The physics of W transport illuminated by recent progress in W density diagnostics at ASDEX Upgrade

Abstract

Due to the high mass and charge of the heavy ions, centrifugal and electrostatic forces cause a significant variation in their poloidal density. The impact of these forces on the poloidal density profile of tungsten was investigated utilizing the detailed two-dimensional SXR emissivity profiles from the ASDEX Upgrade tokamak. The perturbation in the electrostatic potential generated by magnetic trapping of the non-thermal ions from neutral beam injection was found to be responsible for significant changes in the poloidal distribution of tungsten ions. An excellent match with the results from fast particle modeling was obtained, validating the model for the poloidal fast particle distribution. Additionally, an enhancement of the neoclassical transport due to an outboard side impurity localization was measured in the experiment when analyzing the tungsten flux between sawtooth crashes. A qualitative match with neoclassical modeling was found, demonstrating the possibility of minimizing neoclassical transport by an optimization of the poloidal asymmetry profile of the impurity.

Measuring the plasma composition in tokamaks with metallic plasma-facing components

“…(2018), Odstrcil et al. (2018), Sertoli et al. (2018) and reference therein) and on these results depend the conclusions drawn by complex analysis and modelling of present experiments as well as predictions for future machines such as ITER or DEMO (see e.g.…”

“…2015; Odstrcil et al. 2018), while lighter elements are not. Solving (2.1) with secondary impurities therefore requires a certain number of assumptions driven by the present theoretical understanding of impurity transport.…”

“…Following equation (9) of Odstrcil et al. (2018) and neglecting the asymmetries driven by fast particles (first term on the right-hand side), the impurity density on a flux surface at a generic mayor radius can be calculated with respect to the value at a reference major radius , where the asymmetry parameter is fully determined by knowing the main ion and impurity masses ( and ), the average impurity charge , the effective charge , the electron and ion temperatures and the toroidal rotation frequency , The impurity density at the chosen reference radius on the LFS mid-plane can then be estimated by inverting (2.11) and calculating the flux-surface-averaged value of the exponent, All other quantities in (2.12) are already included in the analysis or self-consistently calculated using the available profile data.…”

Optimized tomography methods for plasma emissivity reconstruction at the ASDEX Upgrade tokamak

“…The presence of such features requires a high dynamic range of the tomographic reconstruction (up to 3 orders of magnitude) in order to resolve the radiation profile from the core to the edge. The best results are typically obtained by employing weakly nonlinear algorithms like the anisotropic Minimum Fisher Information (MFI) 12 and the Minimum normalized gradient method (MNGR) 13 . A common drawback of the existing algorithms used by pixel based methods is the cubic computational complexity.…”

“…The metal wall and the high heating power provided by 8 neutral beam injection (NBI) sources delivering up to 20 MW, 6 MW of ion cyclotron heating, and 4 MW of electron cyclotron heating make this tokamak well suited for power exhaust studies relevant for future fusion reactors. The SXR diagnostic in AUG is regularly used for the analysis of the impurity transport 15,16 , magnetohydrodynamic (MHD) instabilities 17 and fast particles in the plasma 18 . All of these measurements can benefit from the high-quality tomographic methods described in this article.…”