During the implosion of metallic materials, the ejecta formed where the fragments were ejected from the shocked metal surface, and a mixed layer with finite width appears between two fluids, the mixed layer significantly influences the subsequent development of hydrodynamic instabilities and mixing. In inertial confinement fusion (ICF), the shock that passes through the roughened metal interface also causes the formation of ejecta. Therefore, accurate prediction of the growth of Richtmyer-Meshkov (RM) instability involving the mixed layer is crucial for understanding various phenomena in astrophysics and optimizing ICF engineering designs. However, in the experimental studies of RM instability on metal interfaces RM instability with a mixed layer, the opacity difference between the mixed layer and the metal sample is relatively small, this makes it difficult to accurately measure the RM instability growth using traditional X-ray backlighting imaging techniques.

To address this issue, this study applies X-ray fluorescence imaging technology to the measurement of RM instability growth. This technique possesses fluid tracing characteristics and localized diagnostic capabilities. The K-shell fluorescence signals of titanium atoms in foam material are obtained by the curved bent crystal imaging system, it enables the measurement of the mixing evolution in low-density regions. More importantly, the signal intensity is approximately proportional to the density of the fluorescing material in X-ray fluorescence imaging, whereas the signal in traditional X-ray backlighting decays exponentially with material density. This allows fluorescence imaging to capture the perturbation structures of the mixing layer under much lower density conditions, obtain higher confidence data on the mixing growth at the interfaces between the mixing layer and the adjacent materials.

In laser direct-drive experiments, the hard X-rays and superhot electrons generated by the direct laser loading of CH samples can heat the metal perturbation sample ahead of the shock. Preheating modifies the initial state of the perturbed interface before the shock arrives, thereby complicating the analysis of hydrodynamic instability evolution that depends on these original conditions. A preheat calibration experiment is conducted at the Shenguang-III prototype laser facility, the experimental results show that a 50 μm thick CH (3% bromine doping) layer and reducing the laser power can block the preheat, the increase in temperature before the arrival time of the shock wave is less than 200 K, which has little influence on the initial state of rear interface of the aluminum layer.

In an effort to better understand the RM instability with a mixed layer, a laser-driven RM instability experiment under two mixed layer density conditions (0.2 and 0.5 g/cm³) has been performed at the Shenguang-III prototype laser facility. The experimental results show that under the condition of the same interface roughness, a larger Atwood number leads to a larger θ value during the nonlinear growth stage. The total amount of vorticity deposited by the shock wave determines the growth rate of the instability, the mixed layer with lower density exhibits better compressibility, and there are steeper density and pressure gradients. Therefore, the low-density mixed layer tends to deposit more vorticity at the interface, which in turn leads to a faster growth of RM instability.