Experimental investigation of electrical breakdown in nitrogen and oxygen induced by focused laser radiation at 1.064 μ

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Abstract
The characteristics of laser-induced breakdown in N2 and O2 at a wavelength of 1.064 μ were investigated using 10-nsec pulses for pressures ranging from 1 to 50 atm. The pressure dependence of the experimentally determined threshold field intensities for these two gases is essentially the same except at the lowest pressures where the O2 values are slightly lower than those for N2. Comparison of this data with existing microwave data can be accomplished most directly by means of a ’’universal plot’’ of the type used effectively in the latter case. The general trends of the microwave and optical data are essentially the same; however, actual numerical values for the optical data tend to be lower than the corresponding microwave values by a constant factor of roughly 0.5. The fact that the two sets of data correspond so closely in spite of the large difference in frequencies has to be regarded as providing substantial evidence supporting the validity at optical frequencies of the cascade theory of breakdown used to interpret microwave breakdown measurements. Time-resolved and time-integrated emission spectra were analyzed, from which the electron density and temperature were obtained for the initial stage of spark development. The spectral characteristics, the electron densities, and temperatures are similar to those determined previously by other investigators for air.