Laser-induced Cavitation in Extreme Thermal Conditions*

Ultra-high-speed video imaging is utilized to track cavitation bubble dynamics in liquid helium, at frame rates of up to 7 million frames per second.  Cavitation is induced by focusing a 532 nm pulsed Nd-YAG laser at a spot with a minimum spot size of 150 μm and pulse duration of 6 ns, creating a high-pressure plasma that initiates the formation of a rapidly expanding bubble/void, followed by its collapse.  Detailed description of our optical-access cryogenic setup is outlined in N. Speirs, PRF (2020).  The setup utilizes four windows for different functions: illumination, image capture, laser beam transmission, and laser beam focusing via a parabolic mirror to induce cavitation.   By using liquid Helium we span cavitation dynamics over a full range of extreme thermal conditions, from 1.2 K, where helium behaves as a superfluid, to T > 5K where we reach super-critical conditions. The influence of ambient pressure on the bubble dynamics is examined, along with the observed shock waves. A comparison between experimental results and a non-thermal bubble model is presented. The pressure field surrounding the bubble is estimated analytically, and irregular patterns on the bubble surface are visualized and discussed wrt nucleation theory.