Development of the 140 GHz gyrotron and its subsystems for ECH and ECCD in TEXTOR

A 800 kW, 140 GHz gyrotron has been mounted on TEXTOR. Initial operation was limited by arcs in the transmission line, and by pulse length restrictions due to the expected limited power handling of the window and the launching mirror. Measurements of power absorption in the window and launching mirror, however, showed them to be capable of handling full power, 3 s pulses, while arcing could be prevented by absorption of stray radiation at strategic position in the transmission line. This resulted in the realisation of full power, 3 s pulses on the plasma. Further measures were taken to prevent gyrotron operation in wrong modes or under conditions of low absorption in the plasma. Future improvements, expected to allow further extension of the pulse length to 10 s include: a CVD diamond window to replace the current quartz torus vacuum window, and a fast and accurate launcher.