Differential Laser Interferometry For Dense Plasma Density Measurement

In this dissertation, the background and the significance of differential interferometry for dense plasma density measurement were stated. In order to construct the perfect technique for this measurement, the theory and the experiments were extensively and deeply studied.Theoretically, using the theory of mutual effect between laser and plasma, the rule of laser propagating in plasma, and the principle of differential interferometry, the basic model of differential interferometry for plasma density measurement was built. Numerical simulations were done to show the distortion of the wavefront and the interference pattern under several kinds of typical plasma density distributions. The ray path in plasma was calculated by the numerical solution of the vector equation of light ray, when the plasma density was closing to the critical electron density of the laser frequency. Through the simulations, the factors that influence the measurement were found out under different density distributions and shapes of plasma. The numerical simulations were also applied to provide original data for testing data processes method.In order to reconstruct the distribution of the local plasma density, a sequence of data process should be done. Firstly the phase is extracted from interferogram by filtering and displacing frequency spectrum. Then the wavefront and the line integral density are recovered by using FFT or polynomial fitting method. Lastly the radial local density distribution is calculated from the fitting curve of line integral density by the Abel inversion.We also derived the relationship between the radial local density distribution and the observed line integral density distribution by Fourier integral transform. On this basis, the density inversion calculation is realized through FFT method and is independent of Abel inversion. The FFT inversion method is fit for reconstruction of density distribution with sharp peaks and complex shape. The results of numerical simulations of this method show high inversion accuracy and fast calculation speed. Further the iterative method for reconstructing the density distribution of strongly refracting plasma was theoretically studied.Practically, we designed a laser differential interferometer system for dense plasma density measurement. The laser characteristics such as wavelength, pulse width, pulse energy, etc. were analyzed and the object-image correspondence in the system was studied. The common path defocusing 4f shearing interferometer in the system was constructed, which is fit for the dense plasma measurement. The advantages of this system consist in its stability and easy adjustment as the common path system and respective adjustability of shearing amount and stripe spatial frequency as M-Z interferometer as well.Experimentally, the plasma produced by cable plasma gun (C-Gun) and dense plasma focus (DPF) was measured by the laser differential interferometer system. The clear interference patterns were acquired at different time. The contour maps of line integral density and curves of radial local density at different height above nozzle were presented. The highest local density of C-Gun plasma is 2.6×l017cm-3. The local density of DPF plasma nearby the focus is 1.2×1019cm-3.