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To clarify the suitability of solid tungsten as plasma facing material, a new divertor, DIV-III, entered in operation in ASDEX Upgrade (AUG) in 2014. Since the first experimental campaign, AUG divertor tiles faced crack issues. Trying to identify the reasoning, a new set of tiles was installed in 2016: many precautions were taken to reduce the stresses possibly causing the cracks. The AUG campaign 2017 started in March and the first results were collected during the short summer opening in 2017. This paper reports on the modification of the tile setup, based on Finite Element Modelling, and on the progress achieved in the first operation phase.

Resume Objectifs Evaluer la place de l’allergie en tant que facteur influencant la presentation clinique (phenotype) et le profil inflammatoire (endotype) de la polypose nasosinusienne (PNS). Materiel et methodes Une etude prospective monocentrique etait menee sur 18 mois incluant 57 patients suivis pour PNS en situation d’echec du traitement medical. L’allergie etait diagnostiquee sur la concordance des tests cutanes allergeniques et des manifestations cliniques. L’analyse du profil phenotypique s’appuyait sur le score de severite clinique, la taille des polypes et le score tomodensitometrique de Lund-Mackay. Le profil endotypique etait etabli par la mesure des concentrations dans le sang et les secretions nasales en IgE totales, IgA, interleukine-5, interleukine-9, ECP et EDN. Les taux de polynucleaires eosinophiles dans le sang, les secretions nasales et le polype etaient aussi compares. Resultats Les profils phenotypiques et endotypiques des patients allergiques (n = 15) et non allergiques (n = 42) etaient comparables. Seuls l’asthme et les IgE totales sanguines etaient associes a la presence d’une allergie. Conclusion Ce travail suggere que l’allergie n’est pas directement impliquee dans l’expression clinique et les processus inflammatoires de la PNS. A l’heure de nouvelles approches therapeutiques ciblant les voies de signalisation inflammatoire, l’identification de marqueurs sanguins et tissulaires pertinents constitue l’axe de recherche la plus susceptible de modifier la prise en charge de la PNS.

Four sets of targets were used in this study: (1) Fe targets surrounded with 304 type stainless steel composed of mid-Z elements: Fe, Cr, Ni, and Mn (designated as Fe[304SS] targets), (2) Fe targets surrounded with high-Z tungsten (designated as Fe[W] targets), (3) Cu targets surrounded with mid-Z copper (designated as Cu[Cu] targets), and (4) Cu targets surrounded with high-Z tungsten (designated as Cu[W] targets). The targets were exposed to low-energy (140 and 200 eV), high-flux (about 1022 D/m2s) deuterium (D) plasmas at various temperatures in the range from 355 to 740 K. The surface morphology of the Fe and Cu targets is found to be dependent strongly on atomic number of re-deposited species and on the exposure temperature. For the Fe[W] and Cu[W] targets, due to formation of the W-enriched nano-sized structures on the target surfaces, the sputtering erosion yield is lower than that for the Fe[304SS] and Cu[Cu] targets, respectively. For the Fe[304SS], Fe[W], and Cu[W] targets, the sputtering erosion yield is increased distinctly as the exposure temperature rises from 355 to 740 K.

Plastic deformation of tungsten wire is an effective source of toughening tungsten fibre-reinforced tungsten composites (Wf/W) and other tungsten fibre-reinforced composites. To provide a reference for optimization of those composites, unconstrained pure tungsten wire is studied after various heat treatments in terms of microstructure, mechanical behaviour and fracture mode. Recrystallization is already observed at a relatively low temperature of 1273 K due to the large driving force caused by a high dislocation density. Annealing for 30 min at 1900 K also leads to recrystallization, but causes a rather different microstructure. As-fabricated wire and wire recrystallized at 1273 K for 3 h show fine grains with a high aspect ratio and a substantial plastic deformability: a clearly defined tensile strength, high plastic work, similar necking shape, and the characteristic knife-edge-necking of individual grains on the fracture surface. While the wire recrystallized at 1900 K displays large, almost equiaxed grains with low aspect ratios as well as distinct brittle properties. Therefore, it is suggested that a high aspect ratio of the grains is important for the ductile behaviour of tungsten wire and that embrittlement is caused by the loss of the preferable elongated grain structure rather than by recrystallization. In addition, a detailed evaluation of the plastic deformation behaviour during tensile test gives guidance to the design and optimization of tungsten fibre-reinforced composites.

Deuterium retention in two reduced-activation ferritic/martensitic (RAFM) steels, F82H (8Cr-2W) and EUROFER (9Cr-1W), was measured by nuclear reaction analysis and thermal desorption spectrometry after exposure to low-energy deuterium plasmas. In the RAFM steels, the deuterium retention was 1017–1019 D/m2 and the major fraction of deuterium was retained in the near-surface layer. The surface concentrations of tungsten increased with the D ion fluence and energy due to the selective sputtering of iron and chromium. Scanning electron microscopy images clearly indicated that the surface morphology changed with increasing D fluence (i.e. the grain boundaries became more visible because of erosion by ion bombardment). At a low fluence of about 1023 D/m2, deuterium retention was affected by surface modification in RAFMs.

