Mitigation of cross-beam energy transfer in inertial-confinement-fusion plasmas with enhanced laser bandwidth

Cross-beam energy transfer (CBET) is a significant energy-loss mechanism in directly driven inertial-confinement-fusion (ICF) targets. One strategy for mitigating CBET is to increase the bandwidth of the laser light, thereby disrupting the resonant three-wave interactions that underlie this nonlinear scattering process. Here, we report on numerical simulations performed with the wave-based code lpse that show a significant reduction in CBET for bandwidths of $2–5$ THz (corresponding to a normalized bandwidth of $0.2\%–0.6\%$ at a laser wavelength of $351\mathrm{nm}$) under realistic plasma conditions. Such bandwidths are beyond those available with current high-energy lasers used for ICF, but could be achieved using stimulated rotation Raman scattering in diatomic gases like nitrogen.

Figure 1

The backscatter mode of CBET in direct-drive ICF. The yellow arrow denotes the direction of laser energy exchange.

Figure 2

Results from lpse showing (a) the electric-field magnitude and (b) the absolute value of the electron-number-density perturbation at $4\mathrm{ps}$. In this simulation, two laser beams (red arrows) with equal intensity $I=1.5\times {10}^{15}\text{W}/{\text{cm}}^2$ cross at ${45}^{\circ }$ in an expanding, supersonic CH plasma with $T_e=3\text{keV}$ and $T_i=1\text{keV}$. Resonance occurs near the crossing point where $M\simeq 1.1$ and $\left|\delta n/n_0\right|\lesssim 4\%$. Approximately $79\%$ of the laser power is transferred, as indicated by the yellow and red regions of the diagonal beam in (a).

Figure 3

Plots of the laser-power amplification due to CBET as a function of average peak intensity and bandwidth for the laser and plasma configuration shown in Fig. 2. The symbols denote results from wave-based lpse simulations, while the dashed lines are predictions based on the steady-state theory described in the text. The inset shows the discrete multiline model of laser bandwidth used in this study for the case $\mathrm{\Delta }\nu =2\mathrm{THz}$.