Experimental results on the 140 GHz, 1 MW, CW gyrotrons for the stellarator W7-X

The development of high power gyrotrons (118 GHz, 140 GHz) in continuous wave operation (CW) has been in progress for several years in a joint collaboration between different European research institutes and industrial partners. In this frame, two 140 GHz prototype gyrotrons for CW operation had been constructed and tested at the Forschungszentrum Karlsruhe. According to the results of the prototypes, seven 140 GHz CW gyrotrons were ordered. The first tube was operated at the Forschungszentrum Karlsruhe. A power of 950 kW at efficiency of 43 % (with energy recovery) could be obtained for pulse lengths of 180 s (limited by the available high-voltage power supply). A 30 minute pulse was performed with an output power of 540 kW. During this pulse almost no decrease in performance was found, especially the tube pressure only increased in the range of 10^-9 mbar.