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 $\Delta \omega_m$. Our primary focus lies on scenarios where $\Delta \omega_m$ is on the same order of the TPD growth rate $ \gamma_0$ ( $\Delta \omega_m \sim \gamma_0$), corresponding to moderate laser frequency bandwidths for TPD mitigation. With $\Delta \omega_m$ 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 $\Delta \omega_m$, particularly when $\Delta \omega_m \sim \gamma_0$. With the dynamic TPD behavior, the overall ratio of hot electron energy to the incident laser energy, $f_{hot}$, changes significantly with $\Delta \omega_m$. While $f_{hot}$ drops notably with increasing $\Delta \omega_m$ at large $\Delta \omega_m$ 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 $\Delta \omega_m$ falls below a specific threshold frequency $\Delta \omega_c$. We find this threshold frequency primarily depends on $\gamma_0$ 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 $\Delta \omega_c$ and laser plasma conditions, enabling the potential extention of our findings to more complex and realistic scenarios.