Characterization of Schlieren Light Source Using Laser -Induced Optical Breakdown in Argon

Abstract

Introduction S CHLIEREN imaging systems are widely used for both qualitative and quantitative flow visualization in compressible flows and active index of refraction flowfields such as two-index mixing problems or combustion flowfields. A common implementation of schlieren imaging uses a pulsed light source to provide instantaneous measurements in unsteady flowfields. A variety of pulsed light sources has been used in the past with schlieren imaging systems, including arc lamps, incandescent bulbs, flash tubes, spark gaps, and light-emitting diodes.1 Lasers have been used to provide a narrow linewidth illumination source, which is useful for filtering broad spectral emissions from plasmas or flames, but at the cost of image degradation due to laser speckle. A recent technical note described the use of a laser-induced spark as a point source, which could be inserted in the flowfield avoiding the need to integrate through the density fluctuations associated with the boundary layers on the wind-tunnel walls.2 Recent papers have also described the use of a laser-induced spark as a light source for schlieren imaging in a plasma flow3 and an exploding wire bridge,4 both applications that benefit from a very high-intensity schlieren light source. Our objective in this Technical Note is to characterize a laserinduced spark schlieren imaging technique that provides a very highintensity light source, with short time duration and with repeatable temporal and spatial characteristics. Spatial and temporal variations in intensity are reported for this light source, as well as a comparable light source using a laser discharge in air. This light source has