Studies On Microwave Propagation Characteristics In Subwavelength Plasma Layers

As a special electromagnetic medium,the electromagnetic parameters(such as permittivity and refractive index)of plasmas not only possess features of wide-band and being continuously adjustable,but also can achieve fundamental changes in the properties of electromagnetic media through continuous changes in parameters(such as that the external magnetic field can cause dielectric anisotropy,and a continuous change in plasma density can result in a change in dielectric constant from positive to negative).Therefore,the effect of plasmas on the transmission properties of electromagnetic waves,the modulation of electromagnetic radiation signals and their applications have always been an important frontier in electromagnetic theory and technology,information science,and plasma physics.The modulation and enhancement of electromagnetic waves using sub-wavelength plasma structures is an emerging field in the classic topic of the interaction between electromagnetic waves and plasmas.This article focuses on the propagation characteristics of Giga-hertz band microwaves in the sub-wavelength plasma layer.Based on the relative position of the microwave transmitting/receiving antenna and the plasma layer,we study three typical issues: the near-field modulation characteristics of the sub-wavelength plasma layer on the emitted signal and the near-field modulation characteristics of the sub-wavelength plasma layer on the received signal,as well as the far-field interaction characteristics between the sub-wavelength plasma layer and microwaves.In the study of the near-field modulation characteristics of the sub-wavelength plasma layer on the emitted signal,combining experiment with numerical simulation,we research the radiation characteristics of transmitting antenna under the near-field modulation and enhancement.The influence of key plasma parameters on the enhancement effect of microwave signals are obtained,and the modu lation mechanism is revealed.And thus,effective experiment control methods are proposed.In the experimental study,we explore the application of sub-wavelength plasma modulation and enhancement on antenna radiation.We develop a technology of such a modulation and enhancement,and extend the enhanced frequency band from 0.6 GHz to L band,achieving 10 d B enhancement of 1 GHz microwave signal.We also conduct experiments to research the spatial radiation characteristics,impedance matching characteristics,effective gain bandwidth,and channel characteristics.Based on these experimental studies and the relevant numerical simulation model,the modulation mechanism of microwave radiation enhanced by sub-wavelength plasma layer is further analyzed.The modulation and enhancement characteristics under non-uniform plasma density distribution are studied.The modulation characteristics and effective gain window under actual plasma density distribution are also analyzed.These works provide a scientific basis for the development of key technologies which can significantly increase the gain of the antenna's electromagnetic radiation.In the study of the near-field modulation characteristics of the sub-wavelength plasma layer on the received signal,the enhancement effect on the received signal under sub-wavelength plasma is found through experimental research for the first time.The basic modulation characteristics and influence rules are given.On this basis,a corresponding numerical simulation model is establishe d to further analyze the modulation rules of the received signal based on the key parameters of the sub-wavelength plasma(plasma density,plasma thickness,plasma collision frequency,and effective relative distance).The receiving efficiency and electromagnetic scattering characteristics of the antenna modulated by sub-wavelength plasma and the same-size metal antenna are compared.Based on the local surface plasmon resonance theory,the mechanism and rules of the modulation and enhancement are analyzed.In the study of the far-field interaction characteristics between the sub-wavelength plasma layer and microwaves,a multi-channel cascaded arc plasma source is used to generate and simulate the target plasma environment.The characteristic parameters of plasmas are diagnosed and analyzed.On this basis,further experiments are conducted to research the transmitting characteristics of far-field microwave signals in high-density plasma.The influence of key plasma parameters on the far-field transmitting characteristics is analyzed.A physical model suitable for studying the transmission of far-field electromagnetic waves in sub-wavelength plasma structure is also established.Based on this model,the basic influence rules of plasma electron number density,collision frequency between electrons and neutral particles,and electromagnetic wave frequency on transmission characteristics are studied.The transmission characteristics of commonly used communication bands electromagnetic waves in different sub-wavelength plasma layers are analyzed.