| The communication blackout caused by plasma sheath is one of the main factors that restrict the development of hypersonic vehicle.At present,a large number of theories and experimental studies have been proposed to solve the radio blackout.The studies reveal that the applied magnetic field method is one of the most practical methods for the breakthrough of the communication blackout of hypersonic vehicle.By using this method,the electromagnetic wave can pass through plasma,when the electromagnetic wave frequency is lower than electron cyclotron frequency.However,for the method of studies do not consider the effects of the electron density change caused by the effect of magnetic field on flow field,plasma electron density in the field of dynamics and inhomogeneous characteristics of the actual magnetic field on characteristics of electromagnetic wave propagation.In addition,the required strong magnetic field of the magnetic field method also limits the method in practice.Therefore,the propagation characteristics of electromagnetic wave in plasma are studied by considering the influence of static magnetic field on plasma electron density and the change of plasma equivalent dielectric property in this thesis.Secondly,the influence of nonuniform characteristics of magnetic field on electromagnetic wave propagation in plasma is studied.Finally,a new method is proposed to reduce the magnetic field intensity by using the traveling wave magnetic field.The main research contents and contributions are as follows:1.A theoretical model for the effect of static magnetic field on plasma flow field is established.The influence of magnetic field on electron density distribution in plasma flow field is analyzed.Using the theoretical model,the effects of magnetic field on the charge particles in plasma are analyzed,the effects of magnetic field on plasma electron density are studied,and electron density attenuation ratio under static magnetic field are obtained.The results show that static magnetic field can reduce the plasma electron density.The larger the magnetic field intensity,the lower the electron density or the lower the velocity of the flow field,the more obvious the effect of the magnetic field on the electron density of the plasma.2.The electromagnetic wave propagation model under the interaction between magnetic filed and flow field is established.According to the effect of magnetic field on the plasma flow field,the electron density distribution model at the antenna window is given,and the equivalent permittivity of plasma is derived under this condition.On this basis,using the layered model for the plasma and the hybrid matrix method,the transmission coefficients of typical band electromagnetic wave in plasma are obtain.The propagation characteristics of electromagnetic wave under the interaction between the magnetic field and flow field are analyzed and compared with the propagation characterizes without considering the interaction.The results show that when the magnetic field intensity is small,the transmission coefficient of the electromagnetic wave in the typical band of the magnetic field and the flow field is significantly greater than that of the non-considering magnetic field and the flow field.Meanwhile,the smaller the flow field velocity,the larger the transmission coefficient of electromagnetic wave in the plasma under consideration of magnetic field.3.Electromagnetic wave propagation model is established in dynamic plasma with magnetic field and flow field.The dynamic property of the electron density plasma sheath flow field is analyzed.Using the method of layered space and time,the transmission characteristics of electromagnetic wave in plasma is obtained.The results show that,with applying a magnetic field,the amplitude and phase jitter of transmission electromagnetic wave in low frequency passband are greatly reduced that is without the magnetic field,and with the increase of magnetic field intensity,the degree of transmission of electromagnetic wave amplitude and phase jitter is reduced.4.In this thesis,the stopband effect caused by non-uniform magnetic field is found and the effect of stopband effect on magnetic window method is reduced by optimizing the structure of the magnet to increase the communication bandwidth.The electromagnetic wave propagation model in the plasma under nonuniform magnetic field is established aiming at the nonuniform property of magnetic field,and the propagation characteristics of the right circular polarization wave in the plasma are calculated.The results show that the stopband bandwidth of the wave transmission coefficient increases under the nonuniform magnetic field,called stopband effect.The stop-band effect reduces the frequency range of the electromagnetic wave propagation,reduces the attenuation of the electromagnetic wave in the band and aggravates the ability of magnetic field to attenuate electromagnetic wave attenuation.The lower the minimum magnetic field the magnet produces,the smaller the frequency range of the low-frequency band.The optimal structure of the magnetic field is obtained through the optimization of the magnetic structure under the volume constraint,and the magnetic field generated by the structure can reduce the adverse effect of the stopband effect.5.This thesis presents a method to reduce the attenuation of electromagnetic waves by using the traveling magnetic field.The analytical expressions of the plasma electron density reduction are obtained by establishing the mathematical model of the effect of traveling magnetic field on the plasma.The effect of traveling magnetic field on local electron density of plasma is analyzed.Based on this,the propagation and calculation model of electromagnetic wave in the plasma is established,and the effect of the traveling wave magnetic field on the attenuation of the electromagnetic waves of GPS,S,C and X-band is simulated and analyzed.Meanwhile,using RAM C data as an example,the effects of traveling wave magnetic field on electron density reduction and the radio blackout during reentry are analyzed.The results show the plasma electron density and the attenuation of EM wave propagation in plasma can be reduced significantly by using a low strength traveling magnetic field.By increasing the speed of the traveling wave magnetic field,it can improve the capability of the traveling wave magnetic field to reduce the electron density of the plasma,and enhance the ability of magnetic field to reduce the attenuation of the electromagnetic wave.In conclusion,this thesis research results improved applied magnetic field approach to mitigate radio blackout.The magnet structure optimization enhance the engineering realizability of magnetic window method,and the traveling magnetic field method provides a new way for blackout mitigation. |
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