Near Exit Plane Velocity Field of a 200-Watt Hall Thruster

Abstract

This work presents the near exit plane velocity field of a 200-W laboratory xenon Hall thruster at a single operating condition with a 250-V anode potential. The ionized propellant velocities were measured using laser-induced fluorescence of the 5d[4] 7/2 -6p[3] 5/2 excited state xenon ionic transition at 834.72 nm. Ion velocities were interrogated from the acceleration channel exit plane to a distance 107 mm from the exit plane (3.3 exit plane diameters). Both axial and radial velocities were measured. A nearly uniform axial velocity profile of approximately 13,800 ± 500 m/s (130 ± 10 eV) was measured at the thruster exit plane. The maximum axial velocity, measured 107 mm from the exit plane, was 16,800 m/s (192 eV). The ion flow exiting the thruster acceleration channel mixes downstream due to both the coaxial thruster geometry and a possible ion-acoustic shock. This behavior appears in regions where multiple, or broadly distributed, radial and axial velocity components occur. These regions also exhibit broadened fluorescence line shapes, likely indicative of collisions between the various velocity populations as well as possible ionization of background neutral, and correspond to the brighter, more visible portions of the plume. This region has been previously identified as a possible ion-acoustic shock. This hypothesis appears consistent with the low radial velocity ion populations measured in this more luminous portion of the plume. In addition, a limited study at five off-nominal conditions near a region of high insulator erosion indicates that the impinging ion energy on the protruding center pole boron nitride insulator is predictably changed by flow rates and anode potentials; however, it also appears to vary significantly with applied magnetic field strength.