Efficient generation of divergent and collimated hot electrons via a novel multi-beam two-plasmon decay and stimulated Raman scattering mechanism

TL;DR

This study uses particle-in-cell simulations to uncover a novel multi-beam instability mechanism in inertial confinement fusion, revealing how it generates hot electrons with diverse angular distributions, which is crucial for understanding preheat risks.

Contribution

It introduces a new multi-beam TPD-SRS instability mechanism and establishes scaling relations for hot electron angular characteristics based on shared wave gains.

Findings

STS dominates hot electron generation at incident angles above 44°

Hot electrons exhibit both divergent and collimated components

Gain variations predict asymmetries in electron angular distributions

Abstract

In inertial confinement fusion (ICF) implosions, the preheating risks associated with hot electrons generated by laser plasma instabilities (LPI) are contingent upon the angular characteristics of these hot electrons for a given total energy. Using particle-in-cell simulations, we reveal a novel multi-beam collaborative mechanism of two-plasmon decay (TPD) and stimulated Raman scattering (SRS), and investigate the angular variations of hot electrons generated from this shared TPD-SRS (STS) instability driven collectively by dual laser beams with varying incident angles $\theta_{in}$ ($24^\circ$ to $55^\circ$ at the incident plane) for typical ICF conditions. In the simulations with $\theta_{in}\gtrsim44^\circ$, STS emerges as the dominant mechanism responsible for hot electron generation, leading to a wide angular distribution of hot electrons that exhibit both pronounced divergent and collimated components. The common Langmuir wave associated with STS plays a crucial role in accelerating both components.By properly modeling the STS common wave gains, we establish scaling relations between these gains and the energies of collimated and divergent hot electrons. These relations reveal that the divergent hot electrons are more sensitive to variations in gain compared to the collimated electrons. Additionally, the calculated gains qualitatively predict the asymmetry in hot electron angular distributions when the density gradients deviate from the bisector of the laser beams. Our findings offers insights for hot electron generation with multiple beams, potentially complementing previous experiments that underscore the critical role of overlapped intensity from symmetric beams within the same cone and the dominance of dual-beam coupling.