Design of a multistage depressed collector system for 1-MW CW gyrotrons. I. Trajectory control of primary and secondary electrons in a two-stage depressed collector

Part I of this paper presents the design of a two-stage depressed collector for a 110 GHz, 1 MW gyrotron, taking into account the trajectories of electrons backscattered from the electrode surfaces. The collector geometry and the magnetic circuit were adjusted in order to minimize the effect of backscattered electrons. A computer code was employed which is based on a new algorithm designed to give good representation to a variety of energy levels and angles of emergence of backscattered electrons. The collector efficiency estimated on the basis of primary electrons alone was 68%. The estimated collector efficiency which included also the effect of backscattered electrons was 60%. In the latter case the surface of the collectors was assumed to be that of polished copper. The techniques and codes used in the design are reviewed and results of trajectory tracing are presented for primaries and secondaries. Part II of the paper presents the system aspects, including the mechanical and thermal designs and the solid-state power supply design.