The mercury target vessel for the pulsed spallation neutron source at the Japan Proton Accelerator Research Complex is severely damaged by the cavitation caused by proton beam-induced pressure waves in mercury. Aiming to mitigate the cavitation damage, we adopted a double-walled structure with a narrow mercury flow channel at the beam window portion of the target vessel. The narrow channel expected to deform the cavitation bubbles during growing process by the pressure gradient caused by the high-speed flow. In addition, gas microbubbles were injected into the mercury to suppress the pressure waves. The beam window portion of the target vessel was cut out to demonstrate the effect of those cavitation damage mitigation technologies. The damage depth of the cutout specimens for the original design type and double-walled target vessels were quantitatively evaluated by replicating the damaged surface using a silicone rubber. The results showed that the double-walled target facing the mercury with gas microbubbles operating at 1812 MWh for an average power of 434 kW had equivalent damage to the original design target operating 1048 MWh for average power of 181 kW. The erosion depth due to cavitation in the narrow channel was clearly smaller than it was on the wall facing the bubbling mercury.