Structure of the electron temperature profile around the separatrix

The temperature decay length (λ_Te) in the near scrape-off layer (SOL) reflects the ratio between the transport parallel to the magnetic field lines via Spitzer–Härm electron conduction and the perpendicular mechanisms involving neoclassical and anomalous transport. The implementation of the thermal helium beam diagnostic on ASDEX Upgrade has enabled an excellent spatiotemporal study of the structure of the electron temperature profile (T_e) around the separatrix and the derivation of the near SOL decay lengths. From the analysis of the T_e profile structure of attached H- and L-mode discharges, a self-consistent correspondence between the extrema of the electron temperature curvature profile (∂_R² T_e) and the position of the separatrix (R_sep) is revealed. A 1.5 D model for the power balance from closed to open field lines, including the escaping heat flux from the confined region and the parallel losses to the divertor, as well as results from the plasma edge code GRILLIX, support these experimental results. The evaluation shows that the temperature decay length is not constant over the plasma edge and SOL region, so that the absolute value of λ_Te strongly depends on the analyzed region. Independent of the exact location and in agreement with edge Thomson scattering evaluations, the decay lengths show the known inverse relation to the plasma current (I_p).