Microwave plume measurements of an SPT-100 using xenon and a laboratory model SPT using krypton
The plasma plumes from two stationary plasma thrusters (SPTs), a flight qualified SPT-100 using xenon and a laboratory model SIT using krypton, have been measured using microwave diagnostic systems capable of differential phase, differential amplitude, and spearal measurements. The results have implications in understanding the transition region between near- and far-field for both thrusters. In addition, estimates of potential impact to communication and other electromagnetic satellite systems can be obtained directly from the measurements and from the electron number density distribution models derived from the measurements. Electron density profdes are obtained throughout the plume for both thrusters via microwave differential phase measurements. Spatially resolved integrated phase shifts for a 17 GHz signal radiated through the plume at various offaxis positions are Abel inverted to calculate electron density profiles perpendicular to the main thruster axis. A functional model of plasma density has been developed combining a near-field Gaussian beam term and a far-field point source expansion term. An outcome of this work is the mapping of the transition region between the near- and far-field plume for both thrusters. An indication of plume asymmetry due to the cathode placement is obtained by comparing total integrated measurements on one side vmus the opposite side with respect to the thruster centerline.Additional 17 GHz measUrements have also determined the plasma plume effect on attenuation and spectral characteristics of a transmitted wave through the plume. The attenuation of the 17 GHz signal was small, with slightly over 2 dB loss at 0.09 m along the thruster axis. However, ray tracing attenuation modeling indicate greater effect for lower fxquency operation. The spectral data of the 17 GHz signal transmitted through the plume exhibited 26 kHz harmonic sidebands at up to -29 dBc Bmdbandnoise also addedbehvm 5 dB and 20 dB to the -110 dBm oscillator noise floor 10 kHz to 1 MHz offset from the carrier.
