On-line measurement and tuning of multi-pass recirculation time in the CEBAF linacs

CEBAF is a CW, recirculating electron accelerator, using on-crest RF acceleration and isochronous recirculation arcs. Previously installed diagnostics allow us to monitor the phase difference between the first-pass beam and the linac RR. The error signal is used for feedback correction of residual drifts in the RF timing system. We also experience drift in the path length (recirculation time) for the recirculated beam resulting in beam energy drift of several times 10^-4 for some acceleration passes, even when one beam energy is held fixed by adjusting the RF gradient. We have extended the beam-based RF phase diagnostic system to the recirculating beam, and can monitor and correct the path length at the 50 micron level (timing to 200 femtoseconds). The previous procedure for measuring and tuning the path length was coarser in resolution and required suspension of beam delivery