Laser-Induced Fluorescence of Singly-Charged Xenon inside a 6-kW Hall Thruster

We present laser-induced fluorescence velocimetry measurements of the 5d[4]7/2 → 6p[3]5/2 transition for singly-charged xenon (Xe II) at 834.953 nm (vacuum) inside and in the near-field plume of a 6-kW Hall thruster. The thruster has a nominal operating discharge voltage of 300 V and a nominal anode mass flow rate of 20 mg/s. Axial velocity profiles are obtained along the centerline of the discharge channel. 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 near-field results match those obtained in a previous experiment on the same thruster, validating the new optical fiber setup. The lengths of the acceleration zone are found to vary from 14 to 26 mm. The location of the acceleration zone appears to move downstream as the flow rate increases but moves upstream as the discharge voltage increases. The signal to noise ratio varies greatly and is generally an order of magnitude lower at locations believed to be inside the Hall current. At these locations, the widths of the velocity distributions are several times wider than those found elsewhere and the distributions are often bimodal. This phenomenon is likely to be the result of taking timeaveraged measurements on a highly oscillatory plasma. By deconvolving the pre-accelerated velocity distribution function from the downstream distributions, we can roughly approximate the minimum and maximum instantaneous bulk velocities found at each scan location. The plot of these velocities supports the possibility that the entire acceleration zone is oscillating back and forth in space.