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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.

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.

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.

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.

Fe–W binary mixed layers were prepared as a model system for reduced-activation ferritic–martensitic (RAFM) steel for studying their dynamic erosion behavior resulting from energetic deuterium (D) irradiation. This investigation aims toward an assessment of RAFM steels as plasma-facing material. The surface composition of the model layers is modified by D irradiation. W is enriched at the surface with D irradiation fluence due to the preferential sputtering of Fe. It depends on the D impinging energy as well as the initial W fraction of the Fe–W layer. No significant development of surface topography was observed within the examined conditions. The erosion yield of a Fe–W layer is comparable to that of pure Fe in the low-fluence range and decreases with increasing D fluence. These results indicate that the dynamic change of erosion yield is significantly correlated with the surface W enrichment.

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.

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.

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.

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.

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.

The erosion yield of SiC due to D bombardment has been determined by the weight-loss method as a function of temperature and energy in the range of 20–300 eV up to 1100 K. A temperature dependence exists clearly below 100 eV. For 20 eV D, the yield is 0.5% at 300 K and 1.5% in the maximum at 500 K. Contrary to Si erosion, no formation of silane was observed with mass spectrometry. Hydrocarbons, predominantly CD 4 molecules, are only found at 20 eV. From the erosion of a thin SiC layer investigated using MeV ion beam analysis, segregation and preferential erosion of carbon are ruled out, because the layer thickness decreases while the composition stays constant. As no volatile silane production is detected, it is assumed that sputtering of Si or silane precursors with low binding energy occurs at low ion energies. The mechanism of the chemical erosion of Si in presence of C and absence of C is different.

The composition of co-deposited carbon–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 ion-induced release of D due to the analysing beam (1.2 MeV 3He) was determined. 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 larger 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. The observed properties of the layers are compared with those of hard and soft a-C:D films.

Erosion yields of Si, C and SiC due to D ion impact were determined between 20 and 300 eV up to 1100 K. A temperature dependence of the erosion yield has been observed for Si and C. The temperature of the maximum increases with ion energy from about 350 K for 20 eV to 570 K above 100 eV for Si, which is always about 250 K lower than for graphite. The erosion yields of Si are in general 10 times smaller than those of graphite. Chemical erosion species were observed mass-spectroscopically: silane was found for Si, but not for SiC, where only hydrocarbons were observed.

Total erosion yields of graphite and carbon materials under hydrogen and deuterium bombardment measured with the weight-loss method are presented for ion energies between 15 eV and 8 keV in the temperature range 300 to above 1000 K. The temperature of the maximum of the chemical erosion increases from below 600 to above 850 K with ion energies from 15 to 300 eV. Chemical erosion yields obtained by weight-loss measurements exceed yields measured mass-spectrometrically always by a factor of about two. Collector experiments show that a fraction of the eroded particle sticks to walls and, therefore, reduces the yield measured by mass spectrometry. A synergistic effect of neutrals in the ion beam on the chemical erosion yield can be excluded.

The co-deposition of deuterium with silicon doped carbon for silicon concentrations between 0–100 at.% in the temperature range from room temperature to 1000 K has been investigated. The eroded material from various different targets was caught on collectors together with the reflected D to build up the co-deposited layers, which were analysed with MeV ion beam techniques. The amount of trapped D per re-deposited target atom depends weakly on the Si concentration. The maximum of about 0.7 D/(Si + C) was found at Si/C ≈ 1. For pure C and pure Si the D concentration is about 0.45 and 0.5 D atoms per re-deposited target atom at room temperature, respectively. For increasing deposition temperature the D concentration does not decrease significantly until about 600 K. At about 1000 K the D concentration for pure carbon layers is still about 30% of the concentration at room temperature. Also, co-deposited layers of stainless steel and of titanium–carbon mixtures were investigated.

The total hydrogen isotope inventories in the tungsten coated divertor tiles of ASDEX-Upgrade were measured by thermal desorption spectroscopy (TDS) and - in the near surface region - by nuclear reaction analysis (NRA). The highest amounts of deuterium (≤ 5 x 10 22 D/m 2 ) are measured in the inner divertor region outside the separatrix due to the formation of a C:H layer on the tungsten by codeposition of deuterium with carbon. The inventories in the outer divertor - where erosion dominates - are generally smaller by an order of magnitude.

