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Trapping induced nonlinear behavior of backward stimulated Raman scattering in multi-speckled laser beamsa)
a)Paper KI3 3, Bull. Am. Phys. Soc. 56, 180 (2011).
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Image of FIG. 1.
FIG. 1.

Upper left: SRS reflectivity vs. laser peak intensity scaling for an isolated F/4 speckle showing a sharp onset of nonlinear SRS at a threshold laser intensity ( W/cm2) and saturation at higher intensity. Lower left: Illustration of the simulation (in the right panel) which includes a nonlinear, strong speckle above at peak intensity W/cm2 and a neighboring weak speckle below at W/cm2. Right panel: 3D simulation of two-speckle interaction shows that high intensity speckle can cause nonlinear SRS onset in a nearby speckle, whose intensity is below onset threshold if isolated. Shown are isosurfaces of electrostatic field of EPW with color indicating light wave field. Inset: illustration of the nonlinear speckles above the threshold intensity populating the tail of the intensity distribution in an ensemble of speckles (enhanced online). [URL: http://dx.doi.org/10.1063/1.3694673.1]10.1063/1.3694673.1

Image of FIG. 2.
FIG. 2.

Left: an example of multi-speckled laser pump field for simulation in domain m. Upper middle: time averaged SRS reflectivity vs. laser average intensity scaling for collections of speckles in domains m (black diamonds) and m (black triangles) using density and keV (). Lower middle and lower right: instantaneous reflectivity for the m simulations at average laser intensity 1.2 and W/cm2, respectively. Upper right: time averaged SRS reflectivity scaling vs. laser average intensity using different number of particles per cell (nppc): For simulations with size m (m), black and red triangles (black and red diamonds) are from simulations using 65 and 512 particles per cell, respectively; the blue squares are from simulations with size m using 65 particles per cell.

Image of FIG. 3.
FIG. 3.

VPIC simulations of the NIC 1 MJ experiment using keV and an average laser intensity of W/cm2 and including large scale density variations around 12% as obtained from radiation hydrodynamic simulation. Top: the plasma density variations and the VPIC simulation regions in relation to that used in pF3D modeling of the experiment. Middle frames: instantaneous reflectivity (left) showing sub-ps bursts of enhanced SRS and time averaged reflectivities (right) which saturate at levels between 30% and 40%. Lower: frequency spectrum from Run 2 (in black) measured at the laser entrance boundary and the spectrum from a simulation using 12% uniform density (in red) for comparison.

Image of FIG. 4.
FIG. 4.

Left: time averaged reflectivity scaling with the simulation length for widths of , 80, and 160 m, in black, blue, and red, respectively, at average intensity W/cm2. Middle: time averaged reflectivity scaling with simulation width for a simulation length of m at average intensities W/cm2 (in black) and W/cm2 (in red). Right: the Landau damping rate () calculated at the location of the strong speckles, spatially averaged over and time-averaged over the simulation time (13.4 ps), for a m simulation at average intensity W/cm2 (for which the reflectivity is shown in blue in Fig. 2).

Image of FIG. 5.
FIG. 5.

Top: SRS hot electron flux (left) from the side-loss hot electrons (indicated by the side boundaries in black color close to the laser entrance boundary, shown in the schematic on the right) and from forward hot electrons (indicated by the side boundaries in red color and the right boundary in blue color; the flux of the Maxwellian background is shown by the dotted curve as comparison) from a simulation with size m at average intensity 2.9 W/cm2. Lower frames: side-loss hot electron flux scaling with laser average intensity (left) and the percentage of laser beam energy going into hot electrons with energy above a certain value (right) at average intensity 1.2, 2.9, and 5.8 W/cm2. See Fig. 2 for the corresponding reflectivities from these simulations.

Image of FIG. 6.
FIG. 6.

Trapping induced nonlinear wave structures in multi-speckled laser beams. Top and middle: the EPW bowing occurring in two speckles centered around z = 14 and 23 m and filamentation occurring within one speckle width (EPW propagates in the x direction) from simulations with size of m at average intensity 2.9 and 5.8 W/cm2, respectively. Lower: the field containing both laser and the scattered light wavefronts distinguished by their different wavelengths (as labeled): and, for the laser, (from simulations with size m at average intensity 2.9 W/cm2). SRS backscatter light wave bowing occurring in multiple speckles with two strong speckles centered around z = 14 and 23 m (backscattered light propagates in the −x direction).

Image of FIG. 7.
FIG. 7.

Time-averaged SRS reflectivity scaling with from 2D single-speckle simulations18 (indicated by the circles) and a fit (the dashed curve).



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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Trapping induced nonlinear behavior of backward stimulated Raman scattering in multi-speckled laser beamsa)