Kinetic simulation of secondary electron emission effects in Hall thrusters

TLDR

A particle‑in‑cell code has been developed for kinetic simulations of Hall thrusters, focusing on plasma‑wall interaction, and recent experimental studies indirectly support these results, particularly regarding electron temperature saturation and the channel‑width effect on discharge. The beams produce secondary electron emission themselves depending on their energy at the moment of impact with the wall, which is defined by the electric and magnetic fields in the thruster as well as by the electron transit time between the walls. The simulations reveal that secondary electron emission alters wall losses differently from prior fluid and kinetic studies, with a strongly anisotropic, depleted high‑energy, nonmonotonic electron velocity distribution and two counter‑propagating beam electrons that generate emission based on their impact energy; the space‑charge‑limited condition depends on both bulk and beam electron energies, and the beams can contribute far more to particle and energy wall losses than the bulk plasma.

Abstract

The particle-in-cell code has been developed for kinetic simulations of Hall thrusters with a focus on plasma-wall interaction. It is shown that the effect of secondary electron emission on wall losses is different from predictions of previous fluid and kinetic studies. In simulations, the electron velocity distribution function is strongly anisotropic, depleted at high energy, and nonmonotonic. Secondary electrons form two beams propagating between the walls of a thruster channel in opposite radial directions. The beams produce secondary electron emission themselves depending on their energy at the moment of impact with the wall, which is defined by the electric and magnetic fields in the thruster as well as by the electron transit time between the walls. The condition for the space-charge-limited secondary electron emission depends not only on the energy of bulk plasma electrons but also on the energy of beam electrons. The contribution of the beams to the particles and energy wall losses may be much larger than that of the plasma bulk electrons. Recent experimental studies may indirectly support the results of these simulations, in particular, with respect to the electron temperature saturation and the channel width effect on the thruster discharge.