Towards absolutely calibrated ECE Michelson measurements in EC heated plasmas at W7-X

Abstract

A Michelson Interferometer is in use at Wendelstein 7-X (W7-X) to probe the Electron Cyclotron Emission (ECE) spectrum [1], [2], [3]. During the past operational campaign (OP2.1), 2nd and 3rd harmonic ECE power density spectra have been routinely recorded in the presence of X2and O2-mode Electron Cyclotron Resonance Heating (ECRH). However, combination of the particular notch filter arrangement and high transmission line losses have thus far prevented overall calibration using a hot source cold source exposure at the input antenna. As an alternative, the response of the individual components is measured and summed. While reasonable numbers on electron temperature are obtained in X2-mode polarisation, interaction between front-end components is neglected and large error bars must be assumed. But the information on the individual components, together with synthetic modelling and data from experiment (OP2.1), has been used to design a new front-end with improved S/N. This optimisation is discussed in this paper with focus on notch filter selection, a new transmission line (Tx-line) and a novel combined quasi-optical taper / polarizer tuner.