Electron Beam Facility JUDITH Research Articles

Progress in additive manufacturing of pure tungsten for plasma-facing component applications

In the present paper, recent progress regarding additive manufacturing (AM) of pure tungsten (W) by means of laser powder bed fusion (LPBF) is discussed. In this context, several aspects are highlighted: The influence of the raw powder material characteristics on the resulting W part quality is briefly discussed, examples for complexly shaped additively manufactured W lattice structure samples are shown, the application of an additively manufactured W structure as preform for a tungsten-copper (W-Cu) composite is illustrated and thermal shock experiments on additively manufactured bulk W samples with the electron beam facility JUDITH 2 are described. The latter demonstrates that W material consolidated by means of LPBF is capable of surviving intense thermal shock loads. This is an encouraging result indicating that the thermal performance and stability of W fabricated by means of LPBF is comparable to that of conventionally fabricated W which in turn implies that the further investigation of additively manufactured W as candidate material with regard to applications in highly loaded plasma-facing components (PFCs) of future magnetic confinement thermonuclear fusion devices can be considered worthwhile.

High heat flux testing of newly developed tungsten components for WEST

In preparation of the production of the actively cooled tungsten targets for the lower divertor of WEST (W -for tungsten- Environment in Steady-state Tokamak), the qualification of suppliers is done by exposing small-scale mock-ups to cyclic high heat flux tests in the electron beam facility JUDITH 1 at Forschungszentrum Jülich. Thereby, the small-scale mock-ups are based on the ITER design, consisting of 7 monoblocks mounted on a CuCrZr cooling tube and connected via a soft Cu-interlayer.The results presented herein focus on the high heat flux test results of two mock-ups produced by AT&M in China. The testing was performed by thermal cycling at 10 MW/m2 and 20 MW/m2 with a maximum of 1200 cycles at 10 MW/m2 followed by 500 cycles at 20 MW/m2 without obvious damage formation.The qualification is supported by subsequent hardness tests and microstructural analyses. The latter focus on local variation of tungsten recrystallization and macro-crack formation and deformation induced changes in the heat sink materials.

High pulse number thermal shock testing of tungsten alloys produced by powder injection molding

Abstract The investigation of plasma facing materials (PFM) subjected to a large number (≥10,000) of thermal shocks is of interest to determine long term morphological changes which might influence component lifetime in and plasma performance of a fusion reactor. The electron beam facility JUDITH 2 was used to simulate these conditions experimentally. In this study eight different tungsten grades produced by powder injection molding (PIM) were investigated: Two pure tungsten grades, one with 2 wt% Y2O3, three with 1, 2 and 3 wt% TiC, and two with 0.5 and 1 wt% TaC. Samples of 10 × 10 × 4 mm³ were brazed to a copper cooling structure and subjected to 105 thermal shocks of 0.5 ms duration and an intensity of Labs = 0.55 GW/m² (FHF = 12 MWs½/m2) at a base temperature of Tbase = 700 °C. The PIM grades showed damages in general comparable with a sintered and forged pure tungsten reference grade (>99.97 wt% W) that complies with the ITER specifications. One exception was the 2 wt% TiC doped material which failed early during the experiment by delamination of a large part of the surface. The Y2O3 doped material showed a comparatively good performance with respect to crack width (

High pulse number transient heat loads on beryllium

The experimental fusion reactor ITER will apply beryllium as first wall armor material. In present fusion experiments, e.g. ASDEX Upgrade, it has been detected that up to 25% of the plasma energy loss is deposited in non divertor regions during edge localized mode (ELM) events. Therefore, the impact of transient heating events on beryllium needs to be investigated to reliably predict the performance of the beryllium armor tiles under ITER operational conditions. In the present experiments, the electron beam facility JUDITH 2 was used to exert transient heat pulses with power densities of 0.14–1.0GWm−2, as they can be expected for mitigated Type 1 ELMs in ITER, pulse durations in the range of 0.08–1.0ms, and a number of pulses in the range of 103–107 on S-65 beryllium specimens that were brazed on an actively cooled copper structure. Thereby, a strong drop of the melting threshold was discovered from a heat flux factor FHF = 22–25MWm−2s0.5 for 102 pulses to FHF < 12MWm−2s0.5 for 103 pulses. However, a saturation of the thermally induced damage was observed for FHF ≤ 9MWm−2s0.5 after 105 pulses. This result indicated a promising performance of beryllium under a high number of transient heat pulses in ITER. Nevertheless, the synergistic effects between thermal loads, particle loads, and neutron irradiation might affect the saturation threshold and need to be investigated in future studies.

