e-mail: michael.kuntze@ihm.fzk.de

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 Vélizy. It operates in the TE_28,8 cavity mode and provides a linearly polarized TEM_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^-7 hPa.

The ITER task on development of coaxial cavity gyrotrons ended in 2001. In accordance with the goal of the task the potential of coaxial gyrotrons has been investigated and as a result data necessary for an industrial realization of a 2 MW, CW, 170 GHz tube have been obtained. In addition, first work on tube integration has been done.

The results will be presented and discussed. By biasing the coaxial insert a fast (within 0.1 ms) frequency tuning has been demonstrated. In particular, a fast step tuning between the 165 GHz nominal mode and the azimuthal neighbors at
162.5 GHz and 167.2 GHz have been performed. In addition, at the nominal mode a continuous frequency variation within the bandwidth of up to 70 MHz have been done.

ICOPS2002