Vortex formation and particle transport in a crossfield plasma sheath
Abstract
The timedependent behavior of a transversely magnetized, twodimensional plasmawall sheath has been studied through particle simulations, with the aim of modelling plasma behavior in the vicinity of the limiters and walls of magnetized plasmadevices. The simulations have shown that the crossfield 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 KelvinHelmholtz 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, longlived 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, spatiallyaveraged 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 longwavelength vortices is a spectrum of shorterwavelength 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 crossfield transport. A central result is that the anomalous transport scales like Bohm diffusion, at least when omega (sub pi) greater than or equal to 2 omega (sub ci). At lower densities, omega (sub pi) less than 2 omega (sub ci), the diffusion coefficient has an additional factor, proportional to the density.
 Publication:

Unknown
 Pub Date:
 March 1988
 Bibcode:
 1988vfpt.reptQ....T
 Keywords:

 Crossed Fields;
 Plasma Control;
 Plasma Equilibrium;
 Plasma Sheaths;
 Vortices;
 Diffusion;
 KelvinHelmholtz Instability;
 Limiters (Fusion Reactors);
 Turbulence;
 Turbulent Boundary Layer;
 Wave Dispersion;
 Plasma Physics