Spectroscopic $T_e$ and $n_e$ measurements in a recombining divertor region and in MARFEs in NSTX using D~I and He~II high-$n$ series line emission.

Spatially resolved measurements of Balmer and Paschen series D~I line emission, and for the first time, Pfund and Humphreys series He~II emission, have been performed in the divertor and MARFE regions in 2-6 MW NBI-heated deuterium and helium plasmas in NSTX. We analyze relative intensities and Stark broadening of the ultraviolet, visible and near infrared lines corresponding to the $2-n$, $3-m$ transitions (D~I) and $5-k$, and $6-l$ (He~II) with upper principal quantum numbers $n=$7-13, $m=$5-12, $k=$10-19, and $l=$12-16. Temperatures in the range 0.5-1.5 eV and densities in the range $(0.5-5) \times 10^{20}$ m$^{-3}$ are inferred using collisional-radiative modeling and published tabulated line shape calculations, confirming the important role of the three-body recombination process as an ion momentum loss mechanism in the detachment and radiative instability development. The diagnostic potential of the spectroscopic techniques for a divertor of a burning plasma device will be discussed. This work is supported by U.S. DoE under contracts W-7405-ENG-48 and DE-AC02-76CH03073.