Papers by Christian Linsmeier
Journal of Nuclear Engineering
For the future fusion reactor, tungsten is the main candidate material as the plasma-facing mater... more For the future fusion reactor, tungsten is the main candidate material as the plasma-facing material. However, considering the high thermal stress during operation, the intrinsic brittleness of tungsten is one of the issues. To overcome the brittleness, tungsten fiber reinforces tungsten composites (Wf/W) developed using extrinsic toughening mechanisms. The powder metallurgy process and chemical vapor deposition process are the two production routes for preparing Wf/W. For the powder metallurgy route, due to technical limitations, previous studies focused on short random distributed fiber-reinforced composites. However, for short random fiber composites, the strength and reinforcement effect are considerably limited compared to aligned continuous fiber composites. In this work, aligned long tungsten fiber reinforced tungsten composites have been first time realized based on powder metallurgy processes, by alternately placing tungsten weaves and tungsten powder layers. The produced W...
Tungsten fibre-reinforced tungsten composites (Wf/W) have been in development to overcome the inh... more Tungsten fibre-reinforced tungsten composites (Wf/W) have been in development to overcome the inherent brittleness of tungsten as one of the most promising candidate for the first wall and divertor armour material in a future fusion power plant. As the development of Wf/W continues, the fracture toughness of the composite is one of the main design drivers. In this contribution the efforts on size upscaling of Wf/W based on Chemical Vapour Deposition (CVD) is shown together with fracture mechanical tests of two different size samples of Wf/W produced by CVD. Three-point bending tests according to ASTM E399 for brittle materials were used to get a first estimation of the toughness. A provisional fracture toughness value of up to 346MPam1/2 was calculated for the as-fabricated material. As the material does not show a brittle fracture in the as-fabricated state, the J-Integral approach based on the ASTM E1820 was additionally applied for this state. A maximum value of the J-integral of...
Metals
Self-passivating Metal Alloys with Reduced Thermo-oxidation (SMART) are under development for the... more Self-passivating Metal Alloys with Reduced Thermo-oxidation (SMART) are under development for the primary application as plasma-facing materials for the first wall in a fusion DEMOnstration power plant (DEMO). SMART materials must combine suppressed oxidation in case of an accident and an acceptable plasma performance during the regular operation of the future power plant. Modern SMART materials contain chromium as a passivating element, yttrium as an active element and a tungsten base matrix. An overview of the research and development program on SMART materials is presented and all major areas of the structured R&D are explained. Attaining desired performance under accident and regular plasma conditions are vital elements of an R&D program addressing the viability of the entire concept. An impressive more than 104-fold suppression of oxidation, accompanied with more than 40-fold suppression of sublimation of tungsten oxide, was attained during an experimentally reproduced accident...
Instruments, 2021
The analysis of material composition by ion-beam analysis (IBA) is becoming a standard method, si... more The analysis of material composition by ion-beam analysis (IBA) is becoming a standard method, similar to electron microscopy. A pool of IBA methods exists, from which the combination of particle-induced-X-ray emission (PIXE), particle induced gamma-ray analysis (PIGE), nuclear-reaction-analysis (NRA), and Rutherford-backscattering-spectrometry (RBS) provides the most complete analysis over the whole periodic table in a single measurement. Yet, for a highly resolved and accurate IBA analysis, a sophisticated technical setup is required integrating the detectors, beam optics, and sample arrangement. A new end-station developed and installed in Forschungszentrum Jülich provides these capabilities in combination with high sample throughput and result accuracy. Mechanical tolerances limit the device accuracy to 3% for RBS. Continuous pumping enables 5 × 10−8 mbar base pressure with vibration amplitudes < 0.1 µm. The beam optics achieves a demagnification of 24–34, suitable for µ-beam...
