Mode Transitions in Hall Effect Thrusters
Technical paper June-Jul 2013
AIR FORCE RESEARCH LAB EDWARDS AFB CA
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Mode transitions have been commonly observed in Hall Effect Thruster HET operation where a small change in a thruster operating parameter such as discharge voltage, magnetic field or mass flow rate causes the thruster discharge current mean and oscillation amplitude to increase significantly. Mode transitions in a 6-kW class HET called the H6 were induced by varying the magnetic field intensity while holding all other operating parameters constant and measurements were acquired with ion saturation probes and ultra-fast imaging. A technique was developed that calculated discharge current density oscillations from high-speed images of the discharge channel in order to quantify the fluctuations. Two operating modes are identified as global oscillation mode and local oscillation mode. In global mode the entire discharge channel is oscillating in unison and azimuthal perturbations spokes are either absent or negligible. Downstream azimuthally spaced probes show no signal delay between each other and are very well correlated to the discharge current signal in global mode in local mode they exhibit a clear signal delay between each other indicating the passage of spokes and are not well correlated to the discharge current. Spokes are localized oscillations propagating in the E61620B direction that are typically 10-20 of the mean discharge current density value while the global oscillation mode can be 100 of the mean value. The transition between global mode and local mode occurred at higher relative magnetic field strengths for higher mass flow rate or higher discharge voltage. The thrust was constant through mode transition but the thrust to power decreases by 25 due in increasing discharge current. The plume showed significant differences between modes with the global mode significantly brighter in the channel and the near-field plasma plume as well as exhibiting a plasma spike on thruster centerline.
- Electricity and Magnetism
- Electric and Ion Propulsion