Nitrogen transport in ASDEX Upgrade: Role of surface roughness and transport to the main wall

We have studied the retention of nitrogen in surfaces with varying roughness and the transport of nitrogen from the divertor to the outer midplane in experiments at the tokamak ASDEX Upgrade. To allow for a reliable identification of nitrogen retained from the plasma exposure, 5.3 · 10²¹ atoms of the tracer isotope ¹⁵N were injected into the private flux region of the plasma. On polished W samples exposed to the plasma in the outer divertor, the N content peaks to both sides of the strike line with an areal density of 1.5 · 10^20 15N/m² and drops to a value of 1.0 · 10^20 15N/m² in the strike line region. In contrast, the N content of samples with a rougher surface peaks at the strike line and reaches areal densities of 3.0 · 10^20 15N/m². The N deposition at the outer limiters was measured via samples exposed on the midplane manipulator. At the limiter position the 15N areal density reaches a value of 0.2 · 10^20 15N/m², only a factor of ten smaller than the areal densities in the divertor. However, a comparison to WallDYN simulations shows that the observed N content is roughly a factor of 4 smaller than the value predicted by these simulations. Possible reasons for this discrepancy are the toroidal asymmetric main wall geometry, which currently cannot be fully included in the simulations, or an enhanced re-erosion of deposited N.