Plasma stability in magnetic traps I. Magnetohydrodynamic stability theory

Recent developments in the linear theory of plasma stability in magnetic traps are described from a magnetohydrodynamic or quasi-fluid point of view. The stability against interchange of fairly general plasma distributions in open traps, which form magnetic wells satisfying the minimum-B criteria, is predicted in agreement with experiment, at least at low density. The situation with regard to closed traps is not so clear. Many new instabilities have been predicted as a result of including in a quasi-fluid theory fine-scale effects such as finite resistivity and ion mass. Systems stable according to `ideal' magnetohydrodynamics, either because of sufficient magnetic shear or average-magnetic-well properties, are unstable, at least in some regions, to the new but weaker instabilities.