[en] Based on the hydrodynamic description, the planar expansion of a plasma into vacuum is investigated numerically and analytically. Several dynamical structures are found and explained. In the inviscid case, ion wave collapse is the most striking feature. Viscosity prevents the collapse and allows long-term calculations. Three phases in the evolution can be distinguished. In the time asymptotic regime the self-similar state is approached. This approach is preceded by a state of intermediate asymptotics and applies to charge separation as well. (orig.)