Abstract
Processes of laser energy absorption and electron heating in an expanding plasma in the range of irradiances $I\unicode[STIX]{x1D706}^{2}=10^{15}{-}10^{16}~\text{W}\,\cdot \,\unicode[STIX]{x03BC}\text{m}^{2}/\text{cm}^{2}$ are studied with the aid of kinetic simulations. The results show a strong reflection due to stimulated Brillouin scattering and a significant collisionless absorption related to stimulated Raman scattering near and below the quarter critical density. Also presented are parametric decay instability and resonant excitation of plasma waves near the critical density. All these processes result in the excitation of high-amplitude electron plasma waves and electron acceleration. The spectrum of scattered radiation is significantly modified by secondary parametric processes, which provide information on the spatial localization of nonlinear absorption and hot electron characteristics. The considered domain of laser and plasma parameters is relevant for the shock ignition scheme of inertial confinement fusion.
Highlights
Laser energy absorption in plasma shows two opposite trends, depending on the laser intensity
The results show a strong reflection due to stimulated Brillouin scattering and a significant collisionless absorption related to stimulated Raman scattering near and below the quarter critical density
The obliquely propagating stimulated Raman scattering (SRS) daughter waves cannot escape the plasma because of their small wavenumber and large amplitude density fluctuations. They are strongly coupled to electron plasma waves and drive a secondary parametric decay instability (PDI) which corresponds to excitation of pairs of electron plasma and ion acoustic waves with equal and oppositely directed wavevectors
Summary
Laser energy absorption in plasma shows two opposite trends, depending on the laser intensity. In future experiments with more intense laser pulses and longer wavelengths, nonlinear and collisionless processes will certainly be important. This domain of laser and plasma parameters is of interest for various applications of high-power lasers, in particular, the shock ignition scheme of inertial confinement fusion targets[7]. This paper presents results of laser–plasma interaction studies in the domain of high irradiances I λ2 = 1015–1016 W · μm2/cm and at normal incidence of the laser on a planar inhomogeneous expanding plasma. It is demonstrated that SRS near the quarter critical density is the dominant process responsible for the laser energy absorption, while stimulated Brillouin scattering (SBS), operating in lower-density plasma layers, creates a significant amount of reflected light.
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