Characterisation of Materials and Components under high Heat Loads

HIGH HEAT LOADS

gçÅÜÉå=iáåâÉ=EftsJOF j.linke@fz-juelich.de

Introduction

The safe operation of next step tokamaks and stellarators strongly depends on reliable wall compo-nents which can sustain intense particle and heat fluxes from the plasma. These particle fluxes will induce plasma-wall interaction processes which finally can degrade the materials with respect to their thermal and mechanical properties; in addition wall erosion is another critical issue which has significant impact on the lifetime of plasma facing components and on the contamination of the fu-sion plasma. The plasma facing materials in next step fufu-sion devices are primarily based on beryl-lium, carbon and tungsten in combination with copper as a heat sink.

A major aim of this R&D activity is to develop and fabricate new materials for future fusion de-vices and to characterise and to test them under simulated load conditions, i.e. at thermal loads up to 20 MWm^-2 and at neutron fluences up to approx. 1 dpa. For this purpose, the electron beam facility JUDITH-1 at Research Centre Jülich is a well-approved instrument for high heat flux testing. With the installation of the new machine JUDITH-2, the parameter range of high heat flux simulations is extended and additional testing capacity is available.

The development of plasma-interactive components for next step devices (ITER) and future elec-tricity generating fusion reactors such as DEMO are an essential material related issue also of the EFDA technology programme. Major aim of the research programme at FZJ is the characterization of plasma facing materials and actively cooled components and their assessment with respect to their thermo-mechanical and neutron irradiation behaviour.

Most work is done as technology tasks or physics tasks under EFDA. The work in 2005 addresses the following specific issues:

• test of new manufacturing processes

ƒ casting copper onto carbon fibre composites

ƒ W-Cu joining techniques by explosive bonding

• test of materials under highest transient heat loads

ƒ simulation and modelling, graphite,

ƒ tungsten melting,

ƒ thermo-mechanical behaviour of beryllium

• participation in an EU Integrated Project for the development of materials for extreme envi-ronments (ExtreMat)

• special EFDA contracts with work for ITER, JET and general technology

ƒ mechanical and thermo-physical characterisation of carbon and tungsten based mate-rials

ƒ upgrade of a new electron beam facility (JUDITH-2)

ƒ test of actively cooled divertor mock-ups with beryllium

ƒ plasma spray beryllium mock-ups

ƒ test of brazing techniques for the new ITER-like wall in JET

1. Manufacturing of plasma facing components

Cu-casting on Cr-activated CFC and joining to a CuCrZr heat sink

In cooperation with the Politecnico di Torino, Italy, the casting process of copper onto carbon fibre composites (CFCs) has been studied and optimised. Due to a poor wetting behaviour of Cu on pure CFC (NB31, SNECMA) the carbon surface to be joined has been activated using chromium (car-bide former). This process was invented and provided by the Politecnico di Torino.

The set-up for the casting process consists of a carbon mould in which pure Cu (Goodfellow) is po-sitioned on top of the Cr-activated CFC. Additionally, a carbon plate with a centre-hole has been put on top of copper and auxiliary loaded by “heavy” tungsten pieces. These means are used to compensate the shrinkage of Cu and the correlated formation of a centre-void occurring during the cool-down process. Furthermore a directional re-solidification of the molten copper had to be guar-anteed to avoid the formation of voids within the copper cast. Therefore the full set up has been put on a steel heat sink providing a re-solidification from bottom to top.

In contrast to the actually used casting process of the Politecnico di Torino at a high argon pressure of 3 bar and a temperature of 1200 °C, the as described assembled component has been put into a vacuum furnace (~ 10^-4 mbar) and heated up to 1150 °C. At this temperature it was held for 10 min-utes followed by an active cooling step down to 400 °C and then cooled by thermal radiation. With this technique, a highly dense material and an optimum bond to the CFC has been generated (fig. 1), providing a shear strength of the bond structure of about 27 MPa.

Fig. 1: SEM-image of the CFC-Cu interface with a Cr-carbide interlayer.

Fig. 2: Divertor test module – brazing of CFC/Cu- composite to CuCrZr heat sink with Cu/Ge-braze.

The final goal of this process development was the implementation of the CFC/Cu-composite into a divertor test component for a nuclear fusion device. This has been realised by the production of small test modules by brazing the CFC/Cu composite (thickness of Cu = 2 mm) with a Cu/Ge-braze (GEMCO) to a CuCrZr-heat sink (fig. 2) and by HIPing (still in progress). Both modules will be tested in JUDITH on their thermal performance under steady state and cyclic loading conditions.

