Time-resolved study of laser initiated shock wave propagation in superfluid ⁴He

Intense shock waves in superfluid ⁴He between 1.7 and 2.1 K are generated by rapidly expanding confined plasma from laser ablation of a metal target immersed in the liquid. The resulting shock fronts in the liquid with initial velocities up to ca. Mach 10 are visualized by time-resolved shadowgraph photography. These high intensity shocks decay within 500 ns into less energetic shock waves traveling at Mach 2, which have their lifetime in the microsecond time scale. Based on the analysis using the classical Rankine-Hugoniot theory, the shock fronts created remain in the solid phase up to 1 μs and the associated thermodynamic state appears outside the previously studied region. The extrapolated initial shock pressure of 0.5 GPa is comparable to typical plasma pressures produced during liquid phase laser ablation. A secondary shock originating from fast heat propagation on the metal surface is also observed and a lower limit estimate for the heat propagation velocity is measured as 7 × 10⁴ m/s. In the long-time limit, the high intensity shocks turn into liquid state waves that propagate near the speed of sound.