Wall Forum 2016

Studies of N transport and WNx formation in tokamaks are hindered by the inevitable presence of a 14N background from air. Therefore, we injected 15N labeled N2 in the divertor of ASDEX Upgrade and measured its deposition on samples exposed via manipulator systems. As predicted by WallDYN simulations [1], the 15N areal density at the low field side midplane is only a factor of 4 below the 15N areal density at the outer strike line region. The direct impact of the roughness on the N deposition in the divertor is found to be small, although co-deposition on rough samples increases the 15N areal density by a factor of 2. The experimental results are combined with WallDYN-DIVIMP simulations to study parallel and anomalous transport of N in the SOL. To identify the impact of a Be main wall, WallDYN simulation of the N migration in JET are performed and compared to available experimental results. Finally, we discuss the challenges towards a full understanding of the nitride formation of W and Be which we have identified and the impact of the presented results on the ammonia formation. [mehr]

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Knowledge about deuterium retention and diffusion in tungsten is important for the design of future fusion reactors. A large number of experiments have been carried out to study the retention of deuterium in tungsten after plasma exposure near room temperature, which is dominated by trapping of deuterium near material defects. The results are usually modeled using diffusion data extrapolated from much higher temperatures, since the dynamic effects of deuterium diffusion in tungsten are difficult to measure near room temperature, due to the very low solubility of deuterium in tungsten. It is thus important to test the validity of this extrapolation and the model used by measuring and modeling of the diffusion dynamics near room temperature. In this talk, a method based on deuterium plasma loading on one side and accumulation of permeating deuterium in a getter layer on the other side of the sample, is presented. The amount of deuterium in the getter is analyzed after the plasma exposure using nuclear reaction analysis. By exposing samples to the plasma for different times, information about the temporal evolution of the deuterium permeation flux and thus the dynamic effects mentioned above can be gained. [mehr]

The behavior of hydrogen isotopes in tungsten below room temperature is studied in order to gain a better microscopic understanding and to provide valuable input for predictive modelling. Deuterium implantation into tungsten at temperatures down to 134K is studied in detail regarding the achievable deuterium retention as well as the induced surface morphology e.g. blister by means of in-situ NRA , SEM and EBSD. The influence of the created blister on deuterium transport is also investigated, revealing a strong suppression of deuterium diffusion into the bulk. Furthermore hydrogen isotope exchange in tungsten is performed at low temperature to verify the fill-level dependent de-trapping model proposed by DFT calculations. [mehr]

SIMNRA is widely adopted by the scientific community of ion beam analysis for the simulation and interpretation of nuclear scattering techniques for material characterization. Taking advantage of its recognized reliability and quality of the simulations, MultiSIMNRA is a computer program that uses multiple parallel sessions of SIMNRA to perform self-consistent analysis of data obtained by different ion beam techniques and/or different experimental conditions of a given sample. In this presentation I will show the basic principle of MultiSIMNRA and a few examples of use. [mehr]

Power handling on castellated divertor components is an area of concern for ITER, in particular with respect to transient excursions by ELMs. Protection of leading edges from full parallel flux is realised by appropriate shaping on the cost of increased power load at plasma exposed surfaces. For insufficiently mitigated ELMs melting can still occur. Surface damage by repeated transient melting and its effects on plasma operation have been studied first in JET and recently in a companion experiment at ASDEX Upgrade. The latter experiment provided for the first time ELM resolved measurements of the thermionic emission current, which is assumed to be the main driver for melt motion by corresponding jxB forces. These measurements as well as results of first post-exposure surface analysis will be discussed. [mehr]

A ten-year study on erbium oxide coating in collaboration with IPP is reviewed. Hydrogen isotope diffusion, solution, and permeation behaviors have been elucidated through experimental and computational approaches. Recent progress will be also introduced in the presentation. [mehr]

The blistering and near-surface deuterium retention of Y₂O₃-doped W and two different pure W grades were studied after exposure to deuterium (D) plasma at elevated temperatures (370, 450 and 570 K) in PlaQ. The research shows blistering and deuterium retention were strongly dependent on the implantation temperature. In addition, blistering was sensitively influenced by the used tungsten grade, although the total amount of retained D measured by nuclear reaction analysis was comparable. Among the three different investigated tungsten grades, Y₂O₃-doped W exhibited the lowest degree of surface modification despite a comparable total D retention. [mehr]

