Measurements of Air Breakdown and Scaling to Microwaves Using 193 nm Focused Laser Radiation

Abstract

The measurements and analysis of air breakdown processes by focusing 193 nm, 200 mJ, 10 MW high power UV laser radiation on to a 10 -20 pm spot size that produces laser power densities up to 1012_ 1013 W/cm2, well above the threshold power flux for air ionization will be presented. The breakdown threshold is measured and compared with classical and quantum theoretical models. A universal scaling analysis of these results allows one to predict some aspects of high power microwave breakdown based on measured laser breakdown observations. The air breakdown threshold is being measured for a wide pressure range from 90 Torr to 5 atmospheres. Above atmospheric pressures and their increased collisional frequencies relative to the high laser frequency (1015 Hz) allow enhanced ionization and plasma formation. Multi-photon ionization processes can also play a substantial role at 193 nm due to the high photon energies (6.4 eV). The laser breakdown threshold data for air at 193 nm is correlated with corresponding microwave breakdown values using the concept of universal scaling, for which extensive microwave data is available [1-2] and current microwave breakdown measurements are being obtained at TexasTech University [3]. An extensive range of optical and spectroscopic diagnostics with ns fast gating and 13 ptm ICCD resolution is utilized to characterize the plasma. Laser shadowgraphy technique is used to characterize the spatial and temporal evolution of the laser focused plasma ionization shockwave. Plasma temperature is measure using optical emission spectroscopy method of the N2 second positive system N2 (2+) (0,0) at 337.1 nm band. Initial measurements of vibrational temperatures obtained from nitrogen lines at 500 pts are found to be 0.22 eV with electronic temperatures of 0.8 eV and rotational temperatures of 0.1 eV. Plasma density and neutral pressure variations are measured using two color laser interferometry. Phase shifts produced by the laser focused plasma for red (632 nm) and green (532 nm) wavelengths are being measured, which is used to measure the plasma and neutral pressure densities. The peak plasma densities are anticipated to be in the 1016-18/cc range. A comparison of the laser breakdown case to the microwave breakdown case utilizing the universal scaling law is being carried out and will be presented.