(FT/3-1Rc) Experimental Investigations on the Pre-Prototype of the 170 GHz, 2 MW Coaxial Cavity Gyrotron for ITER

T. Rzesnicki1), B. Piosczyk1), J. Flamm2), J. Jin1), S. Kern1), O. Prinz1), M. Thumm1)2)
1) Forschungszentrum Karlsruhe, Institut für Hochleistungsimpuls- und Mikrowellentechnik, Association EURATOM-FZK, D-76021 Karlsruhe, Germany
2) Universität Karlsruhe, Institut für Höchstfrequenztechnik und Elektronik, D-76131 Karlsruhe, Germany

Abstract.  The development of a 2 MW, CW, 170 GHz coaxial cavity gyrotron at Forschungszentrum Karlsruhe (FZK) is in progress in cooperation with several European research institutions. The gyrotron is foreseen as a source for heating, current drive and stabilization of plasmas in the International Thermonuclear Experimental Reactor (ITER). Tests with a first industrial prototype started recently at CRPP Lausanne. The design of critical components, especially the electron gun, cavity and quasi-optical (q.o.) RF-output system has been verified at FZK in tests with a short pulse gyrotron, the so called pre-prototype. When starting the experiments the gyrotron operation was strongly limited due to the excitation of parasitic LF-oscillations with very high amplitude mainly around 265 MHz. The oscillations have been successfully suppressed due to small modification of the geometry of the gyrotron components. In recent experiments another parasitic oscillation around 160 GHz appears simultaneously with the desired gyrotron working mode. This parasitic oscillation belongs to a multi-moding scenario observed at operating parameters near of the nominal values. In the region with multi mode operation a reduction of the generated power in comparison with calculations, has been observed. The reason for this behavior is currently under investigation. The performance of the q.o. RF-output system was experimentally tested at the low power. The measured pattern at the window position has shown some discrepancies to the calculated one. The Gaussian content of the RF-beam at the window position evaluated from the experimental values was only 77%, whereas from design calculations a value of 86% is expected. The recent activities towards the improvement of the gyrotron operation will be presented and discussed.

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