Studies On Microwave Interference And Microwave Diagnosis Of Laser-induced Plasma

The effect of laser-induced plasma on microwave transmission using experimental measurement and numerical simulation is reported. It is studied that the effect of laser power density, electron density and collision frequency between electron and neutral on the reflection and transmission coefficient and the frequency shift. In additionally, the velocity of critical surface of plasma and the lifetime of plasma are measured using the spectrum of reflected microwave.The reflection and transmission coefficients are got by finite difference time domain. The relationship among the reflection and transmission coefficient and the plasma frequency, the electron collection frequency is analyzed. The results show that because of the high plasma frequency, the laser-induced plasma can reflect the microwave strongly though the thickness is small.The frequency of microwave reflected by laser-induced plasma is measured by experiment. The frequency upshift and downshift corresponding to the plasma expansion process and extinguish process are found. The relationship among laser power density, the frequency of incident microwave and the frequency shift is investigated. In additionally, the effect of plasma frequency and electron collision frequency on frequency shift of microwave reflected by plasma is investigated using FDTD. The results show that the frequency shift of microwave is affected by the change rate of plasma electron density, the change rate of electron collection frequency affects the reflection coefficient.The feasibility of laser-induced plasma interfering a radar homing missile simulator is studied, and the effect of pulse energy of laser, the power of incident microwave and the frequency of laser pulse on the interference are analyzed.The frequency of microwave reflected by plasma is measured by experiment, and the velocity of plasma is got by Doppler formula. The relationship among the velocity of plasma, the laser power density and the plasma electron density is analyzed.A new method of measuring the plasma lifetime is introduced. A continuous microwave is introduced to the laser-induced plasmas and the reflected microwave is measured. From the frequency spectrum of the reflected microwave, the lifetime of plasma with critical density is investigated. The results indicate that the lifetime of plasma with critical density decreases with the increasing laser power density. And the lifetime of plasma with high electron density is shorter than that with low electron density. These research results will provide a theoretical and experimental basis for laser-induced plasma interference and provide a new method for plasma diagnosis.