Sunlight-like lasers that have a continuous broad frequency spectrum, random phase spectrum, and random polarization are formulated theoretically. With a sunlight-like laser beam consisting of a sequence of temporal speckles, the resonant three-wave coupling that underlies parametric instabilities in laser–plasma interactions can be greatly degraded owing to the limited duration of each speckle and the frequency shift between two adjacent speckles. The wave coupling can be further weakened by the random polarization of such beams. Numerical simulations demonstrate that the intensity threshold of stimulated Raman scattering in homogeneous plasmas can be doubled by using a sunlight-like laser beam with a relative bandwidth of ∼1% as compared with a monochromatic laser beam. Consequently, the hot-electron generation harmful to inertial confinement fusion can be effectively controlled by using sunlight-like laser drivers. Such drivers may be realized in the next generation of broadband lasers by combining two or more broadband beams with independent phase spectra or by applying polarization smoothing to a single broadband beam.

Two nonlinear theoretical models are presented to describe the time evolution of a plasma density grating induced by intersecting high power laser beams. The first model is based on the fluid equations, while the second is a kinetic model that adopts the particle mesh method. It is found that both models can describe the plasma density grating formation at different stages, well beyond the linear growth stage. However, the saturation of the plasma density grating, which is attributed to the kinetic effect of "ion wave-breaking", can only be predicted by the second model based on the particle mesh method. Using the second model, we also find that the saturation time of the plasma density grating increases with the plasma density and decreases with the laser intensity. The results from these two nonlinear theoretical models are compared and verified using particle-in-cell simulations.

A physical model is presented for the study of parametric instabilities in inertial confinement fusion (ICF), which considers the coupling of the incident and scattered electromagnetic waves with plasma electrons and ions. Specially, this model is solved numerically with the particle-mesh method, where the plasma is represented by macro-particles both for electrons and ions, and the velocity and position of each macro-particle are numerically updated by using the particle-mesh method. The developed particle-mesh code in one-dimensional geometry (PM1D) is utilized to study the development of parametric instabilities at the nonlinear stages, where electron plasma wave breaking, particle trapping, hot electron generation and density cavity formation can occur. Particle-in-cell (PIC) simulations are carried out to verify this PM1D code. By comparison, it is found that this PM1D code is able to capture the kinetic effects and precisely describe the developments of parametric instabilities at nonlinear stages as the PIC simulations while saving the computation time obviously. Furthermore, a test simulation of the stimulated Raman scattering evolution up to 200 ps verifies the robustness of this PM1D code.

Electromagnetic emission via linear mode conversion from electron plasma waves (EPWs) excited by stimulated Raman scattering (SRS) of an incident laser pulse in inhomogeneous plasma is investigated theoretically and numerically. It is found that the mode conversion can occur naturally in underdense plasma region below the quarter critical density provided that EPWs are generated due to the development of backward SRS when the laser pulse is incident at certain angle with the plasma density gradient. The produced radiation may cover a broad frequency range up to half of the incident laser frequency. The dependence of the radiation conversion efficiency on the laser intensity, incident angle, laser pulse duration, plasma density scale length, and initial electron temperature is analyzed based on one-dimensional particle-in-cell simulation. In two-dimensional geometry, due to the development of sideward SRS, it is found that the mode conversion to occur even at normal incidence of the laser pulse. The radiation frequency, bandwidth, duration, and amplitude can be well controlled by the laser and plasma parameters, suggesting that it may provide a new source of tunable broadband radiation as well as a diagnosis of the development of SRS.