Predicting Hall Thruster Operational Lifetime Using a Kinetic Plasma Model and a Molecular Dynamics Simulation Method

Hall thrusters are being considered for many space missions because their high specific impulse delivers a larger payload mass fraction than chemical rockets. With a low thrust, however, Hall thrusters need to operate for a long period of time to achieve the necessary velocity of the mission. For these missions, the lifetime requirements can reach into tens of thousands of hours. For Hall thrusters, the most important life-limiting process is the erosion of the channel walls. However, experimental verification of lifetime is time-consuming and expensive. Therefore, computational method is a useful tool to predict thruster lifetime. Many of the Hall thruster lifetime models were developed, and some of theses models gave quite promising results. However, while qualitatively interesting, the results did not match well with experiment. The reason of this discrepancy is that these numerical models assume electrons as a fluid. The proposed innovation will provide a better understanding of the erosion physics and will be useful for future Hall thruster development, such as HiVHAc, with low cost and time. This tool also will allow to aid in the acceptance and implementation of Hall thrusters as a primary propulsion device through improving confidence of their long term reliability.

A kinetic molecular dynamic erosion model has numerous applications outside of the scope of NASA. There are a number of companies that develop electric propulsion system in US and other countries. All of these companies spent more than a thousand hours to test their thruster lifetime. They all need an accurate tool which can predict the lifetime for developing Hall thrusters. Recently, Space Systems/Loral has developed the BEPPA code for modeling plume interactions with spacecraft. They chose the detailed electron fluid model developed by Boyd at University of Michigan. It is known that the detailed model results are very sensitive to the channel exit conditions. The model proposed here can provide accurate initial conditions for plasma plume simulations for the hybrid model. The work proposed here is also directly applicable to relevant Hall thruster lifetime issues presently being studied at AFRL/RZSS. The kinetic molecular dynamic erosion tool can be used to model low energy xenon ion impact for fabrication of hard coatings such as cubic boron nitride or titanium nitride films. The tool would also offer a way to understand and help to optimize the deposition process with low cost. It would offer a way to optimize the deposition process and to investigate the plasma surface interaction for future fusion device development such as ITER with low cost

NASA's Science Mission Directorate In-Space Propulsion Technology Project is funding the development of a high specific impulse long life Hall thruster, HiVHAc, a type of Hall thruster systems, as a lower cost electric propulsion alternative for future cost constrained missions. The kinetic molecular dynamics erosion model proposed here can reduce the time and money spent by NASA employees for the Hall thruster development. It also helps the researchers to design that the hBN discharge channel erosion always shields the Hall thruster magnetic circuit elements from ion impingement. The model also would allow for researchers to understand the effect of the operation conditions and the thruster geometry to the channel wall erosion process.

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