Challenge for 1MW operation of mercury target system

Haga, Katsuhiro  ; Naoe, Takashi   ; Wakui, Takashi  ; Kogawa, Hiroyuki  ; Futakawa, Masatoshi  ; Kai, Tetsuya   ; Kinoshita, Hidetaka   ; Takada, Hiroshi   

The pressure waves are generated in the mercury target of J-PARC by the injection of high power pulsed proton beams and induce cavitation damages on the mercury target wall. The damages are the serious threat factor for the lifetime of the target vessel. After the beam operation of 200 kW corresponding to the accumulated power of 470 MWh, damages with the maximum depth of 0.25 mm were found on the inner surface of the mercury target, and the counter measure to mitigate the damage was recognized to be important. The technique to inject micro-bubbles into the mercury target vessel which has been developed to mitigate the cavitation damages was applied to the next target vessel and the beam operation in the range of 200 kW to 300 kW was continued to 2050 MWh. A specimen was cut out from the target vessel and inspected visually by a remote video camera. No conspicuous damages were found on the specimen. This fact demonstrates the efficacy of the micro-bubble injection to mitigate the cavitation damages. The newly installed target vessel has double wall structure at the beam window as the additional technique for the cavitation damage mitigation. The rapid mercury flow in the narrow channel made by the double wall prevent the cavitation bubble from growing and moderate the severity of the cavitation energy. The efficacy of the double wall structure will be investigated by cutting out the specimen from the target vessel in the future.