  We study the structure of reverse shocks formed by the collision of
supersonic, magnetised plasma flows driven by an inverse (or exploding) wire
array with a planar conducting obstacle. We observe that the structure of these
reverse shocks varies dramatically with wire material, despite the similar
upstream flow velocities and mass densities. For aluminium wire arrays, the
shock is sharp and well defined, consistent with magneto-hydrodynamic theory.
In contrast, we do not observe a well-defined shock using tungsten wires,
instead, we see a broad region dominated by density fluctuations on a wide
range of spatial scales. We diagnose these two very different interactions
using interferometry, Thomson scattering, shadowgraphy, and a newly developed
imaging refractometer which is sensitive to small deflections of the probing
laser corresponding to small-scale density perturbations. We conclude that the
differences in shock structure are most likely due to radiative cooling
instabilities which create small-scale density perturbations elongated along
magnetic field lines in the tungsten plasma. These instabilities grow more
slowly and are smoothed by thermal conduction in the aluminium plasma.
