Hall-Effect Thruster Channel Surface Properties Investigation

Surface properties of Hall-effect thruster channel walls play an important role in the performance and lifetime of the device. Physical models of near-wall effects are beginning to be incorporated into thruster simulations, and these models must account for the evolution of channel surface properties due to thruster operation. Results from this study show differences in boron nitride channel surface properties from beginning-of-life and after hundreds of hours of operation. Two worn thruster channels of different boron nitride grades are compared with their corresponding pristine and shadow-shielded samples. Pristine HP-grade boron nitride surface roughness is 9000 +/- 700 angstrom, whereas the worn sample is 110,900 +/- 8900 angstrom at the exit plane. Pristine M26,grade boron nitride surface roughness is 18400 +/- 1400 angstrom, whereas the worn sample is 52300 +/- 4200 angstrom at the exit plane. Comparison of pristine and worn channel surfaces also shows surface properties are dependent on the axial position within the channel. For example, surface roughness increases by as much as a factor of 5.4, and the surface atom fraction of carbon and metallic atoms decreases by a factor of 2.9 from anode to the exit plane. Macroscopic striations at the exit plane angled 10-30 from the axial are found to be related to the electron gyroradius and give rise to anisotropic surface roughness. Smoothing of ceramic grains at the microscopic level is also found.