Oxide segregation and melting behavior of transient heat load exposed beryllium

In the experimental fusion reactor ITER, beryllium will be applied as first wall armor material. However, the ITER-like wall project at JET already experienced that the relatively low melting temperature of beryllium can easily be exceeded during plasma operation. Therefore, a detailed study was carried out on S-65 beryllium under various transient, ITER-relevant heat loads that were simulated in the electron beam facility JUDITH 1. Hereby, the absorbed power densities were in the range of 0.15–1.0 GW m−2 in combination with pulse durations of 1–10 ms and pulse numbers of 1–1000.In metallographic cross sections, the emergence of a transition region in a depth of ~70–120 µm was revealed. This transition region was characterized by a strong segregation of oxygen at the grain boundaries, determined with energy dispersive x-ray spectroscopy element mappings. The oxide segregation strongly depended on the maximum temperature reached at the end of the transient heat pulse in combination with the pulse duration. A threshold for this process was found at 936 °C for a pulse duration of 10 ms. Further transient heat pulses applied to specimens that had already formed this transition region resulted in the overheating and melting of the material. The latter occurred between the surface and the transition region and was associated with a strong decrease of the thermal conductivity due to the weakly bound grains across the transition region. Additionally, the transition region caused a partial separation of the melt layer from the bulk material, which could ultimately result in a full detachment of the solidified beryllium layers from the bulk armor. Furthermore, solidified beryllium filaments evolved in several locations of the loaded area and are related to the thermally induced crack formation. However, these filaments are not expected to account for an increase of the beryllium net erosion.

Investigation of damages induced by ITER-relevant heat loads during massive gas injections on Beryllium

Massive gas injections (MGIs) will be used in ITER to mitigate the strong damaging effect of full performance plasma disruptions on the plasma facing components. The MGI method transforms the stored plasma energy to radiation that is spread across the vacuum vessel with poloidal and toroidal asymmetries. This work investigated the impact of MGI like heat loading on the first wall armor material beryllium. ITER-relevant power densities of 90-260MWm−2in combination with pulse durations of 5-10ms were exerted onto the S-65 grade beryllium specimens in the electron beam facility JUDITH 1. All tested loading conditions led to noticeable surface morphology changes and in the expected worst case scenario, a crater with thermally induced cracks with a depth of up to ∼340µm formed in the loaded area. The level of destruction in the loaded area was strongly dependent on the pulse number but also on the formation of beryllium oxide. The cyclic melting of beryllium could lead to an armor thinning mechanism under the presence of melt motion driving forces such as surface tension, magnetic forces, and plasma pressure.

Surface damage and structure evolution of recrystallized tungsten exposed to ELM-like transient loads

Surface damage and structure evolution of the full tungsten ITER divertor under transient heat loads is a key concern for component lifetime and plasma operations. Recrystallization caused by transients and steady-state heat loads can lead to degradation of the material properties and is therefore one of the most serious issues for tungsten armor. In order to investigate the thermal response of the recrystallized tungsten under edge localized mode-like transient thermal loads, fully recrystallized tungsten samples with different average grain sizes are exposed to cyclic thermal shocks in the electron beam facility JUDITH 1. The results indicate that not only does the microstructure change due to recrystallization, but that the surface residual stress induced by mechanical polishing strongly influences the surface cracking behavior. The stress-free surface prepared by electro-polishing is shown to be more resistant to cracking than the mechanically polished one. The resulting surface roughness depends largely on the loading conditions instead of the recrystallized-grain size. As the base temperature increases from room temperature to 400 °C, surface roughening mainly due to the shear bands in each grain becomes more pronounced, and sub-grains (up to 3 μm) are simultaneously formed in the sub-surface. The directions of the shear bands exhibit strong grain-orientation dependence, and they are generally aligned with the traces of {1 1 2} twin habit planes. The results suggest that twinning deformation and dynamic recrystallization represent the predominant mechanism for surface roughening and related microstructure evolution.

