Lyman line opacities in tokamak divertor plasmas under high-recycling and detached conditions

Radiation transport simulations of the deuterium Lyman series using EIRENE predict a factor of 2 increase of Balmer- α emission in the JET-ILW low confinement divertor plasmas in high-recycling and detached conditions caused by the re-absorption of Lyman- β emission. The photon tracing model in the EIRENE code is applied to SOLPS-ITER pure deuterium plasma solutions of JET-ILW to evaluate the contribution of Lyman- β capture towards the population density of D( p = 3 ) and consequently the Balmer- α emission. The local population escape factors of Lyman- α and Lyman- β lines in the JET-ILW divertor show steep spatial gradients and indicate local regions with high opacity in high-recycling and detached conditions. The relative contribution of Lyman- β capture towards the population density of D( p = 3 ) is significant in high-recycling and detached conditions. This observation is further corroborated by line-integrated calculations using Balmer- α diagnostic line-of-sights. • The EIRENE photon tracing model shows that the JET-ILW divertor volume is Ly- α and Ly- β opaque in high-recycling and detached regimes. • The model predicts a factor of 2 increase of D- α emission in the JET-ILW low confinement divertor plasmas in high-recycling and detached conditions caused by the re-absorption of Ly- β emission.