Аннотация:It is shown that the hydrodynamic model of a one-dimensional collisionless plasma expansion is contained in the kinetic description as a special case. This belongs to a specific choice for the electron distribution function. Moreover, the consequences of the use of the hydrodynamic approach regarding the temporal evolution of the electron phase space density are investigated. It turns out that only the case of a hydrodynamic description with the adiabatic constant κ=3 is physically self-consistent. Numerical simulations confirm this argumentation. The analysis for the case κ=3 is extended to the kinetics of a relativistic electron gas.
