Gap Voltage Feed-Forward Module for PEP-II Low Level RF System*

The Gap Voltage Feed-Forward (GVFF) Module produces adaptively generated inphase (I) and quadrature (Q) reference signals for a single RF station. This module measures the periodic (1-turn) beam induced cavity transients caused by the presence of an ion clearing gap in the beam. A reference waveform is generated to minimize the direct feedback error signal transient caused by the gap. In addition, the GVFF module receives a fiber-optically connected, band-limited ‘kick’ signal from the Longitudinal Feedback System. The kick signal modulates the I and Q components of the RF reference causing the station to act as a ‘sub-woofer’ for the Longitudinal Feedback System. This process dampens low order coupled-bunch instabilities induced by the fundamental mode of the accelerating cavities. The GVFF Module also includes hardware for remote measurement and adjustment of the ‘kick’ transfer function.