The effect of the electron energy distribution function (EEDF) on the population distribution of excited states is investigated in He and Ar plasmas sustained by a 2.45 GHz microwave at a discharge pressure of 1–5 Torr. The EEDF is measured with a Langmuir probe by Druyvesteyn's method and used as the input parameter of the collisional-radiative (CR) model calculation. The calculated population density of the excited states is compared with the results of optical emission spectroscopic measurement. Reasonable agreement is found between them. It is also confirmed that the measured EEDFs was depleted at energies above the first excitation potential of the discharge gas atoms. When we assume a Maxwellian EEDF and calculate the population distribution of excited states of the argon plasma, we find a marked discrepancy between the populations of excited states observed experimentally and those calculated by the CR model, while a discrepancy for the helium plasma is also found but is less marked.

https://iopscience.iop.org/article/10.1143/JJAP.49.036001/pdf

Evaluation of Electron Energy Distribution Function in Microwave Discharge Plasmas by Spectroscopic Diagnostics with Collisional Radiative Model

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 73D, 83D, 93D 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 41S-31P and 43P-3 3D. The degree of inversion is insufficient to be used for lasing. The electron temperature as found by Boltzmann plots of the higher states drops to about 0.2 eV in the downstream expansion. The electrons are cooled by contact with cold residual gas. The magnetic confinement 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 population inversions are discussed in terms of atomic processes.

https://iopscience.iop.org/article/10.1088/0963-0252/4/1/013/pdf

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

Lasing conditions for He I in a cold recombining helium plasma are numerically discussed including optical thicknesses. A collisional radiative model is used to calculate population densities. We discuss the effect of self absorptions by the background residual gas on the lasing conditions quantitatively. The optical thickness is essential in the calculation and affects the population inversions between level pairs of the singlet state drastically, while those of the triplet state are not affected very much. In the optically thin plasma, the laser lower levels of the singlet state depopulate mainly through radiative transitions. On the other hand, they depopulate mainly through electron collisional deexcitations in the optically thick plasma. Results of the calculations are verified by the spectroscopic observations of an arc-heated magnetically trapped expanding plasma jet generator which we developed. The requirement for the background pressure in which the lasing is possible is discussed quantitatively.

Numerical Study on Population Inversion and Lasing Conditions in an Optically Thick Recombining Helium Plasma

Collisional — Radiative Model of Helium Plasma. II. Global Population Coefficients

Physical characteristics of ion extraction simulation system based on gas discharge plasma jet

We present a collisional-radiative model of helium plasma with explicit inclusion of metastable state influences and give results of numerical calculations of population coefficients and effective reaction rates (generalization of effective ionization and recombination coefficients for the case when metastable states are not explicitly included) for a large region of electron densities and temperatures for optically thin and thick helium plasma. The model is described in this paper.

Collisional — Radiative Model of Helium Plasma. I. Description of the Model

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Collisional — Radiative Model of Helium Plasma. I. Description of the Model

Collisional — Radiative Model of Helium Plasma. II. Global Population Coefficients