High-precision beam-based RF phase stabilization at Jefferson Lab

A non-invasive monitor has been implemented for the relative phase of the beam and the accelerating RF of each of the two CW superconducting linacs at TJNAF. Its noise level and resolution are below 0.1° at beam currents as low as 2 microamperes, and it been successfully tested for use in feedback correction for the overall RF accelerating phase of each linac. The 70 MHz frequency reference in each linac is phase modulated by approximately ±0.05° at different frequencies (383 and 397 Hz). A single analog beam position signal from a dispersive region is used to monitor the resulting micron-level position modulation via off-the-shelf lock-in amplifiers, which also supply the modulation signals. The off-crest phase is proportional to the first harmonic coefficient of the energy modulation. This technique can be applied as well to pulsed machines and to systems for which the beam-to-RF relative phase may be non-zero. Other applications are discussed