Particle modeling of radial electron dynamics in a controlled discharge of a Hall thruster

An improved radial particle-in-cell model of an annular Hall effect thruster discharge with secondary-electron emission from the walls and a radial magnetic field is presented. New algorithms are implemented: first, to adjust the mean neutral density to the desired mean plasma density; second, to avoid the refreshing of axially accelerated particles; and third, to correctly weigh low-density populations (such as secondary electrons). The high-energy tails of the velocity distribution functions of primary and secondary electrons from each wall are largely depleted, leading to temperature anisotropies for each species. The secondary-electron populations are found to be partially recollected by the walls and partially transferred to the primary population. The replenishment ratio of the primary high-energy tail is determined based on the sheath potential fall. Significant asymmetries at the inner and outer walls are found for the collected currents, the mean impact energy, and the wall and sheath potentials. Radial profiles in the plasma bulk are asymmetric too, due to a combination of the geometric expansion, the magnetic mirror effect, and the centrifugal force (emanating from the E x B drift). The temperature anisotropy and non-uniformity, and the centrifugal force modify the classical Boltzmann relation on electrons along the magnetic lines.