Investigations On Distribution Characteristics Of The Plasma Sheath Based On Multi-frequency Reflection Method

During the reentry of a hypersonic vehicle,a “plasma sheath” is created on the surface of the aircraft.The plasma sheath will produce different degrees of reflection and absorption attenuation on the electromagnetic wave,which will deteriorate the communication quality and the communication will be interrupted in severe cases,the “blackout” phenomenon will occur.If the sheath sheath's parameter distribution can be measured,the influence of the plasma sheath on signal transmission can be estimated.And then,the signal can be preprocessed,which can weaken the influence of the plasma sheath on the signal transmission.Based on the theoretical study of plasma sheath electromagnetic wave propagation,a method called multi-frequency point reflection was proposed in this paper.By real-time measurement of the amplitude and phase information of multiple frequencies reflected signals,the parameters distribution of the plasma sheath can be estimated.The main research contents and results are as follows:(1)Equivalent wave impedance method is used to investigate the signal transmission law under different plasma sheath parameters distribution,which provides a theoretical basis for the reliability of reflection signal extraction scheme and multi-frequency point reflection method;(2)A directional coupler-based on-line measurement of the reflected signal is designed,and the feasibility of the scheme is verified by the method of ADS and CST co-simulation.Through the law of signal transmission under different plasma sheath parameters,and the conclusion was obtained that the plasma parameters could be measured by transmission and reflection signals.For transmission signals,there are shortcomings of long-distance transmission to reach terrestrial receiving stations,and there is no long-distance transmission after the reflected signal passes through the plasma.A method for estimating the parameters of the plasma sheath through reflection signals is proposed.The plasma and antenna modeling were simulated in the CST to obtain the S parameters of the antenna under different plasma parameter distributions.The two-port network including antenna S-parameters is introduced into ADS signal measurement system.The time domain signal is extracted by ADS in the transient simulation method,and Fourier transform of the time domain signal is used to obtain amplitude and phase information;(3)A multi-frequency point reflection method was designed to achieve plasma electron density distribution.Plasma sheaths with different parameter distributions have different effects on signal transmission.In order to weaken the influence of plasma on the signal,when the signal is preprocessed,it is necessary to measure the parameters distribution of the plasma sheath in real time.Aiming at the problem that the existing microwave reflection method has time delay,a multi-frequency point reflection method is proposed to measure the sheath parameter distribution method.The method is easy to implement with hardware and has time synchronization.Electromagnetic waves at different frequencies will have total reflection at the interface of different electron density,so the amplitude and phase of the reflected signal at each frequency point are different.The amount of phase change of the reflected signal during the entire transmission process is calculated by the geometrical optical approximation method,and the position where total reflection occurs at each frequency point is obtained by inversely inferring the phase change.According to the foreign flight test measurements,the electron density of the sheath is approximately obeying a double Gaussian distribution.The electron density at different positions and locations is fitted by Gaussian distribution to obtain the density distribution of the rising edge of the sheath.