Experimental observation of proton-induced shocks in free surface liquid metal targets

Tests on the response of liquid metal targets to high-power proton beams have been performed at ISOLDE/CERN. During these tests, a so-called thimble geometry and an extended version, the trough, filled with liquid mercury were exposed to a 1.4 GeV proton beam with intensities up to 33×1012 protons/pulse. In order to extrapolate the behaviour of a liquid metal target from the kilowatt to the megawatt-scale as required for a neutrino factory, various measurements were carried out with the aim of establishing scaling laws of the splash velocity as a function of beam size, intensity and time structure. The mercury volume was placed in a steel frame, while the region above the mercury level was observed through two quartz windows with a high-speed camera. For the highest intensity available at the PS Booster (33×1012 protons/pulse), the mercury expanded with velocities up to 50 m/s. The splash velocity scaled with the power density of the proton pulse. Increasing the beam size or the pulse duration reduced the velocity.