When a shock wave propagating in a solid sample reflects from a free surface, geometrical effects predominantly governed by the roughness and defects of that surface may lead to the ejection of tiny jets that may breakup into high velocity, approximately micrometer-size fragments. This process referred to as microjetting is a major safety issue for engineering applications such as pyrotechnics or armour design. Thus, it has been widely studied both experimentally, under explosive and impact loading, and theoretically. In this paper, microjetting is investigated in the specific loading conditions associated to laser shocks: very short duration of pressure application, very high strain rates, small spatial scales. Material ejection from triangular grooves in the free surface of various metallic samples is studied by combining transverse optical shadowgraphy and time-resolved velocity measurements. The influences of the main parameters (groove angle, shock pressure, nature of the metal) on jet formation and ejection velocity are quantified, and the results are compared to theoretical estimates.

A B S T R A C TThis study presents the results of a large experimental campaign conducted on the Luli2000 laser facility. Thin targets of a commercial grade of porous graphite were submitted to high-power laser-driven shocks leading to their fragmentation. Many diagnostics were used such as high-speed time-and spaceresolved imaging systems (shadowgraphy and photography), laser velocimetry (PDV and VISAR), debris collection and post-mortem X-ray tomography. They provided the loading levels into the targets, the spall strength of the material, the shape and size of debris and the localization of the subsurface cracks. The crossed data reduction of all the records showed their reliability and allowed to get a better insight into the damage phenomena at play in graphite. Thereby, four damage regimes, ranked according to their severity and loading level, were identified. It confirms that laser shocks are very complementary to classical impact tests (plates and spheres) since they ally two-dimensional loadings to the possibility of using both, in-situ and post-mortem diagnostics. Finally, the campaign shall be able to provide large and consistent data to develop and adjust reliable models for shock wave propagation and damage into porous graphite.* Corresponding author. CEA DIF, Bruyères-le-Châtel,

Material ejection upon the breakout of a shock wave at a rough surface is a key safety issue for various applications, including pyrotechnics and inertial confinement fusion. For a few years, we have used laser driven compression to investigate microjetting from calibrated grooves in the free surface of shock-loaded specimens. Fast transverse optical shadowgraphy, time-resolved measurements of planar surface and jet tip velocities, and post-shock analysis of some recovered material have provided data over ranges of small spatial and temporal scales, short loading pulses (ns-order), and extremely high strain rates. In the new experiment reported here, picosecond laser irradiation of a thin copper wire generates an ultrashort x-ray burst which is used to radiograph the microjets expanding from plane wedged-shape grooves in tin and copper samples shock-loaded by a longer, nanosecond laser pulse. Such ultrafast radiography provides estimates of the density gradients along the jets and of the total ejected mass at different times after shock breakout. Furthermore, it reveals regions of low density inside the samples deep beneath the grooves, associated with subsurface damage due to tension induced by the interaction of rarefaction waves. Thus, combining this x-ray probe with our former experimental techniques provides a more complete insight into the physics of microjetting at very high loading rates and the ballistic properties of the resulting ejecta.