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.