The edge fluid code EDGE2D-EIRENE was used to compare the calculated low field side mid-plane separatrix temperature (Teu, sep) in the presence of seeding impurities with the two point model. The value of Teu, sep and scalings of Teu, sep with the power crossing the separatrix (Psep) are studied. Two scalings of Teu, sep with Psep can be derived from two point model; (1) Teu,sep∝Psep 2/7 which assumes that the power decay length λq is constant and; (2) Teu,sep∝Psep 4/9 which accounts for the λq dependence on Psep and the presence of prescribed diffusive radial transport. A linear scaling between Teu, sep and Psep is observed in the EDGE2D simulations, although the Teu,sep∝Psep 4/9 scaling captures the variation in Teu, sep (in the simulations) for all but highly seeded simulations. This linear dependence (between Teu, sep and Psep) is due to a stronger than linear dependence on the parallel heat flux entering the divertor (q∥, u, sep) on Psep (for a doubling of Psep a factor six variation in q∥, u, sep is observed). Psep is reduced by a factor of two due to the varying impurity radiation inside the separatrix. However, q∥, u, sep reduces more than a factor two because (i) impurity radiation preferentially removes heat flux above the x-point and near the separatrix and (ii) the variation in λq with Psep due to increasing radial diffusive heat flux (Eq. (5.77) Stangeby 2000). The largely varying q∥, u, sep, interestingly, is successfully captured by the power decay length (λq, Eich) calculated by fitting to the target heat flux density. Accounting for the q∥, u, sep variation by using λq, Eich and Psep (both experimentally measurable quantities), an agreement between the two point model equation and the predicted Teu, sep from EDGE2D-EIRENE was obtained. A variation of Teu, sep from approximately 60 eV to 120 eV, for electron separatrix density range of 2−3×1019m−3, was observed. This separatrix temperature variation from EDGE2D is in contrast to a routinely assumed separatrix temperature of 100 eV used for pedestal stability analysis at JET.
- Separatrix temperature