Simultaneous measurements of axial motion and azimuthal rotation of non-uniformities (“spokes”) in a Hall thruster

Abstract

Low-frequency instabilities are often present in Hall thrusters and are associated with axial and/or azimuthal oscillations of plasma non-uniformities. The axial oscillations are related to the so-called breathing mode (periodic depletion of the neutral atom density due to ionization) and are associated with large amplitude current oscillations. The low-frequency azimuthal instabilities are characterized by local non-uniformities of the light emission rotating in the azimuthal direction and are generally called “rotating spokes.” The possibility of coexistence of these two modes has been discussed in the literature but without clear experimental evidence of their correlation. In this paper, we present for the first time simultaneous measurements of the axial and azimuthal positions of the spoke. These measurements have been obtained with a high-speed camera using a triangulation method based on parallax. This method has proven to be sufficiently sensitive to track the 3D position (r–θ–z) of local non-uniformities of the light emission. The optical method has been synchronized with measurements of the current on a segmented anode. The results show that under some conditions, breathing oscillations and spoke rotation in the E×B direction are coupled. During the current rise, the spoke moves from the anode region toward the channel exhaust region while rotating in the E×B direction and seems to follow the movement of the front of neutral atoms progressively filling the channel. About 60% ± 20% of the total anode current is carried by the spoke.