Experimental Investigation of 193-nm Laser Breakdown in Air

We present the measurements and analysis of laser-induced breakdown processes in dry air at a wavelength of 193 nm by focusing 180-mJ 10-MW high-power 193-nm UV ArF laser radiation onto a 30-mum-radius spot size. We examine pressures ranging from 40 torr to 5 atm, for laser power densities of 1 TW/cm 2 , well above the threshold power flux for air ionization. The breakdown threshold electric field is measured and compared with classical and quantum theoretical ionization models at this short wavelength. A universal scaling analysis of these results allows one to predict aspects of high-power microwave breakdown based on measured laser breakdown observations. Comparison of 193-nm laser-induced effective field intensities for air breakdown data calculated based on the collisional cascade and multiphoton breakdown theories is used successfully to determine the collisional microwave scaled portion with good agreement regarding both pressure dependence and breakdown threshold electric fields. Using a laser shadowgraphy diagnostic technique, the plasma and shock-wave dynamics are analyzed. Blast shock-wave expansion of the plasma and laser-heated neutral gas is observed with average velocities of 47 km/s, and the temporal shock-wave velocity variation is used to determine electron temperature evolution just behind the shock wave.