Picosecond x-ray radiography of microjets expanding from laser shock-loaded grooves

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.