Low environmental pressure MPD arc tests.

Tests of lithium and ammonia magnetoplasmadynamic (MPD) arc thrusters were conducted in the 15-ft-diam, 65-ft-long vacuum tank at NASA Lewis Research Center. The tank pressure was maintained at 3 X 10 ~ torr during lithium performance testing. The thrust efficiency varied from 10% at a specific impulse of 1000 sec to 70% at 5500 sec, in fair agreement with previously published results at three decades higher environmental pressure. The over-all efficiency of the thruster using ammonia propellant was found to improve with increasing mass flow rate and was almost as high as that of the lithium thruster for specific impulse less than 2500 sec. The degree of ionization of the ammonia MPD arc was studied both by spectroscopic means and by estimating the power available for frozen flow losses. At approximately constant specific impulse the degree of ionization, and hence frozen flow Josses, correlated with arc voltage and less strongly with magnetic field. The current distribution of the discharge of an ammonia MPD thruster was studied by means of Rogowski coils. It was found that, under a wide range of conditions, the arc current was concentrated into spokes rotating at a frequency in the hundreds of kilohertz range. Under the conditions studied, the frequency increased with increasing magnetic field or increasing arc current and decreased with increasing propellant flow rate.