Influence of binary Coulomb collisions on nonlinear stimulated Raman backscatter in the kinetic regime

Abstract

The influence of binary Coulomb collisions on trapped particle nonlinearities related to stimulated Raman scatter ͑SRS͒ in a single laser speckle is examined using one-dimensional particle-in-cell simulations. Binary Coulomb collisions are incorporated using a numerical particle-pairing algorithm that reproduces a collision integral of the Landau form. The onset of nonlinearly enhanced levels of SRS reflectivity is shown to coincide with electron trapping in the daughter plasma wave and is sensitive to the collisional scattering rate. Relaxation of trapping-induced perturbations to the electron velocity distribution via collisional velocity space diffusion is predicted to have the largest effect on the onset of SRS when the amplitude of the daughter plasma wave is smallest, and trapping-induced perturbations to the electron velocity distribution function are also small. In the absence of higher dimensional detrapping mechanisms ͑e.g., electron side-loss͒, it is shown that the onset threshold for enhanced SRS reflectivity is determined predominantly by the parallel diffusion of trapped electrons scattering from bulk thermal electrons, and that for the conditions studied here, the contribution to detrapping from perpendicular diffusion is non-negligible. Additionally, inverse bremsstrahlung heating of the bulk electrons is shown to shift the daughter plasma wave spectrum upward along the Stoke's resonance to larger wave frequency and smaller wave number, changing the linear parametric coupling conditions to SRS backscatter as a function of time. The reduction in SRS reflectivity from binary Coulomb collisions is greatest for laser intensities near the collisionless onset threshold, ulimately leading to an increase in the onset threshold laser intensity for enhanced SRS reflectivity in the kinetic regime.

Contemporary particle-in-cell approach to laser-plasma modelling

“…These SRS results have also been confirmed by Rousseaux et al [64] and Mason-Laborde et al [65] who also considered the role of the Weibel instability on the trapped particle current. Collisions have also been shown in 1D simulations [66] to increase the threshold for SRS by detrapping electrons from weak EPWs and thereby increasing the Landau damping rate.…”

Trapping induced nonlinear behavior of backward stimulated Raman scattering in multi-speckled laser beams

“…It was found in 1D, in the absence of side loss, that collisional de-trapping has the largest effect on the onset of SRS when the amplitude of the daughter plasma wave is smallest 45 (when the trapping modification to the electron velocity distribution function is small). Since collision rates scale (classically) as T À3=2 e and are only a tiny correction to the dynamics at these plasma conditions, collisional effects on de-trapping are not included in this work.…”

“…As shown, near the onset threshold, suppressing particle noise leads to decreased onset threshold and increased SRS level in ensembles of speckles (as shown in single-speckle simulations 10,18,33 ). In the saturated regime at intensities well above the onset threshold, however, the side-loss rate ($v e th divided by EPW width in the presence of bowing and filamentation) dominates over that of electron de-trapping from particle noise and binary Coulomb collisions, and thus, SRS does not depend sensitively on either the number of simulation particles or physical collisions 20,45 ).…”

Fluid modeling of stimulated Raman scattering accounting for trapped particles benchmarked against fully kinetic simulations

“…In particular, Vu et al 4 established the so-called kinetic inflation corresponding to a trapping induced decrease of Landau damping, leading to larger EPWs am-plitudes that saturate by dephasing due to non linear frequency shift. This work was extended by taking collisions into account 25 and later in 1D PIC simulations 26 , modifying the inflation threshold. Then Yin and coworkers extended this work to two-(2D) and threedimension (3D), first in different k L λ D regimes 14 exhibiting beam acoustics modes in the trapping regime, then showing the bowing 15,20 of EPW resulting from non spatially-uniform trapping and trapped particle modulational instability, further addressing the threshold of trapping 16 , the destabilization 17,18 of under-threshold speckles by higher intensities speckles, and detrapping 19 mechanisms by side-loss or collisions.…”