飞行器在高超声速飞行过程中，形成一层包覆在飞行器表面的等离子体鞘套，导致通信质量恶化，形成黑障现象。实现等离子鞘套下信息可靠传输的前提是获得鞘套的参数信息，为实时自适应通信提供依据。由于微波反射法具有响应速度快、非接触测量等优点，为了实现等离子鞘套参数的在线诊断，本文提出一种基于宽带微波反射的等离子鞘套参数诊断方法，进行了方法的理论推导和详细论述，完成了模拟实测环境的电磁仿真和实验验证，进一步开展了电子密度空间分布诊断方法研究，通过仿真计算验证了分布方法理论的有效性。本文主要研究内容和贡献如下：

       （1）提出了一种基于宽带微波反射系数分析的等离子鞘套参数诊断方法，可以同时获得平均电子密度和碰撞频率。针对厚层且参数缓慢变化的等离子鞘套，将其建模为均匀等离子模型。利用宽频带天线进行扫频，依据电磁波在等离子体中的传播特性和反射系数扫频曲线的曲率变化，确定有效反解频段。基于波阻抗分析方法，确立反射系数与等离子体电子密度、碰撞频率之间的关系，通过有效效频点的反射系数反推出等离子鞘套参数。该方法相对传统微波反射诊断方法的优势是无需测量反射信号的到达时间，测量设备和诊断过程更加简单。

（2）实际飞行过程中飞行器表面环境复杂，为了提高诊断精度，建立了标定算法，排除诊断过程中天线、测试距离和复杂环境引入的干扰，以提取单纯由等离子体引起的真实反射系数，完善了上述提出的参数诊断方法。为了进一步验证实测环境中算法的有效性，在CST电磁仿真软件中模拟实际诊断环境，建立等离子体和宽带天线模型，设置了三组仿真分别探究了等离子体电子密度、碰撞频率和厚度对电磁波传播特性以及算法诊断精度的影响，完成了不同等离子体状态下的参数诊断。又利用实验室等离子发生装置进行了实验验证。仿真的参数诊断结果与设置值比对，实验的诊断结果与参考值进行比对，得到诊断精度并分析误差。电磁仿真与初步实验都验证了所提出方法的有效性。

（3）在平均等离子鞘套参数诊断方法的基础上，将电子密度建模成非均匀分布，进一步研究基于宽带微波反射系数分析的等离子鞘套参数空间分布诊断方法。进行了基于波阻抗的多层介质电磁波传播理论分析和推导，使用遗传算法将复杂超越方程组的求解转换成给定目标函数优化变量，最后使用三种常用等离子鞘套非均匀模型拟合RAM-C飞行试验的典型高度下电子密度分布情况，探究三种分布下的电磁波传播特性，诊断电子密度空间分布和平均碰撞频率，验证了方法理论的有效性。该方法可以不受分布拟合曲线形状的限制，适合于各种模型下电子密度分布的诊断。

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.

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