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Numerical simulation of wave-particle interaction in laser plasmas

The propagation of intense laser beam through a plasma ablated from a solid target by a focused ns laser beam gives rise to a high-amplitude electron plasma wave, which has the ability to trap and accelerate significant amount of plasma electrons. This can be caused by many physical processes, which laser beam undergoes in the plasma corona. A very important representative of a such process is the stimulated Raman scattering, which can be characterized as a resonant decay of the incident laser light into a scattered electromagnetic wave and into the electrostatic wave. A good understanding of this mechanism is very important for the control of numerous experimental situations including the thermonuclear fusion experiments, particularly in the indirect drive, where the laser beam after entering the hohlraum faces a developed and only weakly inhomogeneous plasma inside [1]. To avoid high energy loses caused by the Raman backscattering, which is dominant process in the well underdense plasma typically occurring near the light entrance holes, carrying significant amount of energy out of the hohlraum. To achieve a high yield of x-ray radiation from the walls of the hohlraum, which is necessary for an efficient compression of fuel capsule, it is essential to set up the experiment with the values of Raman reflectivity kept within reasonable limits. Our recent simulations based on the spatially 1D periodical kinetic model solving the Vlasov equation together with the full set of Maxwell’s equations [2] showed that the growth rate of Raman backscattering can be significantly decreased, when the scattered electromagnetic wave is scattered once again (Raman cascading), which in turn results in the value of Raman reflectivity around 1% [3]. This can occur in the outer part of the laser corona (ne ≤ ncrit/9), where due to the Raman instability the electrostatic wave with a low phase velocity lying well within the bulk of plasma electrons in the phase space rises. The propagation of such wave through the plasma causes very strong non-linear interaction with the electrons. The electrons from the body of distribution are in the first stage of the instability development mostly accelerated forming a huge plateau on the particle distribution. During this process the significant amount of incoming laser energy is deposited in the plasma electrons. Simultaneously, the particle trapping in the potential wells of the plasma wave leads to a growth of the trapped particle instability [4] and consequently to the rise of sidebands of the central electrostatic wave mode, which is finally outgrown. The interaction of preaccelerated electrons with this wave packet causes a randomization of their velocity distribution even in the absence of collisions. The incoming energy is thus used for plasma heating, which in turn stops the growth of the Raman instability and lowers the Raman reflectivity [5]. However, in this contribution, which is aiming at a comparison between the hydrodynamic envelope and the kinetic Vlasov model we avoided the spectral broadening due to the sidebands by suitably tuning the simulation conditions.