Study On The Propagation Characteristics Of Terahertz Wave In Plasma

The propagation characteristics of electromagnetic wave in plasma is always the key to solve the problem of plasma black barrier in hypersonic vehicle.The existence of plasma sheath will hinder or even interrupt the propagation of microwave communication signals.While terahertz wave can propagate in plasma with low loss,so it is of great significance to study the transmission of terahertz wave in plasma.Based on this,our studies will focus on the propagation characteristics theoretical calculation,the vehicles plasma flow field simulation,the transmission experiment of terahertz wave in plasma and the terahertz active imaging of the plasma coated target.The main contents are:1.The propagation characteristics of 0.1-3THz terahertz wave in homogeneous and inhomogeneous plasma are calculated by using analytical Maxwell equations method and scattering matrix method,respectively.And the influence of different parameters is analyzed.For homogeneous plasma with an electron density of 10¹³cm^-3,the terahertz wave with a frequency higher than 1.24THz can penetrate the plasma with transmittance greater than 80%.2.The ANSYS Fluent software is used to simulate the flow field distribution of vehicles flying at hypersonic speeds,and the equilibrium constant method is utilized to calculate the electron density distribution of plasma sheath.The effects of Mach number,flying height of the vehicles and the external magnetic field on the propagation characteristics of terahertz wave in the plasma sheath are discussed.Taking the RAM C-III vehicle as an example,the transmittance of terahertz wave in its plasma sheath will increase by 22.3%with the decrease of speed from Mach 17 to Mach 11,while the transmittance will increase by 63.6%with the increase of flying height from 20km to50km.In addition,the theoretical results show that the external magnetic field can modulate the propagation of circularly polarized terahertz wave.When the applied magnetic field is 30T,the plasma penetration ability of 0.5THz left-handed terahertz wave increases by 18.5%,while the transmission enhancement of right-handed terahertz wave is not obvious,and the resonance absorption peak will appear at0.84THz.3.The plasma jet is produced by gas discharge at room temperature and atmospheric pressure.The transmittance of 0.1-2THz and 2-20THz in different gas plasma jet is measured by THz time domain spectrum system and ultra-wideband terahertz frequency sweep system,respectively.The experimental results show that the electron density of helium and neon plasma increases with the applied voltage,up to3.0×10¹³cm^-3and 3.6×10¹³cm^-3.For the plasma jet,the absorption coefficient decreases from 0.19cm^-1 to 0.12cm^-1 with the increase of frequency in the 0.1-2THz time-domain spectral system,while in the 2-20THz ultra-wideband system,the absorption coefficient decreases from 0.12cm^-1 to 0.05cm^-1 with the increase of frequency.4.A terahertz active imaging detection system with plasma occlusion is built.The terahertz reflection imaging experiments of metal objects were carried out under the conditions of plasma shielding,high temperature heat flow coating and high temperature tail flame.The experimental results show that the shielding and heating conditions of the plasma have no effect on the results of THz active imaging,which verify the ability of THz wave to penetrate the plasma sheath and detect the target under high temperature flow field condition.