Development of W-Cu-joints by explosive bonding

The character of the interface between plasma facing material (PFM) and heat sink has significant influence on the lifetime of plasma facing components under intense cyclic heat fluxes. In particular these are the promising material candidates: tungsten (PFM) and copper (heat sink) which exhibit substantial differences in their thermal and mechanical properties. To reduce inherent stresses which originate from these mismatches, graded W/Cu-interface structures (FGMs) produced by vacuum plasma spraying were developed. For the joint of these FGMs to the copper heat sink a new method, namely explosive bonding, has been studied in cooperation with TNO, Rijswijk, the Neth-erlands.

Fig. 3: Optical microscopy from the interfaces between Cu, FGM and W for an explosively bonded sample a) bonding at room temperature: perfect bonding between Cu and FGM (but cracking in tungsten) b) bonding at elevated temperatures (> 300 °C: detachment and formation of a sponge-like structure.

The used test samples were composed out of a 7 mm thick tungsten base plate with W/Cu-FGM clad with a thickness of 2 mm. Onto these material combination, 2 mm of pure copper were clad at room temperature (RT) with different welding parameters and at higher temperatures with the opti-mum parameters determined at RT. Whereas at RT cracking of tungsten and the FGM is observed due to the brittleness of the material, at higher temperatures (>300 °C) this effect vanished. Since the conditions for a successful RT cladding do not generate an ideal bonding when the base materi-als (tungsten & plasma-sprayed layer) is pre-heated (fig. 3) the welding parameters need to be ad-justed for the hot cladding which will be done in a follow-up project.

2. Material performance during high transient heat loads

Modelling of material erosion under electron beam and laser power loads

Materials in contact with plasma in fusion devices should be able to sustain extremely high heat loads. With regard to next generation fusion devices, it is important to investigate the mechanism of

material damage both during normal operation regimes and during the transient events such as dis-ruptions, ELMs and VDEs. A critical issue for ITER operation will be the erosion of plasma-facing components due to intense energy deposition in off-normal events. The intense heat load can be simulated using electron beam facilities such as JUDITH and a pulsed laser beam with high density and short pulse duration such as a Nd:YAG laser (1.06 μm). To understand the mechanism of mi-croscopic and mami-croscopic erosion during volumetric and surface heating and to allow a reliable prediction of the material erosion in ITER, two models have been developed in collaboration with Forschungszentrum Karlsruhe: (i) the 3-dimensional semi-empirical model "3D-carbon" based on a destruction threshold enthalpy of 10 kJ/g for the macroscopic erosion of carbon based materials and (ii) the MEMOS-1.5D code which takes into account the surface melting and melt motion for met-als. Both codes have been validated by comparison with experiments.

It was found that the surface heating of fine grain graphite results in a higher evaporated layer than volumetric heating. However, since the particle emission starts due to crack formation, the electron beam induced erosion is strongly increased and can exceed the laser beam erosion, fig. 4. Conse-quently, for heat loads in the GW/m² range and pulse durations of a few ms, volumetric heating can result in higher total erosion due to brittle destruction compared with laser beam loading.

0.0 0.5 1.0 1.5 2.0

1

10 evaporation

laser-beam

BD e-beam

Maximum crater depth, μm ^{1.8 GW/m2}

evaporation

time, ms

Fig. 4: Erosion of graphite at room temperature for e-beam and laser beam loading.

The onset of brittle destruction (BD) has been calculated based on the threshold enthalpy of graph-ite ΔHBD = 10 kJ/g. This means that BD occurs as soon as the specific enthalpy of the graphite ex-ceeds 10 kJ/g. The calculations are in a good agreement with experimental data from the electron beam facility JUDITH for various incident power densities and pulse durations, fig. 5. Taking an enthalpy of phase transition of ΔHBD = 10 kJ/g, surface heating results in a significantly higher BD onset compared with volumetric heating. It should be mentioned that in these calculations the plasma pressure created by laser loading near the target was not taken into account.

0 2 10⁰

10¹

e-beam laser

experiments calculations calculations

Power, GW/m2

t_BD, ms RT

Fig. 5: Onset of brittle destruction for graphite at room temperature for e-beam and laser beam loading.

Calculations have been done using the threshold enthalpy of brittle destruction of 10 kJ/g.

Brittle destruction strongly influences the lifetime of carbon-based armour materials; for metals melt motion is the main damage mechanism during off-normal operation regimes. Calculations of the erosion for W heated by an electron beam at 1.9 GW/m² for 5 ms show a crater depth of 30 μm due to melt motion (fig. 6) which is in agreement with the experiments. The erosion due to evapora-tion is much less compared with the erosion due to melt moevapora-tion. Consequently, the gradient of sur-face tension in the melt layer is the driving force for the erosion of metals.

