RF system for bunch lengthening

A harmonic cavity is used in the VUV ring to increase the Touschek lifetime and to help reduce peak current related instabilities. Phase locking servos for such a system normally require comparison of a Fourier component of the beam to a reference or information about the symmetry of the bunch. We have found it simpler to initially set the phase of the drive so that the center of the bunch is positioned at the zero crossing of the harmonic cavity voltage waveform. This phase relationship is then maintained by comparing the amplitude of the harmonic cavity field to the forward power, and adjusting the phase of the drive to keep the two powers equal. A system of this type has been operating in the VUV ring since October, 1993. It consists of a 52.88 MHz accelerating cavity and a 211.54 MHz bunch lengthening cavity providing increased lifetime over all operating currents