Effect of Secondary Electron Emission on Electron Cross-Field Current in $E \times B$ Discharges

This paper reviews and discusses recent experimental, theoretical, and numerical studies of plasma-wall interaction in a weakly collisional magnetized plasma bounded with channel walls made from different materials. A low-pressure E ×B plasma discharge of the Hall thruster was used to characterize the electron current across the magnetic field and its dependence on the applied voltage and the electron-induced secondary electron emission (SEE) from the channel wall. The presence of a depleted anisotropic electron energy distribution function with beams of secondary electrons was predicted to explain the enhancement of the electron cross-field current observed in experiments. Without the SEE, the electron cross-field transport can be reduced from anomalously high to nearly classical collisional level. The suppression of the SEE was achieved using an engineered carbon-velvet material for the channel walls. Both theoretically and experimentally, it is shown that the electron emission from the walls can limit the maximum achievable electric field in the magnetized plasma. With nonemitting walls, the maximum electric field in the thruster can approach a fundamental limit for a quasi-neutral plasma.