A tearing mode energy principle for an axisymmetric force-free plasma

A self-adjoint, gauge invariant stability functional δWTG for determination of a sufficient condition of tearing mode stability is obtained for a plasma with a uniform pressure. The plasma is stable if there is no free energy available to support energy flux into the singular layer. The Δ′ concept, valid in simple geometries, is generalized to reflect variation of equilibrium quantities on the flux surface. The relationship of δWTG to the ideal stability functional and other possible forms of tearing mode stability functionals is clarified through gauge transformations.