Streak-camera measurements of the PEP-II high-energy ring

The third commissioning run of the PEP-II High-Energy Ring (HER, the 9 GeV electron ring), in January 1998, included extensive measurements of single-bunch and multibunch fills using LBNL’s dual-axis streak camera combined with Argonne’s 119.0 MHz synchroscan plug-in. For single bunches, the dependence of bunch length on charge and rf voltage was studied from 0.5 to 2.5 mA and from 9.5 to 15 MV; the measured values ranged from 38 to 49 ps rms. The multibunch work focused on longitudinal instabilities as the current in the ring was raised to 500 mA, and the length of the bunch train was varied from 100 bunches (with 4.2 ns spacing) to a full ring. Large oscillations of up to 180 ps peak to peak were observed for bunches half a ring turn away from the start of the train, especially at higher currents and for trains filling roughly half the ring. These observations led to a new fill pattern with more gaps that allowed us to raise the current to 750 mA by the end of the run.