SUPPRESSION OF RAYLEIGH-TAYLOR AND BULK CONVECTIVE INSTABILITIES IN IMPLODING PLASMA LINERS AND PINCHES

Linear growth of instabilities during implosion of annular and columnar plasma pinches having sharp outer boundaries is analyzed. The dominant modes of the Rayleigh-Taylor instability differ in time, radius, and growth rate from the dominant modes of the bulk convective instability. The spectra of growth rates of perturbations localized azimuthally or axially suggest explanations for filamentation and stratification of plasmas. Localized Rayleigh-Taylor modes grow fastest initially in an implosion. These modes can be suppressed completely, however, by a relatively modest axial magnetic field, which produces magnetic shear in a Z pinch. Similarly, bulk convective modes can be substantially reduced by magnetic shear. The "window of stability" produced by magnetic shear occurs for an axial field magnitude about equal to the azimuthal. Properly shaped current pulses in Z pinches with axial fields may therefore produce stable implosions to high compression ratios, in agreement with recent experiments. The most dangerous Rayleigh-Taylor modes for the symmetry of an annular pinch of initial aspect ratio R0/ δ0 are those with wavenumber k∼R0/ δ02, where R0 is the initial pinch radius. To suppress pinch instabilities optimally, both the liner thickness and axial magnetic field should be chosen properly. © 1990 American Institute of Physics.