Two-photon absorption laser induced fluorescence (TALIF) neutral density measurements and two-fluid (plasma-neutral) 3D Extended-MHD simulations with PSI-TET on the HIT-SI3 experiment

Two-photon absorption laser induced fluorescence (TALIF) measurements were made on the HIT-SI3 experiment at the University of Washington through a collaboration with West Virginia University. Monatomic deuterium neutral densities of below 1 x 10$^{\mathrm{17}}$ m$^{\mathrm{-3}}$ were measured in $I_{p} =$10 -- 13 kA deuterium spheromak configurations that had electron number densities on the order of $n_{e} =$ 1 x 10$^{\mathrm{19}}$ m$^{\mathrm{-3}}$. These measurements suggest an ionization fraction of 99{\%} or more in HIT-SI3 spheromak plasmas. Spatial and temporal information concerning the monatomic deuterium neutral density in HIT-SI3 will be presented. These data are being used to validate a self-consistent, multi-fluid (plasma-neutral) model that is currently being implemented in the PSI-TET 3D Extended-MHD code. Preliminary results from validation efforts will be presented, along with other plans for validation and proposed uses of this multi-fluid model on mainstream fusion devices.