Experimental investigation of magnetohydrodynamic instabilities in a Magneto-Plasma-Dynamic thruster

An extensive experimental investigation has been carried out in order to understand the role of magnetohydrodynamic (MHD) instabilities on the reduced performance of Magneto-Plasma-Dynamic (MPD) thrusters when operating at high current. MPDs are electromagnetic plasma accelerators, currently under investigation as a possible, high-power electric propulsion option for primary space missions, whose thrust efficiency is limited by the onset of critical regimes observed when the current rises beyond a threshold value. Recently, it has been found (ZUIN M. et al., Phys. Rev. Lett., 95 (2004) 225003) that in these devices large-scale MHD helical kink mode instabilities develop, with m/n = 1/1 azimuthal and axial periodicity, and that the critical current condition is well described by the Kruskal-Shafranov criterion. In this paper the spatial structure of the kink has been reconstructed by magnetic and electrostatic probes and the results of two photomultiplier arrays, 16 channels each, collecting total radiation in the range 350-850 nm, confirm the helical structure of a kink with nonuniform pitch.