Developing local high-Z impurity density measurements for impurity transport in DIII-D plasmas using high-n Rydberg transitions

The primary material for plasma facing components proposed for many burning plasma experiments and fusion pilot plants is tungsten, a high-Z metal. The tolerance for sourcing high-Z materials into magnetically-confined plasmas is extremely low (concentrations ∽10^-5) due to the resulting strong radiative cooling. The transport of these impurities due to turbulence and the resilience of different operational scenarios to impurity accumulation through mechanisms like neoclassical screening is being studied to validate transport models. These experiments can benefit from additional fast measurements of multiple charge states using a two-channel high-throughput charge exchange spectrometer. Charge exchange cross sections for highly charged ions can be sufficiently large that highly-excited Rydberg states can be populated for which background light from electron impact excitation is negligible. Such measurements have been used to quantify iron transport in W7-X. Initial observations of such visible line emission correlated with tungsten impurity injection experiments in negative triangularity discharges on DIII-D is presented along with a review of applying universal cross section scaling approximations to develop estimates of the emission strength of transitions from these highly-excited states.