title: Experimental study of population inversion between excited states of Ar I in a recombining Ar plasma by He contact cooling.
abstract: 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, 1.34, and 1.09 microm are strongly enhanced due to the He gas cooling. It is experimentally found that helium gas contact effectively lowers the 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 a simple numerical analysis that 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 nonlocal-thermodynamic-equilibrium levels is qualitative for the case with the gas contact.
authors.affiliation: Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1, O-Okayama, Meguro-ku, Tokyo 152-8550, Japan.
authors.forename: Katsuhiro
authors.initials: K
authors.lastname: Kano
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Akatsuka