Effect of back-scattered electrons on the efficiency of multi-stage depressed collectors for gyrotrons

Summary form only given. Electrons back-scattered from surfaces of depressed collectors influence the collector efficiency, the current distribution among collectors and the heat dissipation density profile. These parameters were studied for 2-stage depressed collectors for a 110 GHz, 1 MW gyrotron. For this purpose 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 back-scattered electrons. The collector geometry and the magnetic circuit were adjusted in order to minimize the effect of back-scattered electrons. A 3-stage case was also considered. Assuming the surfaces of the collectors to be of polished copper, it was seen that typically the reduction in the estimated collector efficiency was of the order of 8%. For further reduction of this effect, the use of surface treatment of copper or the use of other materials would be possible options. Results obtained from trajectory simulations of primary and back-scattered electrons will be presented.