There is a growing recognition of the importance of neutral density fields to the physics, and particularly facility related effects, of Hall-effect thrusters (HETs). This work presents the first high-resolution (mm scale) spatial maps of neutral density in the near-field plume of a HET using two-photon absorption laser-induced fluorescence (TALIF). We employ a 212.6 nm TALIF excitation scheme, based on a pulsed nanosecond dye laser, to map absolute krypton neutral densities in the plume of a SPT-70 HET. Absolute densities are determined via a ratio-based approach using cold-flow measurements. The diagnostic system incorporates active wavelength stabilization via a feedback-controlled dye laser with drift correction algorithms to maintain measurement accuracy through extended mapping campaigns. Neutral density maps are acquired under cold-flow conditions and four plasma-operating states (540 and 720 W at varying background pressures), highlighting the effects of power level and background pressure on the near-field neutral particle profile. These results provide important experimental benchmarks for validating computational models and advancing physical understanding of neutral behavior and electric propulsion facility effects.
Stienike et al. (2026) studied this question.