Search

The cross section for the combined 2D(15N,p)16N and 2D(15N,nγ)16O nuclear reactions of deuterium (2D) with incident 15N ions is evaluated and found to increase by two orders of magnitude between 3.3 and 7.0 MeV. Detecting the γ-rays at 6.1 and 7.1 MeV from these reactions allows quantifying the depth-integrated 2D content in materials in parallel with nanometer-resolved quantitative hydrogen (1H) depth profiling in surface layers through resonant 1H(15N,αγ)12C nuclear reaction analysis (NRA) using 15N ion beams. The information depth and sensitivity of 15N-2D NRA is estimated through energy loss simulations of the 15N projectiles in the analyzed material. Good agreement of the integral 2D quantitation through 2D(15N,p)16N and 2D(15N,nγ)16O NRA with 2D depth profiles obtained independently via 2D(3He,p)4He NRA is demonstrated at the example of 2D plasma-exposed tungsten samples. The simultaneous quantitation of 2D and depth-resolved 1H profiling with a single 15N ion beam promises application potential to investigate hydrogen-isotope exchange and diffusion processes in surface and thin buried layers.

We performed a systematic study of the deuterium retention in the tungsten heavy alloy W-2Ni-1Fe (HPM 1850 from HC Starck, Germany). Consisting of two different phases with generally two distinctly different heats of solution, such alloys represent a special class of materials with respect to hydrogen isotope transport and retention. Deuterium was implanted from a plasma with an energy of 38 eV D−1 at sample temperatures of 100 °C, 175 °C, and 250 °C in the fluence range 1023 D m−2 to 4 × 1025 D m−2. For each combination of implantation temperature and fluence, two alloy samples were exposed together with one sample of bulk hot-rolled tungsten (from Plansee SE). The amount of retained deuterium was determined by thermal desorption with heating up to 850 °C for the alloy and 1050 °C for the bulk tungsten. The outgassed deuterium was detected by mass spectroscopy at the mass channels 3, 4, 19, and 20 corresponding to HD, D2, HDO, and D2O, respectively. The near-surface deuterium concentration was determined by 3He nuclear reaction analysis. The retention in the alloy as compared to the bulk tungsten decreases systematically with increasing implantation fluence for each of the investigated implantation temperatures. For the higher fluences the ratio decreases systematically also, as a function of implantation temperature. This means that especially for high implantation fluences and temperatures the alloy HPM 1850 systematically retains less hydrogen isotopes than bulk tungsten. This property represents an important qualification criterion as a plasma-facing material for a future fusion reactor.

An in situ hydrogen (H) plasma charging and in situ observation method was developed to continuously charge materials, while tensile testing them inside a scanning electron microscope (SEM). The present work will introduce and validate the setup and showcase an application allowing high-resolution observation of H-material interactions in a Ni-based alloy, Alloy 718. The effect of charging time and pre-straining was investigated. Fracture surface observation showed the expected ductile microvoid coalescence behavior in the uncharged samples, while the charged ones displayed brittle intergranular and quasi-cleavage failure. With the in situ images, it was possible to monitor the sample deformation and correlate the different crack propagation rates with the load-elongation curves. H-charging reduced the material ductility, while increasing pre-strain decreased hydrogen embrittlement susceptibility due to the possible suppression of mechanical twinning during the tensile test and, therefore, a reduction in H concentration at grain and twin boundaries. All the presented results demonstrated the validity of the method and the possibility of in situ continuously charging of materials with H without presenting any technical risk for the SEM.

