Study On Key Issues Of Electromagnetic Characteristics Of Electromagnetic Waves In Some Specific Plasmas

Study on the interaction between plasma and incident electromagnetic?EM?waves has always been an important research direction in the fie lds of defense and aerospace.It is of great practical and scientific significance to carry out research on the key issues about transmission and scattering of EM waves in plasma for mitigating the black barrier problem,improving existing stealth methods,and developing new plasma microwave devices.Up to now,the theoretical basis for the propagation of EM waves in the plasma has been relatively complete,while in some special plasma environments,the propagation and scattering issues are still worthy of in-depth study.This dissertation focuses on the key issues in the interaction between plasma and EM waves.The propagation characteristics of EM wave in some special plasma environments is explored by simulation and experiment.The relationship between EM propagation,scattering and plasma parameters is anaylized.The research consists of four sections:Firstly,the propagation characteristics of EM waves in magnetized plasma flow field by permanent magnets are studied.The electron density and collision frequency distribution in the plasma sheath of typical aircraft at several typical flight heights is studied.From the position of antenna installed,the plasma sheath information is extracted.Then,a permanent magnet is fabricated and its magnetic field intensity in the axial direction is measured.The transmission characteristics of EM waves in inhomogeneous plasma sheath with non-uniform magnenetization by magnet is calculated with the finite difference time domain method?FDTD?.Calculation results show that the transmission characteristic of left or right handed circularly polarized waves is not only determined by plasma sheath distribution,but also related to the direction of wave propagation and magnetic field applied.Based on the simulation results,the relationship between transmission characteristics and direction of wave propagation or magnetic field is revealed.When direction of the magnetic field is perpendicular to the aircraft wall and outward,it is proposed to design the antenna in the form of right-hand transmission and left-hand reception in order to reach a good communication effect of the L and S bands.Secondly,the transmission and communicatio n performance of EM waves in time-varying magnetized plasma are studied.When communication EM waves transmit in plasma sheath outside the hypersonic aircraft,a parasitic modulation will occur on EM waves induced by time-varying properties of plasma sheath,which will further deteriorate the communication performance in blackout zone.By theoretical and experimental research,it is found that applying a magnetic field to the time-varying plasma sheath can not only reduce the attenuation of EM waves,but also suppress the parasitic modulation effect?phase jitter?and improve the communication performance.The simulation results show that the phase jitter and attenuation of left-handed circularly polarized waves will decrease with increasing frequency or magnetic field.Nevertheless,as to right-handed circularly polarized waves,the situation become complex,whch depends on the corresponding frequency is located in the low or high frequency pass-band.In the low frequency pass-band of right-handed circularly polarized waves,it is found that the amplitude of phase jitter increases with higher frequency and decreases with stronger magnetic field strength.Thirdly,the interaction between the electrically small antenna and the subwavelength plasma sheath is studied.As the plasma sheath formed at stagnation region of aircraft is approximately hemispherical,with dense electron density and thin thickness.The impacts of electron density,thickness and collision frequency of the sub-wavelength plasma sheath on the radiation enhancement performance of electrically small antenna are studied in this section.It is found that the resonance frequency of the electrically small antenna mounted in subwavelength spherical plasma sheath will increase as the electron density becomes dense.The ratio of resonance frequency to plasma frequency is nearly 1/1.9.And the radiation gain will decrease when improve the collision frequency.The variation range allowance of the antenna size,plasma sheath size and electron density when the electrically small antenna can resonant normally is also studied.In addition,arrays of electrically small antenna is proposed to increase the radiation gain of the electrically small antenna in the subwavelength plasma and verified by simulation.Fourthly,the discharge process of filamentary plasma array is observed and the transmission,scattering characteristics of EM wave when incident into a filamentous plasma array is studied.Filamentary plasma array are generated by high-voltage DC pulse discharge method in low pressure air.The parameters of filamentary discharge plasma array are diagnosed and the phenomenon of discharge contraction and expansion is recorded and analyzed during the whole discharge process.The average electron density and collision frequency are calculated with the voltage,current and diameter of the filamentary discharge plasma.It is found that the electron density can reach 10²¹m^-3.Subsequently,the experiments are conducted to study the transmission characteristics of X-band EM waves with parallel or perpendicular polarization when propagating through the filamentary plasma array.It is found that X-band microwave with perpendicular polarization mode has more attenuation than that of parallel polarization after propagating through the filamentary plasma array.The SO-FDTD method is applied to simulate the interaction between the filamentary plasma array and EM waves and the similar result is obtained as the experiment.In addition,the radar cross section of a metal plate covered by filamentary plasma array is studied.The simulation results show that backscattering of metal plate for perpendicular polarized incident microwaves is significately suppressed when covered by filamentary plasma array.The maximum decrease of backscattering could reach 27dB compared with that absence of plasma.But as to parallel polarized microwave,the backscattering decrease is not obvious.These conclusions can provide a direction to the filamentary plasma stealth technology for the radar antenna of air vehicle.To sum up,this dissertation has carried out a mass of simulation and experimental research on the transmission and scattering issues of EM wave in several special plasma cases.The conclusion proposed in this dissertation could be applied to alleviating the blackout problems and developing new filamentary plasma stealth technology,and is of significance for exploring new applications of filamentary plasma arrays.