R (cm)

Position(μm)

-0.2 -0.1 0 0.1 0.2

-140 -120 -100 -80 -60 -40 -20 0 20

Evaporated Melt pool

t= 5 ms

Melt surface

Fig .6: Calculated melt layer loss of W loaded by an e-beam of 1.9 GW/m² for 5 ms at room temperature.

Thermo-mechanical behaviour of beryllium under intense thermal loads

For the first wall in ITER the low-Z material beryllium has been selected as plasma facing material.

Due to the relatively low melting point of this low-Z material (Tm = 1278 °C), there is a limited risk

that melt layers form during transient thermal loads, such as vertical displacement events, plasma disruptions or edge localised modes. In addition, due to a non-negligible erosion of the individual plasma facing materials in a future fusion facility, there is a distinct tendency to form layers of re-deposited materials on the plasma facing components (e.g. tungsten/carbon layers on beryllium components, beryllium enriched layers on tungsten or carbon surfaces).

Single and multiple shot electron beam tests have been performed to investigate the performance of different monolithic beryllium grades and beryllium rich coatings on ATJ-graphite under transient thermal loads for pulse durations in the millisecond range. Major objectives of the experiments are the quantification of the threshold values for cracking and melt layer formation.

To minimize the safety requirements during thermal heat load tests with beryllium-coated speci-mens a number of different metallic materials have been evaluated for their suitability to mimic the performance of thin beryllium coatings. From the view point of their physical parameters (melting point, boiling point, density, thermal conductivity, etc.) and chemical aspects (alloying with tung-sten, carbide formation in contact with carbon based materials), aluminium, cobalt and nickel have been selected as the most promising candidates. From these materials thin layers in the micron range have been generated by electron beam evaporation on fine grain graphite (CL5890PT) and sintered tungsten. The coated test samples have been exposed to short transient thermal loads – loading conditions matching those on the above mentioned Be coated ATJ-graphites.

3. Development of materials for extreme environments (IP-project “ExtreMat”)

The ExtreMat Integrated Project targets on the creation of new multifunctional materials being be-yond reach with conventional incremental materials development only. Based on an integrated ap-proach, ExtreMat will dramatically push forward the limits in materials technology and will provide and industrialise new knowledge-based materials and compounds for top-end and new applications in extreme environments.

In top-end applications materials often have to fulfil complex functions in extreme environments, which set limits to their performance and lead to their degradation. As there is a diversity of ex-treme environments there is a diversity of the required functionalities and of the type of materials which could fulfil them.

Typically, there is not only a single factor, but combinations of several issues, which lead to ex-treme and complex loading conditions of the materials. The loading of materials may consist of up to four components:

• very high heat fluxes and temperatures

• physico-chemically aggressive media

• complex mechanical loads

• highly energetic radiation fields

New materials and processes to match these most challenging issues can only be found by a generic and multidisciplinary approach, which addresses the common fundamental issues.

Forschungszentrum Jülich (FZJ) is one of 38 partners throughout Europe joined in the ExtreMat project. Concentrating on 4 major aspects, the integrated project is subdivided into sub-projects (SPs, fig. 7) to which FZJ is contributing on a work package level by providing its knowledge on

FEM analysis and thermo-shock and thermal-fatigue testing of materials exposed to high heat load-ing conditions (e.g. within the nuclear fusion area).

Fig. 7: Structure of the integrated ExtreMat project, containing 4 strongly linked and interacting sub-projects.

Within the first year of the project a huge effort on the definition of promising alternative concepts for high-end material applications has been made, to which FZJ, which is involved in all 4 SPs and takes one work package leader position within SP4, contributed by providing its knowledge in the frame of the scientific industrial committee and by lots of other activities:

• Modification and preparation of test facilities and update for the upcoming challenges.

• Determination of test specimen requirements, test conditions and testing capacities for non-irradiated and non-irradiated (SP3) materials. Preparation of the simulation of operating condi-tions based on the experience in testing of divertor and first wall modules in the field of nu-clear fusion.

• First test experiments with reference materials and illustration of first results.

• Preparation of a database for reference materials used in nuclear fusion (CuCrZr, DS-Cu, etc.) as an input to FEM-analysis of thermo-mechanically loaded components.

• Illustration of modelling experience of thermally and thermo-mechanically loaded com-pounds as a supporting tool for high heat flux experiments and for the development of new functionally graded materials. Suggestion of a 2D and 3D-model geometry for compounds in nuclear fusion and electronic devices.

