  The energy distributions of Ta ions produced in a nanosecond laser-heated
plasma at 4$\times$10$^{15}$ W/cm$^{2}$ are experimentally and theoretically
investigated. They are measured far from the target with an electrostatic
spectrometer and charge collectors. Shadowgraphy and interferometry are used to
characterize the plasma dynamics in the first nanoseconds of the plasma
expansion for electron densities ranging from 10$^{18}$ to 10$^{20}$ cm$^{-3}$.
The experimental data clearly show two components in the energy distributions
which depend on the ion charge states. These components are discussed in light
of fluid and kinetic descriptions of the expanding plasma. In particular,
quantitative comparisons with calculations performed with 3D hydrodynamic
(Troll) and 1D3V Particle In Cell (XooPIC) codes demonstrate that a double
layer created at the plasma-vacuum interface plays a crucial role in the
acceleration of the highest charge state ions at high energy.
