## 3 Experimental Results and Analysis
### Energy Distribution with Scattering Angle of BAD  
Theoretically, the distribution of the BAD cloud is rotationally symmetrical along the projectile incident axis (i.e., the \(x\) axis) [23], and the radial scattering probability along the circumferential direction is the same. Based on the rotational symmetry, the 3D spatial distribution of the BAD was simplified into a 2D distribution, and the scattering angle was used as a variable to quantitatively analyze the spatial energy distribution characteristics of the BAD. Based on the calculated fragment energy and the spatial position of the fragment, the relationships between the energy of the EFP fragments and target fragments and the scattering angle were obtained, as shown in Figure 12. Additionally, two sets of test data of EFP with the same \(L/D\) were accumulated. The green dots mean that no fragments were collected in the corresponding hole of recover plate and that the material of the fragments could not be determined. Figure 12 shows that the BAD energy decreased in an approximately exponential relationship with increasing scattering angle. The energy of the fragments from 0-8\({}^{\circ}\) was affected by the residual EFP, decreasing significantly with increasing scattering angle. The energy distribution was concentrated in the range of 0-10e from 8-20\({}^{\circ}\). When the \(L/D\) was 2.4, the maximum energy was almost unchanged as the scattering angle increased, as  
Figure 12: Relationship between energy and scattering angle of the BAD. (**a**) \(L/D\) = 1.66 (**b**) \(L/D\) = 2.4 (**c**) \(L/D\) = 3.86.  
shown in the red box. The energy distribution was more uniform versus the other two types of EFPs. There were mainly target fragments in the scattering angle range of over 25\({}^{\circ}\), which was the rear section of the BAD cloud. With increasing \(L/D\), the energy of the EFP fragments within the small scattering angle (0-5\({}^{\circ}\)) increased, as did the number of fragments, thus indicating that the energy of the EFP fragments concentrated along the EFP incident axis. According to the scatter plot of energy distribution, the average energy of all fragments in each scattering angle range was calculated, and the results of two tests of each EFP type were averaged to obtain the average energy distribution of the fragments, as shown in Figure 13.  
At scattering angles less than 30\({}^{\circ}\), the average energy of the fragments gradually decreased with increasing scattering angle, thus indicating that the energy of the BAD cloud gradually weakened from the front to the middle section. This was because the fragment velocity in the front section of the BAD cloud was high, and therefore, the energy was higher than in the middle and rear sections. The average energy of the fragments increased when the scattering angle was greater than 30\({}^{\circ}\), and there was a peak in the average energy of the fragments. To analyze the causes, the energy of the EFP fragments and target fragments was separated, and the average energy distribution of the projectile and target fragments is shown in Figure 14.  
Figure 14 shows that there was only target fragment with energy beyond 25\({}^{\circ}\). The fragments with a large scattering angle were from the ring fragments, i.e., the spall ring formed by the stretching wave due to the reflection of the shock wave on the back surface of the target. Although the velocity of the fragments was low, their mass was quite large [11; 19]; therefore, the average energy of the fragments in this section increased instead. With an increasing \(L/D\) of EFP, the average energy of the target fragments with a large scattering angle (35-40\({}^{\circ}\)) increased because the spall area reflected the energy absorbed by the target from the EFP [37]. The specific kinetic energy increased when the EFP diameter decreased, and the input energy on the target per unit mass increased, resulting in a small fragment mass and a high fragment velocity. Thus, the average energy of fragments in this region increased.