Tritium permeation through structural materials in fusion reactor blanket systems is a critical issue from the viewpoints of fuel loss and radiological safety. Ceramic coatings have been investigated to prevent tritium permeation; however, corrosion of the coatings by blanket materials, especially corrosive liquid tritium breeders such as lithium-lead is serious. In our previous study, the improvement of permeation reduction performance using an erbium oxide (Er2O3)-zirconium oxide (ZrO2) two-layer coating was confirmed, but it did not show substantial corrosion resistance. In this study, various multi-layer coatings were fabricated by metal organic decomposition and then exposed to lithium-lead under static conditions to investigate precise corrosion behaviors of multi-layer coatings. After lithium-lead exposure for 1000 h at 600 °C, the outermost Er2O3 layer almost disappeared, while the outermost ZrO2 layer remained, indicating that ZrO2 is more suitable as the outermost layer. However, many cracks and peelings were observed on the outermost ZrO2 layer in the cases of the samples having four coating interfaces. The optimization of layer combination and the control of adhesion between the coatings are required to reduce the degradation of multi-layer coatings.

EUROFER is a Reduced Activation Ferritic Martensitic (RAFM) steel developed as structural material for future fusion power plants and is considered as first-wall material in recessed areas of DEMO. Recent work has shown a fluence-dependent decrease of the sputter yield for bombardment with deuterium ions in the energy range of 100 to about 500 eV. This decrease was previously attributed to preferential sputtering of the lower mass constituents in EUROFER, such as Fe and Cr, compared to the higher mass alloying elements. This leads to an increase of the surface tungsten concentration. However, it was also observed that, after sputtering, the samples had developed a very rough surface morphology. In this work, the combined influence of surface roughness and W surface enrichment on the sputter yield of EUROFER under bombardment with 200 eV deuterium ions was studied. The influence of surface roughness was determined with the aid of Scanning Electron Microscopy (SEM) and SDTrimSP-3D simulations. W surface enrichment was investigated applying sputter X-Ray Photoelectron Spectroscopy (XPS) depth-profiling and Rutherford backscattering spectrometry (RBS). After bombardment to a fluence of 1024 D m−2 (at 200 eV per deuterium) a reduction of the sputter yield to 29% ± 5% of the initial value was measured by weight-loss measurements. This reduction is in good agreement with published values. Two distinct surface morphologies, consisting of smooth and spiked surfaces, were observed on the EUROFER sample after sputtering. Based on the experimental results, the combined effect of the two factors, surface roughness and W surface enrichment, is estimated to be responsible for a reduction in the sputter yield to 27% ± 4% of the initial value, which is in excellent agreement with the measured value. Our assessment shows that both surface morphology and W surface enrichment contribute significantly to the reduction of the sputter yield of EUROFER under the given experimental conditions, and are sufficient to fully explain the experimentally observed reduction in the sputter yield.

Uranium–molybdenum (UMo) alloy embedded in an Al matrix (UMo/Al) has been considered as a promising candidate for fuel conversion of research reactors. A modified system with a diffusion barrier, UMo/X/Al trilayer (X = Ti, Zr, Nb, and Mo), has been investigated in order to suppress interdiffusion between UMo and the Al matrix. The trilayer was tested by swift heavy ion irradiation, followed by Rutherford backscattering spectroscopy (RBS) and X-ray micro-diffraction (μ-XRD). Atomic mixing at the interfaces was resolved by RBS, indicating that Ti interacts strongly with UMo while Zr does with Al. μ-XRD revealed the formation of intermetallic AlX compounds which can detain further atomic mixing. However, Ti and Zr as diffusion barrier can be controversial because their presence might lead to γ-UMo decomposition. This study presents the effectiveness of diffusion barriers and the irradiation-induced phase impacting on the properties of the UMo/X/Al trilayer.

The erosion of Be and W marker layers was investigated using long-term samples during the first ITER-like wall discharge campaign 2011-2012. The markers were mounted in Be coated Inconel tiles between the inner wall guard limiters (IWGL). They were analyzed using Rutherford backscattering (RBS) before and after exposure. All samples showed strong erosion. The results were compared to the data for Be and W erosion rates for the 2005-2009 and the 2001-2004 campaigns, respectively, when JET was operated with a carbon wall. In 2005-2009 Be and C samples were used, and W samples were used in 2001-2004. The mean W erosion rates and the toroidal and poloidal distributions of the W erosion were the same for the 2001-2004 and the 2011-2012 campaigns. The mean erosion rate of Be during the 2011-2012 campaign was smaller by a factor of about two compared to the 2005-2009 campaign and showed a different poloidal distribution. The mean erosion rate of the inner JET ITER-like wall was about 4-5 times smaller than the mean erosion rate of the carbon wall.

