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Numerical Study on Radiation Trapping of He I Resonance Lines in Arc Plasma Under High-Gas Pressures

Hayato KAWAZOME, Md Anwarul ISLAM¹⁾ and Shinichi NAMBA¹⁾

Department of Information Engineering, National Institute of Technology, Kagawa College, 551 Kohda, Takuma-cho, Mitoyo 769-1192, Japan

1)

Graduate school of Advanced Science and Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-hiroshima 739-8527, Japan

(Received 15 November 2020 / Accepted 25 December 2020 / Published 10 February 2021)

Abstract

In high-gas pressure helium arc plasmas, a forbidden line (1s ¹S-2p ³P : 59.1 nm) as well as the resonance lines 1s ¹S - np ¹P lines of He I have been observed. The intensity ratio of the 1s ¹S - 2p ¹P line of He I to forbidden line calculated from the Einstein A coefficients (NIST database) is ∼10⁻⁷, whereas the value obtained experimentally was as small as ∼20. The reason for the discrepancy between the experiment and database can be interpreted from that the photoabsorption (self-absorption) of the resonance lines 1s ¹S - np ¹P lines can cause the drastic change in radiative processes in high-pressure plasmas. In order to validate our interpretation on this mismatch, we investigated the influence of self-absorption of the resonance lines 1s ¹S - np ¹P lines by numerical simulations. The simulation code calculated the photoabsorption process by He atom along a line-of-sight by using coupled rate equations incorporated with the radiation trapping effect. As a result, the simulation yielded the line intensity ratio of 25 because of the strong self-absorption.

Copyright (c) 2021 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords

radiation trapping, helium arc plasma, spectral line shape

DOI: 10.1585/pfr.16.2401012

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References

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• [5] https://www.nist.gov/pml/atomic-spectra-database