We have studied the cooling dynamics, formation process, and geometric structure of mesoscopic crystals of
externally produced magnesium ions in a Penning trap. We present a cooling model and measurements for a
combination of buffer gas cooling and laser cooling which has been found to reduce the ion kinetic energy by
eight orders of magnitude from several hundreds of eV to μeV and below within seconds. With ion numbers
of the order of 1 × 103 to 1 × 105, such cooling leads to the formation of ion Coulomb crystals which display
a characteristic shell structure in agreement with the theory of non-neutral plasmas. We show the production
and characterization of two-species ion crystals as a means of sympathetic cooling of ions lacking a suitable
laser-cooling transition.