The Study of Photon Acceleration Driven by the Near-forward Scattering in the Intense Laser Under-dense Plasma Interaction

The mechanism of photon acceleration driven by the near-forward scattering (NFS) in intense laser under-dense plasma interaction has been studied by particle-in-cell (PIC) simulation. This mechanism utilizes tunneling ionization effect to stimulate electron plasma waves when the intense laser pulse propagates in under-dense plasmas. The electron plasma density is inhomogeneous both in longitudinal and transverse direction. In longitudinal direction, a steep ionized electron density front is generated by incident laser ionizing the helium gas. Around the ionization front, the incident laser interacts with electron plasma waves and generates the first kind of NFS waves. The frequency of NFS waves increases compared to the laser frequency. This is the first characteristic peak in the frequency spectrum. In transverse direction, the electron plasma waves have different phase velocities which makes the incident laser pulse undergoes NFS process and upshifts its frequency. This is the second characteristic peak in the frequency spectrum. Due to that the electron density inhomogeneity is much larger than the electron density perturbation of electron plasma wave, the scattering model and dispersion relationships, which are based on perturbation theory like the stimulated Raman scattering, are no longer applicable in this case. Our further study shows that the incident laser, electron density plasma waves and NFS waves still satisfy the energy conservation and momentum conservation which leads to the three-waves matching conditions of NFS process for inhomogeneous electron density. This can explain the appearance of two characteristic peaks in the frequency spectrum and their growth in the wave-vector space. This study has significant reference for the spectrum evolution when the intense laser pulse propagates in under-dense plasmas.