A predictive formula for the H-mode electron separatrix density: Bridging regression and physics-based models across C-Mod, AUG and JET tokamaks

D. Silvagni; O. Grover; A. Stagni; J. W. Hughes; M. A. Miller; B. Lomanowski; L. Balbinot; G. Ciraolo; W. Dekeyser; M. Dunne; L. Frassinetti; C. Giroud; T. Happel; I. Jepu; A. Kallenbach; A. Kirjasuo; A. Kuang; T. Luda; D. Moulton; O. Pan; C. Perez von Thun; T. Puetterich; G. Rubino; S. A. Silburn; H. J. Sun; D. Umezaki; H. Zohm; the ASDEX Upgrade team; JET contributors; the EUROfusion tokamak exploitation team

arXiv · 2601.23140

A predictive formula for the H-mode electron separatrix density: Bridging regression and physics-based models across C-Mod, AUG and JET tokamaks

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Abstract

The electron density at the separatrix ($n_{e,\mathrm{sep}}$) plays a central role in balancing energy confinement, detachment achievement, and ELM suppression in tokamaks, thereby influencing core-edge integration. To study what determines this key parameter, a database of H-mode separatrix density measurements from Alcator C-Mod, ASDEX Upgrade, and JET tokamaks has been assembled using a consistent analysis method across all devices. This dataset is used to derive a regression scaling expression based solely on engineering parameters, and the results are compared to predictions from the two-point model. The agreement found is remarkable: both the regression and model provide similar parameter dependencies and tokamak-specific multiplicative constants. Building on this agreement, a fully predictive formula that combines the regression dependencies and the two-point model multiplicative constant is proposed. This formula is able to estimate $n_{e,\mathrm{sep}}$ across the three machines within a factor of 1.5, and provides projections to next-step devices (ITER, SPARC, DTT, JT-60SA and COMPASS-U) that are in agreement with available SOLPS simulations.

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D. Silvagni, O. Grover, A. Stagni, J. W. Hughes, M. A. Miller, B. Lomanowski, L. Balbinot, G. Ciraolo, W. Dekeyser, M. Dunne, L. Frassinetti, C. Giroud, T. Happel, I. Jepu, A. Kallenbach, A. Kirjasuo, A. Kuang, T. Luda, D. Moulton, O. Pan, C. Perez von Thun, T. Puetterich, G. Rubino, S. A. Silburn, H. J. Sun, D. Umezaki, H. Zohm, the ASDEX Upgrade team, JET contributors, the EUROfusion tokamak exploitation team. 2026-01-30. A predictive formula for the H-mode electron separatrix density: Bridging regression and physics-based models across C-Mod, AUG and JET tokamaks. https://arxiv.org/abs/2601.23140

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