Surface topography and deuterium retention in polycrystalline hot-rolled W and W–5%Re have been examined after exposure to a low-energy (76 eV), high flux (around 1022 D/m2 s) deuterium plasma to an ion fluence of 1026 D/m2 at various temperatures. The methods used were confocal laser scanning microscopy and the D(3He, p)4He nuclear reaction at 3He energies varied from 0.69 to 4.0 MeV. During exposure to the D plasma at temperatures in the range from 348 to 673 K, small blisters of size in the range from about 1 to about 15 μm, depending on the exposure temperature, are formed on the W and W–5%Re surfaces. In the W–5%Re, the deuterium retention demonstrates its maximum at exposure temperature of 463 K, while in the W this maximum is shifted to 523 K. A difference in the temperature dependence of the D retention for the W and W–5%Re is explained, as a rough approximation, by temperature dependences of the ductility of these materials.

In 2013 a redesigned solid tungsten divertor, Div-III, was installed in ASDEX Upgrade. In two experimental campaigns more than 2700 plasma discharges with up to 10s duration, 110MJ plasma heating, and reaching a Psep/R of 10MW/m were conducted. Div-III is an adiabatically loaded divertor with tungsten target tiles clamped onto a water cooled structure. The tungsten target tiles were inspected and characterized during the shut-down period in 2014. As a result of the target inspection characteristic modifications of the plasma exposed tiles were detected that were not found during the target qualification in a high heat load test facility. 126 out of 128 tiles reveal deep cracks through the target. A lot of tiles show shallow cracks in the high heat load region and finally protruding tiles with strong local damages were found. It should be noted that none of these target damages have caused an unscheduled opening of ASDEX Upgrade. Keywords: Solid tungsten, Divertor, ASDEX Upgrade, Plasma energy

Simulation of neutron irradiation and sputtering on ITER-grade tungsten was studied. The effects of neutron-induced displacement damage have been simulated by irradiation of tungsten target with W6+ ions of 20 MeV energy. Bombardment by Ar+ ions with energy 600 eV was used as imitation of impact of charge exchange atoms in ITER. The sputtering process was interrupted to perform in between measurements of the optical properties of the eroded surface and the mass loss. After sputtering was finished, the surface was thoroughly investigated by different methods for characterizing the surface relief developed due to sputtering. The damaging to, at least, the level that would be achieved in ITER does not lead to a decisive additional contribution to the processes under impact of charge exchange atoms only.

The mapping of hydrogen distributions in 3 dimensions and its correlation with structural features allow further insight into mechanisms of hydrogen trapping in tungsten. We studied hydrogen distributions in 25 μm thick polycrystalline tungsten foils by 3D hydrogen microscopy using a proton–proton-scattering method. Two types of tungsten samples were prepared: (i) at 1200 K annealed foils and using 1.8 MeV implantation energy (ii) at 2000 K annealed foils using 200 eV implantation energy. It has been found that large variations of surface hydrogen contamination occur within different samples. Nevertheless, a statistically significant variation of the hydrogen content across grain boundaries has been observed.

We devised a method to measure erosion in the micrometer range on actively cooled tungsten samples. The method is based on applying micrometer-sized markers onto the side faces of the samples using a focused ion beam. With this method we measured the erosion of tungsten samples exposed to a hydrogen beam with 6% helium content. At 10.5 MW/m 2 the samples were exposed to particle fluences between 2 × 10 25 m −2 and 7 × 10 25 m −2 . Up to 630 pulses with a pulse duration of 30 s were employed. The actively cooled samples reach an equilibrium surface temperature which was varied from about 600 °C to 2000 °C. We find an amount of erosion which clearly exceeds the value computed from physical sputtering yields by roughly a factor of two. This is possibly correlated with the formation of a complex nanometer-sized morphology, which was observed for all temperatures.

The effect of heavy sputtering and of neutron irradiation simulated by displacement damaging with of 20 MeV W 6+ ions on the optical properties of tungsten mirrors was studied. Ar + ions with 600 eV of energy were used as imitation of charge exchange atoms ejected from fusion plasma. The ion fluence dependence of the surface topography and the optical properties of polycrystalline, recrystallized tungsten (grain size 20–100 μm) were studied by optical microscopy, interferometry, reflectometry and ellipsometry. Furthermore, after sputtering in total a layer of 3.9 μm in thickness, the orientation and the thickness of the eroded layer of many individual grains was determined by electron backscattering diffraction and confocal laser scanning microscopy. Concluding from the obtained data the neutron irradiation, at least at the damage level would be achieved in ITER, has not to make an additional contribution in the processes developing under impact of charge exchange atoms only.

Tungsten-doped amorphous carbon films with 0-9.5 at.% W concentration were produced by magnetron sputtering and exposed to deuterium plasmas applying different ion energies and fluences. The partial C and W erosion rates were determined from the C and W areal density changes, respectively, measured by Rutherford backscattering spectrometry. For W-doped films the erosion rate decreases with increasing W concentration and incident fluence. During deuterium plasma exposure carbon is preferentially eroded while tungsten atoms accumulate at the surface leading to the formation of a W-rich layer, which decreases the removal efficiency and leads to a continuous decrease of the erosion rate. At 30 eV/D incident energy a relatively compact W-rich layer is formed on films with higher (≥5%) W concentration which protects the carbon underneath from further erosion. For films with lower (≤2.5%) W concentration the erosion rate decreases, but the erosion process does not stop because the Wrich layer has a high porosity. Reactive neutral species can penetrate through this porous layer and react with carbon atoms below it. At 100 eV/D incident energy the erosion rate is significantly higher compared with 30 eV/D. In addition, slight tungsten sputtering is observed due to the presence of a small fraction of D + plasma ions which impinge with 300 eV/D.

