Collective Thomson Scattering at Wendelstein 7-X: design, commissioning, and first results*

A Collective Thomson Scattering (CTS) diagnostic was installed on the Wendelstein 7-X (W7-X) stellarator for ion temperature measurements. The diagnostic setup features measurement capabilities at two plasma cross-sections with different topology of the magnetic field. CTS uses 140 GHz heating gyrotrons as sources of probing radiation. It also uses a remote steering quasi-optical receiving antenna where the movable parts are located outside the vacuum vessel.

The CTS signal at sub-nW power levels shares a quasi-optical transmission line with the heating beams of ECRH at MW-power levels.

In the commissioning phase, electron cyclotron emission (ECE) measured by CTS showed good agreement with the measurements of the ECE radiometer.

CTS spectra were also measured and the spatial localization of the measurements was demonstrated by sweeping the receiver beam across the probing beam using the moveable quasi-optical components.

We fitted the scattering spectra using the electrostatic CTS code eCTS, which is integrated into the MINERVA system for Bayesian analysis at W7-X. The obtained ion temperatures showed good agreement with X-ray spectroscopy measurements.