Supersonic expansion on non-equilibrium plasmas

In the supersonic expansion of an ionized gas, the dominant factor in describing the atomic processes is the recombination rate constant K_R. Several models describing the recombination process have been reviewed in some detail. It has been found that, depending on the adopted definition, different models will yield different values of K_R for the same electron temperature and number density. A comparison of experimentally and theoretically derived values for K_R has to be done with great care, as in the majority of the experiments K_R is determined from the measured rate of disappearance of free electrons. These measurements give the correct "decay coefficient", but only in certain circumstances will it reduce to the correct recombination rate. In the light of the important role that K_R plays in any numerical solution of nonequilibrium expansion flow of plasmas, details of experiments on a 15-degree corner expansion flow of ionized argon are given. In these experiments the plasma flow which was generated by driving strong normal shock waves into quiescent argon was studied mainly by optical diagnostics. Using a dual-frequency laser interferometer, the plasma properties around a corner expansion were recorded. The analysis of the interferograms has yielded values for the recombination rate constant as a function of the plasma macroscopic properties. The range of shock Mach number, electron number density, temperature and initial channel pressure and temperature were as follows: 13 < M, < 19; 10¹⁶ < n, < 1.5 × 10¹⁷ cm^-3; 9000°K < T < 13,000°K; 2.2 < p₁ < 10 torr; T₁ ≃ 300°K. It was found that the theoretically predicted values for the three-body, electron-ion-electron collisional recombination rate are in good agreement with those measured gasdynamically in a well-defined flow. The measured flow quantities substantiate a previous analysis based on the method of characteristics.