Magnetoplasmadynamic thrusters are known to enter a strongly unstable regime, calledas onset in the literature, under high specific impulse operation. This paper probes the
early signs of onset in relatively moderate specific impulse
operation by a single fluid plasma thruster simulation. The procedure involves solving the combined Maxwell’s-Navier-Stokes equation, with an onset criterion of radial current reaching close to zero values near the electrodes. Thruster parameters are varied starting from voltage potential, plasma temperature and cathodic radius. Onset curves are plotted which can provide important engine-specific information in order to understand the onset performance of the plasma thruster.

MHD arcjet onset boundary of high specific impulse Plasma Thrusters

A magnetoplasmadynamic thruster formulation involving coupled aerothermodynamic-electromagnetic equations and including viscous effects is developed and solved. The effect of various input parameters on the radial current density distribution at the longitudinal cathode surface is studied. In general, radial current concentrations are found at the upstream and downstream ends of the cathode. The upstream current concentration is larger in magnitude and is attributed to large values of the Hall parameter. Higher inlet velocities and densities, lower inlet temperatures and propellants of lower molecular weight, all reduce the upstream current concentration. The downstream current concentration is attributed to large values of the inlet velocity.

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Current density on cathode surface of MPD thrusters

The phenomenon of onset in MHD arcjets is studied. An earlier solution procedure is combined with an onset criterion to obtain the onset boundary curves for hydrogen and argon. The onset boundary curves are in qualitative agreement with experimental results. It is found that the engine-specific impulse at onset can be increased by increasing the propellant low-temperature electrical conductivity. For plasma temperatures in the range 8000-12000 K, it is predicted that a higher specific impulse at onset should be obtained for hydrogen than for argon.

T.S. Sheshadri

Papers

MHD arcjet onset studies