Low-energy hydrogen-isotope (HI) plasma is widely applied for industrial surface processing. Here we demonstrate that HI plasma - even with ion energies far below the threshold for stable Frenkel pair production - can strongly modify crystalline materials by forming heavily lattice-distorted surface layers with a thickness of several nanometers and a HI content of several atomic percent. We experimentally reproduce the identical lattice modification at deuterium and hydrogen plasma-irradiated tungsten (W) surfaces when the sub-threshold HI ion energies are adjusted such as to transfer equal amounts of kinetic energy in collisions with W lattice atoms. A physical model for the low-energy generation of primary defects is proposed, which involves the synergy between temporary Frenkel pair creation by sub-threshold HI ion collisions and vacancy stabilization by trapping of solute HI atoms. Such synergistic defect generation is generally expected upon injection of energetic projectiles (ions, neutrons) into HI-containing solids and likely contributes to material degradation. As a means of surface modification, the fabrication of nanometer-scale hydrogen-rich surface layers on materials (even those with negligible hydrogen solubility) via low-energy H plasma exposure promises utilization potential in catalysis and electrochemistry.

The exposure of metal surfaces to energetic particles originating for example from a plasma can lead to the formation of blisters. These features are caused by hydrogen-filled gas bubbles near the surface and can influence the diffusion and retention of hydrogen atoms in the bulk material during plasma exposure. No comprehensive theory of the underlying processes governing these effects has been developed so far. A major constraint to the investigation of blisters has been the effort that is required to identify and characterize a statistically relevant ensemble of blisters. Therefore, the analysis of a limited number of blisters in isolated observations before and after plasma exposure has been state of the art so far. In this article, a framework is presented that allows to automatically analyze blisters in image data with the aid of convolutional neural networks (CNNs) which are trained with artificial training data. It thereby resolves the limitations of existing analysis methods that prevented the large-scale and in-situ investigation of blistering up to now. Automated routines are used to identify and localize blisters, estimate important blister parameters such as size, and track individual blisters over time. Apart from discussing the principles underlying this approach and describing the implementation of the framework in detail, this article also presents first results acquired with a molybdenum sample that was exposed to a cold deuterium plasma. The results indicate a blister identification accuracy of the framework of more than 80% within the considered parameter ranges and overall demonstrate the feasibility of the approach in general.

The tungsten “heavy alloy” HPM 1850, a liquid-phase sintered composite material with two weight percent Ni and one weight percent Fe, as well as the self-passivating tungsten alloy W-10Cr-0.5Y, a high temperature oxidation resistant alloy with 10 weight percent of Cr and 0.5 weight percent of Y, were investigated with respect to their deuterium retention. The samples were deuterium loaded in an electron cyclotron resonance plasma up to a fluence of 1025m−2. The deuterium retention was then investigated by Nuclear Reaction Analysis and by Thermal Desorption. In HPM 1850 the observed deuterium amount was similar to pure tungsten, however the outgassing behaviour during thermal desorption was considerably faster. In W-10Cr-0.5Y the released deuterium amount during thermal desorption was about one order of magnitude higher; by comparison of nuclear reaction analysis and thermal desorption this was attributed to deeper diffusion of deuterium into the bulk of the material. Keywords: Deuterium retention, Tungsten heavy alloy, Tungsten self-passivating alloy, Nuclear reaction analysis, Thermal desorption

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.

By irradiating a tungsten sample for transmission electron microscopy with deuterium plasma at 230 K, we created a high density of blisters below the plasma-exposed surface. Blisters were even found in the electron-transparent region close to the central perforation. We determined the abundance of blisters depending on the local sample thickness and found that no blisters were formed for thicknesses below about 45 nm for these specific plasma exposure conditions. After removing the approximately 10 nm thick, defect-rich layer at the plasma-exposed surface, which corresponds roughly to the ion implantation zone, by electrochemical polishing, we were able to clearly identify material distortions and dislocation network halos corresponding to blisters by scanning transmission electron microscopy. Compared with unirradiated tungsten of the same grade, we found that in the blister zone the average dislocation density is about 2 orders of magnitude higher, which may, in addition to gas enclosed in blister cavities, explain enhanced deuterium retention in blistered tungsten samples. The proof-of-principle experiments described in this article have the potential to provide constraints for theoretical models for blister nucleation. They also pave the way for direct investigations of the spatial correlation of deuterium-plasma-induced blisters and intrinsic defects. Keywords: Tungsten, Deuterium, Plasma exposure, Scanning transmission electron microscopy, Blister nucleation conditions, Defect creation

