## 6 Applications of Cavitating Jet  
The cavitating jet has been applied for drilling and cutting rocks [16; 105; 106; 113]. The cavitating jet in air can also dig concrete structures for the maintenance of infrastructure [114]. In the case of mechanical surface treatment, the use of a submerged water jet was attempted to mitigate stress corrosion cracking (SCC) of nuclear power plants by using impinging impacts of a water column in the jet center at the beginning [84]. [PERSON] et al. found that the cavitating jet could introduce compressive residual stress into stainless steel by using cavitation impacts [13]; then, the cavitating jet was successfully applied to mitigate SCC in nuclear power plants [115]. Based on experimental results of the introduction of compressive residual stress into metallic materials, the improvement in the fatigue strength of metallic components by cavitation peening was proposed using a pressurized chamber to enhance the aggressive intensity of the cavitating jet [116; 117; 118], and it was demonstrated for forging die [119], gears [120; 121], continuous valuable transmission CVT elements [122] and rollers [123]. After enhancing the aggressive intensity of the cavitating jet by optimizing the nozzle geometry, the improvement in fatigue strength by cavitation peening with an open chamber was demonstrated [124; 125; 126; 127]. Cavitation peening using oil was also proposed [128]. Cavitation peening also improves tribological properties such as fretting fatigue properties [129; 130]. The improvement in fatigue strength of metallic materials using a cavitating jet in air was also demonstrated [36; 131; 132]. In view of environmental-assisted cracking, the suppression of hydrogen-assisted fatigue crack growth in austenitic stainless steel and delayed fracture resistance on chrome molybdenum steel by cavitation peening were reported [133; 134]. The cavitating jet can be applied in the semiconductor industry. It can be used not only for cleaning [135], but also gettering [136]. When erosion rates using ultrasonic vibratory apparatus [137] and cavitating jet apparatus [15] were compared, the erosion rate of the cavitating jet apparatus was larger than that of ultrasonic vibratory apparatus. As conventional ultrasonic cleaning devices use ultrasonic cavitation, cleaning using the cavitating jet is more powerful. The gettering technique is a method to remove unwanted impurities from active device regions in a silicon wafer into the back side of the wafer by introducing oxidation-induced stacking faults (OSF) [138; 139; 140; 141; 142]. In order to produce OSF, the introduction of strain into the wafer is required, and shot blasting is used in a conventional way [143; 144]. However, the cleaning of shots is required. In the case of gettering using the cavitating jet, the cavitating jet can introduce strain for OSF [145; 146] and cleaning at the same time. This is a great advantage for semiconductor processes. One of the applications that was proposed in the bioengineering area is oral cleaning using the cavitating jet [147; 148; 149]. Dental implants have been used as the solution for the loss of teeth;however, peri-implant mucosa and peri-implantitis are new dental diseases affecting implants [150; 151]. The most effective treatment for these diseases is cleaning dental plaque, which is a kind of biofilm that adheres to the surface of teeth and implants [152]. Conventional cleaning methods of the dental plaque are oral brushing, ultrasonic scaling and rubber cup cleaning. Unfortunately, micro-textured roughness is made on the implant surface to improve biocompatibility [153], so oral toothbrushes and the tips of scaling instruments cannot reach the bottom of the rough surface. The cleaning of dental plaque on the rough surface of dental implants using the cavitating jet has been successfully demonstrated [147; 148; 149]. As is well known, sonochemistry is a research area of chemistry using ultrasonic cavitation [154].