Investigation into the use of cathode flow fraction to mitigate pressure-related facility effects on a magnetically shielded hall thruster
The ability to mitigate facility effects by varying the cathode flow fraction of a magnetically-shielded Hall thruster is experimentally investigated. The study is performed on a 9-kW class device operating on both internally and externally mounted cathodes. The cathode flow fraction is varied from 7 to 15% while the facility pressure is changed from background levels 4.5 to 25 µTorr-Xe. A thrust stand and a laser induced fluorescence system are employed to measure changes in performance and the location of the acceleration zone respectively. As has been found in previous studies, the thrust measurements show that the performance with the externally mounted cathode is more susceptible to neutral density changes than with the internally mounted cathode at the nominal cathode flow fraction (7%). The increase in facility pressure leads to a 7.1% change in thrust with the external cathode. When the cathode flow fraction is increased to 15%, the change in thrust is reduced 4.4% for the same facility pressure increase. The thrust for the internal cathode did not change measurably with facility pressure, however it increased 2.2% on average across all pressures by increasing the cathode flow fraction from 7% to 15%. This mitigation with cathode flow fraction occurs despite the fact that the additional xenon flow through the cathode only raises the background facility pressure by 2% compared to an entire order of magnitude change for the pressure studies. The ability for the cathode flow fraction to reduce the impact on performance of facility background pressure is discussed in the context of the local neutral density in the thruster exit plane. Laser induced fluorescence measurements show that the differences in thrust performance are linked to movement of the acceleration zone and that these shifts in acceleration zone asymptote with increasing neutral density in the thruster exit plane. As the cathode is a closer and more efficient source of neutrals, a small increase in cathode flow fraction is capable of saturating the exit plane with neutrals thereby minimizing the effects of the facility pressure increase.
