G. Dammertz31a, A. Arnold31a,2,S. Alberti2, E. Borie31a, V. Erckmann5E. Giguet3, R. Heidinger31b,

S. Illy31, K. Koppenburg31a, M. Kuntze31a, G. Le Cloarec3, F. Legrand3, W. Leonhardt31a, C. Liévin3, G. Mueller4, G. Neffe31a, B. Piosczyk31a,

M. Schmid31a, M. Thumm31a,2, M.Q. Tran5

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

Postfach 3640, D-76021 Karlsruhe, Germany

2also Universitaet Karlsruhe, Institut fuer Hoechstfrequenztechnik und Elektronik,

Kaiserstrasse 12, D-76128 Karlsruhe, Germany

3Thales Electron Devices, 2 rue Latecoere, F-78141 Velizy-Villacoublay, France

4Institut fuer Plasmaforschung, Universitaet Stuttgart, Pfaffenwaldring 31,

D-70569 Stuttgart, Germany

5Centre de Recherche en Physiques de Plasma, Assocation Euratom-Confédération Suisse, EPFL Ecublens, CH-1015 Lausanne, Suisse

In the framework of a collaboration between FZK-Karlsruhe, CRPP-Lausanne and TTETED-Vélizy, with contributions from IPF-Stuttgart, CEA-Cadarache and IPP Garching/Greifswald, a 1 MW, 140 GHz, CW gyrotron for the 10 MW ECRH system of the new stellarator experiment Wendelstein 7-X at IPP Greifswald (Germany) is under development.

The tube operates in the TE28,8 cavity mode and provides a linearly polarised, 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 optimised non-linear uptaper, a highly efficient internal quasi-optical mode converter, a single-stage depressed and a horizontal RF output through a large-aperture, water edge-cooled, single-disk CVD-diamond window.

A first tube has been tested at FZK. In short pulse operation, an output power of 1.15 MW was achieved with an accelerating voltage of 84 kV and a depression voltage of 25 kV (49% efficiency). RF mode purity measurements performed using a thin dielectric target located at different positions across the RF beam showed good agreement with the theoretical predictions. In long-pulse operation, the following performances were finally achieved: 1 MW for 10s (efficiency of 50 %) down to 0.47 MW for 180 s.

Amplitude modulation experiments have been performed both for modulating the depression voltage or the cathode voltage. Power modulation between 100 kW and 900 kW up to a frequency of 50 kHz was possible.

The tube has been sent back to TED Vélizy for completion of an a detailed expertise prior to the fabrication of an improved prototype which is now under test at FZK. These experiments will also be reported.