A theory of MHD instability of an inhomogeneous plasma jet

摘要

A problem of the instability of an inhomogeneous axisymmetric plasma jet in aparallel magnetic field is solved. The jet boundary becomes, under certainconditions, unstable relative to magnetosonic oscillations (Kelvin-Helmholtzinstability) in the presence of a shear flow at the jet boundary. Because ofits internal inhomogeneity the plasma jet has resonance surfaces, whereconversion takes place between various modes of plasma MHD oscillations.Propagating in inhomogeneous plasma, fast magnetosonic waves drive the Alfvenand slow magnetosonic oscillations, tightly localized across the magneticshells, on the resonance surfaces. MHD oscillation energy is absorbed in theneighbourhood of these resonance surfaces. The resonance surfaces disappear forthe eigen-modes of slow magnetosonic waves propagating in the jet waveguide.The stability of the plasma MHD flow is determined by competition between themechanisms of shear flow instability on the boundary and wave energydissipation because of resonant MHD-mode coupling. The problem is solvedanalytically, in the WKB approximation, for the plasma jet with a boundary inthe form of a tangential discontinuity over the radial coordinate. TheKelvin-Helmholtz instability develops if plasma flow velocity in the jetexceeds the maximum Alfven speed at the boundary. The stability of the plasmajet with a smooth boundary layer is investigated numerically for the basicmodes of MHD oscillations, to which the WKB approximation is inapplicable. Anew "global" unstable mode of MHD oscillations has been discovered which,unlike the Kelvin-Helmholtz instability, exists for any, however weak, plasmaflow velocities.