Properties of an argon plasma free jet generated from a continuous optical discharge

Abstract

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.

Inflow and shock formation in supersonic, rarefied plasma expansions

“…If the power density in the source region is high and the pressure in the background is low, the pressure may vary over many orders of magnitude, and a supersonic expansion is formed. A thorough understanding of the plasma-expansion properties is crucial in fields such as astrophysics, 1 high-rate deposition, [2][3][4] and surface modification. 5 In expansions, both plasma and gaseous, the mixing of the expansion with its background is of key importance in a number of applications such as hypersonic propulsion, chemical lasers, plasma deposition, and plasma spraying.…”

Simple model for the electronic relaxation during the expansion of a plasma generated in an Ar–O2 mixture

“…8 The comparison is satisfactory in both cases, and is presented in Fig. 1 for the Saclay experiment.…”

“…Furthermore, such a phenomenon is not expected in jets operated in the conditions of a zone of silence inside a shock structure. 8 Finally, the adjustable parameters can be listed as follows: ͑i͒ the normalization constants ͑respectively K O and K A ) of the Thomson formula of the rate constants for excitation of atomic oxygen and of argon by electrons; ͑ii͒ the degree of dissociation of oxygen at the exit of the nozzle; and ͑iii͒ the relative densities of singly ionized argon and oxygen ͑needed for the populations of the highest levels, as calculated by the Saha Law͒. These parameters are adjusted for minimizing the mean standard deviation between experimental and calculated values of the population densities ͑in logarithmic scale͒.…”

“…Such a task is highly simplified if the problem can be limited to a one-dimensional description, 4,8 or if the overall local properties can be included as parameters measured experimentally 9 or calculated separately. 8 The present work has been applied to both situations and was first performed for a laser sustained plasma free jet of pure argon. 9,10…”

Anomalous Atomic Hydrogen Shock Pattern in a Supersonic Plasma Jet

“…The experimental results show that H atoms escape from the supersonic expansion by a diffusion process due to strong density gradients between the core of the jet and its vicinity. Plasma expansion from a high pressure source region into a low pressure region is a general physical phenomenon [1][2][3] that covers a broad range of physical systems such as solar flares, remote plasma systems, and vacuum arc spots. Apart from the large differences in scale, those phenomena exhibit numerous similarities.…”