Slowly varying envelope kinetic simulations of pulse amplification by Raman backscattering

Abstract

A numerical code based on an eikonal formalism has been developed to simulate laser-plasma interactions, specifically Raman backscatter (RBS). In this code, the dominant laser modes are described by their wave envelopes, avoiding the need to resolve the laser frequency; appropriately time-averaged equations describe particle motion. The code is fully kinetic, and thus includes critical physics such as particle trapping and Landau damping which are beyond the scope of the commonly used fluid three-wave equations. The dominant forces on the particles are included: the ponderomotive force resulting from the beat wave of the forward and backscattered laser fields and the self-consistent plasma electric field. The code agrees well, in the appropriate regimes, with the results from three-wave equations and particle-in-cell simulations. The effects of plasma temperature on RBS amplification are studied. It is found that increasing the plasma temperature results in modification to particle trapping and the saturation of RBS, even before the onset of Landau damping of the plasma wave. This results in a reduction in the coupling efficiency compared to predictions based on the three-wave equations.

Nonlinear kinetic modeling of stimulated Raman scattering in a multidimensional geometry

“…and is thus negligible in Eq. (A.5), whose left-hand side may therefore be approximated by −κ 2 Ẽy,z ≈ −κ 2…”

“…Ref. [2]), which they used in Ref. [3] to show that Raman reflectivity could reach much higher levels than linear theory would predict.…”

Raman backscattering saturation due to coupling betweenωpand 2ωpmodes in plasma

“…To demonstrate how the presence of plasma wave harmonics leads to mode coupling nonlinearities as predicted by the set of equations (3), we first present 1D simulations using the code aPIC [35], where an infinitely long pump beam interacts with a short duration counter-propagating Gaussian seed pulse with full width at half maximum (FWHM) duration of 20 fs. It should be noted that the initial short seed pulse will broaden as it gets Raman amplified [1,3,5], leading to seed FWHM durations significantly larger than the plasma wave period.…”

Pulse width effects on Raman backward laser amplification

“…The novel scheme of laser pulse amplification using Raman backscatter (RBS) in a plasma, which is called Raman backward amplification (RBA), has been arousing much theoretical [1][2][3][4][5][6][7] and experimental [8][9][10] interest due to its possibility of replacing the conventional chirped pulse amplification (CPA) technique. In the CPA systems, the maximum output intensity of the laser pulse is limited by the damage threshold of the gratings.…”