The development of parametric instabilities in a large scale inhomogeneous plasma with an incident laser beam composed of multiple frequency components is studied theoretically and numerically. Firstly, theoretical analyses of the coupling between two laser beamlets with certain frequency difference δω0 for parametric instabilities is presented. It suggests that the two beamlets will be decoupled when δω0 is larger than certain thresholds, which are derived for stimulated Raman scattering, stimulated Brillouin scattering, and two plasmon decay, respectively. In this case, the parametric instabilities for the two beamlets develop independently and can be controlled at a low level provided the laser intensity for individual beamlet is low enough. Secondly, numerical simulations of parametric instabilities with two or more beamlets (N ∼ 20) have been carried out and the above theory model is validated. Simulations confirm that the development of parametric instabilities with multiple beamlets can be controlled at a low level, provided the threshold conditions for δω0 is satisfied, even though the total laser intensity is as high as ∼1015 W cm−2. With such a laser beam structure of multiple frequency components and total bandwidth of a few percentages (≳4%ω0), the parametric instabilities can be well-controlled.
多频激光入射大尺度不均匀等离子体中参数不稳定性的发展，在理论上和数值上进行了研究。首先，针对具有一定频率差δω0的两束子激光在参数不稳定性中的耦合，进行了理论分析。结果表明，当δω0大于某一阈值时，两束子激光将解耦，该阈值分别针对受激拉曼散射、受激布里渊散射和双等离子体衰变推导得出。在这种情况下，两束子激光的参数不稳定性独立发展，并且只要单束子激光的强度足够低，就可以将其控制在较低水平。其次，针对两束及以上子激光（N ∼ 20）的参数不稳定性进行了数值模拟，并验证了上述理论模型。模拟结果证实，即使总激光强度高达 ∼10^15 W cm^−2，只要满足δω0的阈值条件，多束子激光的参数不稳定性仍可被控制在较低水平。采用这种多频率分量、总带宽为百分之几（≳4%ω0）的激光结构，可以很好地控制参数不稳定性。