Observing Laser-Shocked Tin Substrates

When a shock wave generated by a laser beam or similar concentrated force bounces off a free surface, the material can undergo shock-induced fragmentation. Although commonly encountered in engineering and physics, the phenomenon remains poorly understood for fluid media, such as metal melted upon impact with laser light. To capture the events involved in shock-induced fragmentation of tin samples, French investigators have used high-speed imaging techniques.

Motivated by previous studies, the scientists designed a setup incorporating a pulsed Nd:glass laser at 1.06 μm and 1.7 kJ of energy and a vacuum chamber to house samples during laser exposure. For analysis, they chose 50- and 260-μm-thick tin targets backed with copper foil or polycarbonate shields placed 0.8 and 1.2 mm away, respectively. To monitor the behavior of fast particles during and after shocking, they utilized transverse shadowgraphy with two Kentech Instruments gated optical imagers and a pulsed 532-nm laser light source. In a separate experiment, slower fragments were tracked with a Photron video camera with green continuous laser illumination.

Laser-propelled tin fragments have created tiny craters in a plastic substrate, while larger droplets cling to its surface, as seen in this electron micrograph. Courtesy of Thibaut de Rességuier, Centre National de la Recherche Scientifique.

Irradiating a 2.5-mm-diameter zone of the 50-μm-thick tin piece, the investigators observed complete spallation — transformation of the exposed region into submicrometer droplets. In images, the fragmented tin appeared as a fine cloud moving away from the laser at an estimated 2.5 km/s. Electron micrographs of the shield revealed deep and uniform embedding. In the 260-μm sample, 60 μm of the target opposite to the laser underwent similar spallation, while the remaining 200-μm layer turned into large droplets moving at around 16 m/s. The latter eventually settled on the plastic, and after about 2 ms, the group observed no particle movement. Although the spallation results aligned well with expectations, the late fragmentation of the thicker sample did not match the model predictions.

Although the scientists’ work permitted basic characterization of the effects of laser shock and the timing of cloud- and large-particle movement, insufficient image quality will make further research necessary to obtain more accurate ejection velocity approximations. Subsequent studies also may involve foam shields, which could eliminate interference associated with polycarbonate fragment rebound.

(Applied Physics Letters, 3 April 2008, Vol. 92, 131910)