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

Abstract

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.

Velocity and mass density of the ejecta produced from sinusoidal grooves in laser shock-loaded tin

“…When the shock wave breaks out at the opposite free surface, its interaction with the grooves produces the ejection of fast debris, in the form of jets ahead of the main, planar surface. Time-resolved measurements of both jets and planar surface velocities were performed using Photonic Doppler Velocimetry (PDV), while material ejection was observed in transverse optical shadowgraphy, using an ultra high-speed camera with a magnification of 1.93 µm/pixel and exposure times of 5 ns to minimize motion blur [15][16][17]. Finally, at a controlled delay time after the ns-drive, a 1.5 ps-duration laser probe of about 10 J was shot to the tip of a freestanding copper wire of 20 µm-diameter set about 1 cm below the grooved pattern.…”

“…Finally, at a controlled delay time after the ns-drive, a 1.5 ps-duration laser probe of about 10 J was shot to the tip of a freestanding copper wire of 20 µm-diameter set about 1 cm below the grooved pattern. Subsequent emission of a ps-duration X-ray pulse, mainly K-shell radiation (at 8 keV for copper), allows ultra-short in-situ radiography of the ejecta [17,22,23] along the third (vertical) direction, normal to the shadowgraphy axis (Figure 2), using a distant image plate shielded with a 15 µm-thick aluminum foil. The spatial resolution was evaluated at about 20 µm from a static radiograph of a metal grid set at the sample position [22].…”

“…More details on the geometry as well as an example of X-ray spectrum (monitored at each shot inside the vacuum chamber) can be found in Ref. 17. Lasers and diagnostics are synchronized using optical marks, where the time resolution is limited by the bandwidth of the slowest photodiode (≈ 500 MHz).…”

“…For a few years, we have explored how laser-driven shocks can provide some valuable, complementary insights into ejecta physics under pulsed pressure loads of high amplitude (some tens of GPa) and very short duration (a few ns). Our results include measurements of jet velocities using both optical shadowgraphy and Photonic Doppler Velocimetry (PDV) [15][16][17], post-recovery evidence of the jetting process combined with spall fracture [15,17,18], attempts to estimate ejecta size distributions using fast shadowgraphy [16,19,20] or fragment recovery [21], and ultra-fast laser-based X-ray radiography of the jets [17,22,23]. The latter technique was used to evaluate mass ejection from single triangular grooves of controlled depths and angles [17].…”

“…Our results include measurements of jet velocities using both optical shadowgraphy and Photonic Doppler Velocimetry (PDV) [15][16][17], post-recovery evidence of the jetting process combined with spall fracture [15,17,18], attempts to estimate ejecta size distributions using fast shadowgraphy [16,19,20] or fragment recovery [21], and ultra-fast laser-based X-ray radiography of the jets [17,22,23]. The latter technique was used to evaluate mass ejection from single triangular grooves of controlled depths and angles [17]. However, surface roughness resulting from machining processes is usually better represented by periodic perturbations.…”