Investigation of local carbon transport in the ASDEX Upgrade divertor using 13CH4 puffing

To investigate the combined effect of re-deposition, re-erosion and local transport, known quantities of 13CH4 were puffed at the end of the 2007 experimental campaign in the ASDEX Upgrade outboard divertor. Exposed tiles were carefully removed for analysis. The amount of 13C locally deposited was measured by nuclear reaction analysis (NRA) and colorimetry. About 100% of injected carbon is deposited within a ± 15 cm extension in the toroidal direction. In contrast to H-mode results where re-deposition was exclusively downstream, in L-mode, more than one third of the injected hydrocarbon is found upstream. Colorimetric analysis of images taken with different lighting angles to the surface reveals a strong asymmetry in the carbon deposition pattern with respect to the injection facing/averting side of the surface roughness, with 4× thicker layers on the side facing the puffing location. The deposition pattern deviates clearly from the magnetic trajectories showing the effect of downward and radial drifts. ERO modelling of a similar experiment carried out in 2003 in H-mode background plasma can nicely reproduce the toroidal deposition pattern but drifts are not yet satisfactory described.