Azimuthal instability of MPD thruster plasmas and inception of critical regimes

Sebastiano Giannelli, Mariano Andrenucci
2011 47th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit   unpublished
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
more » ... tanding. 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. I. Introduction Magnetoplasmadynamic (MPD) thrusters offer high-thrust densities (∼ 10 3 Nm −2 ) and specific impulse levels in the range 1000 ÷ 5000 s, being then suitable for high-thrust and medium specific impulse space missions, such as Mars manned missions or heavy spacecraft orbit raising. Yet they are affected by a number of flaws, among which limited thrust efficiency (rarely greater than 25 − 30%), high power losses leading to thermal management issues, and the inception of an unstable operating regime, known as onset, at high J 2 /ṁ ≡ k ratios, i.e. squared total current over propellant mass flow rate. 1 This is a major drawback, as the thrust efficiency of these devices (of self-field MPD (SF-MPD) thrusters in particular) is known to increase with increasing k values, and this parameter is directly related to the specific impulse of the thruster and to the specific power put into the flow, i.e. electrical power per unit propellant mass flow rate. When a critical (J 2 /ṁ) crit ≡ k * ratio is reached (usually in the order of 100 kA 2 g −1 s), the structure of the flow undergoes a transition from a homogeneous current attachement over the electrodes to a spotty pattern characterized by the emergence of tiny, bright plasma channels, which we will call filaments, giving indication of azimuthal symmetry breaking. During onset, intense fluctuations of thruster terminal voltage are also observed, and both electrodes and insulators are subjected to increased erosion and damage. Many researchers 2-7 related the onset inception to a number of micro and macro plasma instabilities, as well as to other plasma phenomena, among which the anode starvation is widely accepted to play a major role. 8-12 This is related to the presence of a pinching component, i.e. radially inward directed, of the Lorentz force which becomes higher as the driving parameter k is increased, ultimately leading to an equlibrium configuration in which the density and temperature of the plasma inside the thruster and in the vicinity of the anode in particular, are so low that the current transport to the electrodes is strongly hampered, leading to the so called current saturation, which is supposed to trigger an unstable behavior of the device. Recently Uribarri 13 has highlighted some peculiar aspects of the behaviour of the Princeton Benchmark Thruster operated in the onset regime, among which the absence of characteristic frequencies in voltage traces PFN anode sheath PFN J J 3 PFN J sat J thruster inductance SF-MPD thruster PFN anode sheath PFN J J 3 PFN J sat J thruster inductance SF-MPD thruster PFN anode sheath PFN J J 3 PFN J sat J thruster inductance SF-MPD thruster sat J e J
doi:10.2514/6.2011-5887 fatcat:5ievbhxgvjaglhos76petrw3ie