Plasma-wall transition: The influence of the electron to ion current ratio on the magnetic presheath structure

Abstract

The effect of magnetic field on the plasma-wall transition layer is investigated using the two-fluid formulation. The quasi-neutral, near-sheath plasma (presheath) is examined, with the presence of neutral particles, and a magnetic field parallel to the confining wall. A general approach is used which takes into account the electron momentum equation, including the electric field force, the magnetic force, the pressure gradient, and the drag force. The influence of the electron to ion current ratio on the potential and velocity distribution in the near-sheath plasma is investigated. It is shown that the electron density distribution in the presheath may deviate from the Boltzmann distribution normally used in previous presheath models. Even when the plasma density dependence on the potential corresponds to the Boltzmann distribution, the presheath thickness deviates from that calculated with a model based on this distribution. The potential in the presheath with respect to the plasma–presheath interface can be negative or positive depending on the electron to the ion flux ratio η and Hall parameter βi. In the case of magnetized ions (βi>1) the potential distribution has a positive maximum and is always negative at the wall edge of the presheath. The value and position of the maximum depend on the parameter η. In the case of unmagnetized ions (βi≪1) the potential is positive for large η and is negative for η<100. With large βi the influence of the electrons is significant so that the presheath thickness decreases to the electron Larmor radius and has a strong dependence on the parameter η.