In future fusion reactors, tungsten (W) is considered as the main candidate material for plasmafa... more In future fusion reactors, tungsten (W) is considered as the main candidate material for plasmafacing components. However, the intrinsic brittleness of tungsten is an issue under the extreme fusion environment. To overcome this drawback, W-fiber-reinforced tungsten (Wf/W) composites are being developed relying on an extrinsic toughening principle. Tungsten fibers with extremely high tensile strength showing also ductile behavior are used to reinforce the W-matrix. In previous Wf/W studies, the composite is mainly produced by chemical vapor deposition processes. However, tungsten (and other refractory metals) are industrially produced by powder metallurgical (PM) routes. Therefore, the development of PM routes is important for tungsten composites in view of large-scale production. Within this study, randomly distributed multi-short-fiber Wf/W produced by a PM process with significantly increased mechanical properties has been developed for the first time, which potentially meets the future application requirement. Theoretical understanding of reinforcement mechanisms have also been established. Field-assisted sintering technology (FAST) is the main production process in this study, with which relatively high-density bulk composite materials (92%-96% relative density depending on the sintering parameters and fiber volume fraction) can be produced. The fiber-matrix interface damage during sintering and the fiber embrittlement due to carbon contamination, as the main technical difficulties during the sintering, are investigated. These issues can be solved by increasing the interface thickness and separating the powders and graphite tools with a W-foil, respectively. Apart from the FAST process, pressure-less sintering and hot isostatic pressing (HIP) are also used to prove the sintering possibility of using other PM alternatives.
Metals, 2021
Tungsten (W) has the unique combination of excellent thermal properties, low sputter yield, low h... more Tungsten (W) has the unique combination of excellent thermal properties, low sputter yield, low hydrogen retention, and acceptable activation. Therefore, W is presently the main candidate for the first wall and armor material for future fusion devices. However, its intrinsic brittleness and its embrittlement during operation bears the risk of a sudden and catastrophic component failure. As a countermeasure, tungsten fiber-reinforced tungsten (Wf/W) composites exhibiting extrinsic toughening are being developed. A possible Wf/W production route is chemical vapor deposition (CVD) by reducing WF6 with H2 on heated W fabrics. The challenge here is that the growing CVD-W can seal gaseous domains leading to strength reducing pores. In previous work, CVD models for Wf/W synthesis were developed with COMSOL Multiphysics and validated experimentally. In the present article, these models were applied to conduct a parameter study to optimize the coating uniformity, the relative density, the WF...
Physica Scripta, 2021
Tungsten erosion was quantified during inter/intra-ELM periods in He-dominated JET-ILW plasmas by... more Tungsten erosion was quantified during inter/intra-ELM periods in He-dominated JET-ILW plasmas by optical emission spectroscopy. The intra-ELM tungsten sputtering in helium plasmas, which dominates the total W source, prevails by a factor of about 4 over inter-ELM sputtering in the investigated ELM frequency range from 90 Hz–120 Hz. He ions are mainly responsible for the W erosion during the ELMs in He plasmas. The strong in/out asymmetry of the ELM-induced W erosion is observed in He plasmas even at high ELM frequencies beyond 100 Hz. In Ohmic/L-mode plasmas and during the H-mode inter-ELM plasma phases both He2+ and Be2+ ionic species are major contributors to the W erosion. Their contribution depends on the electron temperature in the divertor: for Te > 15 eV both species cause significant W sputtering, for Te < 15 eV, Be 2+ ions are solely responsible for the W erosion. Tungsten erosion during in both inter and intra-ELM periods in He-dominated plasmas are significantly larger than in deuterium plasmas. It is 15–25 times larger during the inter-ELM phase and in L-mode discharges at Te = 25–30 eV. On the other hand, the ELM-induced W source is by a factor of 3 larger than in D plasmas.
