Fully kinetic study of facility pressure effects on RF-source magnetic nozzles

A fully kinetic 2D axisymmetric Particle-in-Cell (PIC) model is used to examine the effects of background facility pressure on the plasma transport and propulsive efficiency of magnetic nozzles. Simulations are performed for a low-power (150 W class) cathode-less radio-frequency (RF) plasma thruster, operating with xenon, between background pressures up to 10$^{-2}$ Pa and average electron discharge temperatures of 4 - 16 eV. When the electron temperature within the near-plume region reaches 8 eV, a decisive reduction in performance occurs: at 10$^{-2}$ Pa, in-plume power losses surpass 25% of the discharge energy flux. Given that the ionization energy for Xe is 12 eV, the 8 eV threshold indicates that a consistent percentage of electrons has energy enough to trigger ionization. On the other hand, when the temperature is below such threshold, the primary collisions are charge-exchange and inelastic ion scattering, and the power loss remains less than 10%. It is established that losses in the considered HPT are significant if the facility pressure is greater than 10$^{-3}$ Pa, at absorbed powers larger than 130 W. At the nominal 150 W, this results in a 15% thrust reduction. When facility pressure is taken into consideration over ideal vacuum simulations, numerical error is reduced to <30% when compared to experimental thrust measurements at 10$^{-3}$ Pa.