Feedback control of an azimuthal oscillation in the E × B discharge of Hall thrusters

Abstract

Feedback control of a low-frequency azimuthal wave known as a "rotating spoke" in the ExB discharge of a cylindrical Hall thruster was demonstrated. The rotating spoke is an m=1 azimuthal variation in density, electron temperature, and potential that rotates at about 10% of the local E × B electron rotation speed. It causes increased electron transport across the magnetic field and is suspected to be an ionization wave. Feedback control of this wave required special consideration because, although it causes a rotating azimuthal variation in the current density to the anode, it does not show up as a signal in the total thruster discharge current. Therefore, an extra source of information was needed to track the oscillation, which was addressed by using a special anode that was split azimuthally into four segments. The current to each segment oscillates as the rotating spoke passes over it, and feedback is accomplished by resistors connected in series with each anode segment which cause the voltage on a segment to decrease in proportion to the current through that segment. The feedback resulted in the disappearance of a coherent azimuthal wave and a decrease in the time-averaged total discharge current by up to 13.2%.

Theoretical model of suppression of electron instability in hall thrusters by boundary feedback system

“…The carried out experiments showed that the boundary feedback system reduced amplitudes of both low-frequency and high-frequency oscillations in the Hall thruster. Currently, the interest to a suppression of the plasma instabilities in Hall thrusters by boundary feedback system is renewed [8].…”

“…The first of them is in intervals {0, d 1 } and {d 1 , d 2 }, respectively. The second necessary condition, we obtain integrating(8) over an infinitely small interval around the point d 1 .…”

“…(9) in intervals {0, d 1 } and {d 1 , d 2 }, where the values of u y0 are constants. In doing so, we use the boundary conditions(6) and(7)with taking into account Eq (8)…”

Low-frequency azimuthal stability analysis of Hall thrusters

“…In the last few years a significant number of experiments [2][3][4][5][6][7] have been carried out to detect the presence of the spoke and evaluate the influence of this oscillation on the electron mobility. Those experiments make use of high-speed imagery to detect the spoke travelling in the azimuthal direction and a segmented anode in the back of the channel to measure the electron current as a function of the azimuthal coordinate and time.…”

Driving Low Frequency Breathing Oscillations in a Hall Thruster

“…5−8 While these disturbances have received much attention for their possible influence on Hall thruster operation, efficiency, and plume divergence, the underlying physical causes of these phenomena have not been fully understood. 8,9 Extensive experimental campaigns using an annular thruster have been undertaken to characterize the propagation of the instabilities, as well as the various mode transitions that arise from adjustment of the operating parameters. 10,11 Increased background pressure levels have also been found to suppress breathing and spoke oscillations.…”