Measurements of Air Breakdown Process using 193 NM Focused Laser Radiation

Summary form only given. We report the measurements and analysis of air breakdown process by focusing 193 nm, 200 mJ, 10 MW high power UV laser radiation on to a 10-20 mum spot size that can produce a maximum laser intensity of 10¹²-10¹³ W/cm², and well above the threshold flux for air ionization. The breakdown threshold is measured and compared with the theoretical models such as classical and quantum analyses. The air breakdown threshold is measured for wide pressures range from 90 Torr to 5 atmosphere pressures. Above atmospheric pressures are required to increase the collisional frequency comparative to the high laser frequency (10¹⁵ Hz). Multiphoton ionization processes are also expected to play a substantial role at 193 nm due to the high photon energy (6.4 eV). The breakdown threshold data for air at 193 nm is correlated with the microwave breakdown using the concept of universal scaling, for which extensive microwave breakdown data is available at frequencies of 0.99, 2.8 and 9.4 GHz and current microwave breakdown results obtained at Texas Tech University. An extensive range of optical and spectroscopy 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 the shadowgraphy technique. The plasma temperatures can be 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 the N₂ second positive system N₂ (2+) (0,0) at 337.1 nm. Plasma density is measured using two color laser interferometry. Phase shift produced by the laser focused plasma for two different probe laser wavelengths such as red (632 nm) and green (532 nm) wavelengths will be measured, which is used to measure the plasma density.