Nonlinear transmission of laser light through coronal plasma due to self-induced incoherence

The success of direct laser-driven inertial confinement fusion (ICF) relies critically on the efficient coupling of laser light to plasma. At ignition scale, the absolute stimulated Raman scattering (SRS) instability can severely inhibit this coupling by redirecting and strongly depleting laser light. This article describes a new dynamic saturation regime of the absolute SRS instability near one-quarter of the critical density. The saturation occurs when spatiotemporal ion-acoustic fluctuations in the plasma density detune the instability resonance. The dynamic saturation mitigates the strong depletion of laser light and enhances its transmission through the instability region, explaining the coupling of laser light to ICF targets at higher plasma densities.

Figure 1

The amplitudes of laser and Raman light waves at 5.4 ps [(a) and (c), respectively] and 8.4 ps [(b) and (d), respectively]. The amplitudes of (e) plasma waves (in terms of wave potential energy normalized to temperature) and (f) low-frequency density perturbations at 8.4 ps. The vacuum laser wavelength is $0.351\mu \text{m}$.

Figure 2

The scaling of light transmission at the end of the pump depletion stage (red dark symbols) and during the dynamic saturation stage (blue light symbols) as a function of the incident laser intensity, for CH (circles) and Be (squares). Below threshold ($I_0<6\times {10}^{13}{\mathrm{W}/\mathrm{cm}}^2$) neither the SRS instability nor its dynamic saturation occur, so no red dark symbol is plotted.

Figure 3

(a) Transmitted Poynting flux as a function of time and transverse coordinate $y$ for the laser intensity $2\times {10}^{14}{\mathrm{W}/\mathrm{cm}}^2$, and (b) the frequency spectra of transmitted light for three incident laser intensities of ${10}^{14}{\mathrm{W}/\mathrm{cm}}^2$ (solid blue line), $2\times {10}^{14}{\mathrm{W}/\mathrm{cm}}^2$ (dashed green line), and $3\times {10}^{14}{\mathrm{W}/\mathrm{cm}}^2$ (dash-dotted red line).

Figure 4

The spectra of (a) laser light waves, (b) Raman light waves, (c) plasma waves, and (d) low-frequency density perturbations in the dynamic saturation stage (at 8.4 ps) for the parameters of Fig. 1.