High Resolution Laser Diagnostics in Millimeter-Scale Micro Pulsed Plasma Thrusters

摘要

A class of Micro-Pulsed Plasma Thruster (MicroPPT) is being developed to provide precise attitude control and stationkeeping ability to 25-kg class satellites. Operating by means of a surface discharge across a Teflon propellant fuel bar only a few millimeters in diameter, the MicroPPT delivers a thrust-to- power ratio of 5 - 10 Micro-N-s/J. Due to the low pulse energy and size, the MicroPPT produces a spatially confined and diffuse plasma plume that is difficult to analyze with material probes for validation of plume modeling. To this end, a Herriott Cell interferometer is introduced to measure the plume electron and neutral densities. By focusing a large number of laser beams into a small measurement volume, the Herriott Cell can increase the instrument resolution by a factor of about 10, compared to a single-pass interferometer, with minimal degradation of spatial resolution. Comparison of these results with a 2 colors, 2-pass interferometer demonstrates experimental validity of the point measurement technique used. Comparison of the measured electron density with modeling predictions by Keidar and Boyd shows close agreement. Attempts to measure the MicroPPT neutral density with a 2 colors configuration of the Herriott Cell interferometer were inconclusive, in spite of previous successes using this technique on larger PPTs.