Experimental study of ELM-like heat loading on beryllium under ITER operational conditions

The experimental fusion reactor ITER, currently under construction in Cadarache, France, is transferring the nuclear fusion research to the power plant scale. ITER’s first wall (FW), armoured by beryllium, is subjected to high steady state and transient power loads. Transient events like edge localized modes not only deposit power densities of up to 1.0 GW m−2 for 0.2–0.5 ms in the divertor of the machine, but also affect the FW to a considerable extent. Therefore, a detailed study was performed, in which transient power loads with absorbed power densities of up to 1.0 GW m−2 were applied by the electron beam facility JUDITH 1 on beryllium specimens at base temperatures of up to 300 °C. The induced damage was evaluated by means of scanning electron microscopy and laser profilometry. As a result, the observed damage was highly dependent on the base temperatures and absorbed power densities. In addition, five different classes of damage, ranging from ‘no damage’ to ‘crack network plus melting’, were defined and used to locate the damage, cracking, and melting thresholds within the tested parameter space.

Thermal Shock and Thermo-Mechanical Behavior of Carbon-Reduced and Carbon-Free Refractories

The thermal shock behaviour of novel carbon-reduced refractories with maximum grain size of 1 mm was investigated. A wedge splitting test for small specimen geometries (max. 40 × 40 × 20 mm 3 ) was successfully implemented with different loading configurations to determine “work of fracture” and thermal shock parameters. Additionally, heating-up thermal shock tests were performed with an electron beam facility. The addition of 2.5 wt% ZrO 2 and TiO 2 to Al 2 O 3 refractories appears to improve their thermal shock resistance due to microstructural changes that reduce brittleness and inhibit critical crack growth. However, a phase transition of ZrO 2 affects the properties at elevated temperature. For another pure alumina refractory, no geometry-independent value for the work of fracture could be obtained for the sample geometry used, which is probably related to the formation of a large interaction zone of the fracture surfaces. Al 2 O 3 -C materials with addition of semi-conductive Si and nanoparticles revealed a strong effect of the pressing direction on the work of fracture. However, the thermal shock parameter R’’’’ was hardly affected by the different additives. Furthermore, thermal shock tests using the electron beam facility JUDITH 1 did not indicate any significant differences in the damage pattern of the different Al 2 O 3 -C materials.

Impact of combined hydrogen plasma and transient heat loads on the performance of tungsten as plasma facing material

Experiments were performed in three different facilities in order to investigate the impact of combined steady state deuterium plasma exposure and ELM-like thermal shock events on the performance of ultra high purity tungsten. The electron beam facility JUDITH 1 was used to simulate pure thermal loads. In addition the linear plasma devices PSI-2 and Pilot-PSI have been used for successive as well as simultaneous exposure where the transient heat loads were applied by a high energy laser and the pulsed plasma operation, respectively. The results show that the damage behaviour strongly depends on the loading conditions and the sequence of the particle and heat flux exposure. This is due to hydrogen embrittlement and/or a higher defect concentration in the tungsten near surface region due to supersaturation of hydrogen. The different results in terms of damage formation from both linear plasma devices indicate that also the plasma parameters such as particle energy, flux and fluence, plasma impurities and the pulse shape have a strong influence on the damage performance. In addition, the different loading methods such as the scanning with the electron beam in contrast to the homogeneous exposure by the laser leads to an faster increase of the surface roughness due to plastic deformation.

Improvements in electron beam monitoring and heat flux flatness at the JUDITH 2-facility

Three beryllium-armoured small-scale mock-ups and one semi-prototype for the ITER first wall were tested by the electron beam facility JUDITH 2 at Forschungszentrum Jülich. Both testing campaigns with cyclic loads up to 2.5MW/m2 are carried out in compliance with the extensive quality and management specifications of ITER Organization (IO) and Fusion for Energy (F4E). Several dedicated calibration experiments were performed before the actual testing in order to fulfil the testing requirements and tolerances. These quality requests have been the motivation for several experimental setup improvements. The most relevant results of these activities, being the electron beam monitoring and the heat flux flatness verification, will be presented.

