# Laser Cavitation Peening: A Review
## 3 Comparison between Laser Ablation and Laser Cavitation  
As it has been hypothesized that the laser ablation impact is used for submerged laser peening, we compared the pressures and/or impacts produced by laser ablation and laser cavitation collapse. Figure 6 shows (a) images of laser ablation, (b) the signal from the target from a PVDF sensor placed in the target, and (c) the signal from a shockwave sensor in the water [20]. At \(t=0\) ms, both the PVDF sensor in the target and the submerged shockwave sensor detected a pulse-like amplitude increase, which was caused by the laser ablation. After laser ablation, a bubble was initiated and developed, then shrunk as in Figure 4a. When the bubble collapsed at \(t\approx 1\) ms, both the PVDF sensor and the submerged shockwave sensor detected another pulse-like amplitude increase. When the amplitudes induced by the laser ablation and the laser cavitation collapse were compared, the amplitude of the laser cavitation collapse, measured using the PVDF sensor, was larger than that of the laser ablation. However, the amplitude detected by the submerged shockwave sensor with respect to laser cavitation collapse was smaller than that of the laser ablation, the same as in a previous report [18]. The secondary collapse of the bubble at \(t\approx 1.5\) ms was detected by the PVDF sensor, but it was not detected by the submerged shockwave sensor. It was suggested that the PVDF sensor could evaluate the impact more accurately than the submerged shockwave sensor. When glycerol was used for the plasma confinement layer, the amplitude of the signal from a PVDF sensor during laser ablation was nearly equivalent to that of the laser cavitation collapse (see Figure 7).  
As a cavitation bubble collapse can be controlled by boundary conditions around the bubble, in one study, a pipe was placed near a target plate around the focus point and the arc height of the target plate was measured [43]. The arc height has been used for the measurement of the peening intensity [44; 45]. The inverse of curvature can be calculated from the arc height, and it is proportional to the arc height [46]. In Figure 8a, the target plate without a pipe was peened using submerged laser peening; both the laser ablation impact and laser cavitation collapse were used in Figure 8a and without a pipe in Figure 8c. However, a pipe was placed near the target plate around the focus point and peened in Figure 8b. As shown in the schematic above Figure 8b, when the pipe was placed  
Figure 6: Laser ablation and laser cavitation produced by a pulsed laser: (**a**) images showing the development of laser ablation and laser cavitation; (**b**) signal from a PVDF sensor in the target; (**c**) signal from a shockwave sensor in water. Reprinted from [20] with permission from Elsevier, License Number 5532490964237.  
near the wall, the bubble moved upwards and collapsed in the water rather than on the target surface. Thus, the peening intensity, i.e., the inverse of the curvature with the pipe, was nearly half of that without the pipe, as only the laser ablation impact was used; the laser cavitation collapse impact was not used during the peening with the pipe. It can be concluded that both the laser ablation impact and laser cavitation collapse impact are used during conventional submerged laser peening, as one pulsed laser hits the surface twice. Regarding another important paper [47], during optimized laser cavitation peening, the laser cavitation collapse impact energy was 2.2 times larger than that of laser ablation.