Development of a 140 GHz, 1 MW, continuous wave gyrotron for the W7-X stellarator

The development of high power gyrotrons in continuous wave (CW) operation f or heating of plasmas used in nuclear fusion research has been in progress for several years in a joint collaboration between different European resea rch institutes and industrial partners. A recent R&D program aims at the de velopment of 140 GHz gyrotrons with an output power of 1 MW in CW operation for the 10 MW ECRH system of the new stellarator plasma physics experiment Wendelstein 7-X at IPP Greifswald, Germany. The work is performed under re sponsibility of FZK Karlsruhe in collaboration with CRPP Lausanne, IPF Stut tgart, IPP Garching and Greifswald, CEA Cadarache and TED Velizy. The gyrot ron operates in the TE28.8 mode and is equipped with a diode type magnetron injection electron gun, an improved beam tunnel, a high-mode purity low-oh mic loss cavity, an optimized non-linear up-taper, a highly efficient inter nal quasi-optical mode converter, a single-stage depressed collector and an edge-cooled, single disk CVD-diamond window. RF measurements at pulse dura tion of a few milliseconds yielded an RF output power of 1.15 MW at a beam current of 40 A and a beam voltage of 84 kV. Depressed collector operation has been possible up to decelerating voltages of 33 kV without any reductio n of the output power, and an efficiency of 49 % has been achieved. Long pu lse operation of the gyrotron was possible with an output power of 1 MW at a pulse length of 10 s without any signs of a limitation caused by the tube . For this output power the efficiency of the tube could be increased from about 30 % without depression voltage to about 50% with depression voltage. At an output power of 640 kW, a pulse length of 140 s could be achieved.