Progress of the 1 MW, 140 GHz, CW gyrotron for W7-X

The development of high power gyrotrons in continuous wave operation for heating of plasmas used in nuclear fusion research has been in progress for several years in a collaboration between different European research institutes and industrial partners. The 140 GHz gyrotron is designed for an output power of 1 MW in continuous wave operation and is operated in the TE/sub 28,8/ mode. The first tube "Maquette" was tested at the Forschungszentrum Karlsruhe. Long pulse operation of the gyrotron was possible but it was limited by the behaviour of the RF-load and by the pressure increase inside the tube. Modulation experiments with an amplitude between 0.1-0.8 kW were performed up to a modulation frequency of 50 kHz sinusoidally. A visual inspection of the tube showed some melted spots in the mirror box. An improved tube (Prototype 1) with better cooling and an additional relief window at the mirror box has been built and tested. Without optimisation of the parameters in short and long pulse operation, an output power of 0.85 kW could be achieved with a pulse length of 180 s and 0.97 kW for 12 s with an efficiency of 44% (depression voltage: 26 kV). Even for long pulse operation, the pressure increase inside the tube was moderate. The limitations were caused by mode-loss and/or by increased cathode current connected with parasitic oscillations.