Charge separation at a plasma edge in the presence of a density gradient

A fully kinetic code for ions [one dimensional (1-D) in space, and using the three velocity dimensions in velocity space] is used to study the problem of the formation of a charge separation with the self-consistent electric field in a plasma in the presence of a density gradient. Electrons are treated using an adiabatic law. Graphical results are presented which follow the formation of a 1-D steady state showing the formation of an oscillating positive potential bump toward the edge of the plasma. These oscillations are closely associated with the gyration of the ions. It is also shown that the presence of a small fraction of impurity ions at the plasma edge can have a significant effect on the rapid buildup of the potential at the edge, and in increasing the charge separation and the associated electric field at the edge, in comparison to the case when no impurity ions are included. The present results show the importance of a kinetic solution to the problem of the equilibrium electric field and charge separation in the presence of a density gradient, and point to the important role played by the finite ions'gyroradius and the important contribution of impurity ions in this case.