Shock waves in nonequilibrium gases and plasmas

An analysis and assessment of three mechanisms describing plasma/shock wave interaction processes was conducted under conditions typically encountered in a weakly ionized glow discharge. The mechanisms of ion-acoustic wave damping, post-shock energy addition and thermal inhomogeneities are examined by numerically solving the Euler equations with appropriate source terms adapted for each mechanism. Ion-acoustic wave damping is examined by modelling the partially ionized plasma as two fluids in one spatial dimension using the Riemann problem as a basis. Post-shock energy addition in the form of nonequilibrium vibrational energy relaxation is also examined in one spatial dimension using the Riemann problem as a basis. The influence of thermal inhomogeneities on shock wave propagation is examined in two spatial dimensions for both a Riemann shock and a shock generated by a spark discharge. The use of realistic thermal profiles allowed the comparison of measured and numerically predicted shock parameters. Results from time-dependent calculations of the two-fluid plasma under typical weakly ionized conditions, although similar to steady-state results previously reported in the literature, indicate that ion-acoustic wave damping has an insignificant effect on shock propagation. Under strongly ionized conditions, however, ion-acoustic wave damping can increase shock speed and shock front width and reduce the shock strength, each of which is consistent with experimental observation. Results from the analysis of post-shock vibrational relaxation indicate that although this process can lead to increases in the shock speed, the final magnitude of the increase is too small and the time scale for the increase is too large to explain the experimental observations. An analysis of the effects of thermal inhomogeneities reveals that many of the observed plasma/shock anomalies can be explained based solely on this mechanism.