Plasma and Fusion Research, 2020
Fuel retention and hydrogen permeation in the first wall of future fusion devices are crucial fac... more Fuel retention and hydrogen permeation in the first wall of future fusion devices are crucial factors. Due to safety issues and in order to guarantee an economical reactor operation, tritium accumulation into reactor walls and permeation through walls have to be estimated and prevented. Therefore, studies of permeation in the fusion materials are performed and the need for tritium permeation barriers (TPB) is verified. The development of TPB layers is explained. A reliable way of comparing different TPB layers and the estimation of the permeation reduction effect of a TPB layer on different bulk materials is enabled by calculation of the layer permeability.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2020
The dynamic behaviour of thermally driven segregation of Cr to the surface of WCrY smart alloy is... more The dynamic behaviour of thermally driven segregation of Cr to the surface of WCrY smart alloy is studied with low energy ion scattering (LEIS). Sputtering the WCrY sample with 500 eV D + 2 ions at room temperature results in preferential removal of the lighter alloy constituents and causes an almost pure W surface layer. At elevated temperatures above 700 K the segrega
Advanced Engineering Materials, 2020
For future fusion reactors, tungsten (W) is currently the main candidate for the application as p... more For future fusion reactors, tungsten (W) is currently the main candidate for the application as plasma‐facing material due to its several advanced properties. To overcome the brittleness of W, randomly distributed short W fiber‐reinforced W (Wf/W) composites have been developed using field‐assisted sintering technology (FAST). Herein, Wf/W materials with different fiber volume fraction (20–60%) are manufactured by FAST process to study the fiber volume fraction influence on the composite properties. Wf/W with ductile fibers and brittle fibers is produced using different tool setups during the production. Three‐point bending tests on prenotched samples, 4‐point bending tests, and tensile tests have been performed to determine the fracture behavior and flexural/tensile strength of the material. Wf/W materials with 30–40% fiber volume fraction exhibit a promising pseudoductile behavior, similar to fiber‐reinforced ceramic composites. However, Wf/W with 20% and >50% fiber volume frac...
Nuclear Materials and Energy, 2020
The temperature driven segregation of Cr to the surface of the tungsten-based WCrY alloy is analy... more The temperature driven segregation of Cr to the surface of the tungsten-based WCrY alloy is analysed with low energy ion scattering of He + ions with an energy of 1 keV in the temperature range from room temperature to 1000 K. Due to the high surface sensitivity, these measurements probe only the composition of the outermost monolayer. The surface concentration of Cr increases slightly when the temperature of the sample is increased up to 700 K and exhibits a much stronger increase when the sample temperature is further raised. The segregation enthalpy for Cr is obtained from the Langmuir-McLean relation and amounts to 0.7 eV. The surface concentration of Y shows a similar behaviour to the Cr concentration. The temperature thresholds between slow and accelerated surface density increases for Cr and Y are nearly the same. At a temperature of 1000 K the low energy ion scattering detects almost no W on the surface. The modified surface composition due to the segregated species, i.e. the mixed Cr/Y layer, stays stable during cool-down of the sample. Preferential sputtering is investigated using ion bombardment of 250 eV D atoms, resulting in an increase of the W surface density at room temperature. This effect is counteracted at elevated temperatures where segregation replenishes the lighter elements on the surface and prevents the formation of an all-W surface layer. The flux of segregating Cr atoms towards the surface is evaluated from the equilibrium between sputter erosion and segregation.
Contributions to Plasma Physics, 2019
ERO2.0 is a recently developed Monte‐Carlo code for modelling global erosion and redeposition in ... more ERO2.0 is a recently developed Monte‐Carlo code for modelling global erosion and redeposition in fusion devices. We report here on the code's application to ITER for studying the erosion of the beryllium (Be) first wall armour under burning plasma steady state diverted conditions. An important goal of the study is to provide synthetic signals for the design of two key diagnostics: the main chamber visible spectroscopy and the laser in‐vessel viewing systems. The simulations are performed using toroidally symmetric plasma backgrounds obtained by combining SOLPS simulations extended to the wall using the OSM‐EIRENE‐DIVIMP edge code package. These are then further combined with a shadowing model using magnetic field line tracing to provide a three‐dimensional correction for the flux patterns. The resulting plasma wetted area, which amounts to ∼10% of the total first wall area, is in excellent agreement with shadowing calculations obtained with the SMITER field line tracing code. Th...
