On the energy principle for plasma MHD stability in presence of an inhomogeneous magnetic vacuum field

The expression for potential energy changes given in terms of the energy principle by Bernstein et al. was rearranged in order to account for the effects of induced surface currents. This reformulated expression was used to study the influence of an imposed transverse strongly inhomogeneous magnetic vacuum field on the MHD stability of a toroidal magnetically confined plasma column. It was found that electromagnetic modes in toroidal "Extrap" configurations become subject to a strong stabilizing effect from the corresponding inhomogeneous vacuum field and associated induced surface currents. In the special case of a slender toroidal plasma body being immersed in a strongly inhomogeneous vacuum field, these stability properties remain essentially the same as in a corresponding straight geometry. Earlier stability criteria for radial and axial displacements are reconsidered, in particular their relation to the effects of a finite plasma pressure and of electromagnetic induction.