Speaker
Sebastijan Brezinsek
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/I3.111.pdf
Plasma-Surface Interaction and Plasma-Edge Studies in Wendelstein 7-X
Operating with Passively Cooled Graphite Divertor
S. Brezinsek1, M. Jakubowski2 and the Wendelstein 7-X team
1
Forschungszentrum Jülich, Institut für Energie und Klimaforschung – Plasmaphysik,
52545 Jülich, Germany
2
Max-Planck-Institut für Plasmaphysik, Greifswald, Germany
The stellarator Wendelstein 7-X (W7-X) restarted operation in 2017 with ten divertor
modules made of inertially cooled graphite plasma-facing components PFCs [1]. The plasma
exhaust concept and first wall properties are substantially different from those in the initial
campaign which had limiters and explicitly avoided edge magnetic islands. The ten divertors
now intercept an edge island chain. Additionally the inner vessel has been complemented by
adding graphite PFCs to the already installed CuCrZr heat shields.
Before the initial divertor operation started, the vessel was baked up to 150°C, and glow
discharges in helium (He) and hydrogen (H) were performed to condition the device. ECRH
discharges were conducted in He in order to reduce the impurity and H content to acceptable
values for steady and long pulse operation. The main impurities have been identified spectro-
scopically to be carbon and oxygen in the plasma-edge layer (~10%) whereas the post-
plasma outgassing also includes methane, water, and carbon monoxide. H outgassing from
the first wall components, operated at room temperature, largely determined the fueling, but
much less prominently than in the initial limiter phase [2]. Conditions improved with
increased deposition of plasma flux and heat to plasma-facing sides during plasma operation.
A major part of the initial studies was devoted to symmetrisation of the power and particle
load to the divertor modules with the aid of a small external field in order to correct 1/1 error
fields and target module misalignment [3]. Indeed the footprint of particle and heat load was
successfully measured with a multitude of edge diagnostics including interference filtered
cameras, infrared cameras, divertor spectroscopy, and sets of Langmuir probes embedded in
the divertor PFCs. Typical heat fluxes achieved so far are up to 5 MWm-2 and particle fluxes
in the range of a few 1023H+s-1m-2 at the strike line distributed on the horizontal or vertical
target. The edge topology at W7-X depends strongly on the selected magnetic configuration,
i.e. due to different island chains responsible for the divertor function. Indeed divertor
detachment associated with low electron temperature and high density has been observed.
The reduction in the emission of CII and CIII light in the divertor suggests a strong reduction
of the carbon source with higher fueling which is consistent with a reduction of the physical
sputtering, whereas the chemical sputtering still remains. Detailed analysis of the recycling
and radiation distribution in the divertor and associated modelling with EMC3-EIRENE is
progressing [4] and results will be presented,
The global material migration from main chamber locations such as graphite tiles will be
studied by dedicated post-mortem analysis of extracted plasma-facing components with
markers after the initial campaign, i.e. in early 2018, and will be compared with modelling
predictions. In addition dedicated plasmas with tracer injection are foreseen to describe and
quantify the transport from the outer midplane to the divertor target plates. Indicating also
the remote areas where layers will be build-up in time. These studies will aid in predicting
the behavior in longer pulses in W7-X and the expected fuel retention levels by implanted
and co-deposited fuel as well as point to areas where cleaning activities might be required.
[1] T. Sunn Pedersen et al. (2017) presented at the ISHW conference Kyoto
[2] T. Wauters et al. (2017) private communication
[3] P. Drewelow et al. (2018) to be presented as inv. oral at the PSI conference Princeton
[4] F. Effenberg et al. (2018) to be presented as contr. oral at the PSI conference Princeton