Surface topography and deuterium retention in polycrystalline ITER-grade tungsten have been examined after exposure to a low-energy (38 eV/D), high-flux (10 22 D/m 2 s) deuterium plasma with ion fluences of 10 26 and 10 27 D/m 2 at various temperatures. The methods used were scanning electron microscopy equipped with focused ion beam, thermal desorption spectroscopy, and the D( 3 He,p) 4 He nuclear reaction at 3 He energies varied from 0.69 to 4.0 MeV. During exposure to the D plasma at temperatures in the range from 320 to 815 K, small blisters of size in the range from 0.2 to 5 μm, depending on the exposure temperature and ion fluence, are formed on the W surface. At an ion fluence of 10 27 D/m 2 , the deuterium retention increases with the exposure temperature, reaching its maximum value of about 10 22 D/m 2 at 500 K, and then decreases below 10 19 D/m 2 at 800 K.

Deuterium retention in different tungsten (W) coatings on carbon substrates was investigated for various incident ion energies ranging from 20 to 200 eV per deuterium atom and fluences ranging from 1 × 1023 m−2 to 2 × 1025 m−2. The targets were irradiated by deuterium ions at the IPP laboratory with a mass-separated ion beam with a flux of 1019 D/m2 s and with a deuterium plasma providing a flux of 1020 D/m2 s. Irradiation has been done at different sample temperatures ranging from 320 to 650 K. The depth profile of deuterium in the W coatings was measured up to 6 μm depth by nuclear reaction analysis (NRA) and the total retained amount was determined by thermal desorption spectroscopy. It is shown that deuterium retention significantly depends on the microstructure of each W coating. The deuterium retention in W coating of 7 μm thickness produced by combined magnetron-sputtering and ion implantation technique is higher compared to physical vapour deposited W coating of 4–5 μm thickness and 200 μm thickness plasma-sprayed W coating for all investigated energies and sample temperatures up to 650 K.

Powder metallurgy tungsten and plasma-sprayed tungsten samples were exposed to high heat and particle fluxes in order to study changes in the surface morphology. Actively water-cooled and adiabatically loaded samples were irradiated with H, He, and mixed 90% H/10% He neutral beams at power densities of 2 MW m−2 and 10 MW m−2 with fluences up to 3 × 1025 m−2. For He irradiation of adiabatically loaded samples a peak surface temperature of 2100 °C was reached. Under this condition the growth of a coral-like surface structure modification due to bubble formation was the dominant process observed. In all other cases physical sputtering was the surface modification process. For hydrogen irradiation of actively cooled samples with much lower surface temperatures no formation of blisters was observed. Instead, we found small nano-sized pores.

Tungsten-doped amorphous carbon films (a-C:W) are used as model system to study the deuterium (D) retention behavior and the erosion behavior of metal containing co-deposited layers. In our recent studies, the implantation temperature was increased towards an ITER relevant range. Pure and 7.5 at% tungsten-doped carbon films have been exposed to a deuterium beam of 200 eV/D at different implantation temperatures up to 1300 K and at a fluence of 10 24 D/m 2 . Rutherford backscattering spectrometry with a 1.5 MeV + H beam was performed to obtain the total erosion yield. For each implantation condition and for each structure of doped films, the total erosion yield is reduced significant, compared to pure films. The temperature dependence of the total erosion yield is less pronounced for doped films.

Most of spherical blisters formed by deuterium (D) bombardment (38 eV/D) up to 3 × 1024 D/m2 at 300 K on polycrystalline tungsten are fully elastic deformations. This has been proven by opening individual blisters with a focused ion beam and in situ observation of their complete relaxation by scanning electron microscopy. The D2 gas filling is confirmed by observing simultaneously the D2 puff. The gas pressure is causal for the stability of such spherical blisters after implantation and the gas release leads to sudden relaxation. The dilatation of the blister cap by trapped D can be excluded as cause for the blisters.

Bonding structure of carbon and metal as well as nanostructural changes of metal-doped amorphous carbon films (a-C:Me) were investigated depending on metal type (W, Ti, V, and Zr), concentration ( 2 C, and WC 1 − x ) of several nanometers. The VC particles reach the largest size up to 1300 K. All metal dopings reduce the erosion rate against oxidation (expect V) and hydrogen impact.

