Opacity Effect on Visible Neutral Helium Line Emissions and its Impact on Line Ratios

Full text: In this work an effort has been made to investigate the intensities of neutral helium lines and their ratios taking into account the influence and consequences of opacity. For this purpose, helium escape factors have been calculated on the basis of some simplifying assumptions in the ADAS code, which is part of the well-known Atomic Data Analysis Structure (ADAS) database. The change in excited state population and ionization balance due to reabsorption of radiation is then derived using a collisional-radiative model based on calculations of the ADAS code. The opacity affected photon emissivity coefficients (PECs), basically effective rate coefficients, have been estimated for routinely detected visible neutral helium lines and their ratios have been derived for a specified plasma length and geometry of the system. It is found that the spectral lines 4921.9 Å (2p¹P°-4d¹D) and 7281.3 Å (2p¹P°-3s¹S) are slightly affected by opacity whereas the lines 4713.1 Å (2p³P°-4s³S), 5047.7 Å (2p¹P°-4^s1S) and 7065.7 Å (2p³P°- 3s³S) are strongly affected by opacity. Under opacity effects, the density sensitive helium line ratio He I 4921.9 Å/He I 5047.7 Å and temperature sensitive He I 7065.7 Å/He I 7281.3 Å can't be used accurately. (author)