Investigation of Inverse Bremsstrahlung Heating Driven by Broadband Lasers

TL;DR

This work addresses whether spectral broadbanding of lasers used in inertial confinement fusion alters collisional inverse bremsstrahlung (IB) heating in subcritical Au plasmas. The authors employ 1D3V PIC simulations with a Nanbu collision model to isolate bandwidth effects while keeping the mean intensity and plasma parameters fixed, and they validate the IB absorption against classical Beer–Lambert theory. The key finding is that bandwidth-induced temporal incoherence produces transient ps-scale oscillations in the heating rate $dT_e/dt$, but the long-term averaged heating and net IB absorption are essentially unchanged, with the electron temperature saturating near $T_e \approx 1.25$ keV. This implies that, for radiation-hydrodynamic simulations on timescales of hundreds of picoseconds or more, broadband illumination can be treated as an equivalent monochromatic source with the same mean intensity, simplifying modeling and informing ICF design.

Abstract

Broadband lasers have become a key strategy for mitigating laser plasma instabilities in inertial confinement fusion, yet their impact on collisional inverse bremsstrahlung (IB) heating remains unclear. Using one-dimensional collisional particle-in-cell simulations, we systematically examine the effect of bandwidth-induced temporal incoherence on IB absorption in Au plasmas. The simulations are first benchmarked against classical absorption theory, verifying that the implemented Coulomb collision model accurately reproduces the theoretical IB heating rate. A direct comparison of the electron temperature evolution in the broadband and monochromatic cases shows that, although spectral broadening introduces transient picosecond-scale oscillations in the heating rate driven by stochastic intensity fluctuations, the long-term averaged heating and net IB absorption remain essentially unchanged.

Figures (3)

• Figure 1: Comparison between the simulated and theoretical inverse bremsstrahlung (IB) absorption. The simulated absorption $A_{\mathrm{PIC}}$ obtained from EPOCH is plotted against the theoretical prediction $A_{\mathrm{th}}$ as a function of the electron--ion collision frequency $\nu_{ei}$.
• Figure 2: Temporal evolution of the instantaneous and averaged $E^2$ for broadband ($E_b$) and monochromatic ($E_m$) lasers at equal mean intensity, illustrating how different averaging windows ($10\tau_c$ and $50\tau_c$) smooth the broadband intensity fluctuations.
• Figure 3: Temporal evolution of (a) the electron temperature $T_e$, (b) its temporal growth rate $dT_e/dt$, and (c) the difference in heating rate between broadband and monochromatic cases $\Delta(dT_e/dt)$ compared with the normalized laser intensity variation $(E^2_{\mathrm{b}}-\langle E_\mathrm{m}^2\rangle)/\langle E_\mathrm{m}^2\rangle$. Here $T_e$ is evaluated every $\Delta t = 10^4/\omega_0 \approx 2.8\,\mathrm{ps}$, $(E^2_{\mathrm{b}}-\langle E_\mathrm{m}^2\rangle)/\langle E_\mathrm{m}^2\rangle$ is estimated with the same time window.