Bunch-Length Measurements in PEP-II

We measured the lengths of colliding e⁺e^-bunches in the PEP-II B Factory at SLAC using various techniques. First, at several RF voltages and with both single-bunch and multibunch beams, a synchroscan streak camera measured synchrotron emission through a narrow blue filter. With 3.8 MV of RF, the length of a single positron bunch was 12 mm at low current, rising to 13 mm at 1.5 mA and 14.8 mm at 3 mA. The electrons measured 12.2 mm at 16 MV with little current dependence. Both are longer than the expected low-current value of 10 mm (e⁺) and 10.5 mm (e^-), derived from the energy spread and the measured synchrotron tune. We also determined the length for multibunch fills from measurements between 2 and 13 GHz of the bunch spectrum on a BPM button. After correcting for the frequency dependence of cable attenuation, we fitted the measured spectrum to that of a Gaussian bunch. At 3.8 MV, the positron measurement gave 13.2 mm at 1.5 mA/bunch in a full ring, shorter than the 15.6 mm found with the streak camera under these conditions, but we found 11.4 mm for the electrons at 16.7 MV and 1 mA/bunch, in good agreement with the 11 mm from multibunch streak measurements.