The thermal stability of the 8–10 at.% Si doped 3D CFC material NS31 was investigated using scanning electron microscopy (SEM) and ion beam analysis. Total erosion yields and CD4 production yields were determined by bombarding with monoenergetic deuterium ions. During heating to 1800 K SiC crystallites of a few μm size are created on the surface. A Si depletion at the surface and an enrichment layer (20–50 at.% Si) beneath the surface of about 5–10 μm thickness were observed. They are explained by the distribution of the crystallites of μm size and the evaporation of Si from the surface. After heating a NS31 mock-up tile to temperatures above 2300 K in a power load test, the Si concentration in a surface layer of more than 5 μm thickness drops below 3 at.% Si. The chemical erosion is reduced for the initial and heated samples by a factor of 2–3 compared to pure graphite. NS31 shows sputtering yields between those of graphite and silicon carbide depending on the Si concentration of the individual surface.

Tungsten coated graphite tiles were mounted in the divertor of the ASDEX Upgrade tokamak and exposed to approximately 800 plasma discharges with varying average heat fluxes of up to 6 MW/m 2 . After the experimental campaign, cracks were detected on almost all tiles in accordance with results of electron beam tests. The W-coated tiles in the inner divertor were found to be covered by deposited layers of low-Z material with thicknesses in the μm range and only a few percent tungsten remaining on the surface. In contrast the outer divertor is dominated by erosion. Here, the tungsten concentration at the surface was found to be close to the initial level. The global W-migration pattern was determined by analyzing the deposition of tungsten on a complete poloidal set of plasma facing wall components. Both divertor regions display broad deposition peaks with deposited layers at least one order of magnitude thicker than the main chamber values.

The retention of 1 keV and 8 keV deuterium implanted in B, Si, and Ti doped graphites and in the carbides B 4 C, SiC and TiC up to 6.7 × 10 19 D atoms/cm 2 has been studied at room temperature with thermal desorption spectroscopy (TDS) and ion beam analysis. Compared to pyrolytic graphite all doped graphites showed a tendency to increased retention of deuterium, especially the materials with high porosity. For the carbides the saturation concentration in the implantation zone in B 4 C is about 0.4 D per target atom and therefore comparable to the saturation concentration of D in graphite. SiC exhibits a significant higher saturation concentration of 0.6 D per target atom, whereas TiC shows a significant lower saturation concentration of 0.15 D per target atom.

Tungsten heavy alloy (97 wt% W, 2 wt% Ni, 1 wt% Fe) was investigated as an alternative for tungsten (W) as plasma facing material. It is produced commercially by several companies and compared to bulk W it is readily machinable and considerably cheaper. In order to qualify the material for use in the divertor of the mid-size tokamak ASDEX Upgrade (AUG) dedicated laboratory investigations as well as high heat flux tests in the neutral beam facility GLADIS were performed. These investigations revealed that the thermal conductivity at high temperature is close to that of W, the magnetisation is small and saturates already at low magnetic field and the hydrogen retention is similarly low as that of W. In high heat flux tests at power densities up to 20 MWm−2 no failure was observed up to the melting temperature ( ≈ 1500 ∘ C) of the binder phase. Even at surface temperatures of up to 2200 °C the mechanical integrity was sustained. Mechanical tests confirm the ductile behaviour of the W heavy alloy at room temperature and finite element analyses using the aforementioned data suggest a lower tendency for cracking. The increase of the long term dose-rate resulting from the activation of Ni under neutron irradiations appears to be moderate. During the 2017 campaign more than one fifth of the AUG divertor tiles consisted of W heavy alloy. Under nominal operation conditions the tiles showed no macroscopic failure and no increased Fe/Ni influx into the plasma was detected. Even though a few tiles showed strong melting at the edges due to accidental misalignment no failure due to cracking was observed.

The blistering and near-surface deuterium retention of a Y2O3-doped tungsten (W) and two different pure W grades were studied after exposure to deuterium (D) plasma at elevated temperatures (370, 450 and 570 K). Samples were exposed to a deuterium fluence of 6 × 1024 D m−2 applying a moderate ion flux of about 9 × 1019 D m−2 s−1 at an ion energy of 38 eV/D. Morphological modifications at the surface were analyzed by confocal laser scanning microscopy and scanning electron microscopy. The D depth profiles and the accumulated D inventories within the topmost 8 μm were determined by nuclear reaction analysis. Blistering and deuterium retention were strongly dependent on the implantation temperature. In addition, blistering was sensitively influenced by the used tungsten grade, although the total amount of retained D measured by nuclear reaction analysis was comparable. Among the three different investigated tungsten grades, Y2O3-doped W exhibited the lowest degree of surface modification despite a comparable total D retention.