Radiation processes play an important role for deviations from ionization equilibrium in thermal plasmas. Beside continuum radiation, the reabsorption of resonance radiation may play a role in the establishment of plasma parameters. A conventional approach to account for radiation trapping in collisional-radiative modeling is based on an effective lifetime of resonance atoms and the spontaneous transition probability for highly excited states [1].

In the present work, the impact of the spatial redistribution of resonance atoms within the plasma column of a free-burning arc in argon due to trapping of radiation is considered. The method is based on a solution of the Holstein-Bibermann equation applying a matrix method [2]. The results show significant broadening of the radial distribution and enhanced values of the density of resonance atoms close to the arc axis as compared to the effective lifetime approximation.