Divertor plasma-facing components of future fusion reactors should be able to withstand heat fluxes of 10–20 MW/m2 in stationary operation. Tungsten blocks with an inner cooling tube made of CuCr1Zr, so-called monoblocks, are potential candidates for such water-cooled components. To increase the strength and reliability of the interface between the W and the cooling tube of a Cu-based alloy (CuCr1Zr), a novel advanced W-fibre/Cu metal matrix composite (MMC) was developed for operation temperatures up to 550 °C. Based on optimization results to enhance the adhesion between fibre and matrix, W fibres (Wf) were chemically etched, coated by physical vapour deposition with a continuously graded W/CuPVD interlayer and then heated to 800 °C. The Wf/Cu MMC was implemented by hot-isostatic pressing and brazing process in monoblock mock-ups reinforcing the interface between the plasma-facing material and the cooling channel. The suitability of the MMC as an efficient heat sink interface for water-cooled divertor components was tested in the high heat flux (HHF) facility GLADIS. Predictions from finite element simulations of the thermal behaviour of the component under loading conditions were confirmed by the HHF tests. The Wf/Cu MMC interlayer of the mock-ups survived cyclic heat loads above 10 MW/m2 without any damage. One W block of each tested mock-up showed stable thermal behaviour at heat fluxes of up to 10.5 MW/m2.

Er2O3 sesquioxide thin films in the uncommon, monoclinic phase were produced for the first time by filtered cathodic arc deposition on Eurofer steel substrates. This was achieved by applying a sample bias of −250 V and deposition temperatures ⩽400 °C. X-ray diffraction texture analysis revealed a strong preferred orientation of the B-phase crystallites. Deposition at 600 °C without bias voltage resulted in thin films showing the regular cubic phase. A dense, columnar structure was found for both phases by scanning transmission electron microscopy.

In-vessel inspections of AUG reveal arc tracks at different locations. This paper presents investigation of arcing at the inner transition and retention module where arc tracks were found around the whole toroidal circumference. A representative tile was investigated with profilometry, SEM, EDX, RBS, and colourimetry. Type 2 arc tracks with an area fraction of 12% were found in a 10 mm wide region near to the leading edge. Here the tungsten coating (3–4 μm) is removed and a small amount of carbon (max. 1–2 μm) is eroded. Droplets are detected at the surface. The region of the inner divertor where the arcs are observed shows a local enhancement of plasma density, n e ≈ 10 20 m −3 , and neutral pressure, 0.1 mbar, favouring the arc ignition. A high sheath potential at the beginning of an ELM might trigger the arc. The arc pattern and the target load as measured with a fast IR-camera coincide. 2 g of tungsten are eroded in the inner divertor. If 10% of the material is eroded as droplets, 10 8 droplets (5 μm diameter) are ejected.

Finely dispersed Ti-doped isotropic graphites with 4 at.% Ti have been manufactured using synthetic mesophase pitch ‘AR’ as raw material. These new materials show a thermal conductivity at room temperature of ∼200 W/mK and flexural strength close to 100 MPa. Measurement of the total erosion yield by deuterium bombardment at ion energies and sample temperatures for which pure carbon shows maximum values, resulted in a reduction of at least a factor of 4, mainly due to dopant enrichment at the surface caused by preferential erosion of carbon. In addition, ITER relevant thermal shock loads were applied with an energetic electron beam at the JUDITH facility. The results demonstrated a significantly improved performance of Ti-doped graphite compared to pure graphite. Finally, Ti-doped graphite was successfully brazed to a CuCrZr block using a Mo interlayer. These results let assume that Ti-doped graphite can be a promising armour material for divertor plasma-facing components.

W fibre-reinforced Cu shows great promise to improve the mechanical performance at high-temperatures compared to conventional Cu-based alloys. Focus was placed on the optimization of the interface to achieve an enhanced adhesion between W fibre and Cu matrix. The interfacial properties were investigated through pull-out measurements of single matrix-coated fibres for different interfacial concepts. The interfacial adhesion of W and Cu is determined solely through mechanical interlocking. Interdiffusion and segregations experiment showed that there are no interface reactions between W and Cu at elevated temperatures. From the investigated interfacial concepts, a stepwise graded transition interface with additional heat treatment was found to achieve the highest interfacial shear strength. The thermal stability of the MMC in thermal cycling tests can be assured by depositing a stepwise graded transition between W fibre and Cu matrix.

Amorphous carbon layers doped with 8.5 at.% titanium (a-C:Ti) were produced by dual-source magnetron sputter deposition. To investigate the local atomic environment of Ti, EXAFS analysis was performed for as-deposited layers and after annealing to 700, 1100 and 1300 K, as well as for Ti and TiC bulk samples. After deposition of the a-C:Ti films, most Ti atoms are homogeneously distributed in an amorphous carbon matrix. Annealing to 700 K slightly increases the order in the Ti environment, but TiC-like crystallization is hardly existent. After annealing to 1100 K, clear correspondence to the TiC standard is observed. Annealing to 1300 K further increases the order and TiC crystallite size. Fitting of EXAFS data to a theoretical model was used to derive quantitative parameters for a-C:Ti layers, e.g., Ti–C and Ti–Ti distances and ordering parameters.

