Vortex Formation and Particle Transport in a Cross-Field Plasma Sheath

摘要

The time-dependent behavior of a transversely magnetized, two-dimensional plasma-wall sheath has been studied through particle simulations, with the aim of modelling plasma behavior in the vicinity of the limiters and walls of magnetized plasma-devices. The simulations have shown that the cross-field sheath between a wall and a plasma is a turbulent boundary layer, with strong potential fluctuations and anomalous particle transport. The driving mechanism for this turbulence in the Kelvin-Helmholtz instability, which arises from the sheared particle drifts created near the wall. Provided it is replenished by an internal flux of particles, the sheath maintains itself in a dynamic equilibrium, in which the linear edge instability, the nonlinear dynamics of the particles and the outward particle diffusion all balance each other. The sheath assumes an equilibrium thickness of order 1/sub x/ /approximately/ 5 /rho//sub i/, and maintains large, long-lived vortices, with amplitudes delta/phi/ /approximately/ 2T/sub i/e, which drift parallel to the wall at roughly half the ion thermal velocity. The sheath also maintains a large, spatially-averaged potential drop from the wall to the plasma, with lambda/phi/ approx. /minus/1.5T/sub i/e, in sharp distinction with the unmagnetized sheath, where the plasma potential is higher than at the wall. Accompanying the long-wavelength vortices is a spectrum of shorter-wavelength fluctuations, which extend to /vert bar/K/vert bar//rho//sub i/ /approximately/ 1 and omega /approximately/ omega /sub ci/, and which induce an anomalous cross-field transport. A central result is that the anomalous transport scales like Bohm diffusion, at least when omega /sub pi/ />=/ 2 omega /sub ci/. At lower densities, omega /sub pi/ < 2 omega /sub ci/, the diffusion coefficient has an additional factor, proportional to the density. 45 refs., 30 figs., 1 tab. (ERA citation 13:035393)