Investigation of higher harmonics of electron cyclotron emission using Fourier transform spectroscopy in Wendelstein 7-X

Wendelstein 7-X stellarator has a high aspect ratio which makes the electron cyclotron harmonics spectrally well separated and more straightforward to interpret compared to a tokamak with a low aspect ratio. For fusion reactor relevant plasma operations, Wendelstein 7-X is planned to operate at high plasma densities. For these plasma conditions, the classical electron cyclotron emission fails as a diagnostic tool for electron temperature because optically thick O1 and X2 mode are in cut-off. In this paper, we report on the radiation temperatures of X-mode electron cyclotron emission measured with Fourier transform spectrometer from the high-density plasmas. The radiation temperatures of X2 and X3 mode are compared to electron temperature measured with Thomson scattering diagnostic. This comparison suggests that X3-mode can be used as a diagnostic tool to provide electron temperature for high-density plasmas beyond the cut-off of X2-mode.