High Accuracy Beam Current Monitor of the CEBAF Hall A

A beam current monitor system was developed to measure the CW electron beam current delivered to the CEBAF Hall A. The experiments require non-invasive measurements during beam delivery, with an absolute accuracy better than 0.2% in the current range from 1 to 180 mu-A. Two redundant cavities, and their associated electronics, monitor the beam current continuously. They are periodically calibrated with respect to a Parametric Current Transformer (PCT) manufactured by Bergoz, having an accurate and stable sensitivity of 4 mu-A/V, but having offset drifts at the mu-A level that prevent its direct use as a non-invasive device. A beam current of at least 50 mu-A is repetitively turned on and off during the calibration process to reduce the error due to the PCT offset drifts. This is invasive to beam delivery, and one of the goals is to minimize the calibration time without compromising accuracy. This paper describes the beam current monitor system, identifies the sources of error, and reports on the system's performance during actual machine operations