Experimental studies of wall material effects on the Hall thruster discharge

The effects of secondary electron emission (SEE) properties of the channel wall material on the electron temperature and the electron cross-field current are studied for a conventional annular geometry Hall thruster. The linear growth of the maximum electron temperature with the discharge voltage, observed in the channel with a low SEE yield, suggests that SEE is responsible for the electron temperature saturation in the thruster with the channel walls having a higher SEE yield. The temperature saturation is directly associated with a decrease of the Joule heating, rather than with the SEE-enhanced electron energy loss at the walls [Raitses et al., Phys. Plasmas \textbf{12}, 073507 (2005)]. The reduction of the Joule heating is due to the decrease of the electric field, which, in its turn, occurs because of a local increase of the electron cross-field mobility. These results may support the recently predicted kinetic regime of plasma-wall interaction with counter-streaming beams of secondary electrons from the opposite walls of the thruster channel [Sydorenko et al., Phys. Plasmas \textbf{13}, 014501$^{ }$(2006), and I. Kaganovich, invited talk, this conference].