Research On Interaction Between Plasma Sheath And Electromagnetic Wave Based On Finite-Difference Time-Domain Method

In recent years,the near-space hypersonic vehicles have attracted more and more attention from domestic and foreign scholars due to their potential strategic value in the military field and potential application value in the field of commercial communications.When the hypersonic vehicle re-enters the Earth's atmosphere,gas is compressed to form an arcuate off-body shock at the head of the aircraft,and at the same time,by the intense friction with the surrounding air,the large amount of kinetic energy of the aircraft is converted into heat energy,which causes the temperature of ambient air to rise sharply,causing chemical reactions such as dissociation and ionization between the components of the air.In addition,the surface insulation of the aircraft is ablated under heat,and the resulting ablation products are released with the fluid flow into the flow field around the aircraft.Finally,a plasma sheath or re-entry plasma consisting of a large amount of free electrons,ions and neutral particles is formed around the aircraft.The plasma sheath is non-magnetized,low temperature,weakly ionized,and electrically neutral on a macroscopic scale,which affects electromagnetic communication between the aircraft and the ground station,causing telemetry and communication failures and even "black out".At the same time,the reflection,refraction and attenuation of electromagnetic waves caused by the plasma sheath change the radar scattering characteristics of the hypersonic vehicle,which affects the target recognition and tracking of the reentry vehicle.Therefore,it is of great significance to study the interaction mechanism between electromagnetic waves and plasma sheaths.Based on the traditional FDTD iterative formula,the difference scheme of high-order FDTD is given.The time-discrete interval,Courant stability condition,numerical dispersion and numerical anisotropy of high-order FDTD(2,4)are analyzed.The results show that compared with the traditional FDTD method,the high-order FDTD(2,4)not only has an advantage in numerical accuracy,but also has less anisotropy effect on the numerical value.After comparing the advantages and disadvantages of the PLRC-FDTD method and the ADE-FDTD method for dealing with electromagnetic wave propagation problems in dispersive media,the high-order FDTD difference scheme is introduced into ADE-FDTD to obtain the high-order ADE-FDTD(2,4)method.The calculation accuracy of ADE-FDTD and high-order ADE-FDTD(2,4)is compared and analyzed by calculating the backscattering results of plasma.The numerical results show that the latter agrees well with the calculation results of Mie theory.When establishing the FDTD electromagnetic mesh of the target model,the centroid coordinate intersection method based on ray tracing is used to detect whether the FDTD mesh ray intersects with the triangular facet of the model surface,and then the conventional FDTD mesh and the conformal FDTD mesh are generated.Compared with the previous methods,the method used in this paper does not need to calculate the plane equation of the triangular face element in advance,so it can determine whether the ray and the triangular face element intersect more quickly.Moreover,since only the vertex information of the triangular facet needs to be stored,and the information such as the face normal vector of the triangular facet is not required,the memory saved by the centroid coordinate intersection method ranges from about 25% to 50%,according to the size of the model and the vertices shared by the triangular facets.Taking the three-dimensional metal blunt cone and the metal sphere model as the reentry target,the two-temperature model and the seven species chemical reaction model were used to solve the flow field around the hypersonic reentry target.The distribution characteristics of various plasma characteristic parameters such as ionization degree,Debye length,plasma frequency and collision frequency in the flow field around the reentry target at different flight heights and flight speeds are analyzed.It provides a basis for the subsequent study to analyze the influence of hypersonic flow field on the electromagnetic scattering of reentry targets.The interaction mechanism between electromagnetic waves and plasma is studied from the relationship between the real and imaginary parts of the relative dielectric constant of the plasma and the real and imaginary parts of the refractive index and the incident electromagnetic wave frequency,plasma frequency and collision frequency.The three-dimensional metal blunt cone model and the metal sphere model were used as hypersonic reentry targets to study the effects of plasma sheaths on the electromagnetic scattering characteristics of reentry targets in different flight conditions.The numerical results show that the influence of the plasma sheath on the electromagnetic scattering characteristics of the reentry target can be roughly divided into three stages: the enhanced section,the attenuated section and the stationary section.In the enhancement section(frequency below about 1.5 GHz),the plasma sheath increases the electromagnetic scattering characteristics of the target.In the attenuation section(within the frequency range of 1.5 GHz to 14 GHz),the refraction and attenuation of the electromagnetic wave caused by the plasma increases as the frequency of the incident wave increases,so that the plasma around the sheath reduces the electromagnetic scattering characteristics of the target.This also means that the plasma sheath has a certain stealth effect on the reentry target.In the stationary section(above 14 GHz),the plasma sheath has less effect on the electromagnetic scattering characteristics of the reentry target.