High-precision Diagnosis Of Non-uniform Plasma Parameters By Microwave Method

When the aircraft is flying at hypersonic speed,a plasma sheath will be formed on the surface of the aircraft,which will deteriorate the communication quality,and in severe cases will form a black barrier.Simulating the plasma sheath environment on the ground is an important means to study the problem of black barriers,and the plasma electromagnetic scientific experiment device for high-speed targets in near-space is designed for ground simulation experiments.The plasma in this device is generated by a megawatt-level power supply through inductive coupling.The plasma has the characteristics of large electron density span,non-uniformity,fast time change,high temperature,large size,and long duration.In view of the above plasma,traditional diagnostic methods have encountered many new problems.In order to diagnose plasma with high precision and high spatial resolution,new ideas and methods must be developed.In order to improve the accuracy of microwave transmission diagnosis,a high-temperature resistant focusing lens antenna is designed,which can greatly reduce the diffraction of microwave to plasma.At the same time,considering the reflection of electromagnetic waves on the plasma,the diagnostic accuracy can be greatly improved.When electromagnetic waves propagate in large-scale,high-electron-density plasma,there is a problem of phase ambiguity.Therefore,a laser-microwave combined diagnosis method is proposed,which can restore the true phase shift.Without the assistance of other diagnostic methods,a method of solving the whole week ambiguity by group delay was proposed for the first time.The radial electron density distribution of cylindrical plasma is non-uniform,so the microwave diffraction diagnosis method and the microwave directional probe diagnosis method are proposed for the first time,both of which can obtain the electron density distribution.The above diagnosis methods are verified by simulation or other diagnosis methods.The research content and innovations of this article mainly include the following four aspects:(1)A new dual-frequency joint diagnosis method to solve the whole-period ambiguity problem.When a traditional antenna transmits plasma,three parts of microwave energy will propagate through the plasma,namely transmission,reflection and diffraction.Of the above three parts,only the transmission part is needed for diagnosis.In order to solve the diffraction problem,a high-temperature-resistant focusing lens antenna is designed in this paper.After testing,the diffraction isolation of plasma of similar size>50d B.Due to the periodicity of the phase of microwave propagation,there is a problem of ambiguity in the phase shift during plasma propagation,and microwave transmission diagnosis cannot be performed.This department proposes a laser-microwave dual-frequency joint diagnosis method,which can restore the true phase shift,so as to diagnose the plasma electron density and collision frequency with high accuracy.(2)A new type of diagnosis method based on group delay to solve the whole cycle ambiguity problem.When the plasma size is large(150mm),the electron density is high(1.5×10¹⁹m^-3),and Ka-band microwaves are used for transmission,the phase shift generated by the plasma will exceed 3000°.The phase measured by the vector network analyzer is the complement of 360°,so the phase has the problem of ambiguity.Without the aid of other diagnostic methods,this paper proposes for the first time a method to recover the true phase shift by using the phase shift group delay.This method can solve the problem of phase full cycle ambiguity,so as to diagnose electron density and collision frequency with high precision.In addition,by analyzing the influence of reflection,the diagnostic accuracy of collision frequency can be increased to 85%within a specific attenuation range.Through the diagnosis of the known dielectric constant of the medium,the diagnosis accuracy of the electron density of the system obtained by the analysis is greater than 96%.(3)A novel diagnostic method of microwave diffraction that can obtain electron density distribution.In the device,the radial electron density of the cylindrical plasma is non-uniform,the center electron density is high,and the edge electron density is low.And the plasma center temperature reaches tens of thousands K,the interventional diagnosis method is no longer applicable,and the transmission method cannot give the electron density distribution.Based on the above problems,this department proposes a non-invasive microwave diffraction diagnosis method.The microwave diffraction method is proposed based on the diffraction phenomenon of microwaves on metal cylinders.Through the diffraction diagnosis of the cylindrical plasma by microwave,the electron density of different radii can be obtained,and then the electron density distribution can be obtained.In order to verify the results of diffraction diagnosis,the light intensity distribution of plasma was compared and verified by analysis.(4)A novel diagnostic method of microwave directional probe that can obtain electron density distribution.For the microwave transmission diagnosis method,the diagnosis result is the average electron density of the microwave propagation path,and the electron density of different positions cannot be obtained.In order to obtain the electron density distribution,the receiving antenna is designed as a small directional probe.The probe is inserted into the plasma to measure the attenuation and phase shift of electromagnetic waves at different plasma thicknesses.Thus,the electron density distribution can be obtained,and the spatial resolution can be higher.In this device,the microwave directional probe method is more suitable for diagnosing the plasma spray column with a smaller diameter than the diffraction method.In addition,the diagnostic results of the microwave diffraction method and the directional probe method are compared to mutually verify the two diagnostic methods.In summary,this article has carried out a classification study on the amplitude attenuation in microwave transmission diagnosis to improve the diagnosis accuracy;A new solution to the problem of phase ambiguity is proposed to restore the true phase offset,and then diagnose with high progress.In order to obtain the spatial distribution of electron density,the microwave diffraction method and the microwave directional probe method are proposed for the first time.The above diagnosis methods have greater universal applicability and provide new ideas for the high-precision and high-spatial resolution diagnosis of plasma.