Broadband electromagnetic emission via mode conversion mediated by stimulated Raman scattering in inhomogeneous plasma

Abstract

Electromagnetic emission via linear mode conversion from electron plasma waves (EPWs) excited by stimulated Raman scattering (SRS) of an incident laser pulse in inhomogeneous plasma is investigated theoretically and numerically. It is found that the mode conversion can occur naturally in underdense plasma region below the quarter critical density provided that EPWs are generated due to the development of backward SRS when the laser pulse is incident at certain angle with the plasma density gradient. The produced radiation may cover a broad frequency range up to half of the incident laser frequency. The dependence of the radiation conversion efficiency on the laser intensity, incident angle, laser pulse duration, plasma density scale length, and initial electron temperature is analyzed based on one-dimensional particle-in-cell simulation. In two-dimensional geometry, due to the development of sideward SRS, it is found that the mode conversion to occur even at normal incidence of the laser pulse. The radiation frequency, bandwidth, duration, and amplitude can be well controlled by the laser and plasma parameters, suggesting that it may provide a new source of tunable broadband radiation as well as a diagnosis of the development of SRS.

Anomalous hot electron generation via stimulated Raman scattering in plasma with up-ramp density profiles

“…To understand the hot electron generation via SRS in an inhomogeneous plasma, it is necessary to know how EPWs propagate and evolve. It has been shown before, when an EPW is excited in inhomogeneous plasma, its local wave number will change with time [30]. It is assumed that BSRS instability generates a large-amplitude Langmuir wave everywhere in space, which then evolves in time.…”

“…On the other hand, for the EPW excited by SRS propagating in an inhomogeneous plasma, its wavenumber and phase velocity change with space and time [28][29][30], which may have important effects on electron trapping and acceleration. For example, for absolute SRS developed near the quarter critical density in an inhomogeneous plasma, it is shown that when the excited EPW propagates towards the turning point, some trapped electrons can be further accelerated by the primary EPWs as its phase velocity increases [31].…”