Relation between separatrix electron density and divertor neutral pressure in Alcator C-Mod, ASDEX Upgrade and JET H-mode plasmas

The separatrix electron density n_e,sep is an important plasma parameter that influences H-mode confinement, detachment achievement, ELM avoidance and sets a limit to H-mode operation. Therefore, quantitative prediction of n_e,sep is crucial for a meaningful assessment of a core-edge-SOL integrated scenario solution in next-step devices. For this purpose, assembling a multi-machine database is pivotal to both developing simple physics models that could explain the main parameter dependencies of n_e,sep and benchmarking state-of-the-art SOL codes.

In this work, a machine-agnostic procedure to evaluate n_e,sep for ELMy and no-ELM H-mode plasmas is presented and used to assemble a n_e,sep and T_e,sep database which includes Alcator C-Mod, ASDEX Upgrade and JET data. The analysis procedure, which is applied in the same way in all devices, relies on Thomson Scattering (TS) measurements and on the assumption of SOL parallel transport being dominated by Spitzer-Härm electron conduction, see e.g. [1, 2, 3]. It is found that, across all devices, a similar relationship between n_e,sep and the divertor neutral pressure p_0,div is observed, namely n_e,sep ∝ p_0,div^0.25±0.06. Additionally, upon normalizing n_e,sep to the obtained p_0,div dependency, no remaining correlation with the plasma current is found. These observations are in agreement with the expected dependencies from the 2-point model [4, 5, 6], and suggest that in the present dataset the upstream separatrix density is mainly determined by divertor target recycling.

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[3] T. Eich et al., Nucl. Fusion 61, 086017 (2021)

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[6] A. Kallenbach et al., Plasma Phys. Control. Fusion 60, 045006 (2018)