Short-Circuit of Flute Disturbances at a Plasma Boundary

A plasma boundary is defined by magnetic field lines which are tangent to a conducting wall. For a sufficiently good contact between the outermost layers of the plasma and the wall, the boundary will become an equipotential surface. The joint effect of line-tying at the boundary and a finite ion Larmor radius then produces a stabilization of flute disturbances which is stronger than that arising from the finite Larmor radius alone. This effect is especially pronounced when the density increases steeply in a narrow boundary region and is nearly constant within the plasma interior. The present mechanism should have a marked influence on rotating plasma devices, but should hardly affect the rotational instability of theta pinches. An additional stabilization is provided by a velocity field which is sheared in the equilibrium state.