Interplay of Laser-Plasma Interactions and Inertial Fusion Hydrodynamics

The effects of laser-plasma interactions (LPI) on the dynamics of inertial confinement fusion hohlraums are investigated via a new approach that self-consistently couples reduced LPI models into radiation-hydrodynamics numerical codes. The interplay between hydrodynamics and LPI—specifically stimulated Raman scatter and crossed-beam energy transfer (CBET)—mostly occurs via momentum and energy deposition into Langmuir and ion acoustic waves. This spatially redistributes energy coupling to the target, which affects the background plasma conditions and thus, modifies laser propagation. This model shows reduced CBET and significant laser energy depletion by Langmuir waves, which reduce the discrepancy between modeling and data from hohlraum experiments on wall x-ray emission and capsule implosion shape.

Figure 1

Sketch of hohlraum LPI. Arrows give wave propagation direction, and color darkness indicates intensity. Outer beams transfer power to inner beams where they overlap. SRS light from inner beams grows continuously along path, with little absorption. Langmuir waves are driven by beating of inner-beam laser and SRS light.

Figure 2

Left: measured 30° cone SRS spectrum for NIF shot N121130, with cone incident (white) and escaping SRS (red) powers. Right: SRS gain exponent spectrum from Lasnex simulation with inline SRS model (magenta: wavelength of maximum gain).

Figure 3

Energetics of Lasnex simulations with (a) inline SRS model and (b) SRS removed at lens. “Outer post CBET”: incident outer-beam energy not transferred to inners. “Inner transmitted”: incident inner energy minus energy to SRS channels. Inner SRS: escaping inner SRS light. SRS abs: SRS light absorbed. Langmuir: energy to LWs. Energies from 10.5 ns (start of SRS) to 14.8 ns (end of laser pulse), given as percent of incident laser (1120 kJ).

Figure 4

Synthetic x-ray images for Lasnex simulations with inline SRS model (left) and SRS at lens model (right). Images are symmetrized in azimuth like the simulations. Reduced CBET to inner beams with the inline SRS model gives brighter outer-beam spots. Detector in NIF lower hemisphere 19° to hohlraum axis. X-ray emission integrated over all time and energies 3 to 5 keV. $y$ is roughly parallel to hohlraum axis.

Figure 5

Spatial profiles of power densities [$\mathrm{W}/{\mathrm{cm}}^3$] from Lasnex simulations at 12.6 ns (time of peak escaping SRS). All panels are from SRS-inline model, except top of (d), which is from SRS-at-lens model. (a) laser absorption, (b) LW deposition, (c) SRS light absorption, (d) total deposition in SRS-at-lens model (top) and SRS-inline model (bottom). (e) SRS light power density, white ellipse indicates seed points where SRS originates. All panels except (e) use same logarithmic colormap. Dashed magenta contours are helium gas boundaries.

Figure 6

Electron temperature [keV] at 12.6 ns from Lasnex simulations shown in Fig. 3. $r>0$ has SRS removed at lens; $r<0$ uses inline SRS model with LW deposition and is significantly hotter around the entrance hole. Magenta contours are fill gas boundaries as in Fig. 5. White dashed contours are $n_e/n_{cr}=0.25$.

Figure 7

$P_2$ moment of x-ray deposition at ablation front, as a fraction of total deposition $P_0$, for x-ray energies 0.5–2 keV. $P_2<0$ for stronger drive from the equator than the pole. For the inline SRS model, reduced CBET to the inner beams and depletion by LWs both reduce the laser intensity at the equator, which makes $P_2$ less negative.