High accuracy beam current monitor system for CEBAF´S experimental Hall A

The CEBAF accelerator delivers continuous wave (CW) electron beams to three experimental Halls. In Hall A, all experiments require continuous, non-invasive current measurements and a few experiments require an absolute accuracy of 0.2 % in the current range from 1 to 180 μA. A Parametric Current Transformer (PCT), manufactured by Bergoz, has an accurate and stable sensitivity of 4 RAN, but its offset drifts at the RA level over time preclude its direct use for continuous measurements. Two cavity monitors are calibrated against the PCT with at least 50 μA of beam current. The calibration procedure suppresses the error due to PCT´s offset drifts by turning the beam on and off, which is invasive to the experiment. One of the goals of the system is to minimize the calibration time without compromising the measurement´s accuracy. The linearity of the cavity monitors is a critical parameter for transferring the accurate calibration done at high currents over the whole dynamic range. The method for measuring accurately the linearity is described