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Time evolution of filamentation and self-generated fields in the coronae of directly driven inertial-confinement fusion capsules
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View: Figures


Image of FIG. 1.
FIG. 1.

(Color) Experimental setup (a), sample radiograph (b), backlighter spectrum (c), and time history of two laser drive options (d) used on solid CH spheres. In (a), the dashed blue lines represent trajectories of charged particles from the backlighter, and the violet rectangle is the detector (which, with a 10 cm × 10 cm surface, records an area 3 mm × 3 mm at the subject. Darkness in the image (b) is proportional to proton fluence). The backlighter-subject and subject-detector distances shown in part (a), used for the radiograhs in Figs. 2 and 4, give an imaging magnification of 31. Since 10 cm is the size of the detector, the field of view at the subject is about 3 mm. The images in Fig. 3 have similar, but slightly different, magnifications. Image (b) is from OMEGA shot 51237.

Image of FIG. 2.
FIG. 2.

(Color) Monoenergetic charged-particle fluence radiographs of solid CH spheres recorded at different times relative to the laser drive for two drive intensities (6 × 1014 and 2 × 1014 W/cm2) and three particle types (15-MeV p, 3-MeV p, and 4-MeV α). The white diagonal line in each upper left corner is the stalk holding the subject. The size of the field of view at the subject is about 3 mm. The maximum fluence is different for each image, but is generally ∼1 proton per μm2 at the subject. The OMEGA implosions on which the 15-MeV images were recorded were, from left to right, 51243, 51237, 51238, and 51239 in (a); 51240, 51241, and 51242 in (b).

Image of FIG. 3.
FIG. 3.

(Color) 15-MeV-proton fluence radiographs of 860-μm-diameter capsules with 20-μm-thick CH shells and 15-atm H2 fill, recorded at different times relative to the laser drive (6 × 1014 W/cm2). See Ref. 15 for a radiographic study of fields inside the capsules in these kinds of implosions. From left to right, the images are from OMEGA shots 49119, 46536, 46537, 49120, 46535, and 49122.

Image of FIG. 4.
FIG. 4.

(Color) 15-MeV-proton fluence radiographs of 850-μm-diameter capsules with 35-μm-thick CH shells and 15-atm H2 fill. The capsules were driven with 40 laser beams in a shaped pulse (OMEGA type RD1501p). The drive has an initial picket, followed after about 1.3 ns by a plateau with intensity ∼1 × 1014 W/cm2 (∼1.5–2.1 ns) and another with intensity ∼4 × 1014 W/cm2 (∼2.4–2.9 ns). From left to right, the images are from OMEGA shots 51244, 51246, 51247, and 51250.

Image of FIG. 5.
FIG. 5.

(Color online) Pairs of proton-fluence and proton-mean-energy images. In each case, the CH target and the stalk in the upper left leave their signatures in the energy image, while the radial filaments (parts (a) and (c)) and other field-related structures do not. The fluence images are from (a) Fig. 2(a), (b) Fig. 3, and (c) Fig. 3. See also Fig. 13 for more information about part (b).

Image of FIG. 6.
FIG. 6.

(Color) Radiographs made with three different monoenergetic particles during OMEGA implosion 51237, compared with radii for the ¼-critical surface and the outer corona boundary predicted by the 1-D simulation code LILAC.29 This implosion utilized a laser intensity of 6 × 1014 W/cm2.

Image of FIG. 7.
FIG. 7.

(Color) Illustrations showing how E fields (green) around a line charge (red) can deflect trajectories of positively charged ions (blue dashed lines) passing from the particle source to the detector plane (violet). (a) Side view of system. (b) Bottom view when λ > 0, resulting in a fluence-deficit (light) image striation. (c) Bottom view when λ < 0, resulting in a fluence-excess (dark) image striation. As described in the text, φ is the angle between a specific filament and a specific particle trajectory, while Θi is the particle deflection angle.

Image of FIG. 8.
FIG. 8.

