Nonaxisymmetric energy deposition pattern on ASDEX upgrade divertor target plates during type-I edge-localized modes.

ABSTRACT

In the ASDEX Upgrade tokamak, complex power deposition structures on the divertor target plates during type-I edge-localized modes (ELMs) have been discovered by fast (few microseconds), two-dimensional (40 x 40 cm(2)) infrared thermography. In addition to the usual axisymmetric power deposition line near the separatrix, there appear, statistically distributed, several laterally displaced and inclined stripes, mostly well separated from each other and from the main strike zone. These structures are interpreted as footprints of approximately field aligned, helical perturbations at the low field side of the main plasma edge related to the nonlinear ELM evolution. Based on this picture, the ELM related mode structure can be derived from the target load pattern, yielding on average toroidal mode numbers in a range of 8-24.