Review on the study of electron near wall conductivity in hall thrusters

Summary form only given. One of the most important open questions in the physics of Hall thrusters concerns the electron cross-field mobility. Classical diffusion theory based on collision-induced transport, i.e. electron-neutral collisions underestimates the cross-field transport. Therefore, the near-wall conductivity (NWC) theory proposed based on thruster structure and its operating feature has become the research focus. The NWC theory proposed by Morozov is reviewed in this paper. It is a type of conductivity induced by electron-wall interactions. The works studying the influence of magnetic field, thruster structure and wall conditions et al on NWC are summarized firstly. Then, a survey of existing theory on NWC is presented, which demonstrates electron-wall collision induced electron conductivity have less contributions to electron current when considering traditional stable sheath. After that, a new interpretation on dynamic sheath and its induced non-collision electron conductivity is introduced. The electric field component parallel to the wall in the dynamic sheath breaks down the steady Hall drift of slow electrons and reflects them to transport across the magnetic field lines. It is the majority of those slow electrons that results in the characteristics of measured electron near-wall current profile. This new viewpoint provides direct theoretical evidence on the importance of NWC. There are great mounts of theoretical works on NWC. However, compared with Bohm conductivity, the experimental proof of NWC is extreme lack. At the last of this paper, the future experimental works are depicted, which include finding the direct experimental evidence of oscillating sheath, systematic measurement on NWC and distinguishing the difference between wall-collided and non-collided NWC et al.