Investigation Of EM Wave Propagation And Parameter Inversion In Plasma Sheath

Space science and technology progressed with each passing day and the scope of human activities expanded continuously with the help of various advanced aircraft.Various types of spacecraft flight from space back into the atmosphere and high-speed aircraft cruise near space,the plasma sheath is formed since the severe friction between the aircraft and the atmosphere and the appearance of the ablation effect,which lead to the emergence of communication blackout phenomenon,threats to the safety of the aircraft as well as restricts the development of the super high speed aircraft.Thus,alleviating the blackout phenomenon become one of the major problems in the field of space.Information transmission and radar detection of near-space aircraft are important research fields.However,due to the large attenuation of electromagnetic waves during the flight,the effective communication,accurate positioning,rapid recovery and space security have been seriously disturbed.One of the key factors causing this problem is the presence of the plasma sheath,which causes the change of the electromagnetic(EM)properties of the target.Aircraft flying at extremely high speeds,it has a strong compression of the air molecules and a severe friction with the air,leading to a sharp increase in the temperature near the surface of the vehicle.Under the combined high temperature and high pressure,air molecules will be ionized,resulting in a layer of plasma shock fluid with a large number of particles,including free electron,ions and neutral molecules,namely plasma sheath.The particles in the plasma sheath will collide spontaneously,while the charged particles will collide under the driving of the electromagnetic wave since the changeable electrical and magnetic field.The external field-driven collision will cause the strong scattering of electromagnetic waves to cause the decay of energy,and eventually appear the blackout phenomenon.The aircraft cannot receive ground commands in time,which is extremely unfavorable to flight safety.On the other hand,the blackout phenomenon can be fully utilized to avoid the detection from ground radar and realize the purpose of aircraft stealth.Therefore,to realizing the purpose of aircraft safety and target stealth,it is very important to fully recognize and understand the interaction relationship between the plasma sheath and electromagnetic waves.Thus,the electromagnetic wave propagation and parameter inversion in the plasma sheath has been studied to give a further understanding of the interaction mechanism between plasma and the electromagnetic wave,and master the parameter distribution inside the plasma sheath and make full use of it.To find new ways and new methods that can effectively alleviate the impact of plasma sheath on radar detection and communication.The main work of this thesis is as follows:1.In order to research the influence of inhomogeneous flow field changes on EM waves propagation characteristics under different working conditions,the non-uniform flow field model of the high-speed target plasma sheath under different flight conditions is established.The transmission and attenuation coefficients of the high-speed target plasma sheath are calculated numerically under different flight states by using the ADE-FDTD algorithm.The phase shift characteristics of EM waves in plasma sheath are investigated.The influence of different plasma parameters and the incident frequency on the phase shift change is analyzed.For the continus change of the plasma flow field in real situation,we discuss the influence of the different electron density distribution types on the phase shift characteristics of the EM wave,and find that the phase shift is closely related to the mean electron density under various electron density distributions.Meanwhile,the influence of the change of high-speed target plasma flow field on EM wave phase shift under different flight parameters is explored.2.Based on the BGK collision model and dust charging theory,a modified complex dielectric parameter model for the weakly ionized dusty plasma is constructed with the nonzero electron minimum speed and the contribution of the second term of the collision cross section simultaneously considered.By comparing the transmission properties of EM waves in the weakly ionized dusty plasma between traditional and modified models,we show that this two parts have greatly influence on the transmission properties of EM waves in the plasma.The effect of the weakly ionized plasma with and without dust particles on the EM wave transmission properties under the modified model is analyzed.The influences of the different dust parameters on the transmission properties of the EM waves are discussed.Further,the attenuation coefficients of oblique incident EM waves in dusty plasma with different reentry heights are calculated.3.Considering the impact of the plasma sheath on radar detection,the influence of highspeed target plasma sheath on echo characteristics is investigated.According to the characteristics of spatial non-homogeneity and time-variation of the plasma sheath itself,the space-time inhomogeneous plasma sheath model is established.The effects of spatially homogeneous plasma,spatially inhomogeneous plasma,spatially inhomogeneous and time-varying plasma on echo are calculated and compared by using the Z-FDTD method.Then,the influence of different time-varying parameters(peak coefficient,pulse width)on the echo characteristics is analyzed,indicating that the electron density and echo strength are positively correlated.Finally,the influence of different plasma parameters on the EM echo characteristics is discussed and the influence law of the plasma on echo is revealed.4.Based on the EM wave attenuation results in plasma with different spatial positions,the expression between the electromagnetic wave attenuation and the electron density is derived theoretically.By applying differential to the obtained formula,we establish a function between the attenuation gradient and the electron density,and this can be used to preliminarily predict the spatially distribution trend of electron density.Further,the BP neural network algorithm is used to predict the mean electron density results for the entire plasma based on attenuation results.Combining the spatially distribution trent and mean electron density,the spatial distribution of electron density in plasma is obtained.The feasibility of the proposed method to solving the spatial distribution of electron density in plasma is verified by comparing with the desired results.