Two-photon LIF on the HIT-SI3 experiment: Absolute density and temperature measurements of deuterium neutrals

Two-photon laser-induced fluorescence measurements were performed on the helicity injected torus (HIT-SI3) device to determine the density and temperature of the background neutral deuterium population. Measurements were taken in 2 ms long pulsed plasmas after the inductive helicity injectors were turned off. Attempts to measure neutrals during the main phase of the plasma were unsuccessful, likely due to the density of neutrals being below the detection threshold of the diagnostic. An unexpectedly low density of atomic deuterium was measured in the afterglow; roughly 100 times lower than the theoretical prediction of 10¹⁷ m⁻³. The neutral temperatures measured were on the order of 1 eV. Temporally and spatially resolved neutral density and temperature data are presented.