Electron beam current monitoring system

Abstract

A beam current monitoring system used with the NBS electron linac is described in detail. The system consists of a water-cooled Faraday cup designed to dissipate the energy in a 1 cm diameter, 100 kW beam of 100 MeV electrons, and a non-intercepting ferrite loaded toroidal coil. The ferrite is used for continuous-duty monitoring and can be calibrated in absolute units with the Faraday cup. The Faraday cup errors have been measured with an absolute accuracy of better than 0.1% by bucking the Faraday cup current against the beam current, using the ferrite monitor as a null detector. The Faraday cup losses are about 0.5% at 30 MeV and decrease to less than 0.1% at 120 MeV. These losses are attributed to backscatter of high energy electrons from the front face of the cup. In addition, it is shown that secondary electrons produced by Møller scattering in the Faraday cup entrance foils may lead to spurious measurements of beam current, and a correction for this effect is derived.