High-intensity single bunch instability behavior in the new SLC damping ring vacuum chamber

New low-impedance vacuum chambers were installed in the SLC damping rings for the 1994 run after finding a single bunch instability with the old chamber. Although the threshold is lower with the new vacuum chamber, the instability is less severe, and we are now routinely operating at intensities of 4.5/spl times/10/sup 10/ particles per bunch (ppb) compared to 3/spl times/10/sup 10/ ppb in 1993. The vacuum chamber upgrade is described, and measurements of the bunch length, energy spread, and frequency and time domain signatures of the instability are presented.