Influence of ambient air pressure on the energy conversion of laser-breakdown induced blast waves

Influence of ambient pressure on energy conversion efficiency from a Nd : glass laser pulse (lambda = 1.053 mu m) to a laser-induced blast wave was investigated at reduced pressure. Temporal incident and transmission power histories were measured using sets of energy meters and photodetectors. A half-shadowgraph half-self-emission method was applied to visualize laser absorption waves. Results show that the blast energy conversion efficiency eta(bw) decreased monotonically with the decrease in ambient pressure. The decrease was small, from 40% to 38%, for the pressure change from 101 kPa to 50 kPa, but the decrease was considerable, to 24%, when the pressure was reduced to 30 kPa. Compared with a TEA-CO2-laser-induced blast wave (lambda = 10.6 mu m), higher fraction absorption in the laser supported detonation regime eta(LSD) of 90% was observed, which is influenced slightly by the reduction of ambient pressure. The conversion fraction eta(bw)/eta(LSD) approximate to 90% was achieved at pressure > 50 kPa, which is significantly higher than that in a CO2 laser case.