Research On Electromagnetic Characteristics Of Plasma And Microwave Diagnostic Approaches

The reentry of a hypersonic vehicle is a complex physical process involving multiple disciplines and stages.The research on the electromagnetic wave propagation characteristics of plasma during the process has important guiding significance for alleviating the reentry blackout problem.At present,the theoretical research on the electromagnetic wave propagation characteristics of plasma tends to be complete.Due to the high cost and complexity of the reentry laboratory experiment,the corresponding experimental research develops relatively slowly.Based on the above research background,microwave measurement approaches are intensively studied for plasma diagnostic,and a series of experimental researches are carried out on the shock tube to analyze the transmission and reflection characteristics of electromagnetic wave propagation in plasma and the related nonlinear phenomena.The detailed research contents are presented as follows:1.A microwave reflection approach utilizing open-ended rectangular waveguide(ORW)is studied.Based on the analytical model that an airgap exists between ORW and material under test(MUT),a calibration technique is proposed to reduce the effects of higher-order mode fields in dominant mode analysis and undesired reflections caused by the finite-sized flange,achieving dielectric constant characterization of low-loss slab materials.A small perturbation is introduced in calculating the reflection coefficient to eliminate the effects of singularities.Simulation results are utilized to validate the proposed approach.The selection of airgap thickness and the effects of lossy MUT are also discussed.The measured dielectric constants of MUT made of PMMA and PTFE are presented,which agree excellently with the results achieved from the free-space method.2.Based on the lens-horn antenna,a microwave reflection measurement method is proposed to determine the dielectric constant of dielectric slabs without knowing their exact thickness,and the proposed method is suitable for the thin-slab materials with low loss and low dielectric constant.By analyzing the field distributions at the feed port and radiation aperture of the lens-horn antenna,the equivalent analytical model is established and the corresponding calibration is proposed to measure the reflection coefficient at the reference plane.By neglecting the loss of material,the expression between the dielectric constant and the reflection coefficient of lens-horn antenna is derived according to transmission line theory.Error analysis for lossy dielectric slabs are conducted.The measured results of PMMA,PTFE,and FR4 samples are presented to validate the proposed method.Excellent agreement are achieved between the proposed method and free-space method.3.A microwave reflectometry is proposed to extract the permittivity of time-varying plasma generated inside the shock tube.To remove the influence of parasitic reflections caused by the surroundings,a calibration process is introduced and the unknown calibration coefficients are determined by utilizing microwave interferometry as the reference technique.The shock tube is modeled as a three-layered medium to calculate the reflection coefficient.A time-dependent reconstruction algorithm is applied and theoretically validated to eliminate the multiple solutions in the inverse problem.By comparing the permittivities extracted with microwave reflectometry and interferometry,the effects of plasma diffusion are demonstrated with a modified analytical model in the beginning time region of experiments.In addition,the nonuniform flow in the generated plasma located near the end time region is also observed.The determination of the effective time region for electron density Ne and collision frequency ve extraction is discussed as well.Finally,the differences between microwave reflectometry and interferometry in terms of averaged Ne<1 × 1017m3 and averaged ve<1.5 × 109 s-1 are investigated in the effective time region.The influence of shock velocity on Ne and ve is also discussed with the measured results of microwave interferometry.4.Nonlinear electromagnetic wave phenomena induced by transient time-varying plasmas inside the shock tube are investigated.Microwave interferometer and reflectometer are installed at Test Section ?,and the millimeterwave reflectometer is installed at Test Section ?.All the systems have trapped the nonlinear waves induced by the plasmas located at the front of the shock wave.Based on the spectral analysis of these nonlinear waves,it is found that the nonlinear phenomenon can be divided into two regions in the time domain,and multiple nonlinear frequencies are observed in each region.The measured results of multiple experiments demonstrate that the frequency differences between the nonlinear waves and the incident wave are related to the shock wave velocity and the particle formation of plasma.By assuming the particle arrangements in the plasma,a periodic time-varying medium model for transient time-varying plasma is proposed,and the possible physical mechanism of this nonlinear phenomenon is also analyzed.