Plasma Diagnostic Modeling
Spacecraft propulsion devices referred to as Electric Propulsion (EP) use electrical energy to accelerate a propellant. These devices can achieve very high exhaust velocities, from a few kilometers per second for Hall thrusters to tens of kilometers per second for ion thrusters. EP is ideal for many interplanetary and outer planet missions due to the high exhaust velocity and high specific impulse.
Figure 1: Left to right, Aerojet MR-510 arcjet, AerojetBPT-4000 Hall thruster, UM-NASA LM-4 Ion engine
Experimental development of next-generation Hall thrusters and ion engines includes life tests, performance evaluation, and spacecraft integration. Measurements of plasma properties in the exhaust plume are used to evaluate the performance and characterize the conditions around an operating thruster. Common experimental diagnostics such as the Faraday cup, Langmuir probe, and Retarding Potential Analyzer are immersed in the exhaust plume and may influence the measured plasma condition.
Figure 2. The Faraday probe (right) disturbs the exhaust plume from a Hall thruster (left).
Presently, diagnostic modeling is being performed by Jeremiah Boerner, a graduate student working with Professor Boyd. Elementary sheath theory does a good job of predicting the plasma conditions in 1-D situations (such as in front of a large planar probe), but it is not sufficient for the plasma flow around small probe geometries. Our axisymmetric particle simulation uses a combination of Particle In Cell (PIC) methods for determining electric fields and moving ion particles, and Direct Simulation Monte Carlo (DSMC) methods for handling particle collisions. The simulated flow field near a 1.9 cm diameter Faraday probe is shown below.
Figure 3. Left to right, contours of potential, electron density, and ion density near a Faraday probe in the plume of a 200 W xenon Hall thruster.
One interesting application of this work is to simulate the measurement of plasma properties. An experimental Faraday probe collects ion current, corresponding to an integration of ion flux to the probe surface. An equivalent process is easily performed in the simulation by recording an area-weighted sum of ion collisions with the probe surface, giving the total collected ion current.
Continuing efforts will attempt to generalize this result for Faraday probes and eventually reproduce planar Langmuir probe traces. Ultimately, this research will help design new plasma diagnostics and improve diagnostic techniques.