Relating the near SOL transport with plasma properties of the confined edge region in ASDEX Upgrade

An analysis of ASDEX Upgrade plasma is performed to understand the near SOL transport and develop the predictive basis for the electron temperature gradient, λ T e , u . All of the unseeded L- and H-mode attached and N seeded H-mode discharges studied are shown that the analyzed ASDEX Upgrade dataset is in the conduction-limited regime, i.e. the parallel transport in the near SOL is dominated by Spitzer–Harm conduction. By studying a H–L back transition, it is shown that the ‘bifurcation’ in the core plasma between H- and L-mode regimes also exists in the perpendicular transport in the near SOL region. Through power balance and the Spitzer–Harm condution, the SOL perpendicular transport can be derived as χ ⊥ ∝ C H , L χ n e − 1 T e 3 / 2 , with C L χ / C H χ ≈ 2 . For detached plasmas, the SOL upstream electron profile is found to be broader than an equivalent attached plasma under certain conditions. By comparing λ T e , u with global energy confinement, it is found that the discharges with broadened profiles also have degraded confinement, while those with unchanged profiles have similar confinement to attached plasma. The widening of the SOL is also found to coincide with the dropping of upstream temperature. Finally, comparisons of a N seeded H-mode with high pedestal top pressure and an I-mode plasma with an L-mode reference are both found to break the generally observed correlation between the Te SOL decay length and the pedestal top pressure. Thus, the relationship is shown to be non-causal and must instead be due to similar dependences on other plasma parameters. This means that higher global energy confinement is not necessarily imply larger heat flux in the divertor and motivates the search for regimes that optimize both. The implications of these results for SOL transport are discussed.