Abstract
Absolute instability modes due to secondary scattering of stimulated Raman scattering (SRS) in a large nonuniform plasma are studied theoretically and numerically. The backscattered light of convective SRS can be considered as a pump light with a finite bandwidth. The different frequency components of the backscattered light can be coupled to develop absolute SRS instability near their quarter-critical densities via rescattering process. The absolute SRS mode develops a Langmuir wave with a high phase velocity of about $c/\sqrt{3}$ with $c$ the light speed in vacuum. Given that most electrons are at low velocities in the linear stage, the absolute SRS mode grows with very weak Landau damping. When the interaction evolves into the nonlinear regime, the Langmuir wave can heat abundant electrons up to a few hundred keV via the SRS rescattering. Our theoretical model is validated by particle-in-cell simulations. The absolute instabilities may play a considerable role in the experiments of inertial confinement fusion.
Highlights
Stimulated Raman scattering (SRS), the decay of incident laser into scattered light and electron plasma wave[1,2,3], remains to be one of the major obstacles to direct-drive, indirect-drive and possible hybrid-drive schemes[4,5,6,7,8,9]
We present a mechanism of hot electron production via the absolute instabilities in a large nonuniform plasma due to rescattering of SRS
Our work suggests that the absolute instability modes induced via rescattering of SRS are the important mechanisms for hot electron production for long time interactions between laser and large scale inhomogeneous plasma, even if the maximum plasma density is less than 0.2nc
Summary
Stimulated Raman scattering (SRS), the decay of incident laser into scattered light and electron plasma wave[1,2,3], remains to be one of the major obstacles to direct-drive, indirect-drive and possible hybrid-drive schemes[4,5,6,7,8,9]. For the given plasma electron density shown, the secondary backward SRS is developed in [0.01, 0.0625]nc, which can induce the third-order absolute instabilities in [0.034, 0.16]nc. The linear analysis suggests that a laser propagating in large inhomogeneous plasma can generate a large region of absolute instabilities by cascaded scattering and coupling of different frequency components of scattering light Both the second- and third-order rescattering of SRS contributes to the development of absolute instabilities. Based upon the above two simulations as well as the simulation with density range [0.08, 0.1]nc, we calculate the energy ratio of electrons heated by absolute SRS to the whole electrons with energy >60 keV when the electron temperatures are saturated, and the result is ∼30% This calculation indicates that absolute instabilities in inhomogeneous plasma have a crucial effect on the production of hot electrons. This further proves that the absolute instabilities induced via SRS rescattering do contribute significantly to the hot electron productions
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