Relaxation phenomena in thermal molecular plasmas

We present the results of analysis of relaxation phenomena in thermal molecular plasmas. The physical assumptions and the general scalar moment equations, obtained in the 17-moment approximation of the Grad method, are given. By using these equations, we derive the expressions for the relaxation pressure and bulk viscosity coefficients associated with heavy plasma components (atoms, molecules and their ions) and electrons. To gain a deeper understanding of how the physical parameters of particles and inter-particle interactions influence on the plasma relaxation properties, we also employ the semi-qualitative model of energy relaxation for plasma components. Within this model, the expressions for the partial relaxation pressures and bulk viscosity coefficients are derived and analyzed. It is demonstrated that depending on the plasma degree of ionization and the ratio of the characteristic timescales of energy exchange between particles, the partial relaxation pressures and bulk viscosity coefficients can not only vary but also change their signs.