Imaging Ejecta and Debris Cloud Behavior Using Laser Side-lighting

Abstract

The Caltech Small Particle Hypervelocity Impact Range (SPHIR Facility) utilizes a two-stage, light gas gun to accelerate Nylon 6/6 right cylinders (d = 1.8mm, L/D=1, 5.5mg) and spheres (d = 1.8mm, 3.6mg) to impact speeds of 5km/s and above. The projectiles impact aluminum 6061-T6 plate targets. An optical technique was employed to produce images of the hypervelocity impact event with short exposure times (20ns) and short inter-frame times (<1μs). The technique uses coherent illumination, orthogonal to the projectile flight direction, to provide a series of shadowgraph images of the impact on the target. An expanded beam from a 532nm continuous wave laser is used as the illumination source. The beam is expanded to illuminate a 10cm diameter area and is then directed to a gated, intensified high-speed CCD camera. The front ejecta and debris clouds created behind the target are simultaneously imaged with this system. An edge-finding algorithm has been developed to provide a consistent method for identifying the position of the debris-front in sequential images. This technique enables a regular method to investigate the debris cloud evolution and to characterize its asymmetrical features. Furthermore, with the Laser Side-lighting system atmospheric waves emanating from the impact site are also visible. Increasing the atmospheric pressure in the target chamber (above the nominal 1.5 Torr) significantly increases the observable features of these shock waves. The behaviour of these waves provides an improved understanding of the temporal sequence of the impact phenomena.