Investigation of Stimulated Brillouin Scattering Driven by Broadband Lasers in High-Z Plasmas

TL;DR

This work addresses stimulated Brillouin scattering (SBS) in high-Z plasmas under broadband laser illumination relevant to indirect-drive ICF. Using 1D3V PIC simulations with collisional Nanbu dynamics and a multi-frequency beamlet model, the study maps how laser bandwidth and intensity shape SBS growth, saturation, and suppression in Au and AuB plasmas. The key finding is that broadband lasers reduce the SBS growth rate by decoupling the pump from the three-wave resonance, but saturation levels remain similar until the bandwidth exceeds a critical threshold; when normalized by the monochromatic growth rate, the suppression follows a universal scaling with the ratio $Δω/γ_0$ (threshold ≈ 70 in the studied regime). The results provide practical guidance for designing broadband laser drivers to minimize SBS backscatter and improve energy coupling in indirect-drive ICF, with AuB plasmas showing earlier onset of suppression due to enhanced ion-acoustic damping.

Abstract

The evolution of stimulated Brillouin scattering (SBS) driven by broadband lasers in high-Z plasmas is investigated using one-dimensional collisional particle-in-cell simulations. The temporal incoherence of broadband lasers modulates the pump intensity, generating stochastic intensity pulses that intermittently drive SBS. The shortened coherence time weakens the three-wave coupling and continuously reduces the temporal growth rate, while the saturated reflectivity remains nearly unchanged until the bandwidth exceeds a critical threshold. Simulations with varying laser intensities and bandwidths reveal a consistent scaling behavior, indicating that effective suppression occurs only when the laser bandwidth exceeds the temporal growth rate of SBS by several tens of times. Comparative simulations in Au and AuB plasmas exhibit similar suppression trends, with AuB showing reduced SBS growth rate and reflectivity, and the onset of suppression occurring at a lower bandwidth. These findings elucidate the coupled dependence of SBS mitigation on bandwidth and laser intensity in high-Z plasmas, offering useful guidance for optimizing broadband laser designs in inertial confinement fusion.

Figures (6)

• Figure 1: Temporal and spectral characteristics of the incident lasers with different bandwidths. The left panels show the time-dependent electric field profiles for bandwidths of 0 %, 0.3%, 0.6%, 1.2%, and 2.4% relative to the central frequency. The right panels present the corresponding power spectra obtained by Fourier transform, confirming that the broadband laser fields used in the simulations reproduce the designed bandwidth.
• Figure 2: Comparison of the temporal evolution of SBS reflectivity and the corresponding laser intensity for (a) a monochromatic laser and (b) a broadband laser with $\Delta\omega / \omega_0 = 0.6\%$ in Au plasmas at a laser intensity of $I = 1\times10^{15}\,\mathrm{W/cm^2}$.
• Figure 3: Dependence of SBS reflectivity and growth rate on laser bandwidth in Au plasmas at a fixed laser intensity of $I = 1\times10^{15}\,\mathrm{W/cm^2}$. (a) Temporal evolution of SBS reflectivity for different bandwidths. (b) Time-averaged SBS reflectivity $R$ and (c) temporal growth rate $\gamma/\omega_0$ as functions of laser bandwidth $\Delta\omega/\omega_0$.
• Figure 4: Temporal evolution of SBS reflectivity in Au plasmas for different laser bandwidths and incident laser intensities.
• Figure 5: Dependence of normalized SBS reflectivity $R/R_0$ on the scaled bandwidth $\Delta\omega/\gamma_0$ for Au and AuB plasmas at different laser intensities. Here, $R_0$ and $\gamma_0$ denote the saturated reflectivity and the temporal growth rate of SBS under monochromatic conditions, respectively, with each intensity having its own corresponding $R_0$ and $\gamma_0$. The symbols $5I_{14}$, $I_{15}$, $2I_{15}$, and $4I_{15}$ correspond to incident laser intensity of $5\times10^{14}$, $1\times10^{15}$, $2\times10^{15}$, and $4\times10^{15}\,\mathrm{W/cm^2}$, respectively.