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High-Z impurities such as tungsten must be kept out of the plasma, to avoid excessive radiation or plasma collapse. Previous assessments of the neoclassical impurity transport expected at the edge of ITER plasmas has indicated favorable outward convection of tungsten, due to the expected high pedestal ion temperatures and lower density gradients [R. Dux et al, PPCF 2014; NME 2017] leading to temperature screening. While the edge plasma conditions expected on ITER cannot be reproduced in current devices, this effect has been experimentally observed at the periphery of optimized, high performance hybrid scenario pulses at JET [J. Hobirk et al, NF 2023; A.R. Field et al, NF 2023].
In the full W-wall ASDEX Upgrade, experiments to study the neoclassical temperature screening of tungsten in the plasma periphery were undertaken. The required plasma conditions are strong edge $\mathrm{T_i}$ gradient, low edge $\mathrm{n_e}$ gradient, as well as low collisionality and strong rotation [D. Fajardo et al, PPCF 2023]. To this end, the “improved H-mode scenario” [J. Hobirk et al, IAEA-FEC 2012] was optimised by tuning the heating and gas waveforms. High plasma performance was achieved $\mathrm{(H_{98(y,2)} \sim 1.3}$, $\mathrm{β_N \gt 3.0}$, $\mathrm{W_{MHD} \sim 1.3\;MJ)}$, while the radiation was maintained at low levels $\mathrm{(f_{rad}=P_{rad,tot}/P_{in} \sim 0.3)}$. The tungsten concentration remains low, and bolometry and soft X-ray measurements indicate a significant reduction of impurities in the ‘mantle’ region, indicating that tungsten is effectively kept out at the plasma edge. Transport modelling with FACIT [P. Maget et al, PPCF 2020; D. Fajardo et al, PPCF 2022] indicates outward convection of tungsten in the region just inside the pedestal top.
To assess this scenario in detail, the tungsten density profiles inferred from bolometry and soft X-rays are compared with the modelling and the tungsten sources are qualitatively assessed. The analysis shows a strong poloidal asymmetry of the tungsten density in the pedestal. The impact of the ELMs on the tungsten density, as well as its inter-ELM evolution are discussed. Furthermore, small amounts of neon were injected in selected pulses to study the neon transport using CXRS following the method developed in [T. Gleiter et al, NF 2025]. This aims to compare the transport of impurities with different charge, and to assess the effectiveness of impurity seeding for power exhaust in these conditions.
Therefore, this work enables a comprehensive investigation of tungsten transport, demonstrating experimentally its neoclassical temperature screening in the plasma periphery, relevant to ITER.