Comparison of Directly and Inductively Coupled Pulsed Electromagnetic Thrusters

Pulsed electromagnetic plasma accelerators and, more specifically, those that are inductively coupled potentially have high thrust density, high specific impulse, and long thruster lifetime since there is limited contact between the plasma and coil(s). One-dimensional electromechanical models of these thrusters, which are important for identifying important relationships among circuit parameters, coil geometry, and plasma, usually make the same underlying assumption about the mutual inductance between the primary coil and induced plasma current, based on empirical evidence. This approach is inconvenient for arbitrary coil geometries and if trying to make comparisons with directly coupled plasma accelerators, such as coaxial plasma guns. A generalized slug model was developed in which RLC circuit equations are coupled to a mechanical force equation. The acceleration term was made to be thruster and geometry dependent, so that the same model could be used to study both coaxial plasma guns and conical theta pinches. The calculated mutual inductance from the coil model exhibits the same exponential falloff with distance as in previous models and experiments. Efficiency (plasma kinetic energy divided by initial capacitor bank energy) and exhaust velocity were calculated for the conical theta pinch coil over a 5-D parameter space. For the cases with highest efficiencies, peak coaxial plasma accelerator efficiencies were calculated under certain operating constraints. It was found that, while coaxial plasma accelerators are generally more efficient, under certain conditions, the conical theta pinch performance is higher.