Nitrogen migration in ASDEX Upgrade: The role of surface roughness and SOL transport

Studies of N transport and WNx formation in tokamaks are hindered by the inevitable presence of a 14N background from air. Therefore, we injected 15N labeled N2 in the divertor of ASDEX Upgrade and measured its deposition on samples exposed via manipulator systems. As predicted by WallDYN simulations [1], the 15N areal density at the low field side midplane is only a factor of 4 below the 15N areal density at the outer strike line region. The direct impact of the roughness on the N deposition in the divertor is found to be small, although co-deposition on rough samples increases the 15N areal density by a factor of 2. The experimental results are combined with WallDYN-DIVIMP simulations to study parallel and anomalous transport of N in the SOL. To identify the impact of a Be main wall, WallDYN simulation of the N migration in JET are performed and compared to available experimental results. Finally, we discuss the challenges towards a full understanding of the nitride formation of W and Be which we have identified and the impact of the presented results on the ammonia formation.