Exploration of high-n Balmer spectral diagnostics for T_e and n_e in DIII-D divertor detached plasmas using the HRUVS

An electron temperature and density spectral diagnostic is in progress using high-n Balmer series measured by the High-Resolution Ultraviolet Spectrometer (HRUVS) during detached DIII-D plasmas. The diagnostic capabilities are in the range: T_e = 0.7-10 eV and n_e = 6e13-8e14 . Langmuir probes cannot reliably measure T_e and n_e without a contribution by a population in the non-Maxwellian tail during detachment. A Python based code has been developed to generate a synthetic spectral feature. A suite of Fortan codes, PPP, are implemented to model the line shapes using Stark broadening. For electron shell n > 7, the electron contribution dominates over the ion contribution to the line shape allowing for an independent electron diagnostic. Atomic populations are computed using the ADAS 310 collisional radiative code. Impurity lines are filtered from the HRUVS spectra leaving the hydrogenic emissions. Resulting synthetic features are compared to the HRUVS spectra in cold divertor conditions. The fits show good agreement and reproduce expected T_e , n_e dependences. This work will be coupled with a model of lower-n lines where broadening effects by ions and molecules cannot be neglected. Future work is planned with the multichord upgrade of HRUVS to map the T_e and n_e of DIII-D divertor.