Mathematical Modeling of Laser Ablation in Liquids with Application to Laser Ultrasonics

Conant, R. J.
Garwick, S.
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The use of pulsed lasers to produce ultrasonic waves in materials has proven to be attractive in many applications. However, one of the limitations of laser ultrasonics is the weak signal strength produced by thermoelastic sources. One way to improve signal strength is to use laser intensities that are high enough to ablate the material surface. While ablation leads to surface damage in solids, it is generally not a problem in liquids. Consequently ablation is a viable means of enhancing signal strength for laser ultrasonics applications such as high temperature materials processing involving molten metals. Experiments carried out at the Idaho National Engineering Laboratory (INEL) on liquid mercury, using the experimental setup shown in Fig. la, indicate that the signal strength can be increased two orders of magnitude through ablation [1]. Mercury is a nearly ideal material with which to study ablation in molten metals because it is liquid at room temperature and its properties are well known. The results of the experiments carried out at the INEL are shown in Fig. lb. For laser intensities below about 6 MW/cm2 the ultrasonic signal results from thermoelastic sources (rapid thermal expansion) while at intensities above about 20 MW/cm2 ablation is the dominant mechanism. A transition between rapid thermal expansion and ablation occurs between 6 MW/cm2 and 20 MW/cm2.