Development and Application of Multidimensional Computational Models for Hall Thrusters

Abstract

The goal of this work is to aid the development of high-power Hall-effect thrusters through modeling and simulation. The focus is both on improving the state-of-the-art in the field of Hall thruster numerical simulation, as well as studying several physical processes that are important to Hall thruster development and application. Since Hall thrusters have been in use for more than half a century, they have built a reputation of reliability, however they are known for low power operation with primary applications such as station-keeping and orbit raising. Within the past decade there has been a significant effort to increase the power levels for these electric propulsion devices, but when considering such recent developments, several problems become apparent.

First, as we scale these devices to higher power, higher flow rates and more propellant are needed. This translates into increased costs for ground testing, as well as in-space operation. These issues are addressed through a study of an alternative and less ex- pensive option to the ubiquitous xenon gas: krypton. This new chemical species was added to the Hall2De simulation framework and two thrusters were simulated with krypton propellant. Computed thrust values were found to be within 6% for xenon, and within the 2% experimental measurement error for krypton.

Next, scaling to higher power leads to more energetic ions impacting the thruster surfaces that may in turn lead to higher observed erosion rates. Therefore, we must consider the problem of discharge channel erosion, which is investigated by simulating an optical experimental diagnostic that is meant to non-invasively determine the erosion rate: cavity-ring-down spectroscopy. The simulation result over predicts the boron number density in the plume by a factor of 3, and this may be attributed to the significant (±50%) uncertainty in the thruster operation time.

Further, the desire to scale Hall thrusters to higher power has led to the idea of con- centrically nesting multiple discharge channels into a single thruster. This novel con- figuration has yielded anomalous thrust gains which have been investigated through a cold gas (neutral) simulation of dual channel operation. In conjunction with significant experimental work performed by colleagues at the Plasmadynamics and Electric Propulsion Laboratory (PEPL) it was found that the anomalous thrust gains may be explained based on the near-plume pressure distribution. In an effort to fully characterize the thruster, a plasma simulation of the single channel mode operation was performed, and thrust was matched to within 9%, while discharge current was matched to within 5% of the measured values. Moreover, it was determined that improved modeling capabilities are required in order to simulate the dual-channel or even independent outer-channel operating modes. Therefore, a new Cartesian 2D axisymmetric electron fluid model is developed, verified and then integrated within an existing state-of-the-art hybrid-particle-in-cell framework.