Laser-induced cavitation near absolute zero

When a short duration pulsed laser is focused into a liquid, the liquid will break down into a plasma once the laser-power density exceeds some threshold. After the plasma is extinguished, the remaining hot gas expands forming a bubble whose ultimate size depends on the thermo-physical and transport properties of the fluids and the total energy absorbed. Herein, we use ultra-high-speed imaging at frame rates up to 5 million frames per second to experimentally investigate laser-induced cavitation bubble dynamics in liquid helium between 1.25 K and the liquid-vapor critical point at 5.19 K. This small temperature range results in a wide variation of fluid properties including the density, viscosity, vapor pressure and thermal conductivity. A 532 nm, pulsed Nd:YAG laser with duration of approximately 6 ns and variable pulse energy up to 100 mJ is focused into the volume of interest using a parabolic mirror submerged in the liquid with a minimum beam width at the point of cavitation of 150 µm. We present the bubble growth and collapse dynamics and its dependence on temperature and pressure. We compare our results with well known bubble dynamic theory such as the Rayleigh-Plesset equation. We also present the measured cavitation threshold for our entire experimental range.