Excitation of nonlinearly coupled harmonics by linearly independent plasma waves

Time-dependent flows in a plasma produce electromagnetic (EM) waves. The EM waves perturbed by the flows and the intrinsic waves in the plasma can excite the nonlinearly coupled harmonics.¹ The nonlinear modes observed from a plasma contain spatiotemporal information about the plasma flow, so understanding the wave-wave nonlinearity is essential to interpret the plasma behavior. The Vlasov-Maxwell equations are solved to describe the nonlinear coupling between the linearly independent plasma waves propagating parallel to the magnetic field or in an unmagnetized plasma. Extended dispersion relations obtained from the kinetic analysis show the excitation of coupled harmonics with the frequency selection rule. Additionally, the wavenumber selection rule is suggested for the harmonics. Both the selection rules are verified by the fully kinetic particle-in-cell simulation². The nonlinear coupling between the flow-induced perturbation and the background plasma wave may explain observed harmonics in various plasmas and the wave propagating perpendicular to the magnetic field with the whistler wave frequency³.

 

¹M. U. Lee, J.-Y. Ji, and G. S. Yun, Nucl. Fusion 60 126036 (2020)

²T. D. Arber et al, Plasma Phys. Control. Fusion 57 113001 (2015)

³M.H. Kim et al, Nucl. Fusion 60 126021 (2020)