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 Δ ω _m using theoretical and numerical approaches. Our primary focus lies on scenarios where Δ ω _m is on the same order of the TPD growth rate γ _0 ( $\Delta \omega_m \sim \gamma_0$ ), corresponding to moderate laser frequency bandwidths for TPD mitigation. With Δ ω _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 Δ ω _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, $f_\mathrm{hot}$ , changes significantly with Δ ω _m . While $f_\mathrm{hot}$ drops notably with increasing Δ ω _m at large Δ ω _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 Δ ω _m falls below a specific threshold frequency Δ ω _c . This anomaly arises from the pronounced sensitivity of $f_\mathrm{hot}$ to variations in laser intensity. We find this threshold frequency Δ ω _c 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 Δ ω _c and laser plasma conditions, enabling the potential extention of our findings to more complex and realistic scenarios.

Keywords

• two-plasmon decay
• hot electrons
• broadband laser
• intensity modulation
• laser plasma instability
• inertial confinement fusion