Rotating spokes and self-organization in an inductive pulsed plasma thruster

Abstract

An experimental study of azimuthally rotating spokes in the plasma sheet of an inductive pulsed plasma thruster is presented. High-speed imaging reveals the formation of spokes with mode number ≈ 18 for a range of background pressure and coil discharge conditions. The spoke widths increase and the corresponding spoke orders decrease through an inverse cascade of energy flowing from higher-order to lower-order spokes, resulting in a more uniform plasma sheet. Secondary plasma sheets form in the half-cycle of the current discharge following primary sheet formation. It is observed that the plasma sheet forms earlier in the discharge as the pre-ionizer discharge energy increases, and increasing the background pressure results in higher uniformity when the other conditions are held constant. Spokes rotate predominantly in the drift direction at speeds on the order of critical ionization velocity, and orders of magnitude smaller than the drift speed. Simultaneously counter-rotating spokes are observed under various operating conditions.