Plasma profiles in the inner divertor of ASDEX Upgrade

On ASDEX Upgrade, the inner leg of the divertor is diagnosed by Langmuir pr obes, spectroscopy, and thermography. All of these observe a narrow peak ne ar the strike point and a second, broad peak located 10-20 cm higher. The p rofile 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 pea ks, depending on plasma conditions, but never both peaks at the same time, so an explanation based on axisymmetric physics appears unlikely. The obser vation in a discharge where the strike point is swept that the upper peak r emains fixed in space while the lower peak follows the strike point favors a geometric origin, even though the observations by different diagnostics a re similar and the probes were designed to be outside the shadows of the ne ighboring divertor tiles. Alternatively, toroidally asymmetric physics coul d produce such a profile. (C) 2001 Elsevier Science B.V. All rights reserve d.