Measurement of Hydrodynamic Growth near Peak Velocity in an Inertial Confinement Fusion Capsule Implosion using a Self-Radiography Technique

L. A. Pickworth, B. A. Hammel, V. A. Smalyuk, A. G. MacPhee, H. A. Scott, H. F. Robey, O. L. Landen, M. A. Barrios, S. P. Regan, M. B. Schneider, M. Hoppe, Jr., T. Kohut, D. Holunga, C. Walters, B. Haid, and M. Dayton

Phys. Rev. Lett. 117, 035001 – Published 11 July 2016

Abstract

First measurements of hydrodynamic growth near peak implosion velocity in an inertial confinement fusion (ICF) implosion at the National Ignition Facility were obtained using a self-radiographing technique and a preimposed Legendre mode 40, $λ = 140 μ m$ , sinusoidal perturbation. These are the first measurements of the total growth at the most unstable mode from acceleration Rayleigh-Taylor achieved in any ICF experiment to date, showing growth of the areal density perturbation of $\sim 7000 \times$ . Measurements were made at convergences of $\sim 5$ to $\sim 10 \times$ at both the waist and pole of the capsule, demonstrating simultaneous measurements of the growth factors from both lines of sight. The areal density growth factors are an order of magnitude larger than prior experimental measurements and differed by $\sim 2 \times$ between the waist and the pole, showing asymmetry in the measured growth factors. These new measurements significantly advance our ability to diagnose perturbations detrimental to ICF implosions, uniquely intersecting the change from an accelerating to decelerating shell, with multiple simultaneous angular views.

Received 3 February 2016

DOI:https://doi.org/10.1103/PhysRevLett.117.035001

Physics Subject Headings (PhySH)

Implosion symmetry Indirect drive Inertial confinement fusion Plasma instabilities Rayleigh-Taylor instability

Inertially confined plasmas

X-ray & gamma ray plasma measurements X-ray absorption spectroscopy X-ray imaging X-ray techniques

Plasma Physics

Authors & Affiliations

L. A. Pickworth^1,*, B. A. Hammel¹, V. A. Smalyuk¹, A. G. MacPhee¹, H. A. Scott¹, H. F. Robey¹, O. L. Landen¹, M. A. Barrios¹, S. P. Regan², M. B. Schneider¹, M. Hoppe, Jr.³, T. Kohut¹, D. Holunga¹, C. Walters¹, B. Haid¹, and M. Dayton¹

¹Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, California 94551-0808, USA
²University of Rochester, Laboratory for Laser Energetics, Rochester, New York, USA
³General Atomics, San Diego, California, USA

^*pickworth1@llnl.gov

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Vol. 117, Iss. 3 — 15 July 2016

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Images

Figure 1
(a) A pie diagram of the plastic (CH) capsule, doped with Si and 1.4% Cu. A mode 40 sinusoidal ripple is machined into the ablator with a $(220 \pm 20) nm$ peak to valley amplitude, recessed $3 μ m$ . (b) Cut away diagram of the Au hohlraum showing the pole, equator, and the orientation of the ripples on the capsule. The ripples are visible on both the polar and equatorial imaging systems. The spectrometer views the unperturbed region on the equator perpendicular to the imaging system. (c) hydra [55] simulation $\sim 350 ps$ before peak compression. When the shock rebounds through the capsule gas fill, the presence of 1% Ar enhances the $T_{e}$ and, in turn, the self-emission.
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Figure 2
(a) Time resolved spectrum covering $\sim 1 ns$ around peak compression (PC, $t = 0 ps$ ). Absorption features from the Cu dopant in the shell are visible both in the rebounding shock spectrum and at PC. (b) Spectrally integrated emission in a $\sim 1 keV$ bandpass (8–9.2 keV) in time, showing significantly increased emission $\sim 350 ps$ prior to PC. This is compared to 1D non-LTE results (blue), showing good agreement in the timing of the self-emission enhancement. For reference, the simulated emission without added Ar is shown (green, dashed).
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Figure 3
A series of self-radiographs taken with a $12 \times$ magnification, $12 μ m$ pinhole imaging system onto a GXD detector with $\sim 100 ps$ temporal resolution through a $10 μ m$ Cu filter, from the polar and equatorial line of sight, compared with the simulated self-radiographs attenuated by the same filtration and modulation transfer function as the experimental imaging system. The self-radiograph is formed by the hot rebounding shock self-emission being attenuated by the cold shell, which has the perturbations on its outer surface. The dotted lines indicate the region spatially averaged over to infer the preimposed modulation wavelength and amplitude as a function of time. An example of the analysis for the polar view, PC -368 ps, shows the signal $I$ , solid blue, the self-emission from the rebound shock, $I_{B}$ dashed black, on the left. The calculated modulation amplitude, solid blue, and sine fit, dashed red, is shown on the right.
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Figure 4
(a) Radius vs time for experimental and simulated self-radiographs taken on the pole and equator. (b) Modulation amplitude vs radius for the pole and the equator showing continued growth of the modulation on both LOS with a difference in the measured growth between the equator and pole. Solid lines are proportional to $1 / R^{2}$ . The simulated radiographs use a 1D drive that is an average of the pole and equator. The initial modulation amplitude of the perturbation in the ablator was $7.3 \times 10^{- 5}$ OD.
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