A Model for Computing Equation of State of Plasma of the FST Injector of Electromagnetic Railgun

Fast shock tube (FST) is a launcher, which can be used as the injector of electromagnetic railgun, and its working fluid often chooses the inert gas, which is ionized to high-temperature and high-pressure plasma by strong shock wave in the process of launching. This paper presents a simplified model of computing equation of state (EOS) of plasma of the FST injector; we gain the analytic function of the ionization potential versus the average degree of ionization by fitting the numerical results of the ionization potential calculated with Thomas-Fermi (TF) statistical model. Combined the function of the ionization potential with the Saha equation of multiple ionizations, we establish a simplified model for calculating the average degree of ionization, which is the analytic function of the average degree of ionization of argon versus temperature and density in local thermal equilibrium (LTE) case. The curve of pressure p versus specific volume V in argon plasma is calculated according this simplified model, and the results calculated are basically in agreement with the several sets of experimental date and the results calculated with the Saha-equation theory in the high-density region. This simplified model can also be used to calculate the equation of state of plasmas mixture and is expected to have a wider use in the field of electromagnetic launch technology involving the strongly ionized plasma