Scaling laws for electromagnetic pulsed plasma thrusters

Abstract

The scaling laws of pulsed plasma thrusters operating in thepredominantly electromagnetic acceleration mode (EM-PPT) are investigatedtheoretically and experimentally using gas-fed pulsed plasma thrusters. Afundamental characteristic velocity that depends on theinductance per unit length and the square root of the capacitance to the initialinductance ratio is identified. An analytical model of the discharge currentpredicts scaling laws in which the propulsive efficiency is proportional tothe EM-PPT performance scaling number, defined here as the ratio of theexhaust velocity to the EM-PPT characteristic velocity. The importance of theeffective plasma resistance in improving the propulsive performance is shown.To test the validity of the predicted scaling relations, the performance oftwo gas-fed pulsed plasma thruster designs (one with coaxial electrodes and the other withparallel-plate electrodes), was measured under 70 different operatingconditions using an argon plasma. The measurements demonstrate that theimpulse bit scales linearly with the integral of the square of the dischargecurrent as expected for an electromagnetic accelerator. The measuredperformance scaling is shown to be in good agreement with the theoreticallypredicted scaling. Normalizing the exhaust velocity and the impulse-to-energyratio by the EM-PPT characteristic velocity collapses almost all the measureddata onto single curves that uphold the general validity of these scalinglaws. [12pt]This paper is dedicated to the memory of Dr Daniel Birx