Modelling of nitrogen seeding experiments in the ASDEX Upgrade tokamak

Experiments using nitrogen were conducted in H-mode plasmas at ASDEX Upgrade that has a full-W wall. The edge region of H-mode plasmas is modulated by the edge-localized modes (ELMs) which lead to a loss of energy and particles from the confined plasma. In order to gain a better understanding of the complex physical mechanisms which govern the behaviour of radiation and impurities in the presence of ELMs, the evolution of impurities and radiation has been modelled in a time-dependent way. The simulations have been carried out with the ASTRA-STRAHL package featuring the self-consistent interplay between impurity transport, radiation, heat and particle transport of the background plasma, and the effects of ELMs. ELMs are modelled based on the two different assumptions of a diffusive and a convective transport, respectively. The experimental discharge behaviour was reproduced providing only transport coefficients, heat, and particle source. The results underlie the importance of non-coronal effects through the ELM-induced transport which lead to a strong enhancement of the nitrogen radiation. Taking these effects into account is crucial in order to not underestimate the radiation. The radiation properties of high-Z impurities such as tungsten are instead very weakly influenced by non-coronal effects due to its faster equilibration time. While the nitrogen density does not change significantly decreasing the ELM frequency, tungsten density and consequently the radiation increase strongly. The degree to which W is flushed out depends on whether the ELM transport is diffusive or convective. Simulations show that for the N seeded cases considered here, the diffusive model reproduces more accurately the experimental observations. The different behaviour of N and W can be explained in terms of profile peaking which increases with Z (neoclassical pinch). The strong increase in W radiation when the ELM frequency is decreased is not only due to the lack of a sufficiently strong flush out of this impurity but also to the fact that the long time between two crashes gives the impurities time to penetrate further into the plasma escaping the region where they can be flushed out. This is in agreement with the experimental observations and highlights the importance of maintaining high ELM frequencies for the stability and performance of the discharges.