Carbon layers doped with Ti, V, W, and Zr to about 15 at.% were produced by means of magnetron sputter deposition, which distributes metal atoms homogeneously in the amorphous carbon matrix. The effects of thermal annealing to temperatures of 500–1300 K on the phase, crystallinity and distribution of the dopant were determined. X-ray diffraction (XRD), MeV ion beam analysis (IBA), and extended X-ray absorption fine-structure spectroscopy (EXAFS) were used for characterization. The three techniques deliver complementary information on different length scales of the diffusion and crystallization. All four metal dopants are in carbidic state with crystallites on the nanometer scale after annealing to 1100 K. Dopant diffusion of no more than 20 nm, even after heating at 1300 K for 2 h, was observed.

Carbon films doped with Ti, V, W, Zr, Cr, and Cu were produced by magnetron sputtering. To predict the film composition, the deposition rates were systematically studied as a function of discharge power and working pressure. The achieved dopant concentrations range from 20 down to 1 at.%. The films are laterally homogeneously doped and show columnar growth. The dopant distribution is not thermally stable. After heating at 1100 K, the carbides TiC, VC, WC, ZrC, and Cr 3 C 2 are definitely present and their grain size is on the nanometre scale. Cu segregates out. There are strong indications of the formation of carbides already during deposition.

The oxidative erosion of seven types of graphite has been investigated by heating in air at temperatures between 600 and 1000 K. The specimens include pyrolytic graphite, fine-grain graphites, carbon-fibre composites (CFC), and graphites doped with Si and Ti. The weight loss was measured using a microbalance, the surface morphology by scanning electron microscopy, and the composition of the surface layer by MeV ion beam techniques. Pyrolytic graphite is least affected by erosion, while pure and Si-doped CFCs erode particularly fast. Typical erosion rates for specimens with a surface area of ≃4 cm 2 are below 0.2 μg/m 2 s at 600 K for all graphite types, and at 900 K range from 0.34 mg/m 2 s for pyrolytic graphite to about 9 mg/m 2 s for the strongest eroding types. The temperature dependence of the erosion rate of all types of graphite studied is well described by an activation energy of 1.7 eV. The erosion rates of these graphites are by far lower than the removal rates for deposited amorphous hydrocarbon layers. In contrast to all other types, the Ti-doped graphite absorbs a significant amount of oxygen reaching up to ≃5% of its original mass. Once the oxygen uptake is saturated, it erodes with rates similar to those of the strongest eroding types.

Magnetron-sputtered layers consisting of carbon and metal (W, Ti, V) were produced with 0–20 at.% metal concentration. The mixed layers were characterised by RBS, SEM, XRD and XPS, and exposed to D 3 + ions of 30 eV/D at temperatures between 77 and 1100 K. The chemical erosion yield was investigated by mass spectrometry and RBS. Above RT (∼300 K), the CD4 production yield for pure C layers exhibits a maximum around 750 K, which decreases with increasing metal concentration. For more than ∼3 at.% W, ∼6 at.% V and ∼7 at.% Ti, the maximum vanishes and the CD4 yield continuously diminishes with temperature. A decrease of the activation energy for ion-induced hydrogen release by adding dopants is responsible for the decrease. The CD4 yield at RT increases depending on metal and concentration, because the distribution of the erosion products is changed. Enrichment of metal on the surface with ion fluence is observed.

Numerous traces of arcs have been found on W-covered graphite tiles of ASDEX Upgrade after exposure. The distributions of number density, lengths and orientation are calculated and compared to pure graphite tiles at comparable locations. It was established that arcs perforate a 1 μm tungsten layer down to the carbon substrate. The amount of removal should rise with arc current, but a surface fraction of about 8% is eroded at 10 A already. At tiles of the divertor baffle the layer is continuously removed along the entire track pointing to higher currents. The carbon of the stripped parts is subject to subsequent erosion processes. The distribution of materials in and around arc tracks was investigated by sputter depth profiling (SIMS and AES) and the characteristic geometry was studied using an electron microscope. Observations are interpreted using results from laboratory vacuum arcs on the same material.

The process development and characterization of titanium nitride (TiN) as a diffusion barrier for silver (Ag) metallizations were discussed. The structure of Ag/TiN/SiO"2/Si and Ag/TiN/Si metallizations under N"2/H"2 thermal annealing at 300-650 ^oC were investigated with sheet resistance measurement, X-ray diffraction (XRD), focused ion beam-scanning electron microscopy (FIB-SEM), atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS) and compared to as-deposited multilayers. For 12 nm TiN barrier, no change of sheet resistance was observed after annealing at 600 ^oC, but an abrupt rise appeared at 650 ^oC annealing. FIB-SEM revealed that this is due to the surface morphology change after annealing of the samples at different temperatures and the destruction of the TiN film.

