Threshold laser power density for regime transition of a laser absorption wave in a reduced-density air atmosphere

Shadowgraph visualization experiments provide measurements of the threshold laser power density (Sth) for the regime transition of a laser absorption wave generated using a transversely excited atmospheric CO2 pulse laser with various pulse shapes. Results revealed a great influence of the plasma expansion in the direction lateral to the wave propagation on the regime transition by showing that the threshold increased proportionally to the inverse of the beam cross-sectional radius at which the transition occurred (rf,tr): Sth=Cth∕rf,tr. The proportionality constant, Cth, was insensitive to air densities of 0.2–1.3kg∕m3.