Divertor impurity densities at detachment onset in DIII-D discharges*

Intrinsic (carbon) and seeded (nitrogen) impurity densities are measured in the DIII-D divertor to assess the impurity concentration c_z needed to achieve divertor detachment and benchmark recently proposed detachment scalings and measurement techniques. Impurity densities are derived from calibrated 2D visible emissivity, line-integrated VUV brightness and local T_e, n_e. Charge state abundancies and emissivity localization are inferred from a collisional radiative model and from experimentally constrained fluid simulations with the UEDGE code to account for effects of transport and charge exchange recombination. Carbon concentrations up to a few % are measured from emissivity of singly and doubly ionized carbon ions in attached conditions and reduced to < 1% in detached regimes at the radiation front due to the reduction in sputtering and the radiation front movement away from the divertor plate. Measurements of nitrogen density from singly ionized nitrogen visible emissivity are used to examine proposed scalings of c_z at detachment on separatrix density and scrape off layer power over the range of heating powers 3-10 MW, while varying the fraction of radiated power due to nitrogen.