Two-Axis Velocimetry Study of Neutrals in a Hall Thruster

We present two-axis laser-induced fluorescence velocimetry measurements of the Xe I 6s'[1/2]$^{0}_{1} \quad \to $ 6p'[3/2]$_{2}$ transition at 834.9 nm (vacuum) inside a 6-kW laboratory Hall thruster. The spatial density of the data is higher near the walls than in the center of the channel to better capture the neutral-wall and neutral-sheath interactions. The thruster is operated under seven conditions spanning discharge voltages of 150-600 V and anode mass flow rates of 10-30 mg/s. The magnitude of velocity varies from 200 to 400 m/s along the channel centerline of the interrogated region but can go up to a few km/s near the downstream end of the channel walls. These high speed neutrals are speculated to be products of charge-exchange reactions and are only detectable over very small regions. The neutral velocity exhibits greater sensitivity to changes in anode mass flow rates than in discharge voltage. Comparison with simulation data shows that the presence of the plasma greatly affects the behavior of the neutral gas in this Hall thruster.