Computational model of the laser induced Schlieren Technique in sodium-nitrogen atmospheres

An evaluation has been made of the feasibility of detecting trace species concentrations in a host gas mixture using a laser induced Schlieren Technique (LIST). A numerical prediction of the deflection of a probe laser beam intersecting the thermal lens caused by a resonantly absorbed laser pump beam which is tuned to a spectral line of the trace species has been found to be proportional to the con centration of the trace species at the intersection point of the two beams. Results from case studies are presented where the gas mixture is Na-H₂O-N₂ using Na as the absorbing trace species and H₂O as a nonabsorbing trace species. It is estimated that for the mixture with Mach numbers less than 0.1, the Na measurement sensitivity is on the order of 1 ppb for a 1 watt pump laser source with a 10^-3 sec pulse width.