Calculation of Equilibrium Plasma Thermodynamics

The thermodynamic characterization of thermal plasma is a fundamental aspect in many fields, such as high-pressure plasma torches, LIBS (Laser induced Breakdown Spectroscopy), lightning physics and so on. Different aspects must be considered to determine equilibrium quantities. In this paper, we will describe the approach implemented in the code EquilTheTA [1], a web tool to calculate equilibrium composition and the corresponding thermodynamic and transport properties of a plasma. The tool is based on the statistical physics of gases, accessing to an accurate core database of single species internal levels [2]. The tool includes ortho-para separation, important at low temperature for light molecules such as H2 (and isotopologues) and O2. For atomic species, multiply ionized atoms are included and to cope with the divergence of the internal partition function, the number of levels, limited according to the Fermi and Griem cutoff criteria [2], is calculated self-consistently with the composition. The tool has been used also to investigate plasma in high density conditions [3], when also the internal structure of the atoms is influenced by the electron density. In these conditions, quantum effects become important and the Fermi-Dirac distribution must be used for the electron gas [4]. To reduce the size of the core database a general expression, based on the lumped level approach [4], allows to accurately determine the internal thermodynamic properties of species in a wide temperature range. To determine the composition of complex plasmas a novel algorithm has been included, finding one reaction equilibrium, consisting in finding the root of a polynomial, in each step, up to convergence. The possibility of using analytical solution for polynomials up to fourth degree speedup the convergence.