Preliminary studies on Multistage Depressed Collectors for fusion gyrotrons

Fusion gyrotrons are high-power microwave sources which are capable to generate RF power in the MW range at millimeter wave and sub-THz frequencies. Today, typically around 35 % electronic efficiency is achieved in the energy transfer (interaction) from the electron beam to the electromagnetic field. Using a single-stage depressed collector (SDC), the overall efficiency of a gyrotron can be increased to around 50 % as planned for ITER. To achieve higher total efficiencies, so-called Multi-stage Depressed Collectors (MDC) need to be introduced. MDCs offer the possibility to sort the electrons by their energy more accurately and to guide the electrons to electrodes on proper voltages. However, the application of the MDC concept to gyrotron is nontrivial. That is due to the small-orbit hollow electron beam, the large variation in energy of the spent beam at the collector entrance and the existence of a relatively strong magnetic field at the collector region, which dominantly constrains and guides the electron beam. According to Busch's theorem, if both electric and magnetic field components are axially symmetric, the cyclotron motion of the electrons cannot be spatially separated. For this case, a new approach for MDC, based on a spatial trajectory modulation caused by non-adiabatic transition is proposed. A design proposal for a two-stage collector is presented which shows 73 % collector efficiency which is already close to the theoretical limit for a two-stage collector. Besides, the MDC has the advantage that the collector wall loading is sufficiently low, thus electron beam sweeping is not necessary.