Azimuthal Spoke Propagation in Hall Effect Thrusters
AIR FORCE RESEARCH LAB EDWARDS AFB CA
Pagination or Media Count:
Spokes are azimuthally propagating perturbations in the plasma discharge of Hall Effect Thrusters HETs that travel in the E x B direction and have been observed in many different systems. The propagation of azimuthal spokes are investigated in a 6 kW HET known as the H6 using ultra-fast imaging and azimuthally spaced probes. A spoke surface is a 2-D plot of azimuthal light intensity evolution over time calculated from 87,500 framess videos. The spoke velocity has been determined using three methods with similar results manual fitting of diagonal lines on the spoke surface, linear cross-correlation between azimuthal locations and an approximated dispersion relation. The spoke velocity for three discharge voltages 300, 400 and 450 V and three anode mass flow rates 14.7, 19.5 and 25.2 mgs yielded spoke velocities between 1500 and 2200 ms across a range of normalized magnetic field settings. The spoke velocity was inversely dependent on magnetic field strength for low B-field settings and asymptoted at B-field higher values. The velocities and frequencies are compared to standard drifts and plasma waves such as E x B drift, electrostatic ion cyclotron, magnetosonic and various drift waves. The empirically approximated dispersion relation yielded a characteristic velocity that matched the ion acoustic speed for 5 eV electrons that exist in the near-anode and near-field plume regions of the discharge channel based on internal measurements. Thruster performance has been linked to operating mode where thrust-to-power is maximized when azimuthal spokes are present so investigating the underlying mechanism of spokes will benefit thruster operation.
- Direction Finding
- Electricity and Magnetism
- Electric and Ion Propulsion