Advanced high power gyrotrons

Summary form only given. Gyrotrons at high frequency with high output power are mainly developed for microwave heating and current drive in plasmas for thermonuclear fusion. For the stellarator Wendelstein 7-X now under construction at IPP Greifswald, Germany, a 10 MW ECRH system is foreseen. A 1 MW, 140 GHz gyrotron with synthetic diamond window for continuous wave operation and with a single stage depressed collector for energy recovery and improvement of efficiency has been designed, constructed and tested in collaboration with TED Velizy. It operates in the TE/sub 28.8/ cavity mode and provides a linearly polarized TEM/sub 0.0/ Gaussian RF beam. In short pulse operation at the design current of 40 A, an output power of 1 MW could be achieved for an accelerating voltage of 82 kV without depression voltage, an output power of 1.15 MW at an accelerating voltage of 84 kV with a depression voltage of 25 kV. These values correspond to an efficiency of 49%. After some problems with the RF-load, long pulse operation was performed. The power measurements were done by the calibrated signal of the diode detector placed at the second mirror. Output powers of 1 MW could be achieved for 10 s, and an energy as high as 90 MJ per pulse has been produced with an output power of 0.64 MW. The pulse lengths were mainly determined by the preset values. Only for the 100 s pulse, a limitation was found due to a pressure increase beyond about 10/sup -7/ hPa.