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

1-MW continuous-wave (CW) gyrotrons operating at 140 GHz are being develope d for heating fusion plasmas [1], Energy recovery using a depressed collect or 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, depres sed-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 coi ls with time-varying current values. The time-varying magnetic field produc ed by the collector coils sweeps the beam trajectories on the collector sur face lowering the average power density. The second design employs axial be am sweeping using axisymmetric collector coils, whereas the third design em ploys transverse beam sweeping, making the instantaneous fields and electro n trajectories nonaxisymmetric and three-dimensional (3-D). Relative advant ages and disadvantages of these three schemes are discussed.