Within the present work, a device for manufacturing tungsten fiber reinforced copper metal matrix composite pipes by means of centrifugal infiltration has been designed and tested. Such composite pipes are of direct interest with regard to application as advanced heat sinks in highly loaded plasma facing components due to their high thermal conductivity and considerable material strength. Particularly, the approach for the design, analytical calculations, FEM simulations and the detailed design using CAD is presented. [mehr]

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Magnetron sputtering is commonly used vapor deposition technique for the preparation of thin films and coatings. Until recently it was believed that magnetron plasma is homogeneously distributed in a ring-shaped region above the cathode. Investigations by ICCD cameras and other time-resolved techniques changed this view. Namely, plasma is concentrated in dense regions that are called "ionization zones" or "spokes". Ionization zones were first observed in pulsed discharges (i.e., HiPIMS) [1] and later in continuously run discharges (i.e., DCMS) [2]. They are usually organized in semi-periodic patterns and exhibit an arrowhead-like shape. In general, ionization zones in DCMS have a longer azimuthal length, whereas in HiPIMS they are more numerous and azimuthally shorter. Dynamics of zones strongly depends on the discharge conditions. In a low-current DCMS discharges zones move in the -E×B direction, while in a high-current DCMS or HiPIMS discharges they move in the E×B direction [3].In the talk we will present our understanding of the ionization zone phenomenon. The formation, sustainability, organization and dynamics of ionization zones will be reviewed [4]. Measurements of the plasma potential by emissive probe show highly non-uniform potential distribution with strong electric fields at the edge of the ionization zone. Such fields strongly affect motion and energy of charged particles. A self-sustaining feedback loop exists between the potential structure, electron heating and ionization processes as electrons drift in the magnetic trap of the magnetron. We suggest that a moving double layer plays a crucial role in the energization of electrons and is in large part responsible for sustaining the discharge [5]. (References: [1] A. Anders et al., J. Appl. Phys. 111 (2012) 053304; [2] M. Panjan et al., Plasma Sources Sci. Technol. 24 (2015) 065010; [3] Y. Yang et al., Appl. Phys. Lett. 105 (2014) 254101; [4] M. Panjan et al., Plasma Sources Sci. Technol. 23 (2014) 025007; [5] M. Panjan and A. Anders, J. Appl. Phys. accepted for publication) [mehr]

Tungsten is planned to be the main material for the first wall armor of future fusion reactors. Therefore, the investigation of tungsten oxidation addresses one safety aspect in an accident case, in which the cooling system fails and the temperature rises to about 1300 K at the wall of the fusion reactor vessel for several days. In such a situation, the ingress of air into the reactor vessel would lead to the formation of tungsten oxides, i.e. WO3 . WO3 as well as activated WO3 evaporate into the gas phase which should be avoided.The grain orientations have an influence of the oxidation and arestudied in this thesis in more detail. The grain orientations oftungsten (W) were analyzed using electron backscatter diffraction (EBSD)and were followed by oxidations in a range between 720 K and 870 K. Thegrain dependent oxidation rates were determined by measuring thethickness of the oxide layer. The data of the thickness measurement weretransformed into oxidation rates in mg^2/(cm^4 sec.) to compare themwith results obtained by gravimetric measurements.Tungsten grains with {100} orientation have the highest oxidation ratein a range of 720 K to 870 K, which is a factor of two higher than theoxidation rate of W grains with {111} orientation. The {110} orientationexhibits an oxidation rate in between the {100} and {111} orientation. [mehr]

The exhaust of power and particles is regarded as a major challenge in view of the design of a magnetic confinement nuclear fusion demonstration power plant (DEMO). In such a reactor, highly loaded plasma facing components (PFCs), like the divertor targets, have to withstand both severe heat flux loads and considerable neutron irradiation. Existing divertor target designs make use of monolithic tungsten (W) and copper (Cu) material grades that are combined in a PFC. Such an approach, however, bears engineering difficulties as W and Cu are materials with inherently different thermomechanical properties and their optimum operating temperature windows do not overlap. Against this background, W-Cu composite materials are promising candidates regarding the application to the heat sink of highly loaded PFCs. The present contribution summarises recent results regarding the manufacturing and characterisation progress of such W-Cu composite materials produced by means of liquid Cu melt infiltration of open porous W preforms. On the one hand, this includes composites manufactured by infiltrating powder metallurgically produced W skeletons. On the other hand, W-Cu composites based on textile technologically produced fibrous reinforcement preforms are discussed. [mehr]