Anomalous hot electron generation from two-plasmon decay instability driven by broadband laser pulses with intensity modulations

Abstract

We investigate the hot electrons generated from two-plasmon decay (TPD) instability driven by laser pulses with intensity modulated by a frequency $Δω_m$. Our primary focus lies on scenarios where $Δω_m$ is on the same order of the TPD growth rate $ γ_0$ ( $Δω_m \sim γ_0$), corresponding to moderate laser frequency bandwidths for TPD mitigation. With $Δω_m$ conveniently modeled by a basic two-color scheme of the laser wave fields in fully-kinetic particle-in-cell simulations, we demonstrate that the energies of TPD modes and hot electrons exhibit intermittent evolution at the frequency $Δω_m$, particularly when $Δω_m \sim γ_0$. With the dynamic TPD behavior, the overall ratio of hot electron energy to the incident laser energy, $f_{hot}$, changes significantly with $Δω_m$. While $f_{hot}$ drops notably with increasing $Δω_m$ at large $Δω_m$ limit as expected, it goes anomalously beyond the hot electron energy ratio for a single-frequency incident laser pulse with the same average intensity when $Δω_m$ falls below a specific threshold frequency $Δω_c$. We find this threshold frequency primarily depends on $γ_0$ and the collisional damping rate of plasma waves, with relatively lower sensitivity to the density scale length. We develop a scaling model characterizing the relation of $Δω_c$ and laser plasma conditions, enabling the potential extention of our findings to more complex and realistic scenarios.