Modeling of magnetically-guided plasma flows from a gigawatt-level magnetoplasmadynamic source

Critical features of hypersonic plasma flow through applied magnetic fields are evaluated as part of a program sponsored by the NASA Glenn Research Center to design a fusion propulsion system. To evaluate the concept an experiment has been under construction for the past few years at The Ohio State University. The main components of the experiment are two magnet coils and a magnetoplasmadynamic (MPD) high-speed-plasma source (u>80 km/sec, M>6). The coil nearest to the source is to supply a magnetic field that will serve as a diffuser to the plasma exhaust. Simulations using an extended computational region, while the coil was "turned off", show that the desired conditions at the MPD ring gap are reached at approximately three times the current rise time. The time at which the coil is "turned on" relative to breakdown initiation is therefore a design issue. Indeed, when the coil magnetic field was applied at t=0 choked flow was observed near the desired throat location. The reflected high-pressure front reached the MPD chamber at approximately 25 ∝sec and permanently disrupted the flow from the plasma source. Numerical simulations with the coil "turned on" after steady state flow conditions were attained suggest that with the present configuration, if most of the flow energy is to be utilized to balance off the magnetic pressure at the desired throat point, the coil must be placed so close to the electrode gap that the magnetic pressure would greatly over exceed the plasma pressure at that location. When the coil was placed 1.1 m from the inner electrode more than one order of magnitude increase in plasma pressure was computed at the MPD ring gap, shortly after the coil was "turned on". Plasma temperatures exceeded 20 eV close to the ceramic insulator. In this configuration the ratio of plasma pressure to magnetic pressure at the gap, before "turning on" the coil, was approximately 0.03. The results suggest that if the bulk of the flow is re-directed further downstream from the inner electrode, unfavorable interference between the applied magnetic field and the source flow may be minimized.