Hall Effect Thruster

Investigator: Lubos Brieda

Hall thruster is a type of spacecraft electrion propulsion device that utilizes crossed electric and magnetic fields to trap electrons near the thruster exit. This region of increased electron density ionizes the working gas, and also creates virtual "optics" that accelerate the ion beam to generate thrust. Unlike in ion thrusters, the Hall thruster discharge is at all times quasi-neutral, and thus not subjected to a current limit given by the Child Langmuir law. Small HETs produce thrust near 0.1N at exhaust velocities on the order of 20km/s. Current research on HETs is aimed at two main areas: increasing the lifetime, and improving their efficiency.

Electron Transport

The efficiency of these devices is directly related to the magnetic field's ability to capture the electrons. Trait common to all HETs is that the diffusion measured experimentally is greater than what is predicted by classical theory. The transport of electrons across the mangetic field line should be driven by collisions - upon colliding with another particle, the orbit of the electron is perturbed, and the particle will complete orbit about a different guiding center. However, the rates of collisions is too low to account for the electron current measured in experiments at the anode.

Figure 1. Plasma density for the Princeton Cylindrical Hall Thruster as computed by HPHall. The solid line indicates the magnetic field line used for subsequent analysis.

This discrepancy is dealt with in numerical codes by including an anomalous diffusion coefficient to arbitrarily increase the cross-field transport. Our goal is to obtain a better understanding of this process from first-principle laws. Transport in Hall thrusters is complicated by the presence of chamber walls, and also by oscillations in the background fields. Both of these effects contribute to transport, with some evidence suggesting that transport could be driven by near wall conductivity (NWC) induced by emission of secondary electrosn from the chamber walls.

1D Kinetic Code

In order to improve our understanding of the processes controlling the anomalous diffusion, we are developing a one-dimensional kinetic particle in cell (PIC) code to directly simulate the electron motion. The code is being applied to the Princeton 2.6cm cylindrical HET, and is being used to resolve the motion of electrons about a single magnetic field line. Interactions with other particles and wall collisions are also taken into account, and so is the curvature of the magnetic field. The code solves the electric field in the direction parallel to the field line. Properties in the direction normal to the line are obtained by first performing simulation of the discharge channel using a hybrid (kinetic ions, fluid electrons) code called HPHall. Upon impacting walls, the probability of secondary electron emission (SEE) is calculated, and the primary electron is either absorbed, reflected elastically or inelastically, or it generates SEE. Even though the plasma properties are calculated only in one spatial direction, the code resolves all three components of the particle position and velocity. This allows us to export particle data in 3D for improved visualization.

Figure 2. Simulation results from the 1D kinetic code, showing the particle positions (SEE shown in red and green), as well as the particle densities visualized as contour flood. Transport of electrons towards the anode is clearly visible.