Research On The Nonlinear Interaction Between The Electromagnetic Wave And Plasma Sheath

A vehicle at hypersonic velocity or a reentry spacecraft moving through the stratosphere is covered by the plasma sheath.On one hand,the plasma sheath considerably blocks the communication between the aircraft and the ground station,namely blackout.On the other hand,it affects the radar detection of the aircraft.Thus,correct understanding about the interaction mechanism between the electromagnetic wave and the plasma sheath has been a very important and urgent research subject.The interaction is extremely complex and can be affected by lots of factors,leading to kinds of nonlinear effects during the reentry process.So the research about nonlinear effects is of great concern.The main content of the thesis is carried out from four aspects,namely the production mechanism of the plasma sheath and the simulation of the plasma sheath flow field,the mechanism of nonlinear effects,and how nonlinear effects influence the propagation and scattering of the electromagnetic wave in the plasma sheath.Firstly,the production mechanism of the plasma sheath and the simulation of the plasma sheath flow field.The production mechanism of the plasma sheath is summarized and the models which are used in the simulations are given and then its dielectric constant is derived.The geometric model of the aircraft at hypersonic velocity is built and its surrounding space is subdivided by Gambit.By analyzing the production factors of the plasma sheath,setting up the proper conditions of the hypersonic flow field and solving the control equation via Fluent,the flow field distribution of the plasma sheath surrounding the aircraft and the distribution of electron density are obtained,which lays the foundation for further research on the interaction between the EM wave and the plasma sheath.The magnitude of the result is almost equal to that of the blunt cone in RAM-C.In addition,the result indicates that the electron density of the aircraft's top is two orders greater in magnitude than that of the bottom and the thickness of the plasma sheath is thinner,but the distribution of electron density of the underpart is gentler.Secondly,nonlinear effects in the plasma sheath.Based on the principle of the thermal nonlinear effect,the collision frequency,nonlinear attenuation constant and dielectric constant are obtained.The comparison between linear and nonlinear attenuation constant with the change of frequency is given.The simulations of the collision frequency and dielectric constant perturbation along with the change of electric field intensity and frequency are analyzed.The result illustrates that the thermal nonlinear effect can't be ignored when the electric field intensity is far larger than the characteristic field of plasma.By generalizing the nonlinear electric polarization item,the nonlinear attenuation constant is derived.The simulations of the nonlinear attenuation constant along with the change of frequency under different electron densities and collision frequencies are obtained,so is the comparison between linear and nonlinear results.The result indicates that the nonlinear attenuation constant is smaller than the linear one.Thirdly,the mechanism of how to use the three-wave interaction to affect the propagation of the electromagnetic wave in the plasma sheath in order to mitigate blackout is introduced in this thesis.By applying the Ginzburg equation in the plasma sheath,the magnetic field distributions of three kinds of EM waves are simulated,which proves the method feasible.The influence of related parameters on the three-wave nonlinear effect is established by comparing the simulation results under different distributions of the electron density and frequencies of the signal.In addition,the actual effect of the method is testified by simulating the three waves' magnetic distributions of RAM-C at different space heights.The result shows that the method has a good actual application prospect.Fourthly,the influence of the nonlinear effect on the electromagnetic scattering characteristics of the aircraft covered by the plasma sheath is analyzed.Based on the PO and FDTD method for coated targets,the bi-static RCS of the electric square column which is coated with the uniform magnetized plasma,the blunt-cone-shaped and the HTV-similar-shaped aircrafts at hypersonic velocity are simulated,and then the influence of the thermal nonlinear effect on the RCS is analyzed.The results indicate that the RCS of three targets is smaller and the bi-static RCS of the blunt cone under different frequencies is changed because of the thermal nonlinear effect.