To minimize radiation losses of the plasma during long pulse operation, the first wall protection of WENDELSTEIN 7-X demands low-Z plasma facing materials. In addition to carbon materials on high heat flux loaded components, 300–500 μm thick vacuum plasma-sprayed (VPS) layers of boron carbide (B4C) are considered as coatings on large actively cooled stainless steel panels. In order to evaluate the behaviour relevant to the expected plasma wall interactions, an extensive material characterization and test programme was executed. Also included is the examination of the industrial manufacturing and the suitability of these coatings as 70 m2 first wall plasma facing material for W7-X. To improve the adhesion of thick B4C layers on stainless steel, different interlayers have been investigated. Based on the results, a VPS-based coating technique was identified which is suitable to manufacture the B4C protection layers on stainless steel wall panels of W7-X.

Surface roughening of polycrystalline stainless steel mirrors due to hydrogen bombardment was studied as a function of ion fluence and energy (1–4.3 × 10 24 H/m 2 , 300–1500 eV/H). A strong micro-relief (100 nm scale) on single grains and a strong variation of the erosion depth between different grains were observed by scanning electron microscopy (SEM). A broad variation of the in-grain micro-relief was found to be independent of impact energy. The sputtering yield (Y) depends on the grain orientation, varying by a factor of about 2 for all impact energies. The correlation between surface morphology and orientation of the single grains was investigated using electron back-scattering diffraction (EBSD). Grains with nearly (1 1 1) surface orientation do not show any significant in-grain micro-relief, although they have nearly the highest Y. For other orientations close to low indexed surfaces, e.g., (3 1 1), no correlation was found between Y and roughness.

Using a filtered vacuum arc deposition device, stoichiometric aluminum oxide (Al2O3) films, with thickness ranging from 20 nm to several microns, were produced under various substrate bias voltages and temperatures. Analysis of the resulting alumina crystal structures was performed with transmission electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction. Depending on the negative substrate bias voltage, the deposition temperature required to form α-Al2O3 could be reduced. A crystal phase diagram showing the effect of bias and temperature is presented. Also, preliminary hydrogen permeation measurements of these coatings deposited on thin palladium foil show a good barrier performance as compared with uncoated samples.

Plasma sprayed B4C-layers considered as wall coatings for the W7X stellarator have been studied during and after exposure to TEXTOR and after arcing experiments in vacuum. Arcing through the B4C layer occurred favoured by high power fluxes and not restricted to less stable phases. But this arcing implies an especially noisy scrape-off layer (SOL). Instead of moving retrograde in the external magnetic field, the arc spot on the B4C-layer sticks to the same location for its whole lifetime. Consequently, the arc erodes the entire B4C layer, finally burning down to the Cu substrate. In the neighbourhood of craters the surface contains Cu originating from those craters. This material, hauled to the surface by the arc, is subject to subsequent erosion, transport, and redeposition by the SOL-plasma. The behaviour of arcs on B4C is most probably caused by the peculiar temperature dependences of the electrical and heat conductivity of B4C. 2003 Elsevier Science B.V. All rights reserved.

In ASDEX Upgrade, tungsten was employed as plasma facing material at the central column heat shield in the plasma main chamber. The campaign averaged tungsten erosion flux was determined by measuring the difference of the W-layer thickness before and after the experimental campaign using ion beam analysis methods. The observed lateral variation and the total amount of eroded tungsten are attributed to erosion by impact of ions from the scrape-off layer plasma. Migration and redeposition of eroded tungsten were investigated by quantitative analysis of deposited tungsten on collector probes and wall samples. The obtained results, as well as the spectroscopically observed low tungsten plasma penetration probability, indicate that a major fraction of the eroded tungsten migrates predominantly through direct transport channels in the outer plasma scrape-off layer without entering the confined plasma.

The possibilities for optimization of doped fine grain graphites with high thermal conductivity and high thermal shock resistance are demonstrated at laboratory scale. A mixture of MCMB powder and different carbides (B4C, TiC, VC, ZrC and WC) was used as starting material. VC acts as catalyst of the graphitization at the lowest temperature, and ZrC is the most effective catalyst of all investigated carbides. A direct proportionality between the mean crystallite height, Lc, and the thermal conductivity at room temperature was found for all materials except for the B4C- and the ZrC-doped graphites. With increasing graphitization temperature the open porosity of all doped materials becomes gradually closed, suggesting the existence of a diffusion mechanism responsible for both the catalytic effect and the closing of the open porosity. The addition of carbides does not strongly influence the mechanical properties of pure graphite. A high ratio flexural strength to Young’s modulus was achieved.

In the tokamak experiment ASDEX Upgrade, the investigation of tungsten as a first wall material is an ongoing research project. In a step-by-step strategy, the tungsten covered surface area is increased from campaign to campaign. For this purpose an industrial-scale method for coating graphite with micrometer tungsten films had to be identified. Test coatings deposited by magnetron sputtering and by plasma-arc deposition were compared. By X-ray analysis it was found that sputter-deposited coatings suffer from high compressive stress (1.7 GPa). This leads to delamination when a film thickness of about 3 μm is exceeded. For arc-deposited coatings, a compressive stress value of 0.5 GPa was determined and no delamination occurred up to the maximum film thicknesses investigated, i.e. 10 μm. Upon thermal loading, none of the arc-deposited coatings failed up to the melting condition, while one sputter-coating delaminated. First results on similar investigations employing CFC substrates are presented.