(Color) Illustrations showing how B fields (green) around a line current (red) can deflect trajectories of positively charged ions (blue dashed lines) passing from the particle source to the detector plane (violet). (a) Side view of system. (b) Bottom view when , resulting in a light (fluence-deficit) image striation. (c) Bottom view when , resulting in a dark (fluence-excess) image striation.

Image of FIG. 9.
FIG. 9.

(Color) Cartoon showing how the scenario of either Fig. 7(c) or 8(c) might result in a fluence deficit surrounded by fluence excess regions, but only if a strong deflecting field is restricted to a small region around the filament.

Image of FIG. 10.
FIG. 10.

Simple Monte Carlo simulations of images formed when a solid, 880-μm-diameter CH sphere is assumed to be surrounded by 200 filaments that are randomly placed in angle and extend from the CH surface to an outer radius of 1600 μm (all simulations use the same filament angles). The imaging geometry was as shown in Fig. 1, and backlighting protons were assumed to come from a 10-keV D3He plasma with a FWHM of 45 μm. The fields were truncated at a lateral distance of 100 μm from each filament and were simply set to zero beyond the end.

Image of FIG. 11.
FIG. 11.

(Color) Cartoons showing why it may be easier to explain one aspect of image striations with B fields than with E fields, using the observation that fluence-deficit striations appear to be formed by filaments that are closer to the detector plane than those associated with fluence-excess filaments. Current filaments can generate this characteristic if they all have the same current direction (a), while line-charge filaments can do so only in an unphysical arrangement in which filaments closer to the detector have one charge sign while filaments farther than the detector have the other (b).

Image of FIG. 12.
FIG. 12.

(Color) Diagram illustrating how lines of sight of an observer could make radial filaments appear to an observer to intersect the projection of the surface of a sphere even if they did not actually touch it.

Image of FIG. 13.
FIG. 13.

(Color) Radial lineout (b) of fluence image (a) from Fig. 3, and the corresponding mean-proton-energy image (c). Note that only the innermost circle of fluence deficit in (a), from the CH shell, has a counterpart in (c), so the radial oscillations outside the capsule shell seem to be due to radial electric fields.

Image of FIG. 14.
FIG. 14.

(Color) Monte Carlo simulations illustrating how the images in Fig. 13 have the signature of a CH shell plus charge shells. In this case, a charge shell with 5 × 1010 protons was placed outside a hollow CH capsule.

Image of FIG. 15.
FIG. 15.

A spectrum of accelerated ablator protons from an experiment at OMEGA, as presented and discussed in Ref. 19. This spectrum was recorded during an implosion with drive intensity 9 × 1014 W/cm2, 50% higher than that used in Fig. 13.

Image of FIG. 16.
FIG. 16.

Radiographs of CH-shell capsules in laser-driven Au hohlraums at OMEGA, recorded along the hohlraum axis with 15-MeV protons at 0.85 ns (a) and 1.6 ns (b) after the onset of a 1-ns laser drive. The light-colored outer ring is the holraum wall, and the 5-fold pattern between wall and capsule is due to electric fields generated by laser-hohlraum interactions (see Ref. 33 for more information about these experiments).

Image of FIG. 17.
FIG. 17.

(Color) Two different images of a CH sphere driven by 10 OMEGA laser beams. Part (a) shows a cartoon of the field of view of an exploding-pusher imaging system as described in Sec. II, and part (b) shows an image recorded with 15-MeV protons about 1 ns after the beginning of laser drive. Part (c) shows the nearly simultaneous field of view of the OMEGA-EP-based imaging system, and part (d) shows the corresponding image recorded with a wider spectrum of protons with an average energy of ∼15 MeV.

Image of FIG. 18.
FIG. 18.

(Color) 15-MeV-proton radiograph of an 840-μm-diameter capsule with 2-μm-thick glass shell and 15-atm H2 fill. The capsule was driven with 40 laser beams, with a peak intensity of ∼ 6 × 1014 W/cm2. The sample time of 0.5 ns is close to what would be bang time if the capsule were filled with D3He. This was OMEGA shot 51251.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Time evolution of filamentation and self-generated fields in the coronae of directly driven inertial-confinement fusion capsules