  We describe an experimental concept at the National Ignition Facility for
specifically tailored spherical implosions to compress hydrogen to extreme
densities (up to $\sim$800$\times$ solid density, electron number density
n$_e$$\sim$4$\times$10$^{25}$ cm$^{-3}$ ) at moderate temperatures (T$\sim$200
eV), i.e., to conditions, which are relevant to the interiors of red dwarf
stars. The dense plasma will be probed by laser-generated x-ray radiation of
different photon energy to determine the plasma opacity due to collisional
(free-free) absorption and Thomson scattering. The obtained results will
benchmark radiation transport models, which in the case for free-free
absorption show strong deviations at conditions relevant to red dwarfs. This
very first experimental test of free-free opacity models at these extreme
states will help to constrain where inside those celestial objects energy
transport is dominated by radiation or convection. Moreover, our study will
inform models for other important processes in dense plasmas, which are based
on electron-ion collisions, e.g., stopping of swift ions or electron-ion
temperature relaxation.
