An experimental study of stationary population inversion in a cold recombining expanding helium plasma jet

Abstract

A stationary cold recombining helium plasma is generated in an arc-heated magnetically trapped expanding plasma jet that was developed in the authors' laboratory. The plasma can be generated very stably and spectroscopic analysis is accomplished with sufficient accuracy. Population densities of the He I levels whose principal quantum numbers are 3-6 and 7 9.8 3D, 9 3D levels are observed spectroscopically. When the plasma comes downstream, the electron temperature and density become low and population inversion is obtained between some level pairs, such as 4'S-3'P and 43P-33D. The degree of inversion is insufficient to be used for lasing. The electron temperature as found by Eoltzmann plots of the higher states drops to about 0.2 eV in the downstream expansion. The electmns are cooled by contact with cold residual gas. The magneticconfinement of the plasma jet is adequate to cause large population inversions without decreasing the electron density. The experimental results are explained in terms of a calculation based on a collisional radiative model including an optical thickness. The mechanisms of formation of DoDulation inversions are discussed in terms of atomic processes.

Acceleration Phenomenon Of Helium Arc Jet Through A Magnetic Nozzle

“…Figure 4(d) shows that the measured electron temperature T e takes its minimum at the position of maximum Mach number (z = 420 mm), 0.1 eV, which is considered to be a typical value of a cold recombining expanding plasma jet [8][9]. After passing through this position, the velocity begins to decrease and the electron temperature becomes increasing.…”

Spectroscopic Characteristics of Supersonically Expanding Nitrogen Arc-Jet Plume—Strong Atomic Lines and Weak Molecular Band Spectra

“…For example, we found the supersonic velocity of the plasma, up to about Mach 3, and the electric potential drop at the end of the hollow magnets, where the magnetic field becomes diverging [4]- [7]. We also found remarkable population inversion of energy levels of hydrogen atoms [8] and those of helium atoms [9], due to rapid cooling of electrons with Manuscript residual gas molecules in the expanding region [10] as well as adiabatic expansion [4]- [6]. In this paper, as a next step of experimental study of expanding plasma jet, we examine characteristics of nitrogen plasma as a plasma plume, particularly of its spectroscopic characteristics, which gives essential fluid dynamic information of nitrogen plasma jet as a thruster for astronautic applications for artificial satellites.…”

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

“…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.…”

“…Electron temperature T e was determined from population distributions of high-lying levels of the Boltzmann plots obtained by spectroscopic measurements, since it was already shown that these levels of recombining plasmas were in a state of PLTE (partially local thermodynamic equilibrium) [1,2,3,4]. Because the emission is observed along the direction perpendicular to the axis of the plasma jet and the Abel inversion is not applied, electron temperature obtained by spectroscopic measurement approximately corresponds to mean value along the line of sight.…”

“…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].…”