Development of High Power Gyrotrons for Fusion Applications at FZK Karlsruhe

Summary form only given. 140 GHz gyrotrons with a hollow waveguide cavity at 1 MW, CW output power have been developed at FZK Karlsruhe in cooperation with European institutions tor use at the stellarator W7-X at IPP Greifswald, Germany. TE_28.8 is the operating mode. The utilized advanced quasi-optical (qo) mode converter has an excellent performance with respect to low amount of stray radiation (< 3%) and high Gaussian content (> 97%) of the RF output beam. Two prototype tubes have been built and tested successfully. With a first series tube, out of seven, the required specifications have been fully demonstrated. At a pulse length of 30 mm a power of 920 kW has been measured in the Gaussian mode. The corresponding overall RF output efficiency was nearly 45%. The fabrication of the series gyrotrons is in progress. Results and problems with tubes number 2, 3 and 4 are reported and discussed. Based on results obtained on an experimental coaxial cavity gyrotron at 165 GHz at FZK, the development of a 2 MW, CW, 170 GHz coaxial cavity gyrotron for ITER is in progress in a European cooperation. In the meantime a first industrial prototype of the 170 GHz coaxial gyrotron has been fabricated and is ready for operation, which is expected to start in April 2007. In tests with an experimental pre-prototype tube, operated at short (les 5ms) pulses, the design of the main gyrotron components has been confirmed, in general. The mechanism of parasitic low frequencv (LF) oscillations has been identified and the intensity of the LF oscillations has been reduced significantly by performing some minor geometrical modifications on the coaxial insert. The performance of the q.o. RF output svstem is not satisfying mainly with respect to the Gaussian content of the RF output beam. An improved design is in progress. Results are reported and discussed. In addition, work on a high power, multi-frequency gyrotron is going on. A SC magnet which will allow a fast (~sec) varia- tion of the magnetic field has been specified and ordered. The optimization of the q.o. RF output system for a number of different modes is in progress. Results are reported.