Influence of the focusing f number on the heating regime transition in laser absorption waves

Laser plasma was produced in air atmosphere using a transversely excited atmospheric CO2 pulse laser; the influence of the focusing f number (≡ focal length/laser beam diameter) on the threshold of a heating regime transition in laser absorption waves was then investigated. As a result, when the laser energy was 10 J/pulse, the transition threshold was 2.6±0.4 MW/cm2 with f=1.1 optics and 3.7±0.4 MW/cm2 with f=2.2. The difference was explained in terms of the front area and absorption layer thickness in the laser supported detonation regime.