Is the gas-fed PPT an electromagnetic accelerator? An investigation using measured performance

We present a new gas-fed pulsed plasma thruster performance database and use it to investigate the electromagnetic nature of the acceleration process by-identifying trends in the dependencies of the perfor-mance parameters on the mass bit, energy, and ca-pacitance. It was previously observed[l] that perfor-mance measurements for PPTs operating at low mass bits (< 2 μg per pulse) can be influenced by contami-nation from diffusion pump oil. Consequently, facility renovations and a new impulse measurement protocol using liquid nitrogen cooled baffles to reduce diffusion pump oil backstreaming were implemented and are discussed. A portion of the database has been dupli-cated in a cryo-pumped facility at NASA-JPL to verify that the contamination effects have been removed. The intermittent simultaneous operation of the discharge initiation plugs, that was previously observed to degrade the performance, was solved with a new electric circuit and the resulting discharge symmetry verified using a fast-framing camera. The nature of the acceleration process is examined by comparing trends in the performance database with the theoret-ically expected scaling for electromagnetic accelera-tion. The performance was found to transition with decreasing mass bit (or increasing Isp) from a mode (Mode I) where the efficiency and the impulse are respectively independent and dependent on the mass bit (and where the efficiency the thrust to power ratio are respectively independent and dependent on Isp) to another mode (Mode II) where the converse statements hold. Furthermore, in mode II the efficiency is independent of energy level and both the efficiency and the thrust-to-power ratio scale with the square root of capacitance as expected in a predominantly electromagnetic accelerator.