  The irradiation of an atomic cluster with a femtosecond x-ray free-electron
laser pulse results in a nanoplasma formation. This typically occurs within a
few hundreds femtoseconds. By this time the x-ray pulse is over, and the direct
photoinduced processes no longer contributing. All created electrons within the
nanoplasma are thermalized. The nanoplasma thus formed is a mixture of atoms,
electrons and ions of various charges. While expanding, it is undergoing
electron impact ionization and three-body recombination. Below we present a
hydrodynamic model to describe the dynamics of such multi-component nanoplasma.
The model equations are derived by taking the moments of the corresponding
Boltzmann kinetic equations. We include the equations obtained, together with
the source terms due to electron impact ionization and three-body
recombination, in our hydrodynamic solver. Model predictions for a test case:
expanding spherical Ar nanoplasma are obtained. With this model we complete the
two-step approach to simulate x-ray created nanoplasmas, enabling
computationally efficient simulations of their picosecond dynamics. Moreover,
the hydrodynamic framework including collisional processes can be easily
extended for other source terms and then applied to follow relaxation of any
finite non-isothermal multi-component nanoplasma with its components relaxed
into local thermodynamic equilibrium.
