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Description
In the near scrape-off layer (SOL), heat transport is dominated by parallel electron conduction, resulting in a radial heat flux decay length ($\lambda_q$) that is short relative to the machine size. The cross-field extent of the power entering the divertor region is set by $\lambda_q$, which strongly affects divertor performance, including the peak heat flux and access to detachment. In the conduction-limited regime, $\lambda_q$ is proportional to the near-SOL electron temperature decay length ($\lambda_T$). In addition, the near-SOL electron density decay length ($\lambda_n$) strongly impacts particle flux profiles at the divertor target. At high separatrix densities on ASDEX Upgrade, Sun et al. [1] reported broadening of both $\lambda_T$ (and hence $\lambda_q$) and $\lambda_n$ beyond the prediction of the attached $\lambda_q$ scaling [2]. These observations motivated Eich et al. [3] to propose new empirical scalings for $\lambda_T$, $\lambda_n$, and the electron pressure decay length ($\lambda_p$), based on a turbulence control parameter $\alpha_t$. Higher values of $\alpha_t$ correspond to increased cross-field transport driven by resistive-interchange turbulence, leading to profile broadening.
The aim of this contribution is to systematically investigate the dependence of $\lambda_T$, $\lambda_n$, and $\lambda_p$ on $\alpha_t$ in ELMy conditions on JET, including the effects of divertor configuration and isotope fuelling (D, T, and DT) without external impurities. Most empirical $\lambda_q$ scalings are based on experiments with deuterium plasmas, introducing uncertainty when extrapolating to deuterium–tritium (DT) operation. Both isotope [4] and divertor configuration [5] can affect plasma evolution under identical engineering parameters.
The decay lengths were strongly correlated with $\alpha_t$ and showed no significant isotope dependence within $\alpha_t < 0.35$. Vertical–vertical (inner–outer) divertor configurations exhibited broader profiles than vertical–horizontal configurations at the same $\alpha_t$, indicating that target recycling dynamics are not fully captured by the $\alpha_t$ framework. Trends in $\lambda_T$ and $\lambda_p$ showed reasonable agreement with the ASDEX Upgrade empirical scalings [3], providing a valuable inter-machine comparison that validates the machine-size dependence inherent in $\alpha_t$. Larger scatter in the $\lambda_n$ measurements indicates that particle and heat exhaust are governed by distinct mechanisms. For a given divertor configuration, $\alpha_t$ provides a simple parameter to characterise near-SOL broadening, offering guidance for reactor power-exhaust design.
[1] H.J. Sun et al., Plasma Phys. Control. Fusion 57, 125011 (2015).
[2] T. Eich et al., Nucl. Fusion 53, 093031 (2013).
[3] T. Eich et al., Nucl. Fusion 60, 056016 (2020).
[4] C.F. Maggi, Nucl. Fusion (2025).
[5] E. Joffrin et al., Nucl. Fusion 57, 086025 (2017).