Mode conversion and laser energy absorption by plasma under an inhomogeneous external magnetic field

The interaction of a high-frequency laser with plasma in the presence of an inhomogeneous external magnetic field has been studied here with the help of Particle-In-Cell simulation. It has been shown that laser enters inside the plasma as an extraordinary wave (X-wave), where the electric field of the wave oscillates perpendicular to both external magnetic field and propagation direction, and as it travels through the plasma, its dispersion property changes due to the inhomogeneity of the externally applied magnetic field. Our study shows that the X-wave's electromagnetic energy is converted to an electrostatic mode as it encounters the upper-hybrid (UH) resonance layer. In the later stage of the evolution, this electrostatic wave breaks and converts its energy to electron kinetic energy. Our study reveals two additional processes involved in decaying electrostatic mode at the UH resonance layer. We have shown that the energy of the electrostatic mode also converts to a low-frequency lower-hybrid mode and high-frequency electromagnetic harmonic radiations at the resonance layer. The dependence of energy conversion processes on the gradient of external magnetic field has also been studied and analyzed.