Atoms, 2019
In this work, we present a new application for the line shapes of emission induced by reflected h... more In this work, we present a new application for the line shapes of emission induced by reflected hydrogen atoms. Optical properties of the solids in contact with the plasma could be effectively measured at the wavelength of Balmer lines: time-resolved measurements of reflectance and polarization properties of mirrors are performed using the wavelength separation of the direct and reflected signals. One uses the Doppler effect of emission of atoms excited by collisions with noble gases, primarily with Ar or with Kr. In spite of a new application of line shapes, the question of the source of the strong signal in the case of Ar exists: the emission observed in the case of the excitation of H or D atoms by Ar exceeds the signal induced by collisions with Kr atoms by a factor of five, and the only available experimental data for the ground state excitation show practically equal cross-sections for both gases in the energy range of 80–200 eV.
Tungsten, 2019
Tungsten-chromium-yttrium (WCrY) smart alloys are foreseen as the first wall material for future ... more Tungsten-chromium-yttrium (WCrY) smart alloys are foreseen as the first wall material for future fusion devices such as Demonstration Power Plant (DEMO). While suppressing W oxidation during accidental conditions, they should behave like pure W during plasma operation due to preferential sputtering of the lighter alloying elements Cr, Y, and W enrichment of the surface. In this paper, the erosion performance of WCrY and W samples simultaneously exposed to deuterium (D) plasma with the addition of 1% of the projectile ions being argon (Ar) ions at an ion energy of 120 eV is compared. With reference to the previous experiments at 120 eV in pure D plasma, the erosion for both WCrY and W is enhanced by a factor of ~ 7. Adding Ar to the D plasma suppresses significant W enrichment previously found for pure D plasma. To investigate the impact of the plasma exposure onto the oxidation performance, plasma-exposed and non-exposed reference samples were oxidised in a dry atmosphere. Results s...
Fusion Engineering and Design, 2019
Future fusion power plants require the development of a first wall armor material withstanding ex... more Future fusion power plants require the development of a first wall armor material withstanding extreme particle and heat loads. Considering safety, the formation of long-lived radioactive isotopes when irradiated with neutrons and a tritium inventory has to be prevented. As tungsten (W) meets these safety requirements, has a low erosion rate, high melting point, and high thermal conductivity, it is a promising candidate for the first wall armor. There is another important safety consideration: in a loss of coolant accident, cooling systems fail and air ingress into the vacuum vessel may occur. Due to the nuclear decay heat, temperatures in the range from 1200 K to 1450 K for several weeks are predicted in such a scenario: the radioactive W oxidizes and volatilizes, posing a severe hazard for the environment. For a successful future fusion power plant like DEMO, a new material is required. The new material should preserve the advantages of W coupled with suppressed sublimation in case of an accident. Currently an alloy containing W,~12 weight % chromium, and~0.6 weight % yttrium consolidated by Field Assisted Sintering Technology is investigated and shows promising results. During exposures to deuterium plasma, erosion yields are similar to pure W. Moreover, the oxidation resistance is significantly improved: complete mechanical destruction is avoided for at least three weeks. The major goal is to control the sublimation which is responsible for a release of radioactivity. In this work sublimation rates are measured explicitly for the first time in humid air. WO3 sublimates at a rate of 5 g/m²/h at 1273 K in humid air. At the same conditions the alloy suppresses sublimation by a factor of 35 as compared to that of pure W. The results and consequences are discussed. Further, the mechanisms yielding the improved oxidation resistance are analyzed and discussed.
