Theoretical description of heavy impurity transport and its application to the modelling of tungsten in JET and ASDEX upgrade

Abstract

Recent developments in theory-based modelling of core heavy impurity transport are presented, and shown to be necessary for quantitative description of present experiments in JET and ASDEX Upgrade. The treatment of heavy impurities is complicated by their large mass and charge, which result in a strong response to plasma rotation or any small background electrostatic field in the plasma, such as that generated by anisotropic external heating. These forces lead to strong poloidal asymmetries of impurity density, which have recently been added to numerical tools describing both neoclassical and turbulent transport. Modelling predictions of the steady-state two-dimensional tungsten impurity distribution are compared with experimental densities interpreted from soft Xray diagnostics. The modelling identifies neoclassical transport enhanced by poloidal asymmetries as the dominant mechanism responsible for tungsten accumulation in the central core of the plasma. Depending on the bulk plasma profiles, neoclassical temperature screening can prevent accumulation, and can be enhanced by externally heated species, demonstrated here in ICRH plasmas.

ICRH for core impurity mitigation in JET-ILW

“…This is corroborated by dedicated impurity transport studies using W and Mo laser blow-off injection [16]. Theoretical studies suggest that collisions between the W ions and the RF-accelerated ions with moderate energies also contribute to the observed core impurity screening in this case [17]. The impact of ICRF heating on the spatial redistribution of the high-Z impurities in the plasma is illustrated by the tomographic reconstruction of the soft X-ray radiation (g,h) for the two discharges discussed.…”

Modification of impurity transport in the presence of saturated (m,n) = (1,1) MHD activity at ASDEX Upgrade

“…The results from the codes have been used to model the impurity photon emission in the soft X-ray spectral region and compared to the experimental profiles as detected by the soft X-ray (SXR) diagnostic [8,9].…”

Heavy impurity confinement in hybrid operation scenario plasmas with a rotating 1/1 continuous mode

“…However, experiments often show strong peaking on the axis even in the presence of strong flows 4 . This section investigates the possibility of axial neoclassical peaking due to 3D effects in the core despite the presence of centrifugal effects.…”