A comparison of the impact of central ECRH and central ICRH on the tungsten behaviour in ASDEX Upgrade Η-mode plasmas

摘要

A comparison of the impact of additional central electron cyclotron resonance heating (ECRH) and ion cyclotron resonance heating (ICRH) on the behaviour of the tungsten (W) density in the core of Η-mode plasmas heated with neutral beam injection (NBI) is performed in ASDEX Upgrade. Both localized and broad profiles of the power density of the ECRH have been obtained, where broad profiles reproduce the profile shape of the ICRH power density, which is applied with a hydrogen minority heating scheme. In contrast to ECRH, which produces direct electron heating only, ICRH eventually heats both electrons and ions in almost equal fractions. It is found that both additional RF heating systems reduce the peaking of the W density profile with increasing central RF heating power. Approximately the same values of W density peaking are obtained when the same values of electron heating are produced by the two RF heating systems, which implies that less total heating power is required with ECRH than with ICRH to reduce the W density peaking. A related modelling activity shows that an important ingredient to explain the experimentally observed trend is the variation of the turbulent W diffusion as a function of the electron to ion heat flux ratio. Additional effects are connected with the more favorable W neoclassical transport convection in the presence of ICRH, produced by the combination of stronger central ion temperature gradients and the impact of the Η minority on the W poloidal density asymmetry.