Regimes of expansion of a collisional plasma into a vacuum

The effect of elastic Coulomb collisions on the one-dimensional expansion of a plasma slab is studied in the classical limit, using an electrostatic particle-in-cell code. Two regimes of interest are identified. For a collision rate of few hundreds of the inverse of the expansion characteristic time τ_e, the electron distribution function remains isotropic and Maxwellian with a homogeneous temperature, during all the expansion. In this case, the expansion can be approached by a three-dimensional version of the hybrid model developed by Mora [P. Mora, Phys. Rev. E 72, 056401 (2005)]. When the collision rate becomes somewhat greater than 104 τ e -1, the plasma is divided in two parts: an inner part which expands adiabatically as an ideal gas and an outer part which undergoes an isothermal expansion.