Measurements of 193 NM laser air breakdown and scaling to the microwave regime

We report the measurements and analysis of air breakdown process by focusing 193 nm, 200 mJ, 10 MW high power UV laser radiation onto a 20-60 mum spot size that produces a maximum laser intensity of 10¹²-10¹³ W/cm², well above the threshold flux for air ionization. The breakdown threshold is measured and compared with theoretical models including classical (collisional cascade) and quantum (multi-photon) ionization analyses. The air breakdown threshold is measured for a wide range of pressures ranging from 90 torr to 5 atmospheres. Higher pressure enhances the effective electric field due to the increased collisional frequency relative to the high laser frequency (10¹⁵ Hz). Multiphoton ionization (MPI) (n = 3) processes play a substantial role at 193 nm due to the high photon energy (6.4 eV). We examine regimes for which substantial MPI is present and analyze the plasma temperature and density evolution. The breakdown threshold data for air at 193 nm is correlated with the microwave breakdown regime using the concept of universal scaling, for which extensive microwave breakdown data is available as well as current microwave and mm wave breakdown experiments at Texas Tech University and MIT. An extensive range of optical and spectroscopic diagnostics with 5 ns time scale gating and 13 mum ICCD resolution has been constructed to characterize the plasma. The spatial and temporal evolution of the laser focused plasma is measured using shadowgraphy and two- color laser interferometry techniques. The plasma temperatures are obtained by measuring the velocity of the shock wave front and also by using optical emission spectroscopy. Optical emission spectroscopy is performed to diagnose the plasma temperature using the emission lines of O II ranging from 372.3 to 470.4 nm and the band of the N₂ second positive system N₂ (2+) (0,0) at 337.1 nm. Measurements of the core laser plasma density (n_e= 8time- - s10¹⁷/cc) and electron temperature (25 eV) decay are compared with a dominant two- and three-body recombination model with good correlation.