HF wave propagation through inhomogeneous magnetized plasmas: a simulation of mode conversion and stimulated electromagnetic emissions

Abstract The propagation of high-frequency (HF) waves through inhomogeneous magnetized plasmas, to investigate energy absorption mechanisms, mode conversion, and the resulting stimulated electromagnetic emissions (SEEs) is studied, aiming to illuminate the underlying physical mechanisms. A Fully electromagnetic (EM) particle-in-cell method is employed to simulate the interaction between a high-power left-hand circularly polarized HF wave and a magnetized plasma with a linearly increasing density gradient. Two conditions are considered: magnetic field aligned with the HF wave propagation direction and perpendicular to the HF wave propagation direction. The dynamics of HF wave–plasma interactions, the nature of mode conversion, excited stimulated modes, and the conditions that enhance or inhibit SEEs are studied. Parallel propagation to the magnetic field induces small amplitude plasma modes near the left-hand cutoff frequency. In perpendicular propagation into the magnetic field, the incident circularly polarized HF wave decomposes into two distinct polarization modes: the ordinary mode (O-mode) and the extraordinary mode (X-mode). The O-mode behaves as a linearly polarized EM wave and reflects at the ordinary wave cutoff frequency which leads to excitation of plasma modes. However, the X-mode propagating partly transverse and partly longitudinal through the plasma stops propagating at the right-hand cutoff frequency. At this point, it is absorbed and converted into strong EM and electrostatic modes, resulting in a substantial energy loss. The results indicate that considering an elliptically polarized HF wave instead of a circular one can lead to more efficient heating of plasmas. The generated linear and nonlinear plasma modes are investigated using fast Fourier transform analysis. The suppression of SEEs at integer multiples of electron gyroharmonic frequencies observed in experiments is also investigated. It is shown that the resonance of the transmitted HF wave at the upper-hybrid frequency causes this suppression. These results have important implications for plasma diagnostics and heating experiments.