta function. For the maximum effect,
bunch length has to be also reduced 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 degree cells in four arcs are
replaced by 90 degree cells to reduce momentum compaction factor by 30% and
bunch length by 16%. The increased focusing in 90 degree 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 more than 10 times the rms size of a fully
coupled beam for a horizontal emittance of 30 nm and IP beta function of 1cm.
The lattice modification and results of simulations are presented.