We performed investigations to determine the lifetime of tungsten coatings on carbon under high heat flux with respect to tungsten carbide formation. Tungsten coatings with a molybdenum interlayer were deposited by combined magnetron sputtering and ion implantation. These coatings were heat treated in a vacuum furnace. Subsequently the fraction of the coating thickness which had converted into carbide, was determined by focussed ion beam preparation and electron microscopy. This thickness was found to extend from 10 μm to 18 μm for heat treatments between 2 and 20 h at 1350 °C. Such coatings were then exposed to high heat flux pulses of 16.5 MW/m 2 with pulse durations of 1.5 s. A clear threshold behaviour was found: While after 2 h of heat treatment no coating failure was found, the delaminated surface fraction was up to 5% after treatment for 5 h and up to 25% after treatment for 20 h at 1350 °C.

Doping of carbon leads to a reduction of chemical erosion. This reduction is more effective if dopants are in the form of sub-μm precipitates with a very homogeneous distribution. However, dopants should not degrade but rather improve the thermal conductivity of graphite. Taking into account the catalytic effect of some metals and carbides on the graphitization, the potential for a development of improved doped carbon materials with reduced chemical erosion and optimized thermomechanical properties becomes evident. First results on the development of doped carbon materials starting from mesophase carbon powder and with different fine grain carbides as dopants show that VC acts as an effective catalyst for the graphitization. The materials obtained show high densities, a relatively low open porosity and good mechanical properties.

Many plasma-facing components in today's experiments are inertially cooled. Graphite tiles are mounted on a water-cooled steel plate using a spring and bolt mechanism. When exposed to high heat flux, such a shield can take loads only for a few seconds without becoming overheated. To operate in high intensity long pulsed/steady-state conditions, new actively cooled heat shields of brazed and non-brazed types are under development. With clamped (form-locked) monoblock tiles a prototype of non-brazed heat shield structure underwent its first high heat flux tests for divertor applications in the Julich MARION ion source [1]. The test results are encouraging, and validated a finite element model as discussed in this paper.

The composition of co-deposited hydrocarbon-silicon layers (a-C:Si:D) with varying Si concentrations and their removal by heating in air were investigated using MeV ion beam techniques. The amount of trapped D per re-deposited target atom depends weakly on the Si concentration. For pure C and Si, the D concentrations are about 0.45 and 0.5 D atoms per re-deposited target atom at room temperature, respectively. A maximum of about 0.7 D/(Si+C) was found at Si/C≈1. For increasing deposition temperature the D concentration does not decrease significantly until about 600 K. At about 1000 K the D concentration for pure C layers is still about 30% of the concentration at room temperature. The removal rates of D and C by heating in air increase strongly at temperatures around 550 K for a-C:D layers. With increasing Si content, these temperatures rise to above 650 K for layers with Si concentrations higher than 0.2 Si/(Si+C). The C removal rate is always lower than the D removal rate. Si is not removed by this method. For comparison, the composition of co-deposited stainless steel layers and Ti–C mixtures were investigated.

Tungsten coatings on fine grain graphite with film thicknesses in the micrometer range were prepared by three different methods: electron beam evaporation, magnetron sputtering, and arc deposition. They were compared with respect to properties relevant for the application of such systems in the nuclear fusion experiment ASDEX Upgrade. The content of light impurities was determined by sputter XPS. A deposition parameter study for magnetron sputtered coatings is presented; the formation of compressive stress is identified to be the major drawback of this method. Furthermore, experiments with thermal loading by an electron beam facility are presented.

Several carbide-doped (SiC, TiC, V 8 C 7 , WC, ZrC) graphites have been produced. The erosion of these materials at low-energy (eV) hydrogen ion bombardment has been investigated using the weight-loss method, mass spectroscopy, ion beam analysis, and scanning electron microscopy (SEM). The erosion yields of the WC- and V 8 C 7 -doped graphites are reduced by a factor of 2 for 30 eV D at 300 K compared to pure graphite. This observed reduction is partly attributed to surface enrichment of carbide due to preferential C erosion. The other part is assigned to changes in the chemical erosion process (Y surf ) as well as at elevated temperatures in the thermal activated process (Y therm ). The reduction of both erosion processes is determined for all dopants to be more than 25% of the erosion yield of the undoped graphite.

Doping of carbon generally results in a reduction of its chemical reactivity during hydrogen ion bombardment. Due to preferential sputtering of carbon, the dopants may be enriched at the surface, resulting in an additional reduction of the erosion yield. Dopants in the form of sub-μm precipitates with a very homogeneous distribution lead to in a more effective reduction of both chemical erosion processes, Ytherm and Ysurf. However, dopants may degrade the thermal conductivity of graphite, which has to be avoided. First results on the development of carbon materials doped with different carbides and with optimized microstructure and thermomechanical properties show that VC acts as an effective catalyst for graphitization causing an improvement of the thermal conductivity. It leads further to a reduction of both, the Ytherm and the Ysurf chemical erosion processes, which is partly attributed to surface enrichment but is also the result of a chemical influence.