• First FEM-simulations on the integration of a W-fibre reinforced Cu-composite into an ac-tively cooled divertor module in the field of nuclear fusion.

• Comparison of testing requirements set up by the user requirement specifications with avail-able testing possibilities and capacities provided by the testing institutes, especially for high heat flux testing.

• Collection and illustration of testing equipment in various fields (mechanical, thermo-physical, microstructural, chemical, etc.), which will be provided by each partner of SP4 but also by each partner within the whole ExtreMat project.

• Collection of testing needs within SP4.

• Set up of a work programme and testing time frames for the upcoming phase (3 years) con-cerning high heat flux testing of actively cooled compounds in the field of nuclear fusion with or without integrated diffusion barrier.

4. Mechanical and thermo-physical characterisation of carbon and tungsten based materials

In the frame of tasks TW5-TVM-CFCQ2, the follow-up project of TW2-TVM-CFCQ2, samples for the measurement of physical and mechanical properties were machined from different locations in-side of CFC-blocks delivered by SNECMA:

• 3D-CFC NB31: additional production for EFDA and IPP Garching (Wendelstein 7-X)

• 3D-CFC NB41: pilot production

Additional 3-directional CFC-samples for laser-flash measurements have been supplied by DUNLOP.

The results of the thermo-physical characterisation of the DUNLOP-CFC “P25” in the temperature range between room temperature and 1200 °C are presented in fig. 8. The thermal conductivity has been calculated by using the correlation between the thermal conductivity λ and the density ρ, the specific heat cp and the thermal diffusivity a:

λ = ρ · cp · a

Furthermore the comparison of the measured data of several CFC-grades (P25, K321, P30 and P25HD) showed that P25 offered the best thermal properties.

0

0 200 400 600 800 1000 1200 1400

T [°C]

λ [W/mK]

P25-X P25-Y P25-Z

Fig. 8: Thermal conductivity of the 3D-CFC Dunlop P25 in x-, y- and z-direction.

For the mechanical characterisation stress controlled tensile tests at room temperature were per-formed on NB31 (EFDA) and NB41, both materials providing a density > 1.9 g/cm³. Tensile strength and Young's Modulus were measured (fig. 9), which are comparable in Y-direction for both CFC-grades. In comparison to previous productions of NB31 (pilot production, serial produc-tion) the tensile strength in pitch-direction is situated above those values for the serial production but still below those for the pilot production. The minimum specified values for the pitch direction (= 110 MPa) are partly achieved but they still stay below this requirement mainly. Micro-structural analyses also revealed an intermediate number of pitch-fibres per “unit cell” compared to the pilot

and the serial production which leads to the conclusion that the tensile strength of the material in-creases up to a certain extent with a decreasing number of pitch fibres per unit cell.

0 20 40 60 80 100 120 140

0 20 40 60 80 100 120

E [GPa]

UTS [MPa]

pitch-direction PAN-direction NB41: Y-direction minimum specified value for pitch direction

minimum specified value for pitch direction

Fig. 9: Tensile strength in relation to the Young’s modulus for the pitch and PAN-direction of NB31 (additional production, EFDA) and the Y-direction of NB41.

5. High heat flux testing of plasma facing materials and components New Electron Beam Test Facility JUDITH-2

The electron beam facility JUDITH-1 is a well-approved instrument for high heat flux testing of plasma facing materials and components. With the installation of the new machine JUDITH-2 (fig. 10), the parameter range of high heat flux simulations is extended and additional testing capac-ity is available. Experiments cover normal operational conditions (thermal fatigue) on actively cooled samples and abnormal events (disruptions, VDEs, ELMs) on different plasma facing materi-als. A power of 200 kW and a sweeping angle of 14° for the electron beam opens the possibility to test components up to 0.5 x 1 m². A relatively low acceleration voltage (30 – 60 kV) reduces volu-metric heating for the benefit of a more plasma-like surface heating.

Due to a flexible and individually programmable electron beam sweeping system, a very homoge-nous surface heat distribution can be generated during static load tests and a realistic simulation of ITER relevant transient events becomes possible. A new digital IR-camera offers fast image grab-bing and high resolution combined with a powerful software which provides new evaluation meth-ods.

Especially brittle destruction will be addressed, using a spectrometer in the visible range, a photodi-ode array and acoustic emission. The first two methods serve to analyse the emitted particles, while acoustic emission gives information on the formation of surface cracks and on the onset of brittle destruction.

Fig. 10: Electron beam test facility JUDITH-2.

Testing of Actively Cooled Divertor Mock-Ups with CFC Armour

Testing of Actively Cooled Divertor Mock-Ups with CFC Armour

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