Research On Propagation Characteristics Of Optical/electromagnetic Wave Under Effect Of Plasma Sheath Turbulence

With the increasing development of aerospace technology,the speed of aircraft is enhanced constantly,which can reach more than a dozen or even dozens Mach.The intense friction between the vehicle and the air causes the temperature and pressure of air increases sharply and consequently leads to local ionization of the air enveloping the aircraft.Ultimately,the aircraft is enveloped in a hypersonic flow field.The flow field has the characteristics of plasma,so it is also called plasma sheath.Plasma sheath have an severe influence on optical/electromagnetic wave propagation,such as attenuation,refraction and scatter et al.caused by the shock wave,boundary layer and internal turbulent structure,which will produce the severe effect on the wireless communication system of aircraft.This paper aims at the random fluctuation of optical/electromagnetics wave caused by turbulent structure and researches on the characteristics of optical/electromagnetic wave propagation through plasma sheath turbulence based on four aspects: the basic theory of plasma sheath turbulence;propagation characteristics of Gaussian beams in plasma sheath turbulence;simulation of the fractal phase screen in plasma sheath turbulence;performance of free-space optical link under effect of plasma sheath turbulence.The main work of this paper includes: 1.Introducing the basic concepts,basic parameters and research methods of plasma sheath turbulence.The plasma sheath turbulence is a typical random medium,the amplitude and phase of electromagnetic wave can be changed randomly,therefore,the only wave propagation theory in random media can be considered.Finally,the experimental results of fractal measurements of hypersonic turbulence are used to calculate the scaling exponent,spectral index and establish the power spectrum of plasma sheath turbulence.The results show that the power spectrum of refractive index fluctuation of plasma sheath turbulence satisfies the 4/5 scaling law,and the 3D spectral index is 19/5.2.The second moment of Gauss beam in plasma sheath turbulence is derived based on the Gauss beam propagation theory in random media and the power spectrum of refractive index fluctuation in plasma sheath turbulence.On this basis,the expression for mean irradiance,beam spot size,angle-of-arrival fluctuation,and beam wander of Gaussian beam are obtained in plasma sheath turbulence.Finally,the propagation characteristics of Gaussian beam through plasma sheath turbulence are analyzed.The results show that the larger the variance of refractive index fluctuation,the stronger the influence of the plasma sheath turbulence on the wave field;the larger the outer scale of the plasma sheath turbulence,the weaker the influence on the wave field.3.The fractal phase screen model in plasma sheath turbulence is established based on the band-limited Weierstrass fractal function and the power spectrum of refractive index fluctuation in plasma sheath turbulence.The effects of the parameters of plasma sheath turbulence on the diffracted average-intensity distribution by a fractal phase screen are analyzed.Finally,the influence of plasma sheath turbulence on synthetic aperture radar imaging performance are simulated and analyzed based on Range-Doppler Algorithm.The results show that with the increase in the fractal dimension of hypersonic turbulence,the phase error caused by the turbulence microstructures in this sheath becomes more obvious.4.In order to solve the problem of communication interruption of aircraft,the frequency of communicational electromagnetic wave is increased to optical band.The basic theory of free space optical communication system and the technology of subcarrier intensity modulation are introduced.The scintillation index of Gaussian beam propa gation in plasma sheath turbulence is derived.On the basis,the bit error rate performance of free-space optical link under the effect of plasma sheath turbulence are simulated and analyzed.The results show that the bit error rate(BER)obviously increases with the increase in variance of the refractive index fluctuation and the decrease in the outer scale.