Transition from laser Raman to Bernstein scattering in magnetized plasmas

Abstract

When a laser backscatters from an external perpendicular magnetic field, stimulated Raman scattering transitions to stimulated Bernstein scattering, which we show using kinetic theory and particle-in-cell simulations. For low field strengths, the magnetic field acts as an electron detrapping mechanism, suppressing nonlinear effects of Raman scattering. At stronger fields, when the electron cyclotron frequency ωce becomes non-negligible compared to the plasma frequency ωpe, the Raman resonance splits into discrete resonances mediated by electron Bernstein waves (EBWs). Raman scattering ceases to exist between the EBW resonances, which are narrower and stronger than the Raman resonance they replace. The replacement is sensitive to angle: When the magnetic field is away from perpendicular by an angle α {\gtrsim} ωce/ωpe, the damping of EBW resonances blends them back into a broad single Raman peak. Our results demonstrate that strong magnetic fields, either spontaneously generated or externally imposed in high-energy-density experiments, fundamentally change laser-plasma interactions in a way that has no analog in unmagnetized plasmas, with implications for field diagnostics, plasma photonics, and inertial confinement fusion.