M. Thumm1a,2, S. Alberti3, A. Arnold2, G. Dammertz1a, V Erckmann7, G. Gantenbein6,  E. Giguet4, R. Heidinger1b, J.-P. Hogge3, S. Illy1a, W. Kasparek6, H.P. Laqua7, C. Liévin4, R. Magne5, G. Michel7, B. Piosczyk1a, K. Schwörer6, M.Q. Tran3, X. Yang1a

1Forschungszentrum Karlsruhe, Association EURATOM-FZK, 1aIHM, 1bIMF-I

Postfach 3640, 76021 Karlsruhe, Germany. e-mail:

2IHE Universität Karlsruhe, 3CRPP Lausanne, 4TED Vélizy-Villacoublay,
5CEA Cadarache, 6IPF Universität Stuttgart, 7IPP Greifswald

The program aims towards the development and installation of a 10 MW, continuously (30 minutes) operated ECRH system at 140 GHz for the stellarator W7-X at the Max-Planck-Institute of Plasma Physics (IPP) Greifswald (in collaboration with CRPP Lausanne, IPF Stuttgart, CEA Cadarache and TED). The conventional cavity gyrotron operates in the TE28,8 cavity mode and provides a linearly polarized, fundamental Gaussian output beam. It is composed of a diode-type electron gun, an improved beam tunnel, a high-mode-purity low-ohmic-loss cavity with rounded transitions, an optimized non-linear uptaper, a highly efficient internal quasi-optical mode converter employing an improved launcher together with one quasi-elliptical and two beam shaping reflectors, a large single-stage depressed collector (SDC) with a beam shaping magnet, and a horizontal RF output through a large-aperture, water edge-cooled, single-disk CVD-diamond window. The prototype gyrotron delivered an output power of 970 kW (efficiency: 44%) for a pulse length of 11.7 s, and 890 kW (42%) for 180 s (limit of the HV power supply at FZK). At reduced electron beam current (30 A), the pulse length could be increased to 939 s at 539 kW (506 MJ). For this case the limitation was given by the increase of the internal pressure. At even more reduced electron beam current and lower output power of 257 kW a pulse length of 1300 s could be achieved. The reasons for not achieving completely the specified output power and pulse lengths are understood. To overcome the present limitations the gyrotron design has been slightly modified. A better quality assurance of the cathode emitter ring for homogeneous electron emission is being performed and the ion getter pumps have to be placed outside in order to avoid overheating by stray radiation. With these modifications the first series tube has been tested in short pulse operation and the output power of 1 MW at the specified electron beam current of 40 A (acc. voltage = 77.5 kV, without SDC) could be achieved. At a current of 50 A an output power of 1.15 MW could be obtained. The results of short pulse and the presently performed long pulse operation will be reported.