Computer simulation of the thermodynamic properties of high-temperature chemically-reacting plasmas

The Reaction Ensemble Monte Carlo (REMC) computer simulation method [W. R. Smith and B. Triska, J. Chem. Phys. 100, 3019 (1994)] is employed to predic t the thermodynamic behavior of chemically reacting plasmas using a molecul ar-level model based on the underlying atomic and ionic interactions. Unlik e previous plasma simulation studies, which were restricted to fairly simpl e systems of fixed composition, the REMC approach is able to take into acco unt the effects of the ionization reactions. In the context of the specifie d molecular model, the computer simulation approach gives an essentially ex act description of the system thermodynamics. We develop and apply the REMC method for the test case of a helium plasma. We calculate plasma compositi ons, molar enthalpies, molar volumes, molar heat capacities, and coefficien ts of cubic expansion over a range of temperatures up to 100 000 K and pres sures up to 400 MPa. We elucidate the contributions of the Coulombic forces , ionization-potential lowering, and short-ranged interactions to the therm odynamic properties. We compare the results with those obtained using macro scopic-level thermodynamic approximations, including the ideal-gas (IG) and the Debye-Huckel (DH) approaches. For the helium plasma, the short-ranged forces are found to be relatively unimportant, but we expect these to be im portant for molecular systems. The DH theory is always more accurate than t he IG approximation. The DH theory yields compositions that slightly underp redict the overall degree of ionization. For the molar heat capacity and th e coefficient of cubic expansion, the DH theory is accurate at lower pressu res, but at 400 MPa yields results that are up to 40% in error for the mola r heat capacity. (C) 2000 American Institute of Physics. [S0021-9606(00)502 36-0].