Transverse field collector sweep system for high power CW gyrotrons

The spent electron beam of megawatt-class CW gyrotrons requires magnetic sweeping to keep the specific heat dissipation on the collector surface within technically acceptable limits of about 500 W per cm2. Conventionally this is realised with a solenoid coil placed over the collector, which sweeps the strike point of the electron beam up and down the collector-wall at low frequency (<10 Hz). The low frequency is necessitated by the strong screening effects due to eddy-currents in the copper-collector-wall. This paper describes the realisation and test results obtained with a six-coil 50 Hz transverse field sweep system (TFSS) on the 140 GHz 1 MW CW prototype gyrotron for W7-X. The advantages of this system compared to the conventional scheme are discussed and an outlook on further enhancements is given.