  The propagation of a shock wave in solids can stress them to ultra-high
pressures of millions of atmospheres. Understanding the behavior of matter at
these extreme pressures is essential to describe a wide range of physical
phenomena, including the formation of planets, young stars and cores of
super-Earths, as well as the behavior of advanced ceramic materials subjected
to such stresses. Under megabar (Mbar) pressure, even a solid with high
strength exhibits plastic properties, causing the shock wave to split in two.
This phenomenon is described by theoretical models, but without direct
experimental measurements to confirm them, their validity is still in doubt.
Here, we present the results of an experiment in which the evolution of the
coupled elastic-plastic wave structure in diamond was directly observed and
studied with submicron spatial resolution, using the unique capabilities of the
X-ray free-electron laser. The direct measurements allowed, for the first time,
the fitting and validation of a strength model for diamond in the range of
several Mbar by performing continuum mechanics simulations in 2D geometry. The
presented experimental approach to the study of shock waves in solids opens up
new possibilities for the direct verification and construction of the equations
of state of matter in the ultra-high pressure range, which are relevant for the
solution of a variety of problems in high energy density physics.
