Shock wave driven by high intensity nanosecond laser pulse in atmospheric air and 2-D determination of refractive index variations of hot core air

Abstract

In this paper we present the results of determination of 2-dimensional air refractive index variations associated with hot core air in an optical breakdown process. The optical breakdown was produced by focusing of a 9 ns, Q-switched Nd:YAG laser beam (λ = 532 nm) in atmospheric air. Shock and thermal waves (associated with hot core air) were formed in this process during a finite time after the optical breakdown. Using shadowgraphy technique, last stage of the shock wave passage through the focusing region and the formation of thermal wave of the hot core air in focusing region were monitored. The shadowgrams were processed by a developed computer code for calculating (in 2-D) the temporal and spatial variations of non-uniform refractive index of the disturbed region. From the results typical refractive index variations of 10 − 7 was estimated for the conditions of experiment. The disturbance dimension and velocity of the interaction region were also estimated from the shadowgrams. From the results, a relaxation time of ~ 4.5 ms was estimated for the hot core air to cool down to the room temperature.