Laser-induced melting and cavitation in solid Helium

Superfluid helium becomes a solid at pressures above 25 atmospheres. The interface between the solid and superfluid is known to exhibits intriguing behavior, with wavy time-evolving oscillations on its surface. Herein, we use laser-pulses and ultra-high-speed imaging, at frame rates up to 7 million frames per second, to experimentally investigate the melting and interfacial dynamics of solid helium at temperatures between 1.2 K and 2 K and at pressures starting from the melting pressure, of about 25 atm, up to 39 atm, which spans both HCP and BCC structure of the solid. Our optical-access cryostat [1], has five windows, one of which we use to pass the pulsed laser-beam through and a separate set of windows are used to backlight and image the dynamics. The Nd:YAG laser-pulse is focused by a parabolic mirror within the cell. The laser-induced plasma produces a rapidly growing sphere of vapor as well as melts the solid around this bubble, forming an outer liquid layer. The initial shock dissipates very rapidly, after only a few micro-seconds, thereby traveling a short distance before disappearing. The melting front moves at over 100 m/s before resolidifying after a few seconds.

[1] Speirs, N. B., Langley, K. R., Taborek, P. & Thoroddsen, S. T. Jet breakup in superfluid and normal liquid 4He. Phys. Rev. Fluids, 5, 044001 (2020).