Description
In the context of inertial confinement fusion (ICF), laser-plasma interactions are susceptible to parametric instabilities such as stimulated Raman scattering (SRS), which reduce the coupling of the laser energy to the fuel and generate hot electrons detrimental to the fusion yield. Controlling these instabilities calls for a fine understanding of their growth rate and saturation mechanisms.
The saturation level of SRS depends critically on the stability of the daughter electron plasma wave (EPW). When the EPW reaches a high enough amplitude, electron trapping nonlinearly alters the dispersion of the wave and makes it prone to sideband instabilities. In Refs. [1, 2], the stability of the EPW was analyzed assuming an infinite wave-packet width transverse to the wave vector. However, with a finite width EPW, part of the electrons would exit the wave packet transversely before leading to nonlinear effects, thus reducing the sidebands growth rate compared to a 1D geometry.
To investigate this scenario, we performed 2D simulations of adiabatically driven, monochromatic EPWs using our hybrid fluid-kinetic, time-enveloped HYZAK code [3]. The wave is modeled with a Gaussian envelope in the transverse direction, assuming collisionless electrons and static ions. During the wave-drive phase, electrons follow nonlinear trajectories, but their current is decoupled from the electric field evolution.
For EPWs at least a few wavelengths wide, and except for very low wave amplitudes ($e\phi/T_e$, where $\phi$ is the amplitude and $T_e$ the electron temperature), electrons perform at least one trapped orbit during their transverse crossing of the wavepacket. Thus, immediately after the drive phase, the trapped electron distribution depends predominantly on the EPW amplitude rather than its width. Later on, as the EPW evolves freely, we find that narrower wave packets hamper the growth of longitudinal sidebands while amplifying wave front bowing and nonlinear filamentation. We will discuss the joint influence of the width and amplitude on the EPW stability and the electron distribution released after wave breaking.
[1] M. Tacu, and D. Bénisti, Phys. Rev. E 110, 045205 (2024).
[2] S. Brunner et al., Phys. Plasmas 21, 102104 (2014).
[3] G. Sary, and L. Gremillet, Phys. Plasmas 29, 072103 (2022).