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12th International Congress on Plasma Physics, 25-29 October 2004, Nice (France); In this paper we present recent results of a project concerning UV imaging of a plasma for space applications. Magneto-plasmadynamic (MPD) thrusters are a class of high power electric space propulsion devices that accelerate a plasma to high velocities (> 10 km/s), by exploiting the Lorentz force. This force arises from the interaction between a discharge current and both self-induced and externally applied magnetic field. This device constitutes a promising device for long-range space missions. The imaging system has been realized by inserting 3 arrays of UV-enhanced photodiodes directly into the plastic structure of the anode. The amplifiers are miniaturized, and built into the detector. This advanced diagnostic design allows for a detailed tomographic reconstruction of the emissivity spatial structure, both in the axial direction z (corresponding to a wavenumber n) and azimuthal direction q (wavenumber m) with high time resolution. In fact, recently an intense effort has been devoted to the optimization of the performances of the thruster, which unfortunately shows a limitation of the efficiency at high plasma current Ip and applied axial magnetic field Bz . As recently proposed, this limitation is not merely confined to the field of engineering, but it unveils genuine physics, in that it is connected to the onset of an m=1, n=1 magneto-hydrodynamic (MHD) kink instability. In this paper we will show new results that confirm the existence of a kink, and extend the analysis of the properties of the MPD plasma column.

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)