Low alpha mode for SPEAR3

In the interest of obtaining shorter bunch length for shorter X-ray pulses, we have developed a low-alpha operational mode for SPEAR3. In this mode the momentum compaction factor is reduced by a factor of 21 or more from the usual achromat mode by introducing negative dispersion at the straight sections. We successfully stored 100 mA with the normal fill pattern at a lifetime of 30 hrs. The bunch length was measured to be 6.9 ps, compared to 17 ps in the normal mode. In this paper we report our studies on the lattice design and calibration, orbit stability, higher order alpha measurement, lifetime measurement and its dependence on the sextupoles, injection efficiency, longitudinal stability and bunch lengths.