EXPERIMENTS ON THE EU 2MW COAXIAL CAVITY ITER GYROTRON
S. Kern 1), T.
Rzesnicki 1), O.
Dumbrajs 3), J. Flamm 2), G.
Gantenbein 1), S. Illy 1), J. Jin 1),
1) Forschungszentrum Karlsruhe (FZK), Association EURATOM-FZK, Institut für Hochleistungsimpuls- und Mikrowellentechnik, Postfach 3640
D-76021 Karlsruhe, Germany
2) Universität Karlsruhe (TH), Institut für Höchstfrequenztechnik und Elektronik (IHE)
D-76131 Karlsruhe, Germany
3) Helsinki, University of Technology, Association EURATOM-TEKES
FIN-02150 Espoo, Finland
The 2MW coaxial cavity gyrotron for ITER investigated by the EU EFDA team1,2 is based on results from a pre-prototype short pulse gyrotron with similar cavity, gun and quasi-optical mode converter. This tube is still under investigation at FZK to determine basic properties and to understand and overcome possible problems. Limitations on the magnetic field of the FZK magnet allow for an operation at reduced parameters only, thus the expected output power at 170GHz is only 1.5MW. Still such an operation can be considered as a preliminary validation of the design.
The problems found in first experiments3 were low frequency parasitic oscillations in the gun region at about 300MHz and insufficient Gaussian mode content of the output wave, at no more than 1MW rf output power. After changing the electron gun it was now possible to operate the gyrotron without these parasitic oscillations, and a power of up to 1.3MW at an efficiency of 23% was reached. The efficiency is now obviously limited by another parasitic high frequency oscillation at 160GHz, which appears simultaneously to the desired working mode and which, at higher energy working points, enforces multimoding. This oscillation is believed to be a gyro-backward wave interaction located in the beam tunnel. Experiments with modified beam tunnel are underway, the results will be presented in the paper.
To improve the quasi-optical mode converter and to increase the Gaussian mode content of the output wave, respectively, a different approach to launcher design was investigated, using arbitrarily shaped wall deformations. First results with 86% Gaussian mode content, instead of 76% before, are promising. The redesign work is carried on, and the paper will also give an overview of the performance currently reached.
1. T. Bonicelli et al, “EC power sources: European technological developments towards ITER”, Fusion Engineering and Design, 82, 619-626 (2007)
2. T. P. Goodman et al, “First Experimental Results from the EU 2MW Coaxial Cavity ITER Gyrotron Prototype”, this conference
3. T. Rzesnicki et al, 2MW Coaxial Cavity Gyrotron, “Investigation of the Parasitic Oscillations and Efficiency of the RF-Output System”, 8th IEEE Internat. Vacuum Electronics Conference (IVEC 2007), Kitakyushu, Japan, May 15-17, 2007, Proc page 45-46.