Background Pressure Effects on Ion Velocity Distributions in an SPT-100 Hall Thruster

Abstract

Increased background pressure in vacuum chamber test facilities as compared to on-orbit operation has been shown to influence the operation of electric propulsion devices such as Hall thrusters. This study aims to elucidate the impact of pressure on the ionization and acceleration mechanisms in a stationary plasma thruster, model SPT-100 Hall thruster, using time-averaged and time-resolved laser-induced fluorescence velocimetry. The results are compared for the thruster operating at an applied 300 V (∼4.25 A), with vacuum facility background pressures ranging from 1.7 × 10 −5 to 8.0 × 10 −5 torr. Time-averaged measurements reveal that, in general, an upstream shift in the position of the ionization and acceleration regions occurs as the facility pressure is increased above the nominal 1.7 × 10 −5 torr. Time-resolved measurements, implemented using a sample-hold scheme with 1 μs resolution, emphasize that similar acceleration profiles are present within the Hall thruster discharge channel regardless of background pressure. Measurements taken at 3.5 × 10 −5 torr, where the facility background neutral density is similar to the neutral density emitted from the thruster, unexpectedly show increased ion acceleration over the next highest pressure condition at 5.0 × 10 −5 torr. These results indicate a not-yet well defined balance of the impacts of neutral ingestion, classical and turbulent electron transport on thruster operation, and that the ratio of the background to thruster neutral density is a more relevant benchmark than background pressure alone when evaluating Hall thruster operation.

Structure of the ion acceleration region in cylindrical Hall thruster plasmas

“…This result indicates that on the centerline ions are on average only accelerated in the z-direction over a long distance. The observation of significant ion acceleration from z = -6 mm to z = 70 mm in a cylindrical thruster plasma is remarkable because ion acceleration in conventional annular Hall thruster plasmas usually occurs only within 10 -20 mm from the thruster exit plane [1,[18][19][20]. is the ion mass, 𝑣 𝑧 and 𝑣 𝑟 are the axial and radial ion velocities, and 𝑓 𝑧 (𝑣 𝑧 ) is the axial ion velocity distribution function.…”

Evidence of a free-space ion acceleration layer in the plume of a quad confinement plasma source

“…Extensive literature describes the effects of the background pressure on Hall Effect Thrusters testing re-vealing an influence on the performance, discharge channel physics and plume characteristics [20][21][22] . In particular, relevant to the results presented in this article, a recent study 23 has analysed the effect of the background pressure in the range 1.7 -8 × 10 −5 Torr on xenon ion velocity distribution functions in a SPT-100 thruster plume characterised through laser-induced fluorescence velocimetry. The analysis shows the background pressure influences the ionization and acceleration zones, with an observed upstream shift of the axial ion acceleration front when the pressure is increased; nevertheless, the ultimate ion velocity after acceleration appears unaffected.…”

Wall Ion Loss Reduction by Acceleration Zone Shifting in Anode-Layer Hall Thruster

“…The background pressure effects were investigated for 1.35-kW SPT-100, and it was reported that the backpressure of 6.7×10 −3 Pa caused a non-negligible performance change such as 4% increase in thrust, because a fraction of the background particles worked as propellant [12]. In addition, it was reported that the acceleration zone moved upstream by a few millimeters owing to the backpressure [13]. In this experiment, it was expected that these background pressure effects would increase the measured 𝑖 g .…”