Quasi-neutral particle simulation of magnetized plasma discharges: general formalism and application to ECR discharges

摘要

We have developed an electrostatic particle-in-cell/Monte Carlo (PIC/MC) simulation method for magnetized discharges, in which both internal electric fields and sheath potentials are determined from the requirement of quasineutrality within the bulk plasma, rather than by solving Poisson's equation. Thus the electric field is not sensitive to statistical noise which may occur in the small quantity n/sub e/-n/sub i/. Sheaths are treated self consistently as thin potential barriers, and the Bohm criterion for ion flux into the sheath is imposed as a boundary condition. Electron plasma oscillations do not appear in the model, and the debye length is essentially set to zero. Thus time steps and spatial gridding can be chosen to represent the characteristic macroscopic time and space scales of interest, which may be orders of magnitude larger than the plasma frequency/debye length scales. The simulation technique correctly represents kinetic features such as non-Maxwellian distributions and Landau damping and can be used for either collisional or collisionless plasmas. We present results from an axisymmetric simulation of an electron cyclotron resonance (ECR) discharge in low-pressure argon, which show that the discharge is strongly affected by cross-field ion flows, even when the vessel walls are insulators. We also present analytic calculations based on the model, which afford new insights into cross-field transport in a metallic vessel and show that the classic Simon diffusion can be strongly inhibited by the effect of sheath potentials.