Two-dimensional shock propagation and Mach stem formation induced by a laser-produced annular plasma

We have experimentally studied the formation of shock waves in a laser-induced annular configuration. By locating an aluminum target in the focal plane of a ${10}^{9}\phantom{\rule{0.16em}{0ex}}\mathrm{W}/\mathrm{c}{\mathrm{m}}^{2}$ laser, an annular plasma is formed that acts as a piston for a shock wave in a background gas composed by either argon or nitrogen. According to shock wave dynamics, the annular shock follows a two-dimensional propagation profile that is attributed to the boundary conditions of the piston. In addition, we observe the formation of a Mach stem at the center of the configuration, at a position and timing predictable by the adiabatic index of the background gas and shock wave profile. This controlled laboratory setting for producing Mach stems can be used as a benchmark platform for testing conjectures on the observations of clumpy features in astrophysical objects. Moreover, the observed Mach stem propagates as a one-dimensional shock wave given the boundary conditions imposed by the annular configuration.