Helical Instabilities in Magnetized Cylindrical Liner-Plasmas

plasma instabilities;Nuclear Engineering and Radiological Sciences;Engineering;Nuclear Engineering & Radiological Sciences

Yager-Elorriaga, DavidMcBride, Ryan David ;

University of Michigan

关键词: plasma instabilities;    Nuclear Engineering and Radiological Sciences;    Engineering;    Nuclear Engineering & Radiological Sciences;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/140868/dyager_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

A Z-pinch is formed by driving a large axial current through a cylindrical liner, generating an azimuthal magnetic field so that the resulting Lorentz force implodes the system to high energy density conditions. During the implosion process, the magneto-hydrodynamic (MHD) sausage and kink instabilities may couple to the acceleration-driven magneto Rayleigh-Taylor (MRT) instability. These instabilities are particularly relevant to magnetized target fusion schemes such as the magnetized liner inertial fusion (MagLIF) concept being pursued at Sandia National Laboratories, where a Z-pinch driver is used to generate thermonuclear conditions by imploding a magnetized and preheated fusion fuel within a cylindrical liner.This thesis presents an experimental investigation of helical features that appear in magnetized, ultrathin foil-plasmas driven in a Z-pinch configuration by the 1-MA linear transformer driver at University of Michigan. Three types of cylindrical liner loads were designed to produce: (1) pure MHD modes (defined as being devoid of the acceleration-driven MRT instability) using a non-imploding geometry, (2) pure kink modes using a non-imploding, kink-seeded geometry, and (3) MRT-MHD coupled modes in an unseeded, imploding geometry. For each of these configurations, the effects of axial magnetic fields were determined using external Helmholtz coils that generated relatively small fields of Bz = 0.2-2.0 T (compared to peak azimuthal fields of 30-40 T). The resulting liner-plasmas and instabilities were imaged using 12-frame laser shadowgraphy and visible self-emission on a fast framing camera. A tracking algorithm was developed to trace self-emission minima in order to carefully identify the azimuthal mode number.When no axial magnetic field was applied, the unseeded imploding and non-imploding liners were found to develop an azimuthally symmetric sausage instability. Applying an axial magnetic field excited helically oriented instabilities, which are demonstrated to be a manifestation of discrete eigenmodes. The pitch angle of the helix is governed by the simple equation p =m/kR, from implosion to explosion, where m, k, and R are, respectively, the azimuthal mode number, axial wavenumber, and radius of the helical instability. Thus, the pitch angle increases (decreases) during implosion (explosion) as the plasma radius became smaller (larger). It was found that one (or at most two) discrete helical mode(s) developed for magnetized liners, with no apparent threshold on the applied Bz for the appearance of helical modes; increasing the axial magnetic field from zero to Bz = 0.5 T changed the relative weight between the m = 0 sausage and m = 1 kink modes. Further increasing the applied axial magnetic fields excited the higher order m = 2 helical mode, consisting of two intertwined helices. Finally, the importance of seeding when compared to the intrinsic instability modes was investigated using the kink-seeded support structure. It was found that the seeded kink instability overwhelmed the intrinsic instability modes of the plasma, despite the magnitude and orientation of the applied axial magnetic field. The experimental results in this thesis are corroborated with the Weis-Zhang-Lau analytic theory on the effects of radial acceleration on the classical sausage, kink, and higher m modes.