Localizing C III emission using multi-delay coherence imaging spectroscopy on W7-X

Localizing regions of strong spectral emission in the edge of magnetically confined plasmas can give information on where different atomic and molecular processes occur, as well as provide bounds on plasma parameters in those regions. Measurements of 2D spectral emission distributions are available in tokamaks via tomographic inversions, but this is more challenging in stellarators due to the lack of toroidal symmetry. Spectroscopy can provide an alternative way to infer the location of the emission fronts by analyzing the effect of Zeeman splitting on the spectrum for a known magnetic field geometry. As a proof of principle, multi-delay coherence imaging is used to estimate the location of C III emission in the divertor region of the Wendelstein 7-X stellarator for a variety of experimental conditions and magnetic geometries. This allows increased spatial resolution in both poloidal and toroidal directions compared to conventional spectroscopy and could enable reconstructions of the 3D emission distribution in the divertor. Although a significant uncertainty in the precise location is present, the technique is able to differentiate between conditions with the emission close to the target and near the last closed flux surface. ... mehrTo improve the accuracy for future measurements, a new set of crystals is optimized for simultaneous emission location and ion temperature measurements.