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An argon plasma jet is produced from a continuous optical discharge at gas pressures in the range 2

The expansion of a supersonic free jet generated from a laser-sustained argon plasma is studied experimentally by using two techniques: ͑i͒ the time-of-flight analysis of velocity distributions of the atom beam skimmed from the free jet, leading to the conclusion that the translational relaxation of the heavy particles ͑neutral atoms and ions͒ follows very accurately the classical isentropic laws; ͑ii͒ the fluorescence technique for studying the internal state relaxation process by local analysis of the radiation emitted spontaneously from the plasma free jet. Only the electronic states excited above 14.5 eV are found to be in a Boltzmann equilibrium, defining a local temperature T excit in the structure of the free jet. The axial electronic temperature, calculated using a simple one-dimension thermodynamic model, follows fairly well the experimental values of T excit , while the translational temperature of the heavy particles decreases much lower, in agreement with the asymptotic value deduced from the velocity distributions of the atom beam. Thus appears the uncoupling between the electronic excitation process, governed by the electronic collisions, and the translational cooling of the heavy particles, governed by the collisions between the heavy particles. A more detailed analysis of the excitation process is obtained through the calculation of the state population densities with a collisional-radiative model. The agreement with the experimental measurements is rather good for most of the states investigated.

In the theory, the two-photon resonant third-order nonlinear susceptibility of a molecular gas is calculated. The variations of this susceptibility, with the rotational quantum numbers of the resonant states, are given in the case of two singlet states of different symmetries, Σ and Π. In the experiment, third harmonic generation of XUV light (94.5–97.5 nm) is obtained with and without two-photon resonance. The gaseous nonlinear medium is formed by CO as used either in a cell, or in a continuous supersonic free jet zone of silence. The intensity of the third harmonic wave generated is well related to the laser power, phase-matching conditions, two-photon resonances, and rotational temperature, through the nonlinear susceptibility, as described theoretically.

The temperature map of a laser-sustained argon plasma is calculated in the gas flow field upstream of a sonic nozzle orifice. The discretized equation of energy conservation is solved with a finite difference method. A forced parallel gas flow field is introduced in order to simulate the gas flow through the nozzle. Such a parallel flow does not account for the pre-cooling effect at the nozzle throat, so that the isentropic laws must be applied at the end of each iteration (outlet) for re-introducing the corresponding gas flux into the inlet conditions of the next iteration. In this way, the gas flux calculation is self-consistent and convergence is obtained after about 200 iterations. The results are compared with experimental data. The agreement is rather satisfactory except that the theoretical laser power threshold is about one order of magnitude higher than the experimental measurement. This is due to the parallel gas flow scheme, which overestimates the gas flux in the plasma core. This effect can be corrected by taking a gas flux value to account for the convergence of the stream tubes toward the nozzle.