2D Reconstruction of Laser Plasma Electron Density with FFT Method

A transient Mach-Zehnder interferometer has been developed to investigate laser induced plasma on Al target surface in air. The phase of the reconstructed interferograms are extracted with a Fast Fourier-transform method at first, and then a numerical inverse Abel transformation is applied to determine the radial refractive index distribution under the assumption of the plasma plume having a radial symmetry. Finally, based on the fact that the plasma index of refraction is proportional to the density of free electrons in plasma, the plasma electron density is calculated. In the result, the regions with lower refractive index value than the ambient atmosphere reveal the abundance of free electrons, while higher value shows the increased density of heavy particles. The result of the electron density shows that the value in the vicinity of the target is 4.0 times 10²⁶ m^-3 while reach up to 9.0 times 10²⁶ m^-3 at the tip of the plume behind the ionization front.