Control of the Electron Energy Distribution Function (EEDF) in a Hall Thruster Plasma.

Abstract

Further improvements to Hall-effect thruster (HET) efficiency in the low voltage regime will help enable more extensive space missions. HET efficiency depends on its ability to ionize and accelerate neutral propellant, which can be further improved when electrons with energies and trajectories that contribute to ionization are increased in the right locations. Therefore, electron energy distribution function (EEDF) tailoring is needed. However, predictive EEDF control in plasma devices is a challenging problem due to complex electromagnetic interactions that take place that lead to the turbulent nature of these plasmas.

In an effort to control the HET’s EEDF to boost thruster efficiency, and to also uncover further insights into the operation and dynamics of these devices, various analyses were carried out. The last was a reverse-orientation cathode technique to control the EEDF by placing the cathode downstream and pointing towards the thruster to redirect high-energy electrons to a less circuitous path to ionization zones in the thruster’s channel.

Total efficiency and its components were calculated from non-invasive thrust stand measurements and a suite of downstream thruster diagnostics respectively. For the test thruster’s nominal operating condition, total efficiency increased by 3 percentage points with the external, downstream cathode when compared to the standard, central cathode configuration; however, for an off-nominal condition, thruster efficiency decreased by 1 percentage point. For both operating conditions, the EEDFs revealed that external cathode electron temperatures were on average about half that of the corresponding central cathode values at the downstream data-taking locations.

The results implied that the reverse-orientation cathode placed more control on directing high-energy electrons into the thruster channel since less were in downstream locations. These EEDF changes most likely correlated with an increase in efficiency for only the nominal condition due to larger ion beam divergence in the off-nominal condition. However, to confirm these inferences, internal ion current density and divergence measurements where thrust is produced are needed.

Therefore, this EEDF control quest made clear the need for non-invasive, performance diagnostic measurements inside the channel to draw more direct conclusions not only about this method’s results, but also about thruster performance and dynamics in general.