Impurity Transport Studies at the HSX Stellarator Using Active and Passive CVI Spectroscopy

The transport of intrinsic carbon impurities has been studied in the helically symmetric stellarator experiment (HSX) using active and passive charge exchange recombination spectroscopy (CHERS). For the analysis of the CHERS signals, the STRAHL impurity transport code has been re-written in the python programming language and optimized for the application in stellarators. This optimization is done via additional considerations of 3D effects on impurity fueling and diagnostic line of sight emission. In addition, neutral densities both along the NBI line of sight as well as for the background plasma have been calculated using the FIDASIM code. By using the basinhopping algorithm to minimize the difference between experimental and predicted active and passive signals, significant levels of anomalous impurity diffusion are observed. Comparisons with necoclassical calculations from DKES/PENTA show that the inferred levels exceed the neoclassical transport by about a factor of four in the core and more than 100 times towards the plasma edge. This conclusion is compatible with heat transport calculations and measurements as well as recent experiments measuring the impurity decay times of laser ablation injected Aluminum.