Superthermal-electron production by stochastic laser-plasma interaction in sub-relativistic regime*

In laser-plasma interactions with laser intensities 10^15-16 W/cm² and sub-nanosecond interaction time, electrons with energies above 100 keV are observed in particle-in-cell (PIC) simulations. In this parameter region, parametric instabilities (PIs) such as stimulated Raman scattering (SRS) play an important role in the energy transfer from laser light to plasmas and thus affect the performance of inertial confinement fusion (ICF). In PIs’ processes, the laser light excites plasma waves and makes density perturbations in the underdense plasma. These excited waves and perturbations lead to the generation of hot electrons (HEs). Consequently, the HEs transport their energy into the overdense plasma and affect the hydrodynamics of the plasma, such as shock waves. 

In this study, we performed PIC simulations up to 100 picoseconds, changing laser intensities from 10¹⁴ to 10¹⁶ W/cm² and plasma density scale length from 50 to 800 μm, to investigate the characteristics of HEs. HEs consist of two components: thermal electrons with mean energies of 10s of keV and superthermal ones with 100s of keV. We found that the superthermal electrons undergo stochastic accelerations to reach relativistic energies. We report the details of the acceleration mechanism and its model.