Study on Using Microbubbles to Reduce Surface Damage of Mercury Target for Spallation Neutron Source

A liquid mercury target, which is used to explore the neutrons produced by spallation reactions, has been installed at the Materials and Life Science Experimental Facility (MLF) in the Japan Proton Accelerator Research Complex (J-PARC). As the proton beams bombard the target, pressure waves are generated on the interface between liquid and solid metals due to thermal shock. The negative-pressure-induced cavitation causes severe pitting damage to the vessel surface of the mercury target. To reduce the surface damage of the mercury target and prolong its service life, we developed vibratory horn experiments in bubbly water. In this study, the effect of microbubbles on cavitation damage on the workpiece surface was investigated using ultrasonic erosion tests. Experimental results showed that surface damage was significantly reduced under the condition of injecting microbubbles. Additionally, we developed a simulation code to analyze the change in pressure waves in the water. The analysis results showed that the pressure amplitude of the pressure waves was significantly reduced under the condition of injecting microbubbles, and the fluctuation of the pressure waves became more regular when injecting microbubbles. We also found that the pressure amplitude of the pressure waves was decreased with a decrease in the diameter of the microbubbles.