Gyrotron multistage depressed collector based on E x B drift concept using azimuthal electric field. I. Basic design

Multistage Depressed Collectors (MDCs) are widely used in vacuum tubes to regain energy from the depleted electron beam. However, the design of an MDC for gyrotrons, especially for those deployed in fusion experiments and future power plants, is not trivial. Since gyrotrons require relatively high magnetic fields, their hollow annular electron beam is magnetically confined in the collector. In such a moderate magnetic field, the MDC concept based on E x B drift is very promising. Several concrete design approaches based on the E x B concept have been proposed. This paper presents a realizable design of a two-stage depressed collector based on the E x B concept. A collector efficiency of 77% is achievable, which will be able to increase the total gyrotron efficiency from currently 50% to more than 60%. Secondary electrons reduce the efficiency only by 1%. Moreover, the collector efficiency is resilient to the change of beam current (i.e., space charge repulsion) and beam misalignment as well as magnetic field perturbations. Therefore, compared to other E x B conceptual designs, this design approach is promising and fairly feasible.