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Stimulated Raman scatter analyses of experiments conducted at the National Ignition Facility a)
a)Paper BI2 2, Bull. Am. Phys. Soc. 55, 21 (2010).
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10.1063/1.3577836
/content/aip/journal/pop/18/5/10.1063/1.3577836
http://aip.metastore.ingenta.com/content/aip/journal/pop/18/5/10.1063/1.3577836
View: Figures

Figures

Image of FIG. 1.
FIG. 1.

(Color online) Schematic of a NIF ignition target. Twenty-four quads of laser beams enter each side of a hohlraum through a laser entrance hole in four ring cones: four at 23.5°, four at 30°, eight at 44.5°, and eight at 50°. The beams strike the inside of the high-Z cylinder wall, where they undergo conversion to x-radiation, which implodes a DT capsule, initiating a fusion reaction. A low-Z gas fills the hohlraum to minimize wall motion early in time.

Image of FIG. 2.
FIG. 2.

(Color online) The NIF ignition target and delineation of LPI locations. The upper half of the NIF target shows target materials and expected LPI location prior to commencement of experiments in 2009. To mitigate SBS, ion acoustic wave damping was increased in both the wall blow-off plasma and the gas fill plasma by adding boron to the former and hydrogen to the latter. SRS occurs deep in the target, over the capsule, along the inner beams. The bottom half of the NIF target shows updated NIF target materials (no boron, 100% He gas fill), and where postshot analyses indicate LPI occurrence. Cross-beam energy transfer occurs in the vicinity of the LEH, where the beams are overlapped.

Image of FIG. 3.
FIG. 3.

(Color online) (a) A plot of the electron density normalized to critical density at the end of peak power for the first 1.05 MJ shot in one quadrant of the NIF hohlraum. The inner beam quads propagate from the LEH (on the right) to the wall, toward the label “inner beam path.” (b) A plot of the cross section of three quads of beams at the location z = 0.05 cm, i.e., near the wall. Here, the quads are completely separated. (c) A plot of the cross section of three quads of beams at the location z = 0.1 cm. Here, the quads are just beginning to separate. (d) A plot of the cross section of three quads of beams at the location z = 0.3 cm, close to where SRS occurs (black curve labeled HFM SRS). Here the quads are overlapped. (e) A plot of the cross section of three quads of beams at the location z = 0.45 cm. close to the LEH. Here the quads are nearly completely overlapped, as they must be to get through the LEH. In this region, Landau damping is high and SRS is not generated.

Image of FIG. 4.
FIG. 4.

(Color online) A plot of the evolution of our SRS gain spectrum from preshot to postshot. In panel (a), we show our synthetic SRS spectrum based on expected preshot model plasma conditions. SRS peaks late in time at very long wavelengths (high densities, over the capsule). (b) The experimental SRS spectrum indicates SRS occurs at lower wavelengths than predicted. Late in time SRS occurs near 570 nm. (c) Using improved plasma conditions from the high-flux model, and accounting for the overlap beam intensity, we generate SRS spectrum that are qualitatively consistent with experiment. Late in time, SRS is generated at wavelengths close to those of experiment. Early in time, we get a “blip” of SRS, but not as bright as in experiment. This suggests that other mechanisms not accounted for might be important early in time. (d) We show here a plot of laser power vs time for the first ∼1MJ NIF shot. Ref. 20. The SRS gains are largest at peak power, where Figs. 5(a)–5(c) are relevant.

Image of FIG. 5.
FIG. 5.

(Color online) With the improved plasma conditions and the overlap beam intensity concept, we make a successful preshot prediction for the SRS spectrum for the first 1.25 MJ shot Ref. 22. In panel (a), we show the SRS spectrum without accounting for the overlap beam intensity. In panel (b), we show the SRS spectrum when we account for the overlap beam intensity. In panel (c), we show the experimental SRS spectrum. This further validates the overlap beam intensity concept.

Image of FIG. 6.
FIG. 6.

(Color online) A plot of the SRS FOPAG for the first ∼1 MJ shot Ref. 20. Shown in panel (a) is the SRS FOPAG on the rise to peak power, at 16.5 ns. Shown in panel (b) is the SRS FOPAG at the end of peak power, 19 ns. There is sufficient gain across all of peak power when we account for the overlap beam intensity. Without the overlap beam intensity, only 20% of the power is in gains above the approximate threshold early in time [c.f. panel (a)].

Image of FIG. 7.
FIG. 7.

(Color online) A plot of the laser input to the simulation box depicted in Fig. 4(a). The two quads of beams have been propagated to this plane accounting for refraction and absorption using SLIP (Ref. 10). The 23.5° quad is on the top and the 30° quad is on the bottom.

Image of FIG. 8.
FIG. 8.

(Color online) A plot of the reflectivity fraction vs time for three different simulations: a 23.5° single-quad, a 30° single-quad, and the double-quad simulation containing both a 23.5° and a 30° quad. We find a qualitative difference in the reflectivity. The single-quad simulations show 4%–5% reflectivity, whereas the double-quad simulation exhibits pump depletion oscillations and significantly larger reflectivity ( ∼15% to 20%).

Image of FIG. 9.
FIG. 9.

(Color online) Panels (a) and (b) depict the forward intensity pattern for the 30° and 23.5° quads, respectively, when they are propagated separately. In the upper half of the box, there is evidence of some pump depletion, the reddish region near the end of the simulation box. Panel (c) demonstrates what happens when the overlapped beams are simulated. There is a significant depletion of the pump in the region where the beams are overlapped (in region of “V” in the middle of the box). These figures are shown at 19 ps into the simulation, when the reflectivity is ∼20%.

Image of FIG. 10.
FIG. 10.

(Color online) A plot of the SRS reflected light (central azimuthal slice). In panels (a) and (b), we show the SRS light when we propagate the quads individually. In panel (c), we depict the SRS light when we propagate the two overlapped quads. The region where the SRS is generated (near the back of the box) has not changed. Panel (c) indicates that the SRS light is further resonantly amplified in the region where the two quads are overlapped [near the middle (horizontally) of panel (c)].

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2011-05-26
2014-04-25
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Stimulated Raman scatter analyses of experiments conducted at the National Ignition Facility a)
http://aip.metastore.ingenta.com/content/aip/journal/pop/18/5/10.1063/1.3577836
10.1063/1.3577836
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