The Nonequilibrium Thermodynamics of Plasma Under Collisional-Radiative Equilibrium*

We present a thermodynamically consistent description of plasma under collisional radiative equilibrium (CRE). Many plasmas composed of mid- to high-$Z$ materials are much more accurately described by CRE than local thermodynamic equilibrium (LTE), particularly when plasma properties, such as specific heat and sound speed require accurate accounting of the ionization component of the internal energy. CRE opacity/emissivity tables have been in use for many years in radiation-hydrodynamic simulations. We seek self-consistency by implementing equation-of-state tables based on the same CRE atomic models. CRE extends LTE by accounting for the steady-state atomic-kinetic effects of radiation escaping freely from the point of emission. As under LTE, plasma under CRE is described completely in terms of local thermodynamic variables. Consequently, radiative and thermodynamic properties can be tabulated in terms of local thermodynamic variables, and thermodynamically consistent descriptions of both limits are possible. Conceptual problems arise from CRE not being a true equilibrium, which requires a description in terms of nonequilibrium thermodynamics.