Plume characterization of the SPT-100
The following provides a first year summary of a research program sponsored by the AFOSR to characterize the SPT-100 for far-field plume plasma and transport properties as well as thruster operation impact to electromagnetic signals used for communication, radar, and navigation systems. An extensive array of electrostatic and flux probes are being used to assess particle transport within the thurster's plume. This includes a Retarding Potential Analyzer, a Faraday Probe and Convective Heat Flux Probe. In addition, a new Neutral Particle Flux (NPF) Sensor has been developed and used. A high-fidelity Molecular Beam Mass Spectrometer is under development as well as a new insitu probe to characterize flow velocity, electron temperature, and estimate ion temperature within an order of magnitude using the characteristics of an ion acoustic wave. One of the most striking results of this investigation to date has been the discovery of a high energy annulus surrounding a low energy core in the plume at 0.5m radius from the thruster exit which retains certain characteristics out to at least 1 m. Charge exchange products have also been measured which help quantify energetic neutral return flux. Electromagnetic signal characterization has been able to quantify significant signal attenuation through out the thruster plume for low microwave signals near the local plasma frequency. It has also been possible to demonstrate that noise from the plume is induced into the electromagnetic signal by both amplitude and phase modulation processes.
