Evolution of MHD Instabilities in Plasma Imploding Under Magnetic Field

Abstract

Abstract-Two-dimensional 3-ns-gated visible images, recorded at different times during the implosion of plasma under azimuthal magnetic field (Z-pinch), revealed ringlike instabilities followed by the development of axially and azimuthally nonuniform structures in the imploding plasma. Remarkably, the evolution in time of all structures was found to be highly repeatable in different shots, which should allow, through 3-D magnetohydrodynamics modeling, for systematically studying the development in time of these complex phenomena and correlating them with the initial plasma parameters. The data are also used to infer the time-dependent outer plasma radius and plasma radial velocities. Index Terms-Plasma implosion, visible imaging, Z-pinch.T HE Z-PINCH implosions serve as intense sources of X-ray radiation that are of broad interest in the research toward inertial confinement fusion and the study of matter under high energy density [1]. In such systems, a cylindrical plasma column (typically formed out of an array of fine wires or a puff of gas) radially implodes under the azimuthal magnetic field resulting from axially driven high currents. In this process, acceleration-driven instabilities play a major role in the magnetohydrodynamics (MHD) of the plasma, including causing complex current paths in the plasma, magnetic-field penetration into certain parts of the plasma, and nonuniform disturbances in the implosion [2].In this paper, a neon puff (∼70 µg/cm) was imploded in 500 ns until stagnation on axis under a current pulse that rises to 500 kA [3]. The puff consists of a 38-mm-diameter outer shell and an ∼5-mm-diameter on-axis jet. The distance between the cylindrical nozzle (which serves as the cathode) and the cylindrical anode was 9.5 mm. Planar laser-induced fluorescence measurements on a similar system, but with an additional cylindrical nozzle, showed that the azimuthal uniformity of the initial gas density distribution is better than ±15% [4]. An f/16 optics with 7.5× demagnification and coupled to an intensified CCD, viewing the plasma in the radial direction, provided a submillimeter spatial resolution across the 38-mm-diameter plasma shell, together with a sufficient signal. The filter used transmitted visible light above 6000 Å (T > 40%), thus mainly recording continuum radiation except for the earliest phase of the implosion where NeI-II line emission was significant too. The emission is thus nearly proportional to the square of the plasma density. The 3-ns-gated images were taken at different times along the implosion phase until the plasma stagnates on axis. Shown in Fig. 1 are 14 images of the imploding shell, most of them 50 ns apart, starting at t = −500 ns and ending at t = 0 (when the plasma stagnates on axis). Surprisingly, it was found that the complex structures evolving in the plasma are repeatable in the different shots. Their repeatability even allows for tracking the growth of the structures at specific z-locations down to t ∼ = −100 ns.The initial factors that govern the start of the ...

Stagnation of a gas puff Z pinch

“…Visible light visualization of the imploding Z pinch reveal larger scale features as well, 13 which resemble nonlinear Rayleigh Taylor instability.…”

Application of one-dimensional stagnation solutions to three-dimensional simulation of compact wire array in absence of radiation

“…Although this work focuses on 3D stagnation of wire arrays, our results may be relevant to gas puffs [43][44][45][46][47][48][49] and metallic liners, 50,51 which also develop 3D structures during the implosion phase. Furthermore, recent simulations 52 highlight the important role that 3D hydrodynamics may play during stagnation of inertial confinement fusion capsules.…”

Observation of Rayleigh-Taylor-instability evolution in a plasma with magnetic and viscous effects

“…In a plasma, both magnetic field and a strongly temperature-dependent viscosity can dramatically affect the evolution of RTI [3,8] and its consequences on plasma transport [9,10]. While study of RTI in the context of inertial confinement fusion (ICF) spans many decades, e.g., [5] and references therein, only more recently have studies seriously focused on the effects of magnetic field or viscosity on RTI evolution in plasmas, e.g., [9][10][11][12][13][14][15][16]. Detailed experimental data on the latter are especially needed for validating simulations of magnetized ICF [10, 17, 18] and magneto-inertial fusion (MIF) [19,20].In this work, we present time-resolved observations of unseeded RTI growth and evolution at a decelerating, mostly planar plasma interface, focusing on the role played by magnetic field and viscosity in RTI evolution and stabilization.…”