PFMC – Plasma Facing Materials and Components

The task of the Plasma-Facing Materials and Components Project is to investigate the various aspects related to plasma-wall interaction, changes in the properties of materials, and development and characterisation of new materials. This is accomplished by determining the requirements imposed on materials by specific investigations in fusion devices and appropriate modelling (e.g. of particle fluxes and energies, power fluxes). Systematically produced material samples are subjected in laboratory experiments to loads that approximate the conditions in fusion devices, particularly experiments on material erosion, adsorption and desorption of gases, and mechanical behaviour under thermal loading. In addition, materials are subjected to detailed analysis of their structure, composition, and temperature-dependent physical properties both, before and after testing. These results serve as a basis for deciding whether they can be used in IPP?s fusion experiments (ASDEX Upgrade, W7-X). In addition, the assessment of material behaviour during use and investigation of exposed components after use yield conclusions allowing further optimisation of materials. Besides its direct bearing on IPP?s fusion devices, this work is of significance to the ITER project because materials investigated in ASDEX Upgrade are directly relevant to ITER and the thermal loading of the components in Wendelstein 7-X largely conform to that in ITER.

Present Status and Next Objectives

The project comprises five topics with key functions for clarifying plasma-wall interaction and determining further materials development. The structure of the project was updated. "Tritium Inventory" was defined as a new major topic area since quantitative treatment and questions of limiting the tritium inventory are becoming increasingly important.

1. Surface processes on plasma-facing materials

• The development of composites and phase formation through deposition of plasma impurities on wall materials as a result of chemical reactions and transport processes were examined. Typical examples are the investigations of carbide formation of carbon layers on tungsten and beryllium substrates, resulting in surface coating whose behaviour is radically changed by ion bombardment. The tungsten carbide formation processes were also investigated in the ternary system W-O-C. Gaseous compounds, such as CO and CO₂ or O₂, lead to carbon and oxygen losses.
• Erosion and implantation measurements on various high-Z materials bombarded with C⁺ ions in the temperature range 300 to 1100 K: The model used for interpretation affords a consistent description of the experimental results for the materials investigated hitherto, only material constants such as sputtering yields and diffusion coefficients being used as parameters. The model developed allows prediction of erosion and deposition behaviour in boundary layer plasmas with impurity concentration.
• The appropriate laboratory investigations are conducted in specially designed facilities on the new tandem accelerator, the high-current ion source, and a new XPS setup. Methods of surface analysis are being further developed with a particular view to analysis of light impurities in heavy substrates.
• 
  

• Tungsten coatings applied to the inner heat shield during operation of ASDEX Upgrade were investigated by ion beam analysis to determine changes in layer thickness. The results show that material erosion inflicted by ions on the first wall of fusion devices is also of major importance. High ion fluxes, particularly during the final phase of a plasma discharge (ramp-down), exceed the erosion effect of charge exchange neutrals throughout the entire discharge.
• As in ASDEX Upgrade erosion and migration of wall materials will also be investigated in Wendelstein 7-X. Long-time probes to be installed on the vessel wall are being prepared for this purpose.
• Tungsten-clad tiles for use in JET were tested for layer thickness and installed in 2001. Utilisation of these tiles in the JET campaign till 2004 will allow comparison with erosion data available from ASDEX Upgrade.
• 
  

• Basic investigations are being conducted into deposition of hydrogenous carbon layers (a-C:H) as a function of plasma parameters, surface temperature, and ion energy. These are focused on experimental determination of sticking coefficients of various hydrocarbon radicals and the synergy between hydrogen ions and atoms with simultaneously impinging hydrocarbon radicals.
• Also included under this heading are studies of the principles of hydrogen implantation and inventory in tungsten as a function of surface contamination layers such as oxides and carbides. The experimental results are compared with numerical calculations of hydrogen implantation and diffusion.
• Aluminium oxide layers (Al₂O₃) suitable for diffusion barrier applications were synthesised by a plasma-based arcing method. The crystalline structure of the layers can be selected by means of the substrate temperature and ion energy. Exact temperature control during plasma deposition was ensured with a combined (pyrometers and thermocouples) measuring system.
• 
  

• Tungsten layers were developed for application to highly exposed surfaces in ASDEX Upgrade. These µm-thick layers were applied to carbon-fibre-reinforced carbon (CFC) substrates by plasma arcing. Thermal stress tests in the JUDITH electron beam device at Forschungszentrum Jülich demonstrated that the coatings can withstand heat loads of 25 MW/m² for 2 s and thus seem suitable for limiter applications in ASDEX Upgrade.
• Boron carbide layers on a separate, cooled stainless-steel first wall are envisaged for protecting the vessel wall of Wendelstein 7-X. Vacuum plasma spraying was used for depositing coatings with layer thicknesses of up to 0.5 mm. The residual stresses exerted on these layers as a result of deposition process could be determined for the first time by the so-called borehole method in conjunction with the Institut für Keramische Bauteile und Fertigungsverfahren of the University of Stuttgart. Compressive stresses in the region of 50 to 80 MPa were determined.
• Metal-doped graphite samples synthesised in collaboration with a Spanish research institute (CEIT, San Sebastián) were investigated for their thermal desorption behaviour with respect to hydrogen. The hydrogen retention capability was studied as a function of the graphitisation temperature, the concentration of open to closed pores, and the doping metal. The thermally desorbed deuterium was detected by calibrated residual gas analysis.
• 
  

In this sub-project structure-mechanical study of plasma-facing components (PFCs) is performed. In general, an actively cooled PFC consists of plasma-facing armour (PFA) and a heat sink manifold containing coolant tubes. PFCs are in nature a multi-material system which can be processed either by joining or coating. Materials for PFCs must fulfil many stringent mechanical as well as physical requirements. In the same context, PFCs themselves must satisfy rigorous structural requirements, among which structural integrity, heat removal capability and lifetime (PFA/PFC) are primary issues. Hence, it is a natural consequence that materials selection and component design should be coupled with interacting linkages, that is, analysis and testing. Computational analysis and thermo-mechanical testing on PFCs are the main research activities in this sub-project. PFCs for near-future devices such as Wendelstein 7-X or ITER but also for future power-generating reactors are investigated.

In the framework of theoretical analysis, finite element method (FEM) equipped with implemented user-subroutines is the principal computational tool. Current theoretical studies include extended scientific topics: interfacial fracture mechanics, elasto-plastic micro-mechanics, three-dimensional shakedown analysis based on shakedown theorems, stress analysis based on non-isothermal cyclic visco-plasticity, continuum damage mechanics and indentation simulation, etc. FEM analysis combined with a complicated inelastic constitutive equation should be often preceded by materials parameter identification process, for which a series of mechanical testing and data fitting are necessary. Emphasis is placed on the failure assessment and design optimisation of advanced PFC concepts for reactor application. One of ongoing research objects is dual scale analysis of a PFC being locally reinforced with fibrous metal matrix composite laminate.

Bereichsseminar Materialforschung