Pure and Y2O3-doped tungsten samples were simultaneously exposed to deuterium (D) plasma. The following exposure parameters were investigated: ion energy of 38 eV/D, sample temperatures of 370, 450 and 570 K, and incident fluences of 6 × 1024 and 7.5 × 1025 D/m2. The deuterium retention behavior of these tungsten materials was investigated by nuclear reaction analysis and thermal desorption spectroscopy. After exposure at 450 K Y2O3-doped tungsten shows a D depth profile comparable with that of pure tungsten. But at each investigated exposure condition Y2O3-doped tungsten shows a higher total D inventory than pure tungsten. This is attributed to the presence of intrinsic defects with high trapping energies. Investigation of the D retention behavior of four different Y2O3-doped tungsten materials, which contain the same amount of Y2O3, but have various microstructures, suggests that sub-micron pores are the underlying intrinsic defects responsible for the appearance of a high temperature D release peak. Keywords: Tungsten, Tungsten alloys, Deuterium retention, Plasma-surface interaction, Morphology, Thermal desorption spectroscopy

The ASDEX Upgrade (AUG) programme, jointly run with the EUROfusion MST1 task force, continues to significantly enhance the physics base of ITER and DEMO. Here, the full tungsten wall is a key asset for extrapolating to future devices. The high overall heating power, flexible heating mix and comprehensive diagnostic set allows studies ranging from mimicking the scrape-off-layer and divertor conditions of ITER and DEMO at high density to fully non-inductive operation (q95 = 5.5, ) at low density. Higher installed electron cyclotron resonance heating power 6 MW, new diagnostics and improved analysis techniques have further enhanced the capabilities of AUG. Stable high-density H-modes with MW m−1 with fully detached strike-points have been demonstrated. The ballooning instability close to the separatrix has been identified as a potential cause leading to the H-mode density limit and is also found to play an important role for the access to small edge-localized modes (ELMs). Density limit disruptions have been successfully avoided using a path-oriented approach to disruption handling and progress has been made in understanding the dissipation and avoidance of runaway electron beams. ELM suppression with resonant magnetic perturbations is now routinely achieved reaching transiently . This gives new insight into the field penetration physics, in particular with respect to plasma flows. Modelling agrees well with plasma response measurements and a helically localised ballooning structure observed prior to the ELM is evidence for the changed edge stability due to the magnetic perturbations. The impact of 3D perturbations on heat load patterns and fast-ion losses have been further elaborated. Progress has also been made in understanding the ELM cycle itself. Here, new fast measurements of and Er allow for inter ELM transport analysis confirming that Er is dominated by the diamagnetic term even for fast timescales. New analysis techniques allow detailed comparison of the ELM