Impurity temperatures measured via line shape analysis in the island scrape-off-layer of Wendelstein 7-X

Abstract

Impurity temperatures have been determined by a spectroscopic line shape analysis for several species in the divertor scrape-off-layer of the stellarator Wendelstein 7-X. Examples include spectral lines from intrinsic elements (C II and C III, He I) as well as from seeded impurites (Ar II, N II) through the divertor gas inlet system. Both Doppler broadening and Zeeman splitting are found to contribute significantly to the impurity line shapes. Zeeman splitting arises due to the confining magnetic field in Wendelstein 7-X and complicates the line shape appearance. By attributing Doppler widths to each of the various Zeeman components, however, we demonstrate that reliable ion temperature values can be derived provided that the presence of the magnetic field is properly accounted for. The spectrally highly resolved lines are analysed by means of a multi-parameter, least-squares fit routine, which accounts for Doppler broadening, Zeeman splitting, as well as the instrumental broadening of the spectrometer used to measure the spectral line shapes. By spectral fitting of the Zeeman features, it is also found that the line shape analysis can yield values for the local magnetic field, which can be used to localise the impurity radiation approximately provided that the line emission is dominant in a small area intersected by the lines of sight of the spectrometer.