Drifts, boundary conditions and convection on open field lines

In a number of plasmas of practical interest, including the scrape-off layer of a tokamak with a divertor or toroidal limiter, some gas discharge devices, and in the vicinity of spacecraft, magnetic field lines intersect bounding surfaces at shallow angles. Under these circumstances a number of interesting and important effects arise. Drifts can compete with parallel flows in establishing the boundary conditions for plasma mass-flow and current (sheath current-voltage characteristics). This paper derives the mass-flow constraints including both poloidal and radial drifts, reviews the current boundary conditions, and surveys the consequences, including along-field density and heat-flux asymmetries, convection created by a wavy surface, generation of electric fields, and surface currents associated with shadows from surface structures, and modification of instability growth rates. (C) 1999 American Institute of Physics. [S1070-664X(99)96505-9].