Design of E x B Multistage Depressed Collector Concepts for High-Power Fusion Gyrotrons

The introduction of advanced Multistage Depressed Collectors (MDC) to high-power fusion gyrotrons will be the key to increase the gyrotron overall efficiency above 60 % as required for future DEMOnstration power plant. MDCs which base on the ExB drift concept might solve the fundamental issue of handling the secondary electrons. To consider the effect of secondary electrons in a stationary state, full 3D Particle-In-Cell (PIC) simulations are performed. A novel two-stage collector designed using PIC is presented which shows an excellent collector efficiency of up to 77 %. Moreover, the undesired generation of secondary electrons only reduces the efficiency by 1 %, compared to a typical 10 % efficiency reduction in other known classical concepts.