Investigation of the separatrix ion temperature using impurity Charge Exchange Recombination Spectroscopy

Abstract

The ion temperature of a magnetically confined plasma plays an important roll in that plasma’s ability to initiate and sustain fusion reactions. In the development of fusion technology it is thus critical to be able to accurately characterize the ion temperature profile of a plasma. This work measured ion temperature using Charge Exchange Recombination Spectroscopy (CXRS). This method can involve injecting neutral molecules into the plasma, which can contaminate the ion spectral emissions with molecular line emissions, leading to inaccurate temperature measurements. In this master thesis, a multi-gaussian fit was designed to identify and subtract the molecular lines from the complex spectra detected by CXRS diagnostics at the ASDEX Upgrade tokamak for three discharges in different energy confinement modes (H-mode. L-mode, and I-mode). Impurity temperature and intensity profiles were determined for the three discharges using measurements at the edge of the plasma and in the Scrape-Off layer, as these regions were most likely to be affected by the injected neutral molecules. The results suggest that molecular lines effect measurements for a larger range of radial points in the case of L-mode and I-mode discharges, while the H-mode discharge showed a greater difference in temperature magnitude after subtracting molecular contributions, but in a smaller radial range. 1D kinetic simulations were performed to model the density profile of the molecular neutral. Molecular neutrals were found to penetrate the farthest into the plasma in L-mode, followed by I-mode, with H-mode showing the least penetration.