Effects of axial magnetic field on mhd instabilities in cylindrical liners

Summary form only given. This paper analyzes the effects of an axial magnetic field on the magneto-Rayleigh-Taylor instability (MRT) of an imploding cylindrical liner. In recent MagLIF experiments with a premagnetized axial magnetic field [1], helical perturbations on the outer surface of the liner were found to persist despite the severe winding up of the magnetic field streamline. The inner surface of the liner exhibited improved integrity as a result of the premagnetized axial magnetic field. Here, we propose that the helical structure is the signature of the azimuthal m = 1 eigenmode with axial wavenumber k, so that the pitch angle of the helix, phi = arctan(m/kr) ~ 1/kr. Here, r is the outer radius of the imploding liner at which the helical perturbations were observed [1]. This simple scaling shows that phi increases as the convergence ratio (CR) increases, i.e., as r decreases. It also qualitatively explains the evolution of phi in both the low CR and high CR shot data that were tabulated in [1]. We have extended our MRT theory for the slab geometry [2] to a cylindrical geometry, so as to include the coupling between MRT, the sausage mode (m = 0) and the kink mode (m = 1). The effect of feedthrough suppression by the strong axial magnetic field is noted. Instantaneous equilibrium profiles obtained from 1-D HYDRA runs have been applied to the aforementioned formulation.