Michelson Interferometer design in ECW heated plasmas and initial results

Abstract

The Wendelstein 7-X (W7-X) experiment is equipped with an Electron Cyclotron Resonance Heating installation consisting of 10 gyrotrons capable of delivering up to 7.5 MW of Electron Cyclotron Wave power at the 140 GHz resonance in the plasma. During the plasma discharge the gyrotron power is delivered in a narrow band of several 100 MHz around the gyrotron frequency. At plasma start-up, i.e. initially in the absence of plasma, the non-absorbed gyrotron power in the vessel (‘stray radiation’) is very high. During the plasma discharge the stray radiation may also rise depending on the plasma optical depth or due to the challenge to deliver the gyrotron power at the exact required polarisation. At W7-X an additional issue is that at high electron density the second harmonic extra-ordinary heating scheme (X2) is no longer possible and one switches to second harmonic ordinary (O2) heating. But the ECH absorption is not complete in the O2 heating scheme and stray radiation is significantly increased. The stray radiation issue is a problem for sensitive microwave receivers, typically measuring power levels in the sub μW range. For protection, these generally employ notch filters such as fundamental mode interference cavities [1], [2]. This paper focuses on the effect of non-absorbed 140 GHz gyrotron power on the Michelson Interferometer at W7-X.