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Analyses of laser-plasma interactions in National Ignition Facility ignition targetsa)
a)Paper TI1 4, Bull. Am. Phys. Soc. 52, 273 (2007).
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10.1063/1.2901127
/content/aip/journal/pop/15/5/10.1063/1.2901127
http://aip.metastore.ingenta.com/content/aip/journal/pop/15/5/10.1063/1.2901127
View: Figures

Figures

Image of FIG. 1.
FIG. 1.

Schematic of a NIF ignition target. Twenty-four quads of laser beams enter each side of a hohlraum through a laser entrance hole (LEH) 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.

The three candidate ignition designs for the 2009 ignition campaign. Each design has a DT capsule with a Be ablator. The hohlraum and the laser beam radii scale as 1:1.175:1.235 as decreases from . The capsule scales as with decreasing . Each is filled with a low-Z gas fill. The yield ranges from at at . The capsules in these ignition targets were designed to have similar robustness across the range of radiation temperatures.

Image of FIG. 3.
FIG. 3.

A plot of the material fractions for the point design at peak power. Shown in the upper right, upper left, lower left, and lower right quadrants, respectively, are the 23°, 30°, 44°, and 50° laser beam paths. The outer (44° and 50°) beams have a longer path through the wall liner blowoff on the side of the beam closest to the capsule. Over the capsule, half of the inner beam cross section is in ablator plasma, and half is in gas fill. The inner beams undergo refraction in the ablator plasma.

Image of FIG. 4.
FIG. 4.

Maps of maximum SBS gain across the suite of candidate designs for the 23° beams. As decreases, both the value of maximum gain and the areal fraction of the focal spot at that gain decrease. SBS occurs primarily in the ablator plasma (lower portion of the beam cross section) for the inner beams.

Image of FIG. 5.
FIG. 5.

A plot of fraction of power above gain (FOPAG) vs gain for maximum SBS along a 23° beam. At , 90% of the power is in gains ; at , 90% of the power is in gains ; and at , 90% of the power is in gains .

Image of FIG. 6.
FIG. 6.

Maps of maximum SRS gain across the suite of candidate designs for the 23° beams. As decreases, both the value of maximum gain and the areal fraction of the focal spot at that gain decrease. SRS occurs primarily in the gas fill plasma (upper portion of the beam cross section) for the inner beams.

Image of FIG. 7.
FIG. 7.

A plot of fraction of power above gain (FOPAG) vs gain for maximum SRS along a 23° beam. At , 90% of the power is in gains ; at , 90% of the power is in gains ; and at , 90% of the power is in gains .

Image of FIG. 8.
FIG. 8.

Maps of maximum SBS gain across the suite of candidate designs for the 50° beams. As decreases [compare (a) to (c)], both the value of maximum gain and the areal fraction of the focal spot at that gain decrease. In (b), 20% boron has been added to the gold wall liner, which increases the ion acoustic wave damping, thereby reducing gain. SBS occurs primarily in the gold wall liner, over a few hundred micrometers. The lower portion of the beam cross section has a longer path length through the wall liner, and hence larger gain.

Image of FIG. 9.
FIG. 9.

A plot of fraction of power above gain (FOPAG) vs gain for maximum SBS along a 50° beam. At , 90% of the power is in gains ; at , 90% of the power is in gains (effect of adding B to Au); and at , 90% of the power is in gains at .

Image of FIG. 10.
FIG. 10.

(a) A plot of an “approximate” NIF inner beam, used in massively parallel beam propagation simulations. The dashed green ellipse represents the nominal spot area represented by the radii quoted in Fig. 2. The laser speckles are evident in the flattop. (b) A plot of the “letterbox” beam used in pF3D whole beam simulations for those performed at . This beam samples all of the radial variation of the plasma profiles ( direction), and is wide enough in the azimuthal ( direction) to retain adequate speckle statistics.

Image of FIG. 11.
FIG. 11.

Plots of a 23° beam propagating into the (a) , (b) , and (c) ignition target. The beam is “channeled” between the high-density ablator plasma and the hohlraum wall. At , the total reflectivity is . At , the total reflectivity is . At , the total reflectivity is .

Image of FIG. 12.
FIG. 12.

A cross section in azimuth (at ) of the reflected (a) SBS and (b) SRS light for the beam propagation simulation. As predicted in gain analyses, SBS occurs on the capsule side of the beam and SRS occurs on both sides. SRS light amplified in gas fill plasma exits the plasma at because of refraction. SRS light amplified in Be ablator plasma exits the plasma at , also because of refraction. At lower radiation temperature, SRS occurs only in the gas fill plasma.

Image of FIG. 13.
FIG. 13.

A plot of the angular distribution of SBS reflected light across the suite of designs: (a) , (b) , and (c) . The four white squares in each plot indicate the location of the lenses, i.e., where the reflected light would return if it were directly backscattered. The SBS reflected light returns in the upper half of the quad, indicating that SBS is occurring on the capsule side of the beam cross section. (The upper half of the quad is slightly angled toward the ablator plasma.)

Image of FIG. 14.
FIG. 14.

A plot of the angular distribution of SRS reflected light across the suite of designs: (a) , (b) , and (c) . The four white squares in each plot indicate the location of the lenses, i.e., where the reflected light would return if it were directly backscattered. In all three designs, the SRS light is primarily outside the lenses because it is more strongly refracted than the incident light. At , there is SRS light in both halves of the quad, consistent with SRS occurring on both sides of the beam. At 285 and [(b) and (c)], SRS light returns only in the lower portion of the quad, consistent with SRS occurring only in gas-fill plasma (wall side of the beam).

Image of FIG. 15.
FIG. 15.

Beam transmission is analyzed by calculating the flux of incident light across isocontours of . (a) A plot of the electron density near peak power for the design, with the extent of the beam propagation simulation box shown in white, and a few representative density isocontours. (b) A plot of the transmission vs for the design. The dashed line represents the transmission from the radiation-hydrodynamics simulation, reduced from unity because of inverse bremsstrahlung along the beam path. The solid line represents the pF3D transmission, which additionally includes the effect of backscatter. Internal reflectivity larger than the measured reflectivity impairs beam propagation in this design, but should be less of a factor in the 285 and designs.

Image of FIG. 16.
FIG. 16.

A plot of the electron plasma (red) and ion acoustic wave density fluctuations (blue) for the ignition design after reflectivity has reached a quasisteady state. The solid curves depict the intensity-weighted density fluctuations on each transverse plane, with a maximum value . Also plotted (dashed) curves are the maximum density fluctuations achieved on each plane. Typically, only a few cells out of more than 1.5 million on each transverse plane reach a value near this maximum value.

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2008-04-29
2014-04-21
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Analyses of laser-plasma interactions in National Ignition Facility ignition targetsa)
http://aip.metastore.ingenta.com/content/aip/journal/pop/15/5/10.1063/1.2901127
10.1063/1.2901127
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