Plasma Material Interaction Group
Properties and optimisation of materials facing high temperature plasmas
n The focus of the Plasma Material Interaction Group in 2015 was to finalise the quality assessment of the actively cooled high heat flux com- ponents of the new stellarator Wendelstein 7-X, to study the morphology and composition of dust particles in various fusion devices as well as the production and testing of novel materials for plasma facing components.
Actively cooled plasma facing component during high heat flux test at neutral beam facility GLADIS (Photo: IPP)
Prof. Dr. Rudolf Neu
Contact
www.pmw.mw.tum.de Phone +49.89.3299.1899
A highlight was the successful comple- tion of the quality assessment of the actively cooled high heat flux compo- nents for Wendelstein 7-X, which is the world’s largest stellarator built by the Max-Planck-Institute for Plasma Physics
(IPP) in Greifswald, Germany. From 2005
to 2010 a total of 60 prototypes for the actively cooled components were tested. In this period of time a production tech- nology fulfilling the requirements of long pulse operation was developed in coop- eration with Plansee SE. The prototypes of the pre-series have confirmed the robustness of the finally selected design and technology. Prototypes specified
for long pulse operation at 10 MW/m² were successfully loaded at 30 MW/m² in the high heat flux test facility GLADIS operated by the PMI group.
Owing to its two ion sources of 1 MW power each, its 8 m³ vacuum chamber and powerful water cooling, GLADIS offers the possibility to investigate small scale as well as full scale objects loaded with extremely high heat fluxes. From
2011 to early 2015, 86 of the 970 deliv- ered components were evaluated. For this purpose all CFC (carbon fibre composite) tiles were loaded with 100 cycles of 10 seconds duration at 10 MW/m² (see figure). The surface temperature increase of the CFC tiles during the cycling caused by the thermo-mechanical load was evaluated statistically. It was possible to show that the probability of an undetected defective CFC tile is negligibly low.
Besides withstanding severe thermal and mechanical loads the armour materials
of plasma facing components must be erosion resistant and the retention of hydrogen isotopes must be low. In order to understand and to test material pro- perties, a variety of preparation methods are used, as well as a comprehensive set of techniques for physical, chemical and mechanical characterisation is available within the PMI group.
Investigations on Composition and Morphology of Dust from Fusion Devices
Various erosion processes in fusion devices can lead to the production of dust. Accidental air or steam ingress in future power plants could cause signifi- cant production of explosive hydrogen on
the reactive surfaces of the dust particles.
Furthermore, dust could negatively impact operation when entering the confined plasma. For both issues, its occurrence, morphology and composition is important in order to facilitate safety considerations and to provide input for model calcula- tions.
Whereas in devices with graphite based plasma facing components the large erosion through plasma particles leads to thick deposited hydrocarbon layers which eventually flake off, arcs and excessive power loads can create droplets in devices equipped with metals as plasma facing material (PFM).
In order to investigate and document
the influence of the PFM, dust particles were collected in the three fusion devices ASDEX Upgrade (AUG, Germany), DIII-D (USA) and LHD (Japan) [M. Balden et al.,
15. Conf. on Plasma Facing Materials and
Components, Aix en Provence 2015]. The three devices involved utilise PFMs rang- ing from pure carbon (DIII-D), over carbon and steel (LHD) to full tungsten (AUG).
For gathering and analysing the dust, the
strategy developed over the last years at AUG was applied using Si collectors mounted for one experimental campaign inside the device at various locations.
The dust was analysed automatically by a scanning electron microscope equipped with energy dispersive X-ray spectroscopy (EDX). For all collectors at least ~5000
dust particles were analysed, providing
robust statistics. More than 90% of
Electron microscopic images (left: surface, right:
cross section, produced by focused ion beam) of dust particles extracted from the fusion device ASDEX Upgrade
all particles from DIII-D collectors are composed of C (with contributions of O, B, and N), while in LHD this fraction is
below 50% with about 20% of all particles
containing more than 5 at% metal (Fe). For the AUG collectors, the fraction of tungsten containing particles is in the range between 50% and 80%. Spherical particles, which are significantly present in AUG and indicate molten metal or strong plasma contact (see figure), are almost missing on the DIII-D collectors and are sparsely present on the LHD collectors. The particle size distribution, which could be described for different classes of dusts from AUG by a log-normal distribution, is different for the dust originating from LHD and DIII-D. Specifically, in the fully gra- phite covered device DIII-D the frequency of very small and large dust particles is larger compared to that in AUG and the total amount is also larger by 1-2 orders
of magnitude hinting again at the different
production processes.
Mechanical Properties of
Tungsten-fibre-reinforced Tungsten
Crosssection of Wf/W composite after a Charpy impact test: The tungsten fibres show ductile deformation and pull-out leading to increased toughness of the composite
Its combination of unique properties makes tungsten a promising candidate
for plasma-facing components in a future fusion power plant. However, its inherent brittleness and corresponding lack of damage tolerance considerably limit its use.
