driver is transmitted into the liquid sample, while the shock in the relatively thin flyer plate rapidly reflects from its free surface as an expansion front, which is then transmitted into the driver and liquid. Nonlinearity allows the head of the expansion front from the flyer free surface to overtake the shock wave for sufficiently long travel distances. The thickness of the flyer plate and target capsule in these experiments have been chosen such that the shock is unsupported by the time it reaches the free surface (i.e., the shock is directly followed by an expansion front). The shock in the liquid reflects from the rear free surface as an expansion front. The two expansion fronts then interact with each other and generate a rapidly increasing tension in the liquid, which is eventually relieved by cavitation at a critical tension which corresponds to the spall strength of the liquid. The resulting increase in pressure caused by cavitation forms a compressive wave, known as a *spall pulse*, which travels towards the rear free surface.

![image](wavediagram.pdf){width="1.6\columnwidth"}

The process described above is illustrated in the pressure–particle velocity plot in Fig. \[fig:wavediagram\](b), while a representative example of the time evolution of the liquid free-surface 