Development of the techniques to mitigate the cavitation damages in the J-PARC mercury target

Haga, Katsuhiro  ; Kogawa, Hiroyuki  ; Naoe, Takashi   ; Wakui, Takashi  ; Futakawa, Masatoshi  ; Takada, Hiroshi   

For the mercury target of a pulsed spallation neutron source of J-PARC, cavitation damage of the target vessel wall which is caused by the pressure wave in mercury induced by high power pulsed proton beam of 1 MW is the crucial issue. In order to mitigate the cavitation damage, a microbubble injection technique has been developed. A microbubble generator to generate bubbles with a diameter less than 100m in mercury was developed and has been used in the mercury target system of J-PARC since October 2012. The effect of microbubble injection into mercury was verified by using a laser Doppler vibrometer (LDV). The measured data showed that the displacement velocity of the target vessel was reduced to 1/3 in average by injecting microbubbles. For further development of the high power target, we focused on the mercury flow effect to mitigate the cavitation damage. In order to realize this effect into the target design of J-PARC, we adopted doubled-walled structure to the beam window of the target vessel. The mercury flow channel with a narrow gap of 2 mm was made by adding an inner wall to just inside of the beam window. Numerical simulations were carried out using the conventional code, ANSYS FLUENT. While the mercury velocity outside of the narrow channel was 1.2 m/s, the mercury velocity in the narrow channel increased to almost 4 m/s, which was promising to suppress the cavitation damages.