Tungsten transport in the plasma edge at ASDEX upgrade

The Plasma Facing Components (PFC) will play a crucial role in
future deuterium-tritium magnetically confined fusion power plants,
since they will be subject to high energy and particle loads, but
at the same time have to ensure long lifetimes and a low tritium
retention. These requirements will most probably necessitate the
use of high-Z materials such as tungsten for the wall materials,
since their erosion properties are very benign and, unlike carbon,
capture only little tritium. The drawback with high-Z materials is,
that they emit strong line radiation in the core plasma, which acts
as a powerful energy loss mechanism. Thus, the concentration of
these high-Z materials has to be controlled and kept at low levels
in order to achieve a burning plasma. Understanding the transport
processes in the plasma edge is essential for applying the proper
impurity control mechanisms. This control can be exerted either by
enhancing the outflux, e.g. by Edge Localized Modes (ELM), since
they are known to expell impurities from the main plasma, or by
reducing the influx, e.g. minimizing the tungsten erosion or
increasing the shielding effect of the Scrape Off Layer (SOL).
ASDEX Upgrade (AUG) has been successfully operating with a full
tungsten wall for several years now and offers the possibility to
investigate these edge transport processes for tungsten. This study
focused on the disentanglement of the frequency of type-I ELMs and
the main chamber gas injection rate, two parameters which are
usually linked in H-mode discharges. Such a separation allowed for
the first time the direct assessment of the impact of each
parameter on the tungsten concentration. The control of the ELM
frequency was performed by adjusting the shape of the plasma, i.e.
the upper triangularity. The radial tungsten transport was
investigated by implementing a modulated tungsten source. To create
this modulated source, the linear dependence of the tungsten
erosion rate at the Ion Cyclotron Resonance Heating (ICRH) limiters
on the injected ICRH power was used. The phase and amplitude of the
inwardly propagating tungsten signal was then observed at the
erosion site and at three radial positions in the main plasma, from
which two were identified in the course of this work by a thorough
investigation of the tungsten radiation features in the Vacuum
Ultra-Violet (VUV) spectral range . The newly found observation
sites are located right in the steep gradient region, close to the
Edge Transport Barrier (ETB) and slightly further inside at the
pedestal top of AUG H-mode discharges. Futhermore, the parallel
flows in the SOL have been monitored by spectroscopical means and
Langmuir probes. The experimental results were quite unexpected,
since the ELM frequency had no influence on the tungsten
concentration, and the sole actuator on this quantity was the gas
injection rate. The evaluation of the modulated tungsten signal
revealed that neither gas puffing nor plasma shape had an
measureable influence on the radial tungsten transport processes.
In addition, the tungsten erosion sources were only partially
responsible for the observed tungsten behavior. These observations
inspired a simple model, which balanced the tungsten outflux with
the tungsten influx. In this model the impurity exauhst by ELMs is
not diffusive, but turbulent and linked to the ELM size. The model
predicted a linear dependence between the tungsten concentration
and the parallel velocity in the SOL. This linear dependence was
confirmed by the spectroscopical evaluation of the SOL parallel
flows.