Propellant Surface Temperature and Plume Characteristics of Micro-Pulsed Plasma Thrusters

Abstract

The micro-Pulsed Plasma Thruster is a device capable of supplying discrete impulses for propulsive maneuvering of small satellites. These devices suffer from low propellant utilization related to late-time propellant ablation, causing a drop in specific impulse and thruster efficiency. The exhaust of these thrusters can also contaminate critical spacecraft surfaces. In this work, the microPPT is investigated to experimentally characterize both the ablation physics and plume characteristics. The defining parameter for propellant ablation is the surface temperature of the solid Teflon(TM) propellant. All downstream plasma and neutral properties are dependent on this parameter. Infrared photovoltaic detectors using a p-n junction are used to measure the surface temperature of the Teflon propellant in real time. The detector material is Mercury Cadmium Telluride (HgCdTe), chosen because the maximum detector response occurs in wavelengths where a Carbon-Fluorine stretching mode in the solid propellant emits strongly in the infrared. This paper outlines the design, calibration and construction of the infrared thermographic diagnostic. A theoretical treatment of the expected detector output is proposed and validated allowing an estimate of the wavelength dependent emissivity of Teflon in the IR. This diagnostic is applied to an operating microPPT and real-time surface temperature measurements are made after the current pulse ends. This allows analysis of the expected vapor pressure and therefore performance parameters such as thrust, mass loss, and exhaust velocity associated with late-time vaporization. The microPPT plume is characterized using two-color interferometry to simultaneously measure electron and neutral densities during the discharge. As the microPPT operates, the solid propellant recesses into the outer electrode tube, possibly changing thruster performance. Recession shape and depths are measured and electron density measurements as a function of recession depth are made. These data are compared with numerical modeling predictions from the Keidar-Boyd model, which has developed in a parallel effort to create a theoretical tool for predicting spacecraft contamination issues. Predictions from this model compare favorably with the reported data.