Experimental Observations of Laser-Driven Tin Ejecta Microjet Interactions

The study of high-velocity particle-laden flow interactions is of importance for the understanding of a wide range of natural phenomena, ranging from planetary formation to cloud interactions. Experimental observations of particle dynamics are sparse given the difficulty of generating high-velocity flows of many particles. Ejecta microjets are micron-scale jets formed by strong shocks interacting with imprinted surfaces to generate particle plumes traveling at several kilometers per second. As such, the interaction of two ejecta microjets provides a novel experimental methodology to study interacting particle streams. In this Letter, we report the first time sequences of x-ray radiography images of two interacting tin ejecta microjets taken on a platform designed for the OMEGA Extended Performance (OMEGA EP) laser. We observe that the microjets pass through each other unattenuated for the case of $11.7\pm 3.2\mathrm{GPa}$ shock pressures and jet velocities of $2.2\pm 0.5\mathrm{km}/\mathrm{s}$ but show strong interaction dynamics for $116.0\pm 6.1\mathrm{GPa}$ shock pressures and jet velocities of $6.5\pm 0.5\mathrm{km}/\mathrm{s}$. We find that radiation-hydrodynamic simulations of the experiments are able to capture many aspects of the collisional behavior, such as the attenuation of jet velocity in the direction of propagation, but are unable to match the full spread of the strongly interacting cloud.

Figure 1

(a) A schematic of the OMEGA EP laser platform used to drive tin microjets and collect x-ray radiography measurements of the jet interactions. Two long-pulse lasers drive shocks into tin foils with grooves carved into their rear surfaces. A short-pulse laser on a microwire target generates x rays for the radiography measurement. (b) A picture of the target indicating key elements and dimensions. Tin steps of known thicknesses allow calibration of the radiography diagnostic for density reconstruction. The shields are used to block the intense ablation-plasma emission from the radiography image plates. Masks are offset from the inner surfaces of the tin samples to limit the region of the jet that propagates to the interaction point. (c) Simulated spatial variations of pressure at three time instances, highlighting the decay of shock pressure through the tin sample. Groove schematic is overlaid on simulation results. (d),(e) Two analyzed radiographs from drive pressures of 11.7 and 116.0 GPa, respectively. The higher-pressure drive results in densities that are 5 times higher than the lower-pressure case.

Figure 2

Two sequences of radiographs of interacting planar tin ejecta microjets; the color scales with intensity on the image plate. The sequence on the left shows interaction between microjets with velocities of $2.2\mathrm{km}/\mathrm{s}$ driven by 11.7 GPa shocks, and the sequence on the right shows jets traveling at $6.5\mathrm{km}/\mathrm{s}$ from tin shocked at 116.0 GPa. A difference in interaction behavior is observed. Also shown at the bottom of the sequences are sample images of the side views of single unmasked planar jets.

Figure 3

High-pressure shock data and simulations for particle spread. (a) Density maps of the cloud for the data (left) and simulations (right). Spread as measured along the $R$ and $S$ directions is indicated for the case of the simulation. (b) The spread of the microjet cloud in the original direction of jet propagation as measured from the center point of the interaction, $R$ direction, as observed in simulations (blue solid line) and experimentally (black data points). (c) The projection of jet vertical extent on the center axis of symmetry. The points before collision indicate the vertical extent of a single jet; the white point is the extent of the bulk of the jetting material, and the black point is the extent of small bulbous region near the front of the jet. The dashed orange lines in (b) and (c) represent the hypothetical linear extent of unattenuated jets. The simulation appears to capture the behavior of the spread in the $R$ direction, but there is a mismatch in overall spread and postcollision velocity in the $S$ direction.