Kink Instability and Onset of Critical Regimes in a Magneto-Plasma-Dynamic Thruster

Magneto-Plasma-Dynamic (MPD) thrusters are currently under investigation as they constitute a possible, high power electric propulsion option for primary space mission, ranging from orbit raising to interplanetary mission of large spacecraft. They are essentially electromagnetic plasma accelerators, in which a high current discharge ionizes a gas, that is accelerated by the Lorentz force produced by the interaction between the current and a self induced (or applied) magnetic field. Presently, the performance of this class of thrusters is limited by the onset of critical regimes observed when the current rises beyond a threshold value. In this regime, large fluctuations in the arc voltage signals and damages to the anode are observed along with thruster performance degradation. Our experimental investigation carried out by several arrays of magnetic and electric probes and by two photomultiplier arrays of 16 channel each, has shown that the onset of these critical regimes is accompanied by the growth of large scale MHD instabilities. These instabilities have m/n = 1/1 azimuthal and axial periodicity and have features of a helical kink mode. We have found for the first time[1] that the threshold value for the current is well described by the Kruskal-Shafranov criterion currently applied in Thermonuclear Fusion research to interpret MHD stability of a screw pinch. The spatial structure of the kink has been reconstructed by the magnetic and electrostatic probes and preliminary results of the photomultiplier arrays for total radiation in the range 350-850nm confirms the helical structure of the kink. [1] M.Zuin et al., Phys. Rev. Lett. 92, 225003 (2004)