Nested Channel Hall-effect Thrusters
Recently there has been a growing need for higher power Hall-effect thrusters (HETs), as shown by NASA mission plans such as the unmanned segment of the Asteroid Redirect Mission . Historically, HETs consist of a single annular channel, however scaling to higher powers poses several challenges which may be overcome by nesting multiple channels into a single device. This permits independent or simulatanoues operation of the individual channels. The approach was first published in the open literature by Raymond Liang , as he described the two channel 10kW class X2 thruster developed at PEPL.
While operating the thruster at constant mass flow rate, it was observed that the dual channel mode produced higher thrust than the superposition of thrust values from independent inner and outer channel modes. This suggests a coupling effect between the two channels, which should be studied further.
The next development at PEPL was to build a 100kW class Nested Channel HET (NHT): the X3 described by Roland Florenz in his thesis . An indication of channel coupling was also observed between the three channels of the X3, while operating at constant current: nested channel operation required less fuel than independent operation.
Since the proof of concept X2 has been well characterized, the initial modeling and simulation work will be focused on this thruster. Before investigating the physics of channel interaction, the code will be validated by comparing results with laboratory measurements.
The axisymmetric hybrid-PIC code HPHall will be used to simulate NHTs. The framework was initially developed by Fife at MIT , and the code models the heavy species using a particle-in-cell technique, while assuming a quasi-1D continuum for the electrons. It is impossible to simulate a nested channel device with the current version of HPHall and an improved 2D axial-radial electron model is required.
The first step was to simulate the X2 inner channel. Figure 1 shows the simulation domain, and the computational grid.
Figure 1: Inner channel simulation domain and mesh.
Figure 2 below shows results from an HPHall simulation of the X2 thruster's inner channel. The anode flow rate is fixed at 7 mg/s and the discharge voltage is 200 V. The neutral cathode flow is also included in the simulation, at a rate of 10% that of the anode flow. Liang measured a facility background pressure of 1.5*10-5 Torr and this value is also used in the simulation. The laboratory mesured thrust was 92.0± 3.00 mN, and the simulation produced a value of 81 mN. The measured discharge current was between 5.56 and 5.79 A, while the time averaged value from the simulation was 6.08 A.
Figure 2: Number densities for xenon particles, and streamlines (a) neutrals (b) singly charged ions.
Figure 3 shows a comparison with experimentally determined plasma properties along the centerline of the inner channel. The electron temperature is under-predicted by the simulation, but the plasma potential is matched closely.
Figure 3: Properties along the inner channel centerline (a) electron temperature (b) plasma potential.
- development of a 2D electron model
- implementation of the new model in HPHall
- outer channel simulation
- dual channel simulation
This work is currently supported by a NASA Space Technology Research Fellowship.
Brophy, J.R., Technology for a Robotic Asteroid Redirect Mission, 2014 IEEE Aerospace Conference, DOI:10:1109/AERO.2014.6836392, Big Sky, MT, March 1-8, 2014.
Liang, R., The Combination of Two Concentric Discharge Channels into a Nested Hall-Effect Thruster, Ph.D. Dissertation, Aerospace Engineering Dept., University of Michigan, Ann Arbor, MI, 2013
Florenz, R.E.,The X3 100-kW Class Nested Channel Hall Thruster: Motivation, Implementation and Initial Performance, Ph.D. Dissertation, Aerospace Engineering Dept., University of Michigan, Ann Arbor, MI, 2014.
Fife, J. M., Hybrid-PIC Modeling and Electrostatic Probe Survey of Hall Thrusters, Ph.D. Dissertation, Dept. of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA, 1998.
Dragnea, H. C. and Boyd, I. D., "Simulation of a Nested Channel Hall Thruster", 34th International Electric Propulsion Conference, 6-10 July 2015, Kobe, Japan, IEPC Paper 2015-250.