Resonant absorption is a process that electromagnetic (EM) energy is converted to plasma energy with a mode conversion on the resonant layer where the incident EM wave frequency equals to the local frequency of a plasma normal modes. With a finite collision between charged and background neutral particles in a plasma, the plasma oscillation is dissipated to widen the resonance layer and heat the plasma. In this work a modified scattering matrix methods (SMM) are applied to study the effects of the collision frequency, incident angle, and plasma thickness on resonant absorption. We analyze the energy absorption caused by resonance in comparison with collisional absorption for different parameters. It is found that the resonant absorption dissipates about nearly half of the incident EM energy in an overdense inhomogeneous plasma when the collision is weak, and the rest half portion is reflected. If the collision is strong, however, the collisional absorption is then more significant than the resonant and affects the entire wave propagation process.
In recent years, an innovative technology based on modulation and enhancement effects of subwavelength plasma on RF electromagnetic radiation has been proposed, in which the microwave radiation from an electrically small antenna can be significantly enhanced when the antenna is tightly enclosed by a subwavelength overdense plasma shell. But the exact mechanism is still not entirely clear. In this paper, we first use the theory of hybridized Local Surface Plasmon Resonance (LSPR) for visible light in nanometal to explain this cross-domain work of microwave radiation enhancement modulated by a subwavelength overdense plasma layer, and the results show that the LSPR frequency is in good accordance with the frequency of enhanced radiation signals. Furthermore, the relationship between LSPR and antenna impedance as well as their effects on the impedance matching condition between antenna and power supply are investigated. It is indicated that LSPR on the plasma layer changes the impedance of the antenna, which produces circuit resonance (differing from plasmon resonance) between the antenna and power supply, and thus more power is radiated from the power supply to free space.
The modulation and enhancement effect of sub-wavelength plasma structures on compact antennas exhibits obvious technological advantage and considerable progress. In order to extend the availability of this technology under complex and actual environment with inhomogeneous plasma structure, a numerical simulation analysis based on finite element method has been conducted in this paper. The modulation function of the antenna radiation with sub-wavelength plasma layer located at different positions was investigated, and the inhomogeneous plasma layer with multiple electron density distribution profiles were employed to explore the effect of plasma density distribution on the antenna radiation. It has been revealed that the optical near-field modulated distance and reduced plasma distribution are more beneficial to enhance the radiation. On the basis above, an application-focused research about communication through the plasma sheath surrounding a hypersonic vehicle has been carried out aiming at exploring an effective communication window. The relevant results devote guiding significance in the field of antenna radiation modulation and enhancement, as well as the development of communication technology in hypersonic flight.
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