## 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 (P_{sep}) are studied. Two scalings of T_{eu, sep} with P_{sep} can be derived from two point model; (1) T_{eu,sep}∝P_{sep} ^{2/7} which assumes that the power decay length λ_{q} is constant and; (2) T_{eu,sep}∝P_{sep} ^{4/9} which accounts for the λ_{q} dependence on P_{sep} and the presence of prescribed diffusive radial transport. A linear scaling between T_{eu, sep} and P_{sep} is observed in the EDGE2D simulations, although the T_{eu,sep}∝P_{sep} ^{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 P_{sep}) is due to a stronger than linear dependence on the parallel heat flux entering the divertor (q_{∥, u, sep}) on P_{sep} (for a doubling of P_{sep} a factor six variation in q_{∥, u, sep} is observed). P_{sep} 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 λ_{q} with P_{sep} 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 P_{sep} (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.

Original language | English |
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Article number | 100599 |

Journal | Nuclear Materials and Energy |

Volume | 20 |

DOIs | |

Publication status | Published - 1 Aug 2019 |

MoE publication type | A1 Journal article-refereed |

## Keywords

- EDGE2D-EIRENE
- Separatrix temperature