Stimulated Raman scattering instability of a left-handed circularly polarized laser in strongly axially magnetized plasmas

Abstract

The stimulated Raman scattering (SRS) instability of a left-handed circularly polarized (LH-CP) laser in strongly axially magnetized plasmas is investigated in detail with the help of one-dimensional (1D) and two-dimensional (2D) particle-in-cell (PIC) simulations. Since the LH-CP laser has a larger critical density in the axially magnetized plasmas, the SRS instability could be excited in over quarter-critical density plasmas, which is verified by the PIC simulations. This phenomenon could be used to amplify a seed with a frequency smaller than half of the laser frequency, which is impossible for traditional simulated Raman amplification. The simulation results also show that the scattered laser becomes right-handed circularly polarized. With this conclusion, we re-derive the temporal linear growth rate of the SRS instability of the LH-CP laser in the axially magnetized plasmas. The results show that the larger the external magnetic field is the smaller the temporal growth rate is. The suppression of the SRS by the external axial magnetic field in the linear region is verified by both 1D and 2D PIC simulations. The simulation results also show that the phase velocity of the electron plasma wave (EPW) will be decreased by the external magnetic field as expected by the theory, which makes it easier for the EPW to trap electrons and for the nonlinear frequency shift of the EPW to happen. As a result, not only the linear growth rate of SRS but also the saturation level of SRS is decreased by the external axial magnetic field. When the external magnetic field is strong enough, the saturation level of SRS can be suppressed by several times. So, this work also provides an efficient way of suppressing the SRS instability. Besides, the 2D simulation results show that some transverse instabilities of the electron plasma wave are also suppressed by the external magnetic field and this suppression will provide us with an electron plasma wave with a better structure, which may benefit the simulated Raman amplification.