| Popis: |
Shock propagation into weakly ionized gases show several features differing markedly from conventional, non-ionized gas, shock structure. Phenomenological analysis of general macroscopic features of the previously observed plasma shock effects allows only two possible interpretations: (i) existence of an energy (momentum) flux toward the wave precursor; (ii) volumetric energy release (exothermic phase transition) in the upstream portion of the wave (precursor) followed by the reverse transition in the downstream portion of the wave. It is shown that known microscopic mechanisms are not capable of producing such a flux or energy release: typical processes involving electrons, ions, and excited species do not couple strongly to neutral atoms and molecules, and there is not enough energy stored in these species because of the low ionization fraction. The theoretical basis for phase transitions in low-density nonequilibrium plasmas is also unknown. Analysis of the steady two-wave system created by either of the two effects raises a question as to whether the observed plasma shocks are stable objects. Another question raised is whether there exists phase transition within the plasma shock. Finally, the contribution of thermal nonhomogeneity of the plasma to the experimentally observed shock wave distortion remains unknown. Answering these three fundamental questions requires additional experimental and theoretical studies of the problem. |