Experimental and numerical assessment of normal heat flux first wall qualification mock-ups under ITER relevant conditions

The ITER first wall (FW) panel consists of beryllium in the form of tiles covering its surface, high strength copper alloy as the heat sink material and stainless steel as the structural material. Small-scale normal heat flux FW mock-ups, provided by Fusion for Energy, are tested in the electron beam facility JUDITH 2 at Forschungszentrum Jülich to determine the performance of this design under thermal fatigue. The mock-ups are loaded cyclically under a surface heat flux of 2 MW m−2 with ITER relevant water coolant conditions. In this study, three-dimensional finite element method thermo-mechanical analyses are performed with ANSYS to simulate the thermal fatigue behaviour of the mock-ups. The temperature results indicate that the beryllium surface temperature is below the maximum allowed temperature (600 °C) of beryllium to be tested. The thermal mechanical results indicate that copper rupture and debonding between Be and copper are the drivers of the failure of a mock-up. In addition, the experimental data, e.g. the surface temperature measured using an infrared camera and the bulk temperature measured using thermocouples, are reported. A comparative study between experimental and simulation results is performed.

Performance of yttrium doped tungsten under ‘edge localized mode’-like loading conditions

Spark plasma sintered tungsten grades, with an yttrium content varying between 0.25 and 1 wt%, were characterized and exposed to transient thermal loads. The samples were cyclic tested at room temperature applying 1 ms long heat pulses using a Nd:YAG laser beam and the electron beam facility JUDITH 1. The absorbed power density of these pulses varied between 0.37 and 1.14 GW m−2. The material modifications were analysed with scanning electron microscopy, optical microscopy and laser profilometry. Comparison showed an improvement of the thermal shock resistance with increasing yttrium content. Additionally, three samples were tested at an elevated base temperature at 400 °C. The two materials with highest yttrium content cracked, indicating still brittle behaviour at the elevated base temperature when adding yttrium.

Thermal shock behaviour of tungsten after high flux H-plasma loading

Previous studies have shown that transient thermal shock loads induce crack networks on tungsten samples especially at low base temperatures. To achieve test conditions which are more relevant for the performance of tungsten-armoured plasma facing components in next step thermonuclear fusion devices tungsten tiles were exposed to high flux hydrogen-plasma in the linear plasma generator Pilot-PSI and the high heat flux ion beam test facility MARION. Subsequently, the cyclic transient heat load tests were done in the electron beam facility JUDITH 1. The induced damages after these combined tests were examined by microscopically means, profilometry and metallography. The comparison of the obtained results and damage characteristics with those obtained after thermal shock loading show that the preloading of tungsten targets with high flux hydrogen-plasma has significant influence on the thermal shock behaviour of tungsten in terms of crack distance, width, and depth as well as cracked area. Furthermore the plasma parameters, in particular pulse duration and sample temperature during loading, have strong impact on the damage pattern after thermal shock loading.

Thermal shock response of deformed and recrystallised tungsten

The thermal shock response of tungsten as a plasma facing material (PFM) strongly depends on its mechanical properties and consequently on its microstructure. In order to characterise this influence, deformed tungsten, both in its stress relieved and recrystallised condition, was exposed to 100 ELM like thermal shock events in the electron beam facility JUDITH 1. The induced thermal shock damages were analysed by scanning electron microscopy, optical microscopy and laser profilometry. Tensile tests at different temperatures show that the mechanical properties such as fracture strength and strain depend on the grain orientation and microstructure. Transmission electron microscope images of the as received and the recrystallised material show that the defect density of the recrystallised samples is decreased. Threshold values such as damage and cracking threshold vary with microstructure by a factor of 2. Also the induced thermal shock damages and surface modifications are strongly depend on the microstructure. Surface roughening due to plastic deformation is more pronounced in the recrystallised state and crack parameters as well as crack propagation is influenced by grain orientation due to preferential crack formation along grain boundaries.

Thermal fatigue characterization of CFC divertor modules using a one step brazing process

From the European side, three directional carbon fiber composites (CFCs) are foreseen to be used as plasma facing material for the strike point region of the initial ITER divertor installed for the non-tritium operational phase. For such divertor components two designs, the flat tile and the monoblock concept, are feasible, comprising a joint of the CFC with a Cu/Cu-alloy heat sink.This paper deals with the qualification of a reliable and cheap joining technology for such components, i.e. the simultaneous joining of the CuCrZr heat sink to a compliant Cu layer for the accommodation of thermal stresses and of the Cu layer and the CFC using a non-active Cu–Ge brazing material. For this purpose flat tile and monoblock mock-ups were manufactured, microstructurally analyzed, and subsequently exposed to cyclic high heat flux tests in the electron beam facility JUDITH. Applying hundreds of cycles at up to 20MW/m2 the tested mock-ups underwent partial damaging, which was characterized in post-mortem microstructural investigations to analyze occurring degradation mechanisms, e.g. partial delamination at the CFC/Cu-interface.

