Effects of sheath instability on properties of a Hall thruster

Recent analytical studies and particle-in-cell simulations suggested that the electron velocity distribution function in a Hall thruster plasma is non-Maxwellian and anisotropic. The electron average kinetic energy in the direction parallel to the walls of a thruster is several times larger than the electron average kinetic energy in the direction normal to the walls. Electrons are stratified into several groups depending on their origin and confinement. These kinetic effects lead to many new properties of plasma in a Hall thruster. In particular, the sheath near the electron-emitting surface may become unstable if it is characterized by the negative electron conductivity. In this paper, the negative conductivity effects in Hall thrusters are studied making use of a one-dimensional particle-in-cell code. In case of intense turbulent heating, a new regime with relaxation sheath oscillations is observed. In this regime, the plasma switches between a state with non-space charge limited emission and a state with space charge limited emission. Transition to the latter state occurs when the negative conductivity appears due to modification of the velocity distribution of the plasma bulk.