Azimuthal instability of MPD thruster plasmas and inception of critical regimes

摘要

Magnetoplasmadynamic thrusters are known to enter a strongly unstable operating regime when operated at high ratios of total squared current over propellant mass flow rate. This regime, simply named onset, sets a limit to the propulsive effectiveness of these thrusters, as it leads to thrust efficiency degradation and intense damage of thruster components. Despite various theories have been put forward to explain this phenomenon, we still lack a complete and definitive understanding. In this work we investigate the possibility that this critical regime can be explained in terms of a series of events starting with the breaking of the azimuthal symmetry of the flow and then leading to the experimentally observed oscillations of thruster voltage by means of either variations of thruster inductance or fluctuations of the voltage across the anode sheath. We show that in both cases the so called filamentation of the plasma, i.e. the redistribution of the homogeneous plasma into tiny plasma channels, can be accounted as one of the main triggers of the onset regime.