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Laser-Induced Breakdown in Oxygen Gas at High Pressure T. H.Wiggins, R.V.Wick, D. H. Rank, and A. H. Guenther T. A. Wiggins, R. V. Wick, and D. H. Rank are in the Physics Department, The Pennsylvania State University, University Park, Pennsylvania, and A. H. Guenther is in the Air Force Weapons Laboratory, Kirtland AFB, New Mexico. Received 18 August 1965. We have been investigating gases under pressure to observe the stimulated Brillouin effect and the stimulated Raman effect. We have observed the stimulated Brillouin effect in CH4, N 2 , and CO2 and the stimulated Raman effect in CH 4 and N 2 . Extensive quantitative measurements have been made on all three of the above-mentioned gases. These experiments will be reported in detail in another paper. The pressure cell employed was 6 cm long and had an internal diameter of 2 cm. The windows were 2.5 cm thick and were constructed either from plate glass or quartz. The cell was rendered gas tight by means of neoprene gaskets held under com- pression. Only the edges of the gaskets were in contact with the ompressed gases. The excitation of the stimulated spectra was by means of a Korad ruby laser whose output in a single pulse usually was about 0.5 J with a pulse width of 10-15 nsec. In the experiments with gases the laser usually operated in a single mode. The mode separation was 0.016 cm -1 . The light was focused in the middle of the cell with a 5-cm focal length lens. Whenever the stimulated Brillouin threshold was exceeded it became immediately apparent since the integrated energy (due to stimulation of the ruby at the doppler shifted wavelengths) would rise to a value as great as 1.1J. The time resolution of the dop- pler shifted pulses has been demonstrated by Wiggins et al. 1 We have performed a single experiment with oxygen gas at 395-atm pressure. The oxygen was compressed to the high pres- sure by cryogenic means. The giant pulse caused a breakdown most probably at the focus of the lens. Presumably the plasma or the shock wave initiated by the spark spread to the ends of the cell and ignited the neo- prene gaskets. We were aware of the remote possibility of this happening, and the pressure was released immediately (within a second) upon observance of ignition. The plate glass windows were not shattered but were very badly and deeply crazed on their inner surfaces. The oscilloscope gave a measurement of the pulse energy of 0.9 J with a half-intensity width of about 10 nsec. Unfor- tunately, the Fabry-Perot etalon (40-mm spacing of plates) was not in the best adjustment. The fringes appeared to be about order wide and did not show resolution of either the Brillouin doublet or the mode structure. There is a good probability that the pulse contained a single mode, and the breadth of the fringes was the result of the stimulated Brillouin effect. It is almost certain that the stimulated Brillouin effect was excited, since in our experience with the apparatus with other gases and liquids we never obtain energies in excess of 0.5 J to 0.6 J unless the stimulated Brillouin effect occurs. The laser and the gas cell were separated by 60 cm so that the minimum time between the initial and the stimulated laser pulse must be 4 nsec. Thus it can be seen that the laser pulse width must have been somewhat smaller than the 10 nsec indicated by the oscilloscope. We can estimate that the power level at which the breakdown occurred may be as high as 75 mW. 166 APPLIED OPTICS / Vol. 5, No. 1 / January 1966
