Recent improvements on the 170 GHz, 1.5 MW coaxial cavity gyrotron at FZK

The 2 MW coaxial cavity gyrotron for ITER investigated by the EU EFDA team is based on results from a short pulse gyrotron with similar cavity, gun and quasi-optical mode converter. This tube is still under investigation at FZK to understand and overcome possible problems. Limitations on the magnetic field of the FZK gyrotron magnet allow for an operation at reduced parameters only, thus the expected output power at 170 GHz is only 1.5 MW. The problems found in first experiments were low frequency parasitic oscillations in the gun region at about 300 MHz and insufficient Gaussian mode content of the output wave (76%), at no more than 1 MW rf output power. After changing the electron gun the parasitic oscillations vanished, and an rf output power of 1.3 MW at an efficiency of 23% was reached. The efficiency now seems to be limited by another parasitic high frequency oscillation at 160 GHz, which appears simultaneously to the desired working mode and which, at higher energy operating points, enforces multi- moding. 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 at the conference. To improve the quasi-optical mode converter and to increase the Gaussian mode content of the output wave, respectively, a different design approach for the launcher is under investigation, using arbitrarily shaped wall deformations.First results with 86% Gaussian mode content are promising.