Research On Electromagnetic Characteristics Of The Target Covered With Plasma Sheath In Terahertz Band

The plasma sheaths generated by vehicles flying at hypersonic speed in the near space will affect the communication and detection of vehicles.Compared with microwaves and millimeter waves,terahertz(THz)waves have higher frequency and are easier to achieve large bandwidth.Compared with optical waves,terahertz waves have the ability to penetrate many nonpolar and nonmetallic substances,and can work all-time and all-weather.Therefore,terahertz technology is expected to be an effective new method to solve the blackout problem and the detection problem of hypersonic vehicles.In this dissertation,the electromagnetic characteristics of the target covered with plasma sheath in terahertz band are studied.Taking the plasma sheath as the research object,four key parameters affecting the electromagnetic characteristics of plasma are analyzed,and the public data of radio attenuation measurement C(RAM-C)experiment are introduced.The propagation characteristics are numerically calculated by simultaneously solving the electromagnetic field distribution and the corresponding electromagnetic field boundary conditions.Firstly,a single-layer homogeneous propagation model is established,and the transmittance,reflectance,absorbance and attenuation of terahertz waves in plasma are studied,as well as their variations with electron density,collision frequency and thickness of plasma.Considering the inhomogeneous distribution of plasma sheaths,a layered inhomogeneous propagation model is established.In order to make the model more close to the actual situation,an adaptive layered method is proposed,and the propagation characteristics of terahertz waves are calculated and analyzed based on RAM-C experimental data.Dielectric barrier discharge plasma is generated.The parameters of the plasma and the transmittance of terahertz waves in the plasma are measured by terahertz time domain spectroscopy system.The results show that terahertz waves can effectively penetrate the plasma sheath.The scattering characteristics and imaging of the plasma-covered target are studied.Considering that the solution of radar cross section(RCS)of plasma-covered target is very complicated and difficult to ensure accuracy,a simplified model is established and a calculation method of echo attenuation is proposed.The echo attenuation of RAM-C experiment at different incident frequencies is calculated by the proposed method,and the effects of different incident frequencies,plasma collision frequencies and incident angles on the echo attenuation are analyzed in detail.The plasma-covered target is modeled and simulated by the software,and the effects of different plasma electron densities and thicknesses on the RCS of the target are studied.Experimentally,a plasma generator and a THz RCS measurement system are built,and the effect of plasma on the RCS of the target in THz band is measured.Considering that it is difficult to obtain the echo signal of the plasma-covered target,the imaging model of the plasma-covered target is established,and the effects of different transmitting signals,plasma electron densities,collision frequencies and thicknesses on the imaging results are studied.The imaging experiments of plasma-covered target are carried out by using a radar imaging system and a terahertz camera imaging system.The results show that the effect of plasma sheath on RCS and imaging decreases with the increase of incident frequency.Especially in the high-frequency band,the effect of plasma sheath can be ignored.Taking the hypersonic vehicle as the research object,the three-dimensional models of RAM-C and hypersonic technology vehicle 2(HTV-2)are established,and the objects are simulated by the software.By calculating the RCS of the target at different incident frequencies,the scattering characteristics of the target are analyzed.By simulating the echo signals in different frequency bands and different observation angles,the imaging results of the target are analyzed.Considering the limitations of existing model,the attribute scattering center model is modified appropriately.The attribute scattering center model of RAM-C is established,and the different image characteristics of different scattering centers in one-dimensional image,two-dimensional image and time-frequency image of the target are analyzed.The research results provide some reference and support for the detection of hypersonic vehicles.