Striated filamentary sparks produced by a CO2 TEA laser

Sparks in the form of long, thin filaments having quasi-periodic longitudinal light and dark regions (striations) in time-integrated images have been observed in various gases using a CO(2) TEA laser. Typically, a 50-mJ pulse will produce a filament 1 cm long and 130 microm in diameter, with more than 150 striations spaced 50 microm apart in atmospheric air. Each striation is associated with the formation of a plasma region by one pulse in a train of pulses from the mode-locked laser, and the filament results from the formation of successive (nearly identical) regions, each displaced from the previous one toward the laser. The possible use of these sparks as a light source in diagnostics is noted.