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

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.