crash and are in good agreement with nonlinear MHD modelling. The observation of accelerated ions during the ELM crash can be seen as evidence for the reconnection during the ELM. As type-I ELMs (even mitigated) are likely not a viable operational regime in DEMO studies of 'natural' no ELM regimes have been extended. Stable I-modes up to have been characterised using -feedback. Core physics has been advanced by more detailed characterisation of the turbulence with new measurements such as the eddy tilt angle—measured for the first time—or the cross-phase angle of and fluctuations. These new data put strong constraints on gyro-kinetic turbulence modelling. In addition, carefully executed studies in different main species (H, D and He) and with different heating mixes highlight the importance of the collisional energy exchange for interpreting energy confinement. A new regime with a hollow profile now gives access to regimes mimicking aspects of burning plasma conditions and lead to nonlinear interactions of energetic particle modes despite the sub-Alfvénic beam energy. This will help to validate the fast-ion codes for predicting ITER and DEMO. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 and 2019–2020 under grant agreement No. 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. "Article signat per més de 100 autors/es: H. Meyer, for the AUG Team: D. Aguiam, C. Angioni, C.G. Albert, N. Arden, R. Arredondo Parra, O. Asunta, M. de Baar, M. Balden, V. Bandaru, K. Behler, A. Bergmann, J. Bernardo, M. Bernert, A. Biancalani, R. Bilato, G. Birkenmeier, T.C. Blanken, V. Bobkov, A. Bock, T. Bolzonella, A. Bortolon, B. Böswirth, C. Bottereau, A. Bottino, H. van den Brand, S. Brezinsek, D. Brida, F. Brochard, C. Bruhn, J. Buchanan, A. Buhler, A. Burckhart, Y. Camenen, D. Carlton, M. Carr, D. Carralero, C. Castaldo, M. Cavedon, C. Cazzaniga, S. Ceccuzzi, C. Challis, A. Chankin, S. Chapman, C. Cianfarani, F. Clairet, S. Coda, R. Coelho, J.W. Coenen, L. Colas, G.D. Conway, S. Costea, D.P. Coster, T.B. Cote, A. Creely, G. Croci, G. Cseh, A. Czarnecka, I. Cziegler, O. D'Arcangelo, P. David, C. Day, R. Delogu, P. de Marné, S.S. Denk, P. Denner, M. Dibon, A. Di Siena, D. Douai, A. Drenik, R. Drube, M. Dunne, B.P. Duval, R. Dux, T. Eich, S. Elgeti, K. Engelhardt, B. Erdös, I. Erofeev, B. Esposito, E. Fable, M. Faitsch, U. Fantz, H. Faugel, I. Faust, F. Felici, J. Ferreira, S. Fietz, A. Figuereido, R. Fischer, O. Ford, L. Frassinetti, S. Freethy, M. Fröschle, G. Fuchert, J.C. Fuchs, H. Fünfgelder, K. Galazka, J. Galdon-Quiroga, A. Gallo, Y. Gao, S. Garavaglia, A. Garcia-Carrasco, M. Garcia-Muñoz, B. Geiger, L. Giannone, L. Gil, E. Giovannozzi, C. Gleason-González, S. Glöggler, M. Gobbin, T. Görler, I. Gomez Ortiz, J. Gonzalez Martin, T. Goodman, G. Gorini, D. Gradic, A. Gräter, G. Granucci, H. Greuner, M. Griener, M. Groth, A. Gude, S. Günter, L. Guimarais, G. Haas, A.H. Hakola, C. Ham, T. Happel, N. den Harder, G.F. Harrer, J. Harrison, V. Hauer, T. Hayward-Schneider, C.C. Hegna, B. Heinemann, S. Heinzel, T. Hellsten, S. Henderson, P. Hennequin, A. Herrmann, M.F. Heyn, E. Heyn, F. Hitzler, J. Hobirk, K. Höfler, M. Hölzl, T. Höschen, J.H. Holm, C. Hopf, W.A. Hornsby, L. Horvath, A. Houben, A. Huber, V. Igochine, T. Ilkei, I. Ivanova-Stanik, W. Jacob, A.S. Jacobsen, F. Janky, A. Jansen van Vuuren, A. Jardin, F. Jaulmes, F. Jenko, T. Jensen, E. Joffrin, C.