Stabilization of magnetohydrodynamic modes by applied radio-frequency waves

A kinetic theory describing the nonlinear interaction of radio-frequency waves with low-frequency magnetohydrodynamic modes is presented. The calculation of the nonlinear force density on a fluid element includes both ponderomotive and sideband mode coupling terms and allows arbitrary rf wave polarization. Electromagnetic effects and wave–particle interactions are retained in the analysis. The influence of the nonlinear force on magnetohydrodynamic plasma stability is calculated self-consistently, and a generalized low-mode-number ballooning equation is obtained for paraxial magnetic mirror geometry. The low-frequency stability problem is thereby shown to involve the coupling of four waves and the system of governing equations is presented. Specialized limits of the general equations are discussed, and a physical interpretation of rf stabilization is given. It is shown that stability is enhanced by placing the antenna and conducting cavity walls close to the plasma surface.