Precision intercomparison of beam current monitors at CEBAF

The CEBAF accelerator delivers a CW electron beam at a fundamental frequency of 1497 MHz, with an average beam current up to 200 μA. Accurate and stable non-intercepting beam current monitors are required for a number of applications. These include setup and control of the accelerator, monitoring of both beam current and beam losses for machine protection and personnel safety purposes, and providing beam current information to the experimental users. Fundamental frequency stainless steel RF cavities have been chosen for these beam current monitors. This paper reports on a precision intercomparison between two such RF cavities, an Unser monitor, and two Faraday cups, all located in the injector area. At the low beam energy in the injector, it is straightforward to verify the high efficiency of the Faraday cups, and the Unser monitor included a wire through it to permit an absolute calibration. The cavity intensity monitors have proven to be capable of stable, high precision monitoring of the beam current.