Studies of Thrust Density Limits in Hall Thrusters

Abstract

The rapid advance of deep-space spacecraft technologies has increased the viability of many ambitious missions, many of which require some form of propulsion. Hall thrusters remain one of the most developed technologies for such missions, however there are several fundamental principles of these thrusters that are not studied in-depth and are therefore poorly understood, particularly the amount of thrust that is possible to generate within a given thruster area, otherwise known as the thrust density. Operating a thruster at a higher thrust density has distinct advantages for spacecraft propulsion in reducing thruster size and mass, yet the theoretical limits of this thrust density appear to be well above that of conventional operation for state-of-the-art Hall thrusters. This research describes several methods that attempt to increase this thrust density for Hall thrusters. The first was shown by increasing the total utilized area of the Hall thruster by removing the channel walls and confining the plasma by a magneto-electrostatic trap. This configuration, known in literature as the ``wall-less Hall thruster,'' has the advantage of reducing thruster weight, lowering unfavorable plasma-material interactions, and utilizing the center of the thruster to generate thrust. The second method to improve thrust density was through the use of segmented electrodes in these thrusters, which is shown to improve thrust by focusing ions axially and improve total efficiency. The third method is by the use of modulation of the applied voltage to the discharge in the thruster in resonance with natural plasma instabilities, in particular the low frequency breathing mode. This has been shown to amplify these instabilities. This has been shown to improve both the ionization of the input propellant as well as improve thrust through higher ion energy and ion current. An example large satellite formation mission that demonstrates the utility of such miniaturized Hall thrusters is analyzed, where it is shown that utilizing a high thrust density Hall thruster fits mission requirements and greatly decrease launch costs by removing the need for a dedicated launch.