Effect of impurity ions on the charge separation and velocity shear at a plasma edge

The problem of the formation of charge separation in a plasma in the presen ce of a steep density gradient, the self-consistent electric field and the associated EXB velocity, are studied using a two-dimensional (2D) gyro-kine tic Vlasov code for the ions, with electrons following an adiabatic law. Th e code shows the formation of a one-dimensional (1D) equilibrium charge at the plasma edge. It is also shown that the presence of a small fraction of impurity ions at the plasma edge can have a significant effect in increasin g the effective charge separation and the associated electric field. The pr esent results show that only a kinetic code can solve the problem of the eq uilibrium electric field in the presence of a density gradient, and point t o the important role played by the ions' gyro-radius in establishing a char ge separation at a plasma edge. (C) 1999 American Institute of Physics. [S1 070-664X(99)02605-1].