When a plasma interacts with a material surface subject to an applied voltage, a sheath results. A one-dimensional model, specific to the magnetized ion sheath, is developed and applied to the radiofrequency (RF) sheath which forms near the Faraday screen of an ion cyclotron heating antenna. The RF sheath rectification of the applied voltage is shown to provide a large DC potential drop which can accelerate tons and cause sputtering. Numerical estimates of the high-Z impurity influx are compared with fast wave experiments, and it is concluded that the DC acceleration mechanism is a plausible explanation for the observed high-Z impurity release. Means of controlling the sputtering are examined, including operating with low densities at the Faraday screen.

Particle simulations have been made of an infinite plasma slab, bounded by absorbing conducting walls, with a magnetic field parallel to the walls. The simulations have been either one dimensional or two dimensional, with the magnetic field normal to the simulation plane. Initially, the plasma has a uniform density between the walls and there is a uniform source of ions and electrons to replace particles lost to the walls. In the one-dimensional (1-D) case, there is no diffusion of the particle guiding centers and the plasma remains uniform in density and potential over most of the slab, with sheaths about a Debye length wide where the potential rises to the wall potential. In the two-dimensional (2-D) case, the density profile becomes parabolic, going almost to zero at the walls, and there is a quasineutral presheath in the bulk of the plasma, in addition to sheaths near the walls. Analytic expressions are found for the density and potential profiles in both cases, including, in the 2-D case, the magnetic presheath resulting from a finite ion Larmor radius, and the effects of the guiding center diffusion rate being either much less than or much greater than the energy diffusion rate. These analytic expressions are shown to agree with the simulations. A 1-D simulation with Monte Carlo guiding center diffusion included gives results that are in good agreement with the much more expensive 2-D simulation.

It is known that tokamaks display a second region of stability to ideal magnetohydrodynamic (MHD) internal modes. An important determining factor for MHD properties is the radial profile of toroidal current. Here it is shown that in a low aspect ratio tokamak with high on-axis safety factor (qo ~ 2) and high shear a path to high beta can be obtained that remains completely stable against ideal MHD modes. By maintaining high shear this scenario avoids fixed boundary instabilities for both high and low toroidal mode numbers for beta values well above the Troyon limit (stability was tested up to c = 1.4, Q = 10.8% ). For a close fitting wall (awaul/aplasma ~ 1.2) this configuration is also stable to low toroidal mode number balloon-kink modes. I