Electrothermal Instability Mitigation by Using Thick Dielectric Coatings on Magnetically Imploded Conductors

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Electrothermal Instability Mitigation by Using Thick Dielectric Coatings on Magnetically Imploded Conductors

Kyle J. Peterson, Thomas J. Awe, Edmund P. Yu, Daniel B. Sinars, Ella S. Field, Michael E. Cuneo, Mark C. Herrmann, Mark Savage, Diana Schroen, Kurt Tomlinson, and Charles Nakhleh

Phys. Rev. Lett. 112, 135002 – Published 2 April 2014

Abstract

Recent experiments on Sandia’s $Z$ facility have confirmed simulation predictions of dramatically reduced instability growth in solid metallic rods when thick dielectric coatings are used to mitigate density perturbations arising from an electrothermal instability. These results provide further evidence that the inherent surface roughness as a result of target fabrication is not the dominant seed for the growth of magneto-Rayleigh-Taylor instabilities in liners with carefully machined smooth surfaces, but rather electrothermal instabilities that form early in the electrical current pulse as Joule heating melts and vaporizes the liner surface. These results suggest a new technique for substantially reducing the integral magneto-Rayleigh-Taylor instability growth in magnetically driven implosions, such as cylindrical dynamic material experiments and inertial confinement fusion concepts.

Received 23 January 2014

DOI:https://doi.org/10.1103/PhysRevLett.112.135002

Authors & Affiliations

Kyle J. Peterson^1,*, Thomas J. Awe¹, Edmund P. Yu¹, Daniel B. Sinars¹, Ella S. Field¹, Michael E. Cuneo¹, Mark C. Herrmann¹, Mark Savage¹, Diana Schroen², Kurt Tomlinson², and Charles Nakhleh³

¹Sandia National Laboratories, P.O. Box 5800, Albuquerque, New Mexico 87185-1186, USA
²General Atomics, San Diego, California 92121, USA
³Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

^*kpeters@sandia.gov

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Vol. 112, Iss. 13 — 4 April 2014

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Images

Figure 1
Simulated density and electron temperature contours showing the difference in perturbation growth predicted with several different thicknesses of dielectric coating on an Al rod at identical times when the drive current is approximately 7 MA. The coating is not observable in the temperature plots with the chosen contour levels since the coating carries very little current and remains comparatively cold in these simulations. Note that a small black line on the left side of each image represents the initial position of the coating-liner interface.
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Figure 2
(a): Measured load region $B$ -dot current for the two experiments performed, with vertical lines representing the time x-ray radiographs were taken. (b): Pictorial diagram of the multicomponent rod with a dielectric coating that covers both the Al and Be sections while only extending $π$ radians azimuthally. The experimental field of view was normal to the $R − Z$ plane and encompassed the entire rod. The regions represented by the black rectangles are the portions of the experimental field of view that are analyzed and presented in this Letter.
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Figure 3
Cropped and zoomed monochromatic 6151 eV x-ray transmission images of the instability growth observed on the coated (right) and uncoated (left) sections of the Al rod at times $t = 0 ns$ , $t = 3033 ns$ , $t = 3053 ns$ , $t = 3073 ns$ , and $t = 3093 ns$ . Red lines indicate the initial position of the Al surface and green lines indicate the initial position of the dielectric coating surface. Note that the diameter of the rod used in the first experiment was slightly smaller ( $OD = 6.768 mm$ ) due to a target fabrication problem.
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Figure 4
Fourier transforms of radiograph lineouts from the coated (green) and uncoated (blue) sections taken from the Al section of the rod at times $t = 3033 ns$ (A), $t = 3053 ns$ (B), $t = 3073 ns$ (C), and $t = 3093 ns$ (D). The coated section of the rod exhibits significantly less instability growth by approximately a factor of 10 to as much as a factor of 50 in dominant wavelengths.
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Figure 5
Preshot simulated transmission radiographs of an Al rod with a $70 − μ m$ dielectric coating (bottom) and without a coating (top) shown at the same times as the experimental radiographs in Fig. 3. While the dramatic reduction in instability growth with the thick dielectric coating was correctly predicted qualitatively, these simulations clearly underpredict the fraction of current flowing in the coating which consequently allows the coating and Al to progressively separate.
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