Development of Ar+16 charge exchange recombination spectroscopy measurements at ASDEX Upgrade

Argon is being considered as a radiator for future fusion devices. To support this, experiments in present day devices are needed to assess its effectiveness as a radiator and to study its compatibility with key aspects of reactor operation. For these purposes, accurate measurement of the Ar density in the confined plasma region are required. Charge exchange recombination spectroscopy is capable of providing this information, but requires validated cross-sections to produce accurate density profiles. For typical ASDEX Upgrade plasma parameters Ar+16 is the charge state of the most interest and the ArXVI n = 15-14 was identified as the best target for these measurements. Due to the fine structure splitting, the Ar charge exchange (CX) emission lines are highly asymmetric and detailed modeling is required to extract accurate ion temperatures or rotations. For the evaluation of Ar+16 densities, there are two main sets of CX cross-sections available: the data calculated by Schultz et al (Whyte et al 2010 J. Phys. B At. Mol. Opt. Phys 43 144002, Schultz et al 2010 J. Phys. B At. Mol. Opt. Phys. 43 144002) (ORNL) and by Errea et al (2006 J. Phys. B At. Mol. Opt. Phys. 39 L91) (UAM). These cross-sections differ by over an order of magnitude and have very different energy dependencies. In this work, the validity of these datasets is tested experimentally. The Ar density profiles calculated using the ORNL cross-sections are 10-50x too large while the UAM densities are a factor of 2.73 too small. The UAM data, however, does a much better job at capturing the observed energy dependence. The authors conclude that the CTMC calculations of Errea, which use a hydrogenic distribution to describe the initial target and donor distributions better reproduce the experimental data. However, to produce accurate Ar density profiles, these cross-sections need to be corrected downward and the best profiles are produced with a small correction to the UAM energy dependence.