## 3 Results
### Effect of the Second Shock Pressure on the Second Ejection  
This section examines the effect of the second shock pressure on the second ejection, with a time interval of 145 ns. The second surface jump velocities were 2.0 km/s, 3.0 km/s, 4.0 km/s, 5.0 km/s, and 6.0 km/s, corresponding to second shock pressures of 16.9 GPa, 28.5 GPa, 43.0 GPa, 57.7 GPa, and 74.5 GPa, respectively. Figure 10 presents the density and phase distributions of the second ejecta at 500 ns for various second shock pressures. An evident second ejecta was formed from the disturbed surface at a shock pressure of 16.9 GPa despite the shock pressure being significantly lower than the unloading melting pressure (64.0 GPa). The shock pressure threshold required for the second ejection was significantly reduced compared with the first ejection. The material at the disturbed surface underwent melting, which came from multiple reasons, such as the initial temperature of the material increased due to the loading and unloading of the first shock wave, collisions between material on either side of the disturbed surface during the second ejection converted kinetic energy into internal energy, and the temperature increase caused by the recompression of the spallation material at the bottom of the bubble. There was no distinct boundary between the high-, medium-, and low-velocity ejecta, and the low-velocity ejecta contained some solid structures due to the low second shock  
Figure 8: (**a**) Density and (**b**) cumulative area density distributions of the second ejecta along the shock direction at 500 ns. The black dashed line indicates the free-surface position and the orange dashed line indicates the total mass of the first ejecta. Regions I, II, and III denote the high-, medium-, and low-velocity ejecta, respectively. Figure 9: Source distribution of different velocity ejecta. (**a**) Different velocity ejecta at 500 ns (black dashed line indicates the free surface); (**b**) source distribution at 195 ns. pressure. The solid structures within the low-velocity ejecta gradually decreased with increasing shock pressure. As the second shock pressure continues to increase (57.7 GPa and 74.5 GPa), unloading melting of the material occurs. The structure of the high- and medium-velocity ejecta remains almost unchanged, while the spikes of the low-velocity ejecta gradually dispersed. We further analyzed the density and cumulative area density distribution of the second ejecta along the shock direction at various second shock pressures, as shown in Figure 11. Distinct oscillations were observed in the density distribution at a shock pressure of 16.9 GPa, originating from the solid structure of the low-velocity ejecta. At this pressure, it becomes difficult to distinguish between the various velocity components of the ejecta. The density distributions for the different velocity ejecta exhibited more defined boundaries with increasing second shock pressure. The medium- and high-velocity ejecta displayed minimal variation, while the density of spikes within the low-density ejecta gradually increased. The density of the spikes in the low-density ejecta reached 0.724 g/cm\({}^{3}\) at a second shock pressure of 74.5 GPa. The cumulative area density distribution showed that the total mass of the ejecta was 8.793 mg/cm\({}^{2}\) at a second shock pressure of 16.9 GPa, which was significantly higher than the total mass of the first ejection (3.889 mg/cm\({}^{2}\)). The total mass of the second ejecta continued to increase with increasing second shock pressure; however, the rate of increase diminished progressively. Figure 12 illustrates the source distribution of the different velocity ejecta at varying second shock pressures. The source distribution for all the ejecta appeared consistent at a shock pressure of 16.9 GPa due to the difficulty distinguishing between different velocity ejecta. The source distributions for medium- and high-velocity ejecta at different second shock pressures remained approximately unchanged.