INVESTIGATION OF THE MECHANISMS ASSOCIATED WITH GAS BREAKDOWN UNDER INTENSE OPTICAL ILLUMINATION

Experimentally, the focused high-intensity optical frequency beam from a Q-spoiled laser is used to cause electrical breakdown in a test gas, and the ionization produced is examined as a function of gas pressure. Of the gases studied, breakdown in air required the highest field strengths, with lower field strengths required in helium and in argon. Studies were also conducted of the attenuation of the laser beam by the breakdown plasma. With either ruby or neodymium incident radiation, it is observed that more than half of the laser beam energy can be absorbed in the plasma produced by the breakdown and that over 90% attenuation of the laser beam can occur at later times in the optical pulse. Preliminary measurements were made of the effects of diffusion loss on the breakdown threshold by varying the focal volume within which the breakdown is formed. These data show that, in argon at atmospheric pressure, the breakdown threshold electric field strength is inversely related to the dimensions of the breakdown region; i.e., breakdown within small focus volumes requires a greater optical frequency electric field than is necessary with a larger focus region. This implies that, over the range of breakdown volumes studied, at atmospheric pressure diffusion losses play a significant role in the development of optical frequency breakdown.