Plasma profiles in the inner divertor of ASDEX Upgrade

On ASDEX Upgrade, the inner leg of the divertor is diagnosed by Langmuir probes, spectroscopy, and thermography. All of these observe a narrow peak near the strike point and a second, broad peak located 10–20 cm higher. The profile form in and between ELMs is very similar, so it is not the temporal superposition of two single peaks. Numerical modeling shows each of the peaks, depending on plasma conditions, but never both peaks at the same time, so an explanation based on axisymmetric physics appears unlikely. The observation in a discharge where the strike point is swept that the upper peak remains fixed in space while the lower peak follows the strike point favors a geometric origin, even though the observations by different diagnostics are similar and the probes were designed to be outside the shadows of the neighboring divertor tiles. Alternatively, toroidally asymmetric physics could produce such a profile.