The CEBAF frequency distribution system

The RF system for the CEBAF accelerator requires that two frequencies, 70.000 and 499.000 MHz, be distributed throughout the accelerator site. Proper operation of the multipass beam requires an energy spread of less than 2.5 x 10-5. This imposes stringent constraints on the allowed slow phase error, in turn implying that there must be active regulation of the phase of the distributed reference frequencies. There are several methods already in use that regulate the phase of a distributed signal. A SLAC type pulsed microwave interferometer was not readily applicable, as this method uses the off time in a pulsed accelerator to perform its phase correction and measurement; CEBAF is a continuous device with every RF bucket filled. Since the CEBAF system requires amplifiers in the distribution line, a diplexed reference signal such as that used at LANL would require a series of discrete control loops, tied together with an overall control loop. It was felt that a less complex system would be desirable. To accomplish this, the frequency distribution system uses temperature regulated coaxial lines to provide high level signals to the RF control modules in the klystron galleries, combined with a lower power reference signal carried via phase stable fiberoptic cable