Study On The Influence Of Plasma Sheath On Electromagnetic Wave Propagation And Antenna Performance

With the continuous improvement of the flight speed of aircraft in the atmosphere,aircraft will gradually face important challenges such as sound barriers,thermal barriers,and black barriers.During the process of reentry into the atmosphere,the hypersonic vehicle in near space has a superb flying Mach number,which will produce intense friction with the background atmosphere,resulting in a sharp rise in the temperature of the vehicle surface and the surrounding air.High temperatures will lead to the surface of the aircraft to deform or even melt,so it is necessary to adopt new thermal protection materials to overcome the problems caused by "thermal barrier".Under the action of high temperature,the air surrounding the vehicle will undergo a thermochemical ionization reaction,creating a layer of "plasma sheath".The plasma sheath will affect the normal transmission of electromagnetic signals and interfere with the communication,measurement and control,and navigation process of the aircraft.Therefore,it is of great significance to find practical and effective methods to suppress the interference caused by "black barrier".The study of the propagation characteristics of electromagnetic waves in the plasma sheath is the key to solve the problem of communication black barriers in hypersonic vehicles.In this paper,the propagation characteristics of electromagnetic waves in the plasma sheath are calculated,and the ground experiment simulation device of the plasma sheath is built,the electron density distribution and the antenna transmission coefficient of the experimental device are measured.The influence of the antenna performance in different regions of the plasma sheath is studied,and finally the influence of the ablation effect of the protective material on the antenna radiation performance is analyzed.The main work of the thesis is as follows:1.The propagation characteristics of electromagnetic waves in the plasma sheath are studied.Using the transmission line analogy method,the transmission characteristics of electromagnetic waves in inhomogeneous plasma are calculated,and the influence of plasma electron density,electron collision frequency,incident angle and plasma inhomogeneity on the transmission characteristics is analyzed.The reflectance and transmittance of the blunt cone plasma sheath and the HTV-2-like plasma sheath are calculated respectively,and the influence of the plasma sheath on the electromagnetic wave propagation characteristics at different flight altitudes and flight speeds is discussed.The transmission line analogy method is improved,the first-order differential Gaussian pulse is used as the incident wave,The variation of reflected echo and incident wave in plasma was compared,and the influence of plasma sheath on the echo amplitude,center frequency and spectrum width is analyzed.Finally,Finally,the phase change characteristics of the electromagnetic wave after passing through the plasma sheath are analyzed.2.The electron density distribution and propagation characteristics of plasma sheath ground simulator were measured.An inductively coupled plasma(ICP)experimental device was built,and argon was used as the ionized gas.The electron density distributions of plasma generators at different power levels were measured by using a Langmuir single probe.The transmittance of the plasma generator was measured by using a broadband horn antenna.,and the measured frequency-selective characteristics of the plasma are basically consistent with the law obtained by theoretical calculations.Propagation coefficients of microstrip antennas in different frequency bands in plasma generator were measured,and the influence of different positions,different powers,and different argon gas intakes on the propagation coefficients of the antennas was analyzed.3.The effects of different regions of the plasma sheath on the performance of the antenna are studied.The plasma sheath flow field of the blunt-cone aircraft is divided into stagnation region,middle region and tail region,and the HTV-2-like plasma sheath flow field is divided into back region and abdominal region.The distribution of parameters such as electron density,neutral molecule concentration and plasma temperature in different regions were extracted,and the plasma frequency and collision frequency were calculated and put into the Drude model.The Finite Integration Technique(FIT)was used to simulate the results of the return loss,standing wave ratio and gain pattern of the microstrip antenna after passing through different plasma sheaths,and the changes of the antenna impedance and radiation characteristics under different regions of the sheath,different flight altitudes and different flight speeds were compared.4.The influence of the ablation effect of the plasma sheath on the antenna radiation characteristics is studied.The ablative particles produced by the ablation and peeling of the protective material enter the plasma to generate ablation plasma.In this paper,the influence of plasma under the weak ionization model containing ablation particles on the antenna pattern is calculated by using the ray tracing method,and the influence of different ablative particle concentration and radius on antenna radiation characteristics is analyzed.Finally,the influence of the plasma sheath produced under the coating of carbon-based protective material on the antenna radiation pattern is analyzed.In summary,the propagation characteristics in the plasma sheath studied in this paper can provide an effective reference for the selection of communication frequency bands for hypersonic vehicles.Studying the influence of different region of the plasma sheath on antenna performance is helpful to the layout optimization of the antenna.Studying the influence of ablation effects on antenna performance has certain guiding significance for the design and selection of aircraft protective materials.