Abstract
Radio-frequency (RF) ion thrusters are characterized in vacuum test facilities differentiated by pumping speed and thus subject to varying levels of neutral propellant ingestion that affect plasma plume properties and artificially raise the pressure of neutral propellant available to the thruster. These plasma properties are often used to calculate anticipated thrust values for RF thruster prototypes without consideration of the effects ingested neutral propellant may have beyond increasing the amount of neutral atoms available. This study compares exit plane plasma properties for nominal operation of a replica of the Madison Helicon Experiment operating at a propellant flow rate of 2 standard cm^3/min argon subject to 3.8 cm^3/min ingested argon flow with thruster operation over a range of propellant flow rates (1.3–60 standard cm^3/min argon) subject to a maximum ingested argon flow rate of 0.8 cm^3/min to determine the validity of compensating for neutral ingestion at higher operating pressures by increasing supplied propellant flow rates when operating at lower facility pressures. This study finds that no single operating condition at the 0.8 cm^3/min ingestion condition reproduces all the plasma property values recorded at the nominal flow rate at the 3.8 cm^3/min ingestion condition. The inability of plasma properties to be reproduced at a single adjusted flow rate is a result of the differing magnitudes of influence neutral ingestion effects have on individual plume properties.
Highlights
Interest in helicon plasma thrusters results from the absence of many lifetime-limiting components required by other thruster architectures [1]
In order to evaluate the difference in thruster performance due to ingested vs. supplied propellant flow rates, plasma properties for a nominal operating condition of 2 standard cm3/min argon propellant volumetric flow rate for a replica of the Madison Helicon eXperiment (MadHeX) subjected to 3.8 cm3/min argon ingestion volumetric flow rate are measured
ion velocity distribution function (IVDF) recorded during operation at the “Low-Pressure” condition (Low P) for volumetric flow rates from 1.3 to 60 cm3/min shown in Figure 3B through Figure 3D shift to lower most probable voltages (Vmp) with increasing supplied argon flow rate
Summary
Interest in helicon plasma thrusters results from the absence of many lifetime-limiting components required by other thruster architectures [1]. In order to assess the viability of helicon plasma thrusters as an alternative to the gridded ion thruster, thrust generation must be estimated. Helicon ion thrusters are theorized to generate thrust by accelerating ions across a naturally forming potential drop near the thruster exit plane. Measurement of the resulting force during thruster operation can be achieved via direct thrust measurements but requires the thruster to be immersed in a vacuum environment. In a significant portion of published ion thruster literature, theoretical thrust calculations are presented in place of Ingested vs Injected Propellant direct thrust measurements when the thruster is mounted externally to a facility and exhausted into a vacuum environment [3,4,5] or as validation to thrust measurements recorded using less conventional means [6, 7]. Theoretical thrust calculations make use of experimentally-measured plasma plume properties to estimate thrust [8, 9], but those properties have been shown to be affected by facility background pressure [10]
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