(a) Total time-averaged SRS reflectivity scaling vs peak laser intensity results from 1D PIC simulations for (black diamonds), with like-species (, ) collisions (green squares), and with all species collisions (red triangles). A sharp onset of enhanced SRS reflectivity at a threshold intensity and a saturated level at higher intensities are observed for both collisionless and collisional plasma. However, the onset intensity threshold is increased by binary Coulomb collisions. (b) Measured shift in the onset intensity threshold vs plasma collisionality. Collisional detrapping increases the onset intensity threshold approximately linearly with plasma collisionality.
Electron plasma wave amplitude when vs plasma collisionality for parallel diffusion (black solid line), as well as perpendicular diffusion (red dotted line: , , and ; green dot-dashed line: and ). The threshold for trapping-induced enhancement is predominantly determined by parallel velocity space diffusion resulting from collisions.
(a) Total time-integrated SRS reflectivity vs plasma collisionality for (black triangles), (blue diamonds), and (black squares). (b) Electron velocity distribution function (averaged over space and time) for and (red line) and (green diamonds). Significant electron trapping is observed for enhanced levels of SRS reflectivity, whereas for low levels of SRS reflectivity, the electron velocity distribution is Maxwellian.
(a) SRS reflectivity for collisionless (black) and collisional (green) and (red) plasma. (b) Time-integrated SRS reflectivity for the cases shown in (a). Collisional velocity space diffusion increasingly delays the onset time of the first burst of enhanced SRS with increasing collisionality. Instantaneous SRS reflectivity (c) and electrostatic wave energy (d) (for ) show that individual bursts of enhanced SRS are spatially and temporally correlated with bursts of electrostatic wave energy .
Electrostatic and electromagnetic spectra measured over the entire spatial and time domain. (a) Dispersion contours for the component of the electrostatic field in the collisional case with show the formation of a spectral streak along the Stoke’s resonance as well as trapping-induced BAMs. Spectral features in frequency (b) and wave number (c) are narrowed by collisional detrapping with (black), (green), and (red). (d) Dispersion contours for the component of the SRS filtered electromagnetic field [the same parameters as in (a)]. Collisional narrowing of the electrostatic spectra in (b) and (c) results in narrowing of SRS spectral features in both frequency (e) and wave number (f) .
(a) SRS reflectivity for collisionless (black), and collisions only (green), and , , and collisions (red) for . Electron velocity distribution functions are plotted for the three cases in (a) at (b) and (c). Increased perpendicular velocity space diffusion from collisions further delays the onset time of the first enhanced burst of SRS. Collisional heating is negligible during the development of the first SRS burst and the onset is determined by collisional detrapping. Dispersion contours for the component of the electrostatic field at (d) show peak spectral power for initial linear resonance conditions, while at (d), the peak spectral power has shifted upward along the Stoke’s resonance indicating the collisional heating has changed the linear resonance conditions.
SRS reflectivity for collisionless (black) and collisional (red) plasma with : (a) time-integrated and (b) instantaneous from 1D VPIC simulations using Gaussian (3 ps full width at half maximum) laser profile. As in Fig. 3, collisional velocity space diffusion increasingly delays the onset time of the first burst of SRS. The influence of velocity space diffusion from like-particle scattering in determining the inflation onset threshold is reduced in comparison to the case of a constant temporal laser profile.
Ratio of the parallel to the perpendicular relaxation rate vs change in velocity for collisions (green line) as well as and collisions (red dot-dashed line). Relaxation of small perturbations to the parallel velocity is dominated by parallel diffusion, whereas the relaxation of large perturbations is dominated by perpendicular diffusion.
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