Metals, 2018
Tungsten (W) is currently deemed the main candidate for the plasma-facing armor material of the f... more Tungsten (W) is currently deemed the main candidate for the plasma-facing armor material of the first wall of future fusion reactors, such as DEMO. Advantages of W include a high melting point, high thermal conductivity, low tritium retention, and low erosion yield. However, was an accident to occur, air ingress into the vacuum vessel could occur and the temperature of the first wall could reach 1200 K to 1450 K due to nuclear decay heat. In the absence of cooling, the temperature remains in that range for several weeks. At these temperatures, the radioactive tungsten oxidizes and then volatilizes. Smart W alloys are therefore being developed. Smart alloys are supposed to preserve properties of W during plasma operation while suppressing tungsten oxide formation in case of an accident. This study focuses on investigations of thin film smart alloys produced by magnetron sputtering. These alloys provide an idealistic system with a homogeneous distribution of the elements W, chromium (Cr), and yttrium (Y) on an atomic scale. The recommended composition is W with 12 weight % of Cr and 0.5 weight % of Y. Passivation and a suppression of WO 3 sublimation is shown. For the first time, the mechanisms yielding the improved oxidation resistance are analyzed in detail. A protective Cr 2 O 3 layer forms at the surface. The different stages of the oxidation processes up to the failure of the protective function are analyzed for the first time. Using 18 O as a tracer, it is shown for the first time that the oxide growth occurs at the surface of the protective oxide. The Cr is continuously replenished from the bulk of the sample, including the Cr-rich phase which forms during exposure at 1273 K. A homogenous distribution of yttria within the W-matrix, which is preserved during oxidation, is a peculiarity of the analyzed alloy. Further, an Y-enriched nucleation site is found at the interface between metal and oxide. This nucleation sites are deemed to be crucial for the improved oxidation resistance.
Composite Interfaces, 2018
To overcome the intrinsic brittleness of tungsten (W), a tungsten fiber-reinforced tungsten-compo... more To overcome the intrinsic brittleness of tungsten (W), a tungsten fiber-reinforced tungsten-composite material (W f /W) is a possible solution. The introduction of energy dissipation mechanisms like fiber bridging or fiber pull-out by means of an engineered interface between fiber and matrix mitigate the brittleness of tungsten and lead to a pseudo-ductile material behaviour. The push-out test of single-fiber samples is an experimental method to investigate the properties of the interface between fiber and matrix of composite materials. It is widely used for the investigation of ceramic composites. This method was also used to investigate the debonding and frictional properties of the Er2O3 interface region between fiber and matrix of W f /W single-fiber samples made by CVD-and HIP-processes. In this article finite element calculations are used to get a better understanding of the processes acting in the interface during a push-out test of W f /W. A detailed overview of the debonding progress and of the corresponding stress states of the interface during the different stages of the test is presented. In addition the sensitivity of the push-out behaviour regarding the different interface properties and the plastic flow curve of the tungsten fiber are investigated. KEYWORDS Metal-matrix composites (MMCs); fiber/matrix bond; cohesive interface modelling; finite element analysis (FEA); push-out test; tungsten fiber-reinforced tungsten composites
Nuclear Fusion, 2017
Plasma-facing materials and components in a fusion reactor are the interface between the plasma a... more Plasma-facing materials and components in a fusion reactor are the interface between the plasma and the material part. The operational conditions in this environment are probably the most challenging parameters for any material: high power loads and large particle and neutron fluxes are simultaneously impinging at their surfaces. To realize fusion in a tokamak or stellarator reactor, given the proven geometries and technological solutions, requires an improvement of the thermo-mechanical capabilities of currently available materials. In its first part this article describes the requirements and needs for new, advanced materials for the plasma-facing components. Starting points are capabilities and limitations of tungsten-based alloys and structurally stabilized materials. Furthermore, material requirements from the fusion-specific loading scenarios of a divertor in a water-cooled configuration are described, defining directions for the material development. Finally, safety requirements for a fusion reactor with its specific accident scenarios and their potential environmental impact lead to the definition of inherently passive materials, avoiding release of radioactive material through intrinsic material properties. The second part of this article demonstrates current material development lines answering the fusion-specific requirements for high heat flux materials. New composite materials, in particular fiber-reinforced and laminated structures, as well as mechanically alloyed tungsten materials, allow the extension of the thermo-mechanical operation space towards regions of extreme steady-state and transient loads. Self-passivating Nuclear Fusion Development of advanced high heat flux and plasma-facing materials Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Fusion Engineering and Design, 2017
h i g h l i g h t s • Self-passivating W-CrY shows the best oxidation performance of all tested a... more h i g h l i g h t s • Self-passivating W-CrY shows the best oxidation performance of all tested alloys. • WO 3 formation and evaporation in dry atmosphere is fully mitigated. • A mechanism which makes oxidation in humid atmospheres so challenging for the W-CrY is introduced and a solution is proposed.
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Papers by Christian Linsmeier