A possible solution/improvement is to
incorporate fibres in the material produc- ing a kind of toughness, viz. increased resistance to failure. The embedded fibres can bridge or deflect cracks, or plastically deform.
During the past years W samples rein- forced with coated long W fibres (Wf/W) from drawn tungsten wires could be produced on a laboratory scale (60 x 60 x
3 mm³, 2000 fibres). Charpy impact tests
on the ‘as produced’ samples (conducted at Institute of Materials Science and Mechanics of Materials, TUM) showed increased fracture energy mainly due to the ductile deformation of the tungsten fibres (see figure). Three-point bending tests with ‘as produced’ samples showed stable crack propagation and rising
load-bearing capability still after crack
initiation. Interface debonding and crack bridging could be directly observed. Even after reaching the maximum strength no catastrophic failure appeared, but only
a reduction of load, revealing the ideal
composite behavior.
In the view of fusion application this ductility in the fibres might get lost due to accidental thermal overload and the
embrittlement through neutron irradiation.
In such a fully embrittled case (reached by heating the sample at 2000K for 30min) still crack bridging by the fibres and a rising load-bearing capacity was observed, albeit only up to lower maximum load. These experimental observations prove
that the toughening in Wf/W is still effective
after embrittlement. The results illustrate that the use of Wf/W could broaden the operation temperature window of tungsten components significantly and mitigate problems of deep cracking occurring typically in cyclic high heat flux loading.
Project
n Supported by EUROfusion (2015)
Research Focus
n Detailed understanding of complex interaction processes between plasma and material
n Development of novel materials with
improved properties
n Integration of new materials into
plasma-facing components
Competence
n Measurement and modeling of erosion, surface composition and hydrogen retention of materials
n Laboratory scale production of thin
coatings
n Laboratory scale production of tungs-
ten fibre reinforced materials
n Performance and analysis of high
heat flux tests of inertially and actively
cooled materials and components
n Thermo-mechanical analysis of high heat flux components
Infrastructure
n Accelerator for surface analysis
n High heat flux ion beam test stand
n Manipulator in the fusion device ASDEX Upgrade
n Scanning electron microscopy (SEM
with Focused ion beam (FIB), Energy Disp. X-ray spectrosc. (EDX), Electron Backsc. Diffract.(EBSD))
n Atomic Force Microscope (AFM)
n X-ray Diffraction (XRD)
n Confocal laser scanning microscope n Photo Electron Spectroscopy (XPS) n Magnetron sputter devices
n Vacuum ovens for thermal desorption
Courses
n Plasma Physics for Engineers
n Plasma Material Interaction
Research Group at:
Max-Planck-Institut für Plasmaphysik funded by MPG/HGF and supported by EUROfusion
Management
Prof. Dr. Rudolf Neu
Research Scientists (MPI für Plasmaphysik) Dr. rer. nat. Martin Balden Dipl.-Phys. Stefan Elgeti Dipl.-Ing. Hanns Gietl Dipl.-Ing. Henri Greuner Georg Holzner, B.Sc.
Dipl.-Phys. Till Höschen
Dipl.-Chem. Freimut Koch
Dr. rer. nat. Karl Krieger
Dr.-Ing. Muyan Li
Dr. rer. nat. Hans Maier
Dipl.-Ing. Alexander von Müller
Dr.-Ing. Johann Riesch
Dr. rer. nat. Volker Rohde
apl. Prof. Dr.-Ing. Jeong-Ha You
Technical Staff
Dipl.-Ing. (FH) Bernd Böswirth
Gabriele Matern
Publications 2015
n L. Frassinetti, D. Dodt, M.N.A. Beurskens, A Sirinelli, J. Boom, et al., ‘Effect of nitrogen seeding on the ELM energy losses in JET with the ITER-like wall’, Nucl. Fusion 55 (2015) 023007
n M.J. Leyland, M.N.A. Beurskens, L. Frassinetti, C.
Giroud, S. Jachmich,et al., ‘The H-mode pedestal structure and its role on confinement in JET with
a carbon and metal wall’, Nucl. Fusion 55 (2015)
013019
n F. Romanelli, M.. Abhangi, P. Abreu, M. Aftanas,
M. Afzal, et al., ‘Overview of the JET results’, Nucl. Fusion 55 (2015) 104001
n A. Hakola, S. Koivuranta, J. Likonen, A. Herrmann, H. Maier, M. Mayer,, et al., ‘Erosion of tungsten and steel in the main chamber of ASDEX Upgrade’, Journ. Nucl. Mater. 463 (2015) 162
n M. Turnyanskiy, R. Neu, R. Albanese, C. Bachmann,
S. Brezinsek, et al., ‘A roadmap to the realisation of fusion energy – mission for solutions on heat- exhaust systems’, Fusion Eng. Design 98-99 (2015)
361
n P. Paris, K. Piip, A. Hakola, M. Laan, M. Aints, et al.,
‘LIBS characterization of ASDEX Upgrade samples’, Fusion Eng. Design 98-99 (2015) 1349