Femtosecond laser tagging for velocimetry in argon and nitrogen gas mixtures.

Abstract

Tagging is demonstrated in argon and nitrogen gases using a femtosecond laser with pulse energies of approximately 70μJ through a nonresonant ionization process at 267nm. The signal fluorescence lifetime in pure argon and nitrogen-argon mixtures are measured and found to be long enough to make mean velocity and turbulence measurements in a subsonic flow. In pure argon, the dominating processes involve atomic transitions between 700 and 900nm. In argon-nitrogen mixtures, nitrogen quenches atomic argon species and the dominant radiating processes are transitions in the nitrogen second positive system. In pure nitrogen, emission on the microsecond time scale comes from the nitrogen first positive system. Lower energy density is needed for tagging and narrower tagged lines are produced using 267nm as compared to femtosecond laser tagging in argon and nitrogen using 400nm or 800nm. Velocimetry using the 267nm line is demonstrated in a turbulent argon pipe flow and the Taylor microscale of the flow is determined.