A comparative study of three single-stage, depressed-collector designs for a 1-MW, CW gyrotron

1-MW continuous-wave (CW) gyrotrons operating at 140 GHz are being developed for heating fusion plasmas. Energy recovery using a depressed collector is employed to improve total efficiency and reduce power supply voltage and voltage regulation requirements for various electron beam devices. This paper describes and compares three different types of single-stage, depressed collector designs. The first design is a magnetically shielded, coaxial collector with fixed-current collector coils. The second and third designs use cylindrical collectors with an outer magnetic shield and collector coils with time-varying current values. The time-varying magnetic field produced by the collector coils sweeps the beam trajectories on the collector surface lowering the average power density. The second design employs axial beam sweeping using axisymmetric collector coils, whereas the third design employs transverse beam sweeping, making the instantaneous fields and electron trajectories nonaxisymmetric and three-dimensional (3-D). Relative advantages and disadvantages of these three schemes are discussed.