Theoretical influence of ablator thickness on laser induced hydrodynamics in materials

Abstract The studies on material response to hydrodynamic shock is a topic of large interest because it allows one to determine the material properties including the equation of state and the mechanical behavior, to evaluate the mechanical damage resistance, or to improve mechanical material properties including the adhesion between two materials. For such studies, the hydrodynamic shock can be induced by irradiating the target with a laser pulse where a so called ablator is commonly arranged on the target surface. This ablator allows one not to damage the target surface by the laser irradiation and, owing to other studies, allows one to better control the laser energy deposition and the subsequent amplitude of the induced pressure loading. Such a control of the pressure source term is simpler with a metallic ablator, especially with aluminum for which the laser absorption can be evaluated with a laser-matter interaction code. However, for parametric studies for instance, such an approach may be computationally expensive and possibly prohibitive. The use of an applied loading as boundary condition mimicking the laser-matter interaction then is an alternative. However, due to the various underlying assumptions of the boundary condition approach, it is thus not clear to which extent the latter is equivalent to the loading induced by laser-matter interaction. The present work first addresses this question by performing numerical simulations within various conditions including variations in the ablator thickness and in the laser pulse duration (nanosecond range). Aluminum and alumina are chosen for ablator and target, respectively, as materials of wide interest. Although significant ablation may take place, it is shown that an equivalence between the two loading procedures is obtained when the initial ablator thickness is used for the boundary condition approach. This result is related to the back-and-forth wave dynamics in the ablator initiated by the acoustic impedance mismatch between ablator and target material. This back-and-forth wave dynamics is in particular analyzed through analytical considerations. For a thick enough ablator, it is shown that the primary compression wave is followed by a secondary tensile wave. As a second study, the present work shows how one can take advantage of this generated tensile wave to test the adhesion between two materials. A procedure for laser shock adhesion test (LASAT) based solely on the arrangement of an ablator layer on the target then is proposed.