Effects of sheath modes in the high-voltage Hall effect thruster discharge channel by 2D axial-radial particle-in-cell simulations

We integrate an oscillating-sheath model, obtained from 2D full particle-in-cell (PIC) sheath simulations, into an accelerated 2D particle-in-cell Monte-Carlo collisions (PIC-MCC) Hall-thruster solver. This integration compensates for the inability of conventional acceleration schemes to resolve self-consistent near-wall sheath effects. We investigate high-voltage conditions (500--800 V) and vary wall secondary-electron emission (SEE) by tuning the critical energy in a linear SEE model. Replacing an space-charge-limited (SCL) sheath with a positive-ion (PI) sheath suppresses near-wall conduction while sustaining relatively stronger anomalous transport. In SCL conditions, the mean sheath drop collapses and the electron--wall collision frequency $\nu_\mathrm{ew}$ rises to $\sim10^{8}$ Hz. In PI conditions, $\nu_\mathrm{ew}$ remains about one order of magnitude lower. Compared with SCL, the PI sheath reduces the conduction-electron current. It does so by replacing numerous low-energy carriers with fewer, more energetic ones, increasing their average energy by 40\%--50\%. The PI sheath also reduces the electron-induced wall power density by a factor of 2--5 in high-$\nu_\mathrm{ew}$ regions, whereas the ion-induced loading varies only weakly. These results link SEE-driven sheath weakening to increased wall losses at high voltage. They further suggest that low-SEE walls can mitigate wall loading, limit the anodeward shift of the acceleration layer, and improve current utilization in high-voltage Hall thrusters.