Analysis of structural changes and high-heat-flux tests on pre-damaged tungsten from tokamak melt experiments

The characteristics of W plasma-facing components under high heat fluxes were investigated in view of the material choice for the divertor in future fusion devices. Thin W sheets were exposed in the TEXTOR tokamak to multiple steady-state loads (20 MW at 36°), resulting in substantial W melting. One sample was subjected to post-exposure edge localized mode (ELM) simulation in the electron beam facility JUDITH 1 to determine the modified behavior of the pre-damaged components. The material structure after exposure and the cracking thresholds and patterns under repetitive transient thermal loads were investigated. Grain sizes from the original μm up to the mm range were observed. Void-like structures from the low μm up to the mm range were found at different regions of the resolidified sample. Exposure of the molten and recrystallized specimens to ELM-like thermal loads revealed a shift of the cracking threshold towards 300 °C, indicating an unfavorable increase in the ductile-to-brittle transition temperature.

Comparison of the thermal shock performance of different tungsten grades and the influence of microstructure on the damage behaviour

The thermal shock performances of two new tungsten grades with 1 and 5 wt% of tantalum were characterized with the electron beam facility JUDITH 1. As a reference material, ultra-high-purity tungsten (W-UHP) with a purity of 99.9999 wt% was used. The induced thermal shock crack networks and surface modifications were analysed by a scanning electron microscope, light microscopy and laser profilometry. Damage and cracking thresholds were defined for all materials as a function of absorbed power density and base temperature. The materials showed significantly different thermal shock behaviour, which is, among others, expressed by differences in cracking patterns, i.e. crack distance and depth. These results allow us to quantify the influence of the materials’ mechanical and thermal properties on the thermal shock performance. Furthermore, the specific grain structure of the materials has a significant influence on crack propagation towards the bulk material.

Evolution of tungsten degradation under combined high cycle edge-localized mode and steady-state heat loads

Combined thermal shock and steady-state heat loads (SSHLs) can have an impact on divertor materials and are therefore important for lifetime estimations and evaluations of operational thresholds of divertor components in future fusion devices such as ITER. This paper discusses the results of tests performed in the electron beam facility JUDITH 2 (Forschungszentrum Jülich, Germany) on actively cooled tungsten specimens, loaded with edge-localized mode-like thermal shocks (pulse duration 0.48 ms, power densities 0.14–0.55 GW m−2, frequency 25 Hz and up to 1000 000 pulses) either with or without an additional SSHL of 10 MW m−2. The material showed no damage at 0.14 GW m−2 (independent of the SSHL) for up to 250 000 pulses. At a power density of 0.27 GW m−2 (without SSHL), surface roughening occurred at 100 000 pulses, developing into a crack network at 1000 000 pulses. In general, the additional SSHL resulted in an earlier (in terms of pulse number) and more severe material degradation.

Influence of high flux hydrogen-plasma exposure on the thermal shock induced crack formation in tungsten

The influence of high flux hydrogen-plasma on the thermal shock behaviour of tungsten was investigated in a combined experiment using the linear plasma device Pilot-PSI and the electron beam facility JUDITH 1. Tungsten targets were exposed to high flux hydrogen plasma, cyclic thermal shock tests and a combination of both loading conditions. The induced thermal shock crack networks and surface modifications were investigated and characterised using scanning electron microscopy (SEM), optical microscopy and laser profilometry. Comparisons of the results showed that the combination of hydrogen plasma and thermal shock loading has a significant influence on the crack pattern in terms of crack distance, width and depth. Furthermore the sequence of the different loading types is of importance. Due to the prior loading with hydrogen plasma the thermal shock cracks were not limited to the electron beam loaded area but propagated through the just plasma loaded area and even through the unexposed area at the edges of the tungsten target.