Helium breakdown by DF laser radiation

Abstract
Thresholds have been measured for the breakdown of helium by laser radiation near 3.8 μm in wavelength. Four-μsec chemical laser pulses were focused to a well-defined spot 100 μm in diameter. Values of the instantaneous irradiance qt and the accumulated fluence et occurring at time of breakdown were observed as a function of helium pressure in the range 0.12–0.99 atm. The measured qt was 2.3 ×1010 W/cm2 at 1 atm, in good agreement with results scaled from other wavelengths.