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Asymmetric directly driven capsule implosions: Modeling and experiments—A requirement for the National Ignition Facility
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Image of FIG. 1.
FIG. 1.

A defect target is shown with polar direct drive illumination. A backlighter foil to the left was used for x-ray radiography. The capsule drive beams are reduced in footprint size to enable clear viewing of the equatorial-plane target defect in this OMEGA Visrad model.6 Note the absence of equatorial-plane beams.

Image of FIG. 2.
FIG. 2.

Typical defect target: optical photograph (left) and x-ray image (right), courtesy of General Atomics.

Image of FIG. 3.
FIG. 3.

For the 6 × 6 frame LFC images, time goes from right to left with each row of strip lines below the first triggered sequentially later in time (with some overlap).

Image of FIG. 4.
FIG. 4.

First light on F13 establishes the timing within ±25 ps. The LFC high-voltage optical gate propagates from the right. Note the absorption of backlighter emission in the diffuse dark area by the limb of the target. Self emission on the limb from the start of laser irradiation is seen from 9 o'clock to ∼10:30 with respect to the capsule at the top of the strip line in Fig. 4(a), but it is not seen to the right at 4:30 on the clock face. This indicates that the laser has not turned on when the optical gate passes this point on the image, i.e., first light. (a) The F13 image; (b) the same image with selected radial points and the best-fit circle.

Image of FIG. 5.
FIG. 5.

(a) The best-fit circle for determination of the radius in frame F43 for a defect target; (b) at a later time for the same target (F53). The defect for F53 is more apparent, plus it has some pathology of absorption and emission associated with the presence of the defect. Note that for this and other images the camera view is ∼11° out of the equatorial plane.

Image of FIG. 6.
FIG. 6.

An x-ray streak of an imploding capsule. First light appears at t = 0. At ∼1 ns, the laser turns off, and the plasma cools until about 1.5 ns when the stagnation process heats up the plasma and the Ti in the core. The apparent curvature of the 4.75-keV He-alpha Ti line may be due to a temporally expanding absorption feature resulting from progressive ionization of Ti in the doped layer that is outboard of the shell-gas interface.

Image of FIG. 7.
FIG. 7.

Radii of various imploded capsules from (a) experimental data and (b) simulations for a perfect and defect capsule with the corresponding experiments.

Image of FIG. 8.
FIG. 8.

With PDD, backlit images of a perfect capsule, beginning at ∼1.1 ns, show time-dependent evolution of symmetry. Time goes from right to left and top to bottom with ∼250 ps/strip.

Image of FIG. 9.
FIG. 9.

(a) The elliptical least-squares fit for the outer-most ablator layer of F34; (b) the best fits for the major and minor elliptical radii of the frames in Fig. 8. Both experimental data and 2D Hydra simulation data are shown.

Image of FIG. 10.
FIG. 10.

The Legendre P2/P0 ratio of experimental data for the imploding core shows wide symmetry swings with time.

Image of FIG. 11.
FIG. 11.

(a) Frame F33 from Fig. 8, a perfect target with PDD. The spatial resolution is ∼10 μm. (b), (c), and (d) Equatorial-plane 2D Hydra simulations of x-ray emission of shot 62813 near bang time that show the conversion from oblate toward a prolate geometry, c.f. frames F33–F44 in Fig. 8.


Generic image for table
Table I.

Forty beams are used for PDD. The latitude is equal to 90° minus the polar angle Θ. All beams are directed toward a point on the Z (polar) axis on the opposite side of the equator (Z). The defocus offset is the distance the lenses are moved (<0 pulls the lens away from the target axis).


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Asymmetric directly driven capsule implosions: Modeling and experiments—A requirement for the National Ignition Facility