A liquid mercury (Hg) target system for a MW-scale spallation neutron source is being developed in the world. A proton beam is directed to the Hg target to induce the spallation reaction. As shown in Fig. 4-1, the moment the proton beams bombard the target, pressure waves will be generated in the Hg by the thermally shocked heat deposition. The pressure waves propagate in the Hg to the target vessel and impose an impact load on the target vessel. Negative pressure is generated near the interface between the Hg and vessel through wave propagation, which results in cavitation in the Hg. The approximate dynamic behavior in terms of the pressure wave propagation had been predicted by numerical simulation. Moreover, impact tests, in which pressure waves were mechanically imposed on the interfaces between Hg and metals, were conducted to improve the Hg model for the numerical simulation. Through systematic tests, we discovered many micropits on the interface. The micropits may be formed by the cavitation induced through the pressure wave propagation. These experimental results became a trigger for researchers in the US to perform proton beam incident tests to confirm micropit formation, and they found the same phenomena that we had already observed.
In J-PARC, a pulse proton beam will be injected into the Hg target at 25 Hz. Throughout the expected lifetime of the target, more than 108 pulses will be imposed on the Hg target. The damage due to micropits will be one of the critical issues for determination of the target lifetime. Therefore, an innovative machine, which has an electromagnetic driving system to impose the impact pressure on the interface between the Hg and metals, was developed to investigate the micropit damage of more than 108 cycles. Fig. 4-2 shows the micropit formation behavior observed using a laser microscope. It was found from this observation that the damage process is divided into two phases: the incubation state with microplastic deformation and the steady state with mass loss due to peeling out. As a result, an empirical equation was derived to estimate the micropit damage formation. Additionally, it was confirmed that a surface hardening treatment is effective to suppress the damage formation. These results attracted worldwide attention and provide important information for target lifetime estimation.