Stationary Population Inversion in an Expanding Argon Plasma Jet by Helium Puffing

An experiment of He gas-contact for generating population inversion in a recombining Ar plasma jet is carried out. Population inversion between Ar I excited states 5s′ → 4p′[1/2]1 and 5s′ → 4p[3/2]1,2, [5/2]2,3 is created by helium gas-contact cooling of electrons, whereas it is not created without gas-contact. Ar I lines 1.14 μm, 1.34 μm, and 1.09 μm are strongly enhnced due to the He gas cooling. It is experimentally found that helium gas contact effectively lowers electron temperature of the Ar plasma jet. The mechanisms giving rise to population inversion are discussed in terms of atomic collisional processes of the recombining plasma. The experimental results of electron temperature and population densities are discussed by simple numerical analysis which we previously developed. It is shown that the experimental results are well explained by our modeling quantitatively for the case without gas contact, except that the agreement of number densities of lower lying non-LTE levels is qualitative for the case with the gas contact. INTRODUCTION The authors have been studying fundamental spectroscopic characteristics of hydrogen and noble gas plasmas generated by a plasma jet apparatus [1]. These plasmas are in a typical recombining phase, and stationary population inversions of H I and He I have been observed by spectroscopic examinations [2, 3, 4]. On the other hand, no population inversion of Ar I was observed for the argon plasma in the same discharge apparatus. Collisional radiative model theoretically showed that electron temperature should be sufficiently low in order to create population inversion in recombining plasmas [5]. We found that the electron temperature of the hydrogen and helium plasmas was about 0.1 eV or much lower and cold enough to make population inversion, whereas that of the argon plasma was about 0.5 eV, by spectroscopic measurement. It was supposed that electron gas in the downstream region of the plasma jet becomes relaxed mainly by collisions with residual gaseous molecules in the plasma jet chamber [4]. The authors consider that the result of the Ar plasma is attributed to the fact that the cross section of elastic collision between an electron and an argon atom is reduced owing to the Ramsauer effect in the energy range about 0.2− 0.5 eV [6]. And consequently, electron temperature of the argon plasma does not become lower than 0.5 eV, and no population inversion occurs, whereas electron temperature of hydrogen and helium plasmas was found to be as low as 0.1 eV due to frequent collisions of electrons with residual gaseous molecules in the plasma expansion region, since they have no Ramsauer minimum in their elastic electron scattering cross sections. The authors have numerically investigated an effect of He gas-contact cooling of electrons on generation of population inversion in the Ar plasma [7]. It was shown that the electron temperature of the Ar plasma was significantly decreased by gas-contact cooling, so that the population inversion of Ar I excited states 5s′ → 4p′[1/2]1 and 5s′ → 4p[3/2]1,2, [5/2]2,3 was created. The objective of the present study is to demonstrate the effectiveness of the gascontact cooling of electrons and to generate population inversion of Ar I experimentally. Another objective is to investigate the mechanism of the electron energy transfer in the recombining plasma jet. EXPERIMENTAL SETUP Figure 1(a) shows the schematic view of the plasma jet apparatus [1]. The apparatus is composed of a plasma generator, six magnetic coils, traversing mechanism for moving the optical fiber terminal assembly for spectroscopic