Exploration of electron and ion thermodynamic models in laser-plasma expansion

The expansion of laser-produced plasma (LPP), an important process to be understood to design the debris mitigation system of an extreme ultraviolet (EUV) light source, is governed by its associated electron and ion thermodynamics, the modeling of which is, however, a subject of debate. By applying the polytropic equation of state (EoS) for both electron and ion, we have derived the hydrodynamic-based self-similar solutions for an expanding plasma slab with finite ion temperature. The effects of electron and ion thermodynamics on plasma expansion and ion acceleration are investigated. We show that the unusual negative-correlation thermodynamic model for electrons in the hydrodynamic description of plasma expansion is an outcome of the interactions between the electrons following a nonequilibrium kappa distribution and the inherent plasma-induced electric field from a kinetic point of view. The comparisons between the self-similar solutions and the recent experiment data reveal that the electron is better characterized by the nonequilibrium kappa-based thermodynamic model with suprathermal population than the common equilibrium Boltzmann one. For thermal-ion expansion, it is found that the polytropic index for ion thermodynamics (γ_i) is about 2, in contrast to γ_i = 3 for the adiabatic assumption made in earlier studies.