Investigation of optically grey electron cyclotron harmonics in Wendelstein 7-X

Electron cyclotron emission (ECE) is a standard diagnostic to measure electron temperature profiles in magnetized plasmas. In W7-X with a magnetic field of 2.5T, ECE from the second harmonic X-mode (120-160 GHz) is scanned using a heterodyne radiometer. For the X2 mode, re-absorption of microwaves in the plasma is strong enough for the plasma to be considered as optically thick. Hence ECE can be taken as blackbody emission representing an electron temperature. Other than the optically black X2 mode the ECE spectrum also has higher harmonics. Due to lower absorption coefficients these harmonics are optically grey. In case of overdense plasmas the X2 mode reaches its cutoff. Then higher harmonics provide the only access to ECE emission and hence the physics of core electrons. The aim of this work is to perform a broadband scan of the ECE spectrum using a Michelson Interferometer in W7-X and compare it to the already existing radiation transport calculations (TRAVIS). Stray radiation from non-absorbed electron cyclotron resonance heating is an issue as it is in the middle of the X2 emission spectrum. Therefore a stray radiation filter is required. The design for such a filter consisting of multiple dielectric layers has been tested and will be presented.