Parameters Diagnosis Method Of Plasma Sheath Based On Broadband Reflection Microwave

The process of hypersonic flight will produce a plasma sheath covering the surface of the aircraft,leading to the deterioration of communication quality and a “black barrier” phenomenon may occur.The precondition of transmit data reliably and efficiently is to obtain the parameter information of the plasma sheath and provide information for real-time adaptive communication.Since microwave reflection method has the advantages of fast response and non-contact,this paper proposes a real-time diagnosis method to calculate density and collision frequency of the plasma by broadband reflection microwave,which based on the wave impedance theory.The theoretical derivation of the method,the electromagnetic simulation and experiment are completed and diagnosis of density distribution is explored.The method is verified and the diagnosis accuracy is measured.The main research contents and contributions of this paper are as follows:(1)A diagnosis method of plasma sheath parameters based on the analysis of broadband microwave reflection coefficient is proposed.The density and collision frequency of the plasma can be obtained simultaneously.The thick plasma sheath with slowly changing parameters is regarded as a uniform plasma model.The broadband antenna is used to frequency sweep.The effective inverse frequency band is determined according to the propagation characteristics of electromagnetic wave in plasma and the curvature change of the reflection coefficient sweep curve.The relationship between the complex reflection coefficient with the plasma average density and average collision frequency is established based on the wave impedance analysis,then the parameters can be deduced by the reflection coefficient of effective frequency point.The advantage of this method over the traditional microwave reflection diagnosis method is that it does not need to measure the arrival time of the reflection signal,and the measurement equipment and data process are simpler.(2)In actual flight,the environment of aircraft surface is complex.In order to improve the diagnosis accuracy,the calibration algorithm is established to eliminate the interference of the antenna,the test distance and environment in the diagnosis process,so as to extract the real reflection coefficient caused by the plasma.This part integrate the parameter diagnosis method proposed above.In order to verify the feasibility of the method,the actual diagnostic environment is simulated in the CST electromagnetic simulation software,and the plasma and broadband antenna models are established.Three groups of simulation are set up to explore the influence of plasma density,collision frequency and thickness on the propagation characteristics of electromagnetic wave and diagnostic accuracy of the method,as well as the parameters in different plasma states are calculated.In addition,an experiment is completed with the plasma generator in the laboratory.The simulation results are compared with the set value,and the experimental result is compared with the reference value to get the diagnosis accuracy and error analysis.Both electromagnetic simulation and preliminary experiments verify the proposed diagnosis method.(3)Based on the average plasma parameter diagnosis method,the electron density is modeled as non-uniform distribution,and the diagnosis method of plasma sheath parameter non-uniform distribution is further studied based on the analysis of broadband microwave reflection coefficient.The diagnosis method is based on the theory of wave impedance and analysis of electromagnetic wave propagation in multilayer media.Complex transcendental equations are transformed into the optimization variables of given objective function by using Genetic algorithm.Finally,three commonly plasma sheath non-uniform models are used to fit the typical high-level electron density distribution in RAM-C.The propagation characteristics of electromagnetic wave under three kinds of distribution are explored to diagnose the distribution of density and the average collision frequency.The simulation results are equal to the fitting values,which verifies the effectiveness of the method.This method is not limited by the shape of the distribution fitting curve,and is suitable for the inhomogeneous density distribution under various models.