Femtosecond Resolution Mid-Level RF Phase Jitter and Drift Measurement Using Passive Components

Abstract

The Bates Linear Accelerator uses six 5‐MW 2.856‐GHz RF transmitters. Phase drifts between the transmitters’ 12 klystrons of greater than 0.5 degrees, or any amplitude drifts, compromise beam delivery and are a common occurrence. Instrumenting the phase shifts of these klystrons for machine diagnostics has received high priority. The hardware at the Bates facility that has been developed over the past seven years has proven very flexible. It is employed as a slow monitoring system of 14 phase and amplitude channels with VME hardware under EPICS, and as a fast jitter measurement system. We have recently measured phase jitters between the master oscillator reference line and klystron outputs, RF cavity outputs, drive laser detectors, wave guide loads and beam position monitors at two facilities. The resolution and repeatability of the medium level passive mixer implementation are better than 0.2 degrees for measurements of 5 to 60 seconds. This represents an R.M.S. jitter resolution of 20 fs. Data are presented for the above implementations and measurements. We are investigating the use of harmonic cavities and frequency up‐converters for further improvements in jitter measurement resolution.