PLASMA-WALL TRANSITION - THE INFLUENCE OF THE ELECTRON TO ION CURRENTRATIO ON THE MAGNETIC PRESHEATH STRUCTURE

The effect of magnetic field on the plasma-wall transition layer is in vestigated using the two-fluid formulation. The quasi-neutral, near-sh eath plasma (presheath) is examined, with the presence of neutral part icles, and a magnetic field parallel to the confining wall. A general approach is used which takes into account the electron momentum equati on, including the electric field force, the magnetic force, the pressu re gradient, and the drag force. The influence of the electron to ion current ratio on the potential and velocity distribution in the near-s heath plasma is investigated. It is shown that the electron density di stribution in the presheath may deviate from the Boltzmann distributio n normally used in previous presheath models. Even when the plasma den sity dependence on the potential corresponds to the Boltzmann distribu tion, the presheath thickness deviates from that calculated with a mod el based on this distribution. The potential in the presheath with res pect to the plasma-presheath interface can be negative or positive dep ending on the electron to the ion flux ratio eta and Hall parameter be ta(i). In the case of magnetized ions (beta(i) > 1) the potential dist ribution has a positive maximum and is always negative at the wall edg e of the presheath. The value and position of the maximum depend on th e parameter eta. In the case of unmagnetized ions (beta(i) much less t han 1) the potential is positive for large eta and is negative for eta < 100. With large beta(i) the influence of the electrons is significa nt so that the presheath thickness decreases to the electron Larmor ra dius and has a strong dependence on the parameter eta. (C) 1997 Americ an Institute of Physics.