Partial trapping of secondary-electron emission in a Hall thruster plasma

Secondary-electron emission at the ceramic walls of a Hall thruster modifies the potential jump of the wall Debye sheaths and thus the electron energy losses to the wall. Because of the low plasma collisionality the two counterstreaming beams of secondary electrons are not expected to be totally trapped within the bulk of the discharge. In order to analyze the effects of partial trapping of secondary electrons on the presheath∕sheath radial structure, a macroscopic model is formulated. The plasma response depends on the secondary electron emission yield and the trapped fraction of secondary electrons. The sheath potential and wall energy losses are determined mainly by the net current of secondary electrons in the sheaths. For any practical value of the secondary emission yield, the zero-trapping solution is very similar to the zero secondary emission case. Space charge saturation of the sheaths is unattainable for weak trapping. In all cases, secondary electrons have a weak effect on the presheath solution and the ion flux recombined at the walls.