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Summary form only given. An intense electron beam pulse deposits energy into a thin metal plate creating a region of hot ionized fluid, which vents as an expanding hypersonic flow with nonequilibrium ionization. This paper describes an analytical model and how it compares to experimental data from Los Alamos National Laboratory. The source temperature is proportional to the ratio of Bethe stopping power to source volume, and the shape of this volume depends on the lateral diffusion of beam electrons traversing the plate. Mass flow within the plate is modelled as a one-dimensional unsteady expansion of a perfect gas with clumped pressure waves, and the mass flow into an external vacuum is modelled as a sequence of steady hemispherical flows. The local conditions in the external flow are parametrized by Mach number, and derivatives of these relations give local gradients of temperature, velocity and density. The source volume can be highly ionized, yet on expansion the density and temperature drop so precipitously that the ionization fraction becomes both small and out of equilibrium with local conditions. The ability of the ionization fraction to adjust to local conditions increasingly lags the motion of the accelerating flow, until a point is reached where the ionization fraction is "frozen" at a fixed value. The transition between equilibrium and frozen now occurs in a narrow zone close to the plate, and the location of this zone is given by a criterion first stated by K.N.C. Bray (1959).