-P. Käsemann, A. Kallenbach, S. Kálvin, M. Kantor, A. Kappatou, O. Kardaun, J. Karhunen, S. Kasilov,, Y. Kazakov, W. Kernbichler, A. Kirk, S. Kjer Hansen, V. Klevarova, G. Kocsis, A. Köhn, M. Koubiti, K. Krieger, A. Krivska, A. Krämer-Flecken, O. Kudlacek, T. Kurki-Suonio, B. Kurzan, B. Labit, K. Lackner, F. Laggner, P.T. Lang, P. Lauber, A. Lebschy, N. Leuthold, M. Li, O. Linder, B. Lipschultz, F. Liu, Y. Liu, A. Lohs, Z. Lu, T. Luda di Cortemiglia, N.C. Luhmann, R. Lunsford, T. Lunt, A. Lyssoivan, T. Maceina, J. Madsen, R. Maggiora, H. Maier, O. Maj, J. Mailloux, R. Maingi, E. Maljaars, P. Manas, A. Mancini, A. Manhard, M.-E. Manso, P. Mantica, M. Mantsinen, P. Manz, M. Maraschek, C. Martens, P. Martin, L. Marrelli, A. Martitsch, M. Mayer, D. Mazon, P.J. McCarthy, R. McDermott, H. Meister, A. Medvedeva, R. Merkel, A. Merle, V. Mertens, D. Meshcheriakov, O. Meyer, J. Miettunen, D. Milanesio, F. Mink, A. Mlynek, F. Monaco, C. Moon, F. Nabais, A. Nemes-Czopf, G. Neu, R. Neu, A.H. Nielsen, S.K. Nielsen, V. Nikolaeva, M. Nocente, J.-M. Noterdaeme, I. Novikau, S. Nowak, M. Oberkofler, M. Oberparleiter, R. Ochoukov, T. Odstrcil, J. Olsen, F. Orain, F. Palermo, O. Pan, G. Papp, I. Paradela Perez, A. Pau, G. Pautasso, F. Penzel, P. Petersson, J. Pinzón Acosta, P. Piovesan, C. Piron, R. Pitts, U. Plank, B. Plaum, B. Ploeckl, V. Plyusnin, G. Pokol, E. Poli, L. Porte, S. Potzel, D. Prisiazhniuk, T. Pütterich, M. Ramisch, J. Rasmussen, G.A. Rattá, S. Ratynskaia, G. Raupp, G.L. Ravera, D. Réfy, M. Reich, F. Reimold, D. Reiser, T. Ribeiro, J. Riesch, R. Riedl, D. Rittich, J.F. Rivero-Rodriguez, G. Rocchi, M. Rodriguez-Ramos, V. Rohde, A. Ross1, M. Rott, M. Rubel, D. Ryan, F. Ryter, S. Saarelma, M. Salewski, A. Salmi, L. Sanchis-Sanchez, J. Santos, O. Sauter, A. Scarabosio, G. Schall, K. Schmid, O. Schmitz, P.A. Schneider, R. Schrittwieser, M. Schubert, T. Schwarz-Selinger, J. Schweinzer, B. Scott, T. Semer, E. Seliunin, M. Sertoli, A. Shabbir, A. Shalpegin, L. Shao, S. Sharapov, G. Sias, M. Siccinio, B. Sieglin, A. Sigalov, A. Silva, C. Silva, D. Silvagni, P. Simon, J. Simpson, E. Smigelskis, A. Snicker, C. Sommariva, C. Sozzi, M. Spolaore, A. Stegmeir, M. Stejner, J. Stober, U. Stroth, E. Strumberger, G. Suarez, H.-J. Sun, W. Suttrop, E. Sytova, T. Szepesi, B. Tál, T. Tala, G. Tardini, M. Tardocchi, M. Teschke, D. Terranova, W. Tierens, E. Thorén, D. Told, P. Tolias, O. Tudisco, W. Treutterer, E. Trier, M. Tripský, M. Valisa, M. Valovic, B. Vanovac, D. van Vugt, S. Varoutis, G. Verdoolaege, N. Vianello, J. Vicente, T. Vierle, E. Viezzer, U. von Toussaint, D. Wagner, N. Wang, X. Wang, M. Weiland, A.E. White, S. Wiesen, M. Willensdorfer, B. Wiringer, M. Wischmeier, R. Wolf, E. Wolfrum, L. Xiang, Q. Yang, Z. Yang, Q. Yu, R. Zagórski, I. Zammuto, W. Zhang, M. van Zeeland, T. Zehetbauer, M. Zilker, S. Zoletnik, H. Zohm and the EUROfusion MST1 Team55

