Overview of recent pedestal studies at ASDEX Upgrade

Abstract

Abstract. New or upgraded diagnostics of the edge transport barrier allow investigations of the dominant transport mechanisms in the pedestal. The density build-up after the L-H transition can be explained with a mainly diffusive edge transport barrier. A small inward convection term improves the agreement between modelling and experiment, but its existence cannot be confirmed due to the uncertainty in the neutral sources. Measurements of the impurity ion flow asymmetry as well as the edge current density are in agreement with neoclassical modelling. The inter-ELM pedestal recovery was traced with ideal peeling-ballooning modelling, which shows that the stability boundary moves closer to the operational point as the pedestal becomes wider. Gyrokinetic modelling of the different phases reveal that density gradient driven trapped electron modes are dominant during the early recovery, while electron temperature gradient modes or kinetic ballooning modes determine the temperature gradient in the final phase. Micro tearing modes are modelled and also experimentally determined at the top of the pedestal. Non linear coupling between modes could explain the failure of ideal linear MHD modelling. IntroductionThe edge transport barrier (ETB) is a characteristic of the high confinement mode (H-mode) of tokamaks. It is a narrow region of reduced radial transport, in which steep gradients in both density and temperature are observed, forming a pedestal for the core profiles.Extensive studies have shown that the pedestal radial electric field (E r ) profile in H-mode [1] and asymmetric density and flow profiles of impurity ions are consistent with neoclassical predictions [2]. While the ions set the background flow profile in the pedestal and their transport properties can be described by neoclassical modelling, the mechanisms which determine the electron density and temperature profiles are more varied, as electron heat and particle transport are governed by turbulence. The current emphasis of research concentrates on the determination of the driving force as well as the characteristics of the dominant transport mechanism. The understanding of the types of instabilities and the transport they create in the core plasma has been greatly advanced in recent years [3,4]. Although it has been shown that peeling ballooning (PB) theory in combination with the assumption that the pedestal width is determined by kinetic ballooning modes (KBM) [5,6] describes the pedestal top in a wide range of pedestal pressures [7], the detailed processes which determine the ion and electron heat transport as well as particle transport are not yet clearly [15] and the determination of the edge current density from a combination of magnetic measurements, edge pressure profiles and scrape-off layer (SOL) current measurements [16]. The combinations of these diagnostics deliver a set of profiles consisting of electron density (n e ), electron temperature (T e ), ion temperature (T i ), radial electric field (E r ) and edge current density (j), with temp...

The role of the density profile in the ASDEX-Upgrade pedestal structure

“…This is correlated with the change in the density contained in the HFSHD, which increases by 20 % between steps one and two and by only a few percent during the last step. To test the hypothesis of the density profile location, the experimental data from the AUG edge diagnostic suite [21] were analysed. Shown in figure 3 are the electron temperature (a) and density (b) profiles for the very low (black) and medium (red) fuelled phases.…”

Global and pedestal confinement and pedestal structure in dimensionless collisionality scans of low-triangularity H-mode plasmas in JET-ILW

“…However, part of the recent results show that under some conditions the JET-ILW plasma in the pre-ELM phase might not have reached the peelingballooning stability limit yet and the modelled normalized pressure gradient tends to overestimate the experimental one [4,18,49]. Recent results in AUG-W also suggest a behaviour similar to JET-ILW [50].…”

Plasma shaping and its impact on the pedestal of ASDEX Upgrade: edge stability and inter-ELM dynamics at varied triangularity

“…It is important to notice, that the simple picture of a temporally localised stability boundary in j-α space is incomplete. In addition to the operational point of the plasma equilibrium, which moves according to the evolution of α max and j tor [56], also the stability boundary changes correspondingly [57]. This can be explained by the evolution of the global plasma parameters such as W MHD .…”