Using EDGE2D-EIRENE to simulate the effect of impurity seeding and fueling on the upstream electron separatrix temperature. (August 2019)

• Record Type: Journal Article
• Authors: Simpson, J.; Moulton, D.; Giroud, C.; Groth, M.; Corrigan, G.
• Is Part Of: Nuclear materials and energy. Volume 20(2019)
• Publication Date: 2019-08
• Subjects: Separatrix temperature -- EDGE2D-EIRENE; Nuclear energy -- Periodicals; Nuclear fuels -- Periodicals; Nuclear reactors -- Materials -- Periodicals; Radioactive substances -- Periodicals; 621.4833
• DOI: 10.1016/j.nme.2019.02.002
• Languages: English
• ISSNs: 2352-1791

Abstract

Highlights: Compared electron separatrix temperature between two point model (TPM) and EDGE2D. The simulations included varying neon, nitrogen seeding and density. A factor two variation in EDGE2D electron separatrix temperature ( Te ) was observed. The observed electron temperature in EDGE2D was inconsistent with the TPM. Te was driven by seeding radiation, power crossing separatrix and radial heat flux. Abstract: The edge fluid code EDGE2D-EIRENE was used to compare the calculated low field side mid-plane separatrix temperature ( T eu, sep ) in the presence of seeding impurities with the two point model. The value of T eu, sep and scalings of T eu, sep with the power crossing the separatrix ( Psep ) are studied. Two scalings of T eu, sep with Psep can be derived from two point model; (1) T e u, s e p ∝ P s e p 2 7 which assumes that the power decay length λq is constant and; (2) T e u, s e p ∝ P s e p 4 9 which accounts for the λq dependence on Psep and the presence of prescribed diffusive radial transport. A linear scaling between T eu, sep and Psep is observed in the EDGE2D simulations, although the T e u, s e p ∝ P s e p 4 9 scaling captures the variation in T eu, sep (in the simulations) for all but highly seeded simulations. This linear dependence (between T eu, 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 ( λ q, Eich ) calculated by fitting to the target heat flux density. Accounting for the q ∥, u, sep variation by using λ q, Eich and Psep (both experimentally measurable quantities), an agreement between the two point model equation and the predicted T eu, sep from EDGE2D-EIRENE was obtained. A variation of T eu, sep from approximately 60 eV to 120 eV, for electron separatrix density range of 2 − 3 × 10 19 m − 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.