| During flight of hypersonic vehicle in near space,the temperature around the aircraft increases rapidly because of severe friction,which leads to dense air ionization.As the temperature rises,ionization becomes intense,chemical reactions and ionizing phenomena occur in different air components around the vehicle,thus forming a plasma flow field which contains the free electrons,ions and uncharged neutral particles cover around the vehicle.In addition,the material on the surface of the aircraft will also be abated and produce ablative particles due to the high temperature.Theses ablated particles also have partial ionization,and the electrons,ions and neutral particles produced will be mixed with the plasma flow field,eventually forming plasma sheath.The plasma sheath interferes the radar’s monitoring of the target in the sky,early-warning and affecting the monitoring station on the aircraft,it could even interrupt the connection between the aircraft and the radar on the ground,it is called “blackout”.These factors threaten the life and safety of the pilot and the state’s property.The existence of the plasma sheath seriously restricts the development of the spacecraft near the space,and at present,the main way to deal with the problem is the store-and-forward,but it can only solve the problem of reentry blackout in a short time,not a long-term plan.In order to completely solve the problem of blackout,researchers from the perspective of ballistic reentry test,get a lot of data about the plasma sheath,through theoretical analysis and simulation research,and then carry out the ground experiment simulation.It has formed a set of technical route for the research on the electromagnetic(EM)characteristics of the plasma sheath.The paper researches the interaction mechanism between EM wave and plasma sheath,which is based on the hypersonic with its plasma flow.Mainly on the three aspects of the problem are described: the characteristics of EM wave propagation in plasma sheath,the EM scattering problems of hypersonic vehicle and its plasma sheath,and EM wave echo characteristics which the hypersonic vehicle covered with plasma sheath.Establish the hypersonic vehicle and its plasma flow model,then based on the finite difference time domain method,the results of theoretical analysis are presented the propagation and scattering characteristics of EM wave of plasma sheath.This paper also gives the reasonable suggestions about the selection of aerodynamic shape of the flight target stealth and radar detection.The content researched in this paper provides the theoretical basictheory support of the domestic near space plasma sheath for the EM wave’s propagation and scattering characteristics in plasma sheath.The specific content and author’s main contribution are summarized as follows:1.The propagation characteristics of terahertz(THz)wave in time-space inhomogeneous plasma sheath are studied;the Laplace transform JE convolution-finite-difference time-domain(LTJEC-FDTD)is adapted to calculate the reflection coefficient,transmission coefficient and absorption rate in one-dimensional time-space inhomogeneous plasma sheath.And based on the model,the expression of dielectric coefficients of weakly collisional and fully ionized dusty plasma sheath is adopted.The shift-operator finite difference tome-domain(SO-FDTD)method is used to analysis the EM wave’s propagation properties of dusty particles and plasma parameters in the THz band.The results show that the THz wave can easily penetrate the plasma sheath and dusty plasma,and can reduce the energy of EM wave.These results provide reference and theoretical basis for the mechanism of interaction of THz wave and plasma sheath.2.In two-dimensional case,the blunt and HTV model covered with plasma sheath in different re-entry height,different flight speeds and different attack angles are anglicized.Based on the Z-FDTD method,the two kinds of targets and their spatial inhomogeneous plasma sheath model of double drill RCS are calculated,by comparison the relate results,the reasonable proposals are provided for the hypersonic vehicle in the near space.Otherwise,the magnetic field is applied to analyse the characteristics of RCS,and the“magnetic window” is verified.These results pave the way for the future research of three-dimensional plasma sheath.3.This study analyzes the scattering characteristics of obliquely incident EM waves in time-varying plasma sheath.The three-dimensional model of time-varying plasma is established.According to the empirical formula of the collision frequency in plasma sheath,and combine with the JEC-FDTD method,present the FDTD iterative formula of the electric field,magnetic field and current density.Then,the monostatic RCS of plasma frequency,temperature,pressure are discussed to analyze the EM scattering characteristics in the case of oblique incident.Finally,the laws of the RCS varying with time are summarized at the L and S wave bands.4.In order to analyze the EM wave’s scattering characteristics of three-dimensional time-space inhomogeneous plasma sheath,the three-dimensional model of time-space inhomogeneous plasma sheath is established,and combined with JEC-FDTD method,the results of EM scattering characteristics in time-space inhomogeneous plasma sheath are provided.On this basis,the dielectric coefficient of a weakly ionized dusty plasma is used to establish a three-dimensional time and space inhomogeneous dusty plasma sheath.The effects of scattering on EM waves in this dusty plasma sheath are investigated using the auxiliary differential equation finite-difference time-domain(ADE-FDTD)method.Simultaneously,the RCS of blunt cone,point cone and HTV which covered with plasma sheath are analogized.5.The EM wave echo characteristics of conventional plasma,weakly ionized dusty plasma and fully ionized dusty plasma are studied.For one-dimensional,the RAM-C reentry experimental data of NASA are used to analyze the time-domain and frequency-domain echo characteristics of ordinary plasma,uniformly distributed dust plasma,time-varying dust plasma and time-space inhomogeneous dust plasma.For the three-dimensional,two models of the blunt cone and the point cone are given,and the influence of the plasma on the characteristics of the electromagnetic echo is calculated and analyzed,and the schlieren diagram of the whole process of the EM wave from incident to reflect is given.In addition,the effects of aerodynamic configuration of two hypersonic vehicles on the EM scattering characteristics are compared. |
PDF Full Text Request