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Experimental investigation of bright spots in broadband, gated x-ray images of ignition-scale implosions on the National Ignition Facility
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View: Figures


Image of FIG. 1.
FIG. 1.

Cross-section of capsule used for NIF experiments, with glow-dischargepolymer (GDP) ablator doped with various levels (0%, 0.2%, 0.5%) of Ge (orange layers). For layered targets, the capsule ablator surrounds a layer of DT (or THD) ice (dark blue) that in turn confines the DT (THD) gas (light blue). For symmetry capsules (Symcaps), the ice layer is replaced with a mass equivalent layer of GDP, and the GDP shell is filled with a mixture of He and D gas. Some capsules replace Ge dopant with various levels of Si dopant.

Image of FIG. 2.
FIG. 2.

Simulated x-ray image of the implosion near stagnation for (a) a Gaussian intensity profile signal with FWHM of 80 m describing a homogenous mix of material, (b) simulated image as described in (a) with superimposed off-center bright spot with 30 m FWHM, and (c) simulated image as described in (a) with superimposed off-center bright spot with 12 m FWHM with ∼5× higher emission. (d) Normalized power spectra density (PSD) as a function of wavenumber, for images in (a) (blue curve), (b) (red curve), and (c) (green curve). PSD for simulated image for homogenous case, (a), shows no structure for wavenumbers larger than ∼0.12 m. Superposition of bright spot translates to a change in PSD, having structure above ∼0.12 m. Superposition of a smaller bright spot (FWHM 12 m) with higher emission (∼5×) is also tracked via the PSD, where the spectral distribution extends over ∼0.5 m and normalized power spectral density is higher than in either of the previous cases.

Image of FIG. 3.
FIG. 3.

Compilation of input and Fourier filtered images, with corresponding PSD for a Symcap capsule. The raw input image, spatial intensity envelope of the hot spot, and the bright spot images are shown in (a), (b), and (c), with white line markers depicting φ = 0° and 78°, and red marker showing fill tube location. The white curve around the implosion is the 17% of peak intensity contour determined from the envelope image, validating the use of the envelope as an adequate description of large-scale structure/shape of the implosion. Image (c) highlights the use of Fourier filtering to identify emission features with spatial structure between ∼10 and 50 m. Note that bright spots that are difficult to identify on the input image (a) are evident on the bright spot image (c). The low emission structure seen in (c) is related to the broadband instrument noise. (d) Calculated PSD for input image is shown as solid blue curve and super-Gaussian filters (dotted red curve for low pass filter, and dotted green for notch filter used for bright spot analysis). A cutoff-wavenumber, determined by the resolution element of the diagnostics, is shown as the dashed black line in (d) and (e); it sets the wavenumber upper bound for the notch filter applied to the input image. (e) The normalized PSD for the input image described in (d) is compared with the PSD contributions from the envelope and bright spot images, shown as the solid red curve and shaded green region. The gray shaded region denotes the diagnostic noise level.

Image of FIG. 4.
FIG. 4.

Bright spot emission for Symcap shot N110208 integrated over of 5° azimuthal bins (shown in inset) within the 17% contour as a function of φ at  = 21.34 ns. Data are normalized to total emission of the input image within the 17% contour. Dashed vertical red lines indicate FT location and 180° from FT. A strong correlation between bright spot- and fill tube-location is clearly identified.

Image of FIG. 5.
FIG. 5.

Time integrated (normalized) bright spot emission for shot Symcap shot N110208 shows a large portion of bright spots that are generated close to the fill tube location. Dashed red lines indicate FT location and 180° from FT.

Image of FIG. 6.
FIG. 6.

Normalized PSD (solid blue curve) and contributions to the PSD from envelope (solid red curve) and bright spots (shaded green) for four different types of implosions. Graphs on the left (right) column show Symcap (DT) shots with Ge- and Si-doped GDP ablator at peak x-ray emission. Dashed gray line and shaded gray region denote cut-off wavenumber and instrument noise, respectively. Normalized PSD shows DT experiments have higher power for  > 0.2 m compared with Symcaps. Symcap targets with Si doped GDP ablator show lower bright spot contribution to the normalized PSD in comparison to Ge doped ablator targets for  > 0.1. Differences in bright spot contribution to the normalized PSD between Ge- and Si- doped ablator DT implosion are not as pronounced at those observed in Symcap experiments.

Image of FIG. 7.
FIG. 7.

Bright spot filtered images for a compilation of Ge-doped ablator targets (from left to right: Symcap, DT, and THD). Middle frame corresponds to peak x-ray emission, top and bottom frames are ∼100 ps before and after peak x-ray emission; image markers as described in previous figures.

Image of FIG. 8.
FIG. 8.

Time integrated average bright spot emission (normalized to total input image emission within 17% contour) as a function of degrees from fill tube, for Symcap (a) and layered (b) targets. Data show the fill tube seeds bright spots for both Symcap and DT/THD shots. This feature is most predominant for Symcap experiments. For layered shots, however, comparable features are also measured away from the fill tube.

Image of FIG. 9.
FIG. 9.

Power contribution from bright spots normalized by total power from raw input image PSD (fractional power) for all analyzed shots at peak x-ray time. Results for Symcap, DT, and THD are shown as the green circles, red upright triangles, and dark red downward triangles, respectively. Measurement errors are estimated to be ∼10%, and are smaller than the symbol size. Note fractional power levels differ between different ablator shots for Symcaps, though remain stable for cryogenic DT layered shots.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Experimental investigation of bright spots in broadband, gated x-ray images of ignition-scale implosions on the National Ignition Facility