## 3 Results
### Effect of the Time Interval on the Second Ejection  
This section discusses the effect of the time interval on the second ejection. The second shock pressure was maintained at the same value as the first ejection, 43.0 GPa. Time intervals of 50, 95, 145, 195, and 245 ns were investigated. Figure 14 presents the density distribution of the second ejection for 300 ns after the second shock wave reaches the bottom of the bubble at different time intervals. The high-, medium-, and low-velocity ejecta demonstrated obvious changes with varying time intervals. The material behind the spike in the high-velocity ejecta exhibited significant fragmentation for a short time interval of 50 ns. The structural integrity of the high-velocity ejecta improved as the time interval increased, resulting in a more stable configuration. Similarly, the medium-velocity ejecta displayed pronounced changes in structure. A prominent spike structure was formed at the head of the medium-velocity ejecta at shorter time intervals. The phenomenon occurred due to the limited opening of the spallation region at the bottom of the bubble when the time interval was short. The strong jet convergence effect contributed to forming a distinct spike structure. The spallation region at the bottom of the bubble gradually widened as the time interval increased, leading to a gradual weakening and eventual stabilization of the spike structure in the medium-velocity ejecta. The low-velocity ejecta was also significantly influenced by the time interval, primarily due to the convergence effect of the first ejecta. The development of the first ejecta was insufficient at shorter time intervals, resulting in the absence of a prominent spike structure in the low-velocity ejecta of the second ejection. The enhanced development of the first ejecta contributed to gradually enlarging the spike structure in the low-velocity ejecta of the second ejection with increasing time intervals. Figure 15 illustrates the density and cumulative area density distributions along the shock direction at various time intervals. No clear boundary was observed between the high-, medium-, and low-ejecta for shorter time intervals, resulting in a relatively smooth density distribution. Distinct boundaries began to form between the high-, medium-, and low-velocity ejecta with increasing time intervals. Additionally, the spike density of the low-velocity ejecta gradually increased, reaching a maximum of 0.894 g/cm\({}^{3}\) at a time interval of 245 ns. The cumulative area density distribution revealed that the total mass of the second ejecta at all the time intervals was significantly higher than that of the first ejecta. The total mass of the second ejecta gradually stabilized with increasing time intervals, which was attributed to the gradual stabilization of the bubble region. The total masses for time intervals of 195 and 245 ns were approximately identical, measuring approximately 15.619 mg/cm\({}^{2}\). Figure 13: (**a**) Spike velocity and (**b**) the total mass of different velocity ejecta at different second surface jump velocities. The spatial distribution of the high-velocity, medium-velocity, and low-velocity ejecta is shown in Figure 12.  
Figure 16 depicts the source distributions of different velocity ejecta at various time intervals. The source distributions of medium- and low-velocity ejecta were presented together for the time interval of 50 ns because it was difficult to distinguish between them. As the time interval increased, the source distributions of the high- and medium-velocity ejecta remained approximately constant. However, the low-velocity ejecta gradually extended further into the interior of the sample and saturated as the bubble and spallation regions stabilized. Although the first ejecta detached from the substrate, they were captured by the second ejecta and incorporated into the low-velocity ejecta during the second ejection at the time interval of 245 ns.