Radial variation of laser-induced air plasma parameters

Summary form only given. Laser induced breakdown spectroscopy (LIBS) is a noninvasive technique that needs no sample preparation and even recently, quantitative measurements were done without the need for calibration standards. Much research has been done on the laser induced air plasma to study the spatial variation of plasma parameters in the axial direction of the laser beam. In this work we report investigation on the radial variation of the parameters of air plasma during laser induced breakdown of air. The results show that the radial variations of the plasma frequency and refractive index are caused by the radial electron gradient of the plasma. The radial variation of the electron density shows a noticeable depletion at the centre of the focus which results in plasma phase velocities greater than the speed of light. Further from the plasma core the electron density diminishes to ambient density at the edges of the plasma. The FWHM for these plasma profiles was found to be about 1.2 mm. The results further reveal that the plasma has a concave electron density and a convex refractive index profile near the center of the laser axis, i.e. around a diameter of 0.5 mm. The electron density and electron-neutral collision frequency are crucial in understanding transport coefficients of large scale plasmas as found in thermonuclear fusion devices like tokamaks, ITER, JET, etc