Status of the 2 MW, 170 GHz Coaxial Cavity Gyrotron for ITER

B. Piosczyk¹, F. Albajar², S. Alberti³, P. Benin⁴, W. Bin⁵, T. Bonicelli², S. Cirant⁵, G. Dammertz¹ O. Dumbrajs⁶, D. Fasel³, J. Flamm⁷, G. Gantenbein¹, E. Giguet⁴, T. Goodman³, J.P. Hogge³, S. Illy¹, J. Jin¹, C. Lievin³, O. Prinz¹, T. Rzesnicki¹, M. Schmid¹, M. Thumm^1,7, M.Q. Tran³

¹Forschungszentrum Karlsruhe (FZK), Association EURATOM-FZK, IHM, Postfach 3640,

D-76021 Karlsruhe, Germany,

²European Fusion Development Agreement (EFDA), D-85748 Garching, Germany

³Centre de Recherche en Physique des Plasmas (CRPP), Association Euratom-Confédération Suisse, EPFL Ecublens, CH-1015 Lausanne, Suisse

⁴Thales Electron Devices (TED), F-78141 Vélizy-Villacoublay, France

⁵Instituto di Fisica del Plasma, EURATOM-ENEA-CNR Association, 20125 Milan, Italy

⁶Helsinki University of Technology, Association EURATOM-TEKES, FIN-02150 Espoo, Finland

⁷Universitaet Karlsruhe, IHE, D-76131 Karlsruhe, Germany

e-mail: bernhard.piosczyk@ihm.fzk.de

A 170 GHz coaxial cavity gyrotron with 2 MW output power in continuous wave (CW) operation is under development in cooperation between European research centres together with European industry. A first industrial prototype of such a gyrotron has already been fabricated and delivered to CRPP Lausanne, where a suitable test facility has been constructed. Due to a delay in fabrication the delivery of the gyrotron magnet is expected in May 2007. Thus experimental tests are expected for the second half of this year.

In parallel to the industrial activities, experimental operation with a short pulse (~ few ms) 170 GHz coaxial gyrotron ("pre-prototype") which uses the same main components as designed for the industrial tube has been continued. The mechanism of parasitic low frequency (LF) oscillations around 260 MHz has been identified. Based on this identification, small modifications of the geometry of the coaxial insert have been made. As a result the starting current for the LF oscillations has been increased by a factor of about 3 causing a strong reduction of the LF amplitude. Measurements with a prototype of a microwave load, which has been designed and fabricated for operation with the 2 MW prototype tube, have been performed. In addition to the distribution of the microwave power absorbed on the wall, the amount of power reflected back into the gyrotron has been measured and its influence on gyrotron performance has been investigated. The performance of the quasi optical (q.o.) RF output system presently installed in the industrial prototype tube is insufficient, mainly because of the low Gaussian content of the RF output beam. As a first step a new launcher with a different wall corrugation and a new adapted phase correcting mirror has been designed and fabricated. According to simulations an increase of the Gaussian content to about 87% is expected. This q.o. RF output system has been installed in the pre-prototype tube for performing hot measurements.

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This work was carried out within the framework of EFDA