The effect of near-surface blisters on deuterium transport in tungsten is studied by means of nuclear reaction analysis (NRA) and scanning electron microscopy (SEM). Gentle deuterium plasma loading of different durations and subsequent NRA depth profiling is performed in heavily pre-blistered and unblistered areas on self-damaged tungsten samples. Comparison of the deuterium depth profiles reveals a considerable reduction of the deuterium transport into the bulk due to the presence of near-surface blisters. SEM and NRA results identify the enhanced re-emission of deuterium from the sample due to open blisters as the underlying mechanism, which reduces the deuterium flux into the bulk. Based on a simple analytical hydrogen retention model, the re-emitted deuterium flux by open blisters is determined to be 80% of the implanted deuterium flux in the present conducted experiment. In addition, the deuterium flux into the bulk is reduced by 60% compared to the unblistered case. Hence the presence of blisters is not a general disadvantage in the context of retention, but can be beneficial in slowing down the build up of a certain hydrogen inventory and in reducing the permeation flux.

In the Wendelstein 7-X stellarator with its twisted magnetic geometry the investigation of plasma wall interaction processes in 3D plasma configurations is an important research subject. For the upcoming operation phase i.e. OP1.2, three different types of material probes have been installed within the plasma vessel for the erosion/deposition investigations in selected areas with largely different expected heat load levels, namely, ≤10 MW m−2 at the test divertor units (TDU), ≤500 kW m−2 at the baffles, heat shields and toroidal closures and ≤100 kW m−2 at the stainless steel wall panels. These include 18 exchangeable target elements at TDU, about 30 000 screw heads at graphite tiles and 44 wafer probes on wall panels, coated with marker layers. The layer thicknesses, surface morphologies and the impurity contents were pre-characterized by different techniques and subjected to various qualification tests. The positions of these probes were fixed based on the strike line locations on the divertor predicted by field line diffusion and EMC3/EIRENE modeling calculations for the OP1.2 plasma configurations and availability of locations on panels in direct view of the plasma. After the first half of the operation phase i.e. OP1.2a the probes will be removed to determine the erosion/deposition pattern by post-mortem analysis and replaced by a new set for the second half of the operation phase, OP1.2b.

A method to measure deuterium permeation through tungsten near room temperature under plasma loading is presented. The permeating deuterium is accumulated in a getter layer of zirconium, titanium or erbium, respectively, on the unexposed side of the sample. Subsequently, the amount of deuterium in the getter is measured ex-situ using nuclear reaction analysis. A cover layer system on the getter prevents direct loading of the getter with deuterium from the gas phase during plasma loading. In addition, it enables the distinction of deuterium in the getter and at the cover surface. The method appears promising to add additional permeation measurement capabilities to deuterium retention experiments, also in other plasma devices, without the need for a complex in-situ permeation measurement setup. Keywords: Deuterium, Plasma, Permeation, Tungsten, Getter, Ion-beam

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.

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.

Chemical erosion of carbon can be described as function of the incident ion flux, the ion energy and the surface temperature. Spectroscopy on the hydrocarbon break-up products CH and C 2 is applied to determine in-situ hydrocarbon fluxes and quantify the chemical erosion by means of hydrocarbon injection. Present-day knowledge on critical issues concerning the erosion yield and its determination is presented: Effective inverse photon efficiencies are measured in TEXTOR for different hydrocarbon species and compared with calculations from HYDKIN. The underlying database for the methane break-up, used in different erosion/deposition models, has been put to test. The chemical erosion yield related to higher hydrocarbons is determined to be lower in the JET outer divertor than measured in previous experiments. In ASDEX Upgrade and DIII-D first in-situ calibrations of hydrocarbon fluxes in the detached outer divertor are performed.

The recovery of radiation defects in recrystallized tungsten was studied in the temperature range 400–1150 K. The defects were created by 20 MeV tungsten ions irradiation at room temperature to a maximum damage level of 0.9 dpa. The samples were then annealed to temperatures of 400–1150 K for 1 hour in order to anneal the created defects. After annealing the remaining defects were decorated with D by exposing them to a low-temperature ECR-plasma at 400 K to a fluence of 1 × 10 25 D/m 2 . Deuterium was detected by nuclear reaction analysis and temperature programmed desorption. Annealing of defects is observed already at slightly elevated temperatures. At 820 K about 50% of the initial defects were annealed, while still about 30% of the initial defects were present after annealing at 1150 K.

The paper presents the results of an experimental study of deuterium retention in W and W–Ta alloy that were exposed to first-wall relevant low flux (∼1020 m−2 s−1) deuterium plasma in the ECR plasma generator PlaQ. Subsequent analysis included surface imaging by optical microscopy, deuterium depth profiling by nuclear reaction analysis (NRA) and measurements of deuterium content by thermal desorption spectroscopy (TDS). It was found that under investigated exposure conditions the deuterium content was higher in W–Ta alloy than in W. Combined with the previously reported results showing that under high-flux (∼1024 m−2 s−1) retention is higher in W instead, this gives rise to a peculiar flux effect – dependence of relative retention between different materials on exposure flux. We interpret this effect as evidence that at different flux ranges different populations of trapping sites determine the retention, namely pre-existing microstructural traps at low-flux exposure and plasma-induced ones at high-flux exposure.

In order to provide a solid basis for the correlation of microstructure and hydrogen isotope retention in tungsten, reference samples with different microstructures were prepared from a single batch of polycrystalline tungsten by standardised polishing and heat treatment procedures. Representative samples were analysed by scanning electron microscopy and scanning transmission electron microscopy as well as by electron backscatter diffraction. We show that if the annealing temperature is increased from 1 200 to 1 500 K, practically only the density of dislocations and grain boundaries with very small misorientations of less than 2° is reduced, while for annealing at 1 700 and 2 000 K, also the density of high-angle grain boundaries is reduced due to grain growth. Furthermore, the dislocation density is reduced by nearly two orders of magnitude compared to tungsten annealed at 1 200 K. We also comment on two different textures on the front and rear side of the samples that were observed both by X-ray diffraction and EBSD.

The lifetime of plasma-facing components is critical for future magnetic confinement fusion power plants. A key process limiting the lifetime of the first-wall is sputtering by energetic ions. To provide a consistent modeling of the sputtering process of realistic geometries, the SDTrimSP-code has been extended to enable the processing of analytic as well as measured arbitrary 3D surface morphologies. The code has been applied to study the effect of varying the impact angle of ions on rough surfaces on the sputter yield as well as the influence of the aspect ratio of surface structures on the 2D distribution of the local sputtering yields. Depending on the surface morphologies reductions of the effective sputter yields to less than 25% have been observed in the simulation results.

Up to now, blister formation on rough or technical tungsten surfaces exposed to hydrogen isotope plasma was believed to be completely suppressed. The few dedicated experiments on this issue that can be found in literature appear to support that claim. Using a novel technique of 3D difference imaging of tungsten surfaces, we now demonstrate that roughness introduced by chemical etching, i.e. without the associated mechanical deformation layer introduced by grinding, only moderately reduces blistering. A technical surface with comparable roughness produced by precision grinding (R a ≤ 1.6 µm) led to a strong reduction in blister size and density, but blisters were found nevertheless. In this article we give a detailed description of the investigated rough W surfaces and present a statistical evaluation of blistering on these surfaces after exposure to a low-temperature deuterium plasma.

Blistering of tungsten (W) surfaces due to deuterium (D) implantation was investigated by a sequence of exposures at two different temperatures—230 and 450 K—and by the reversed sequence. A single exposure at 230 K leads to a high areal density of small dome-shaped blisters (up to 3 µm in diameter) together with much smaller flat-topped structures, while a single 450-K exposure produces large dome-shaped blisters up to 40 µm in diameter without the flat-topped structures. Most of the small dome-shaped blisters from 230 K exposure disappeared after annealing at 450 K for 17 h, but survived and even grew in size if the surface was exposed to D plasma during annealing. Sequential exposure at the two temperatures reveals a non-commutative behaviour: after a first exposure at 450 K the second exposure at 230 K leads to superposition of the observed blister structures without changing the large blisters from the first exposure. By contrast, a first exposure at 230 K almost completely suppresses the formation of large blisters during a second exposure at 450 K. Obviously, the presence of the small blisters strongly influences the penetration of D into the W bulk.

The influence of nitrogen (N) pre-implantation on the deuterium (D) retention in tungsten (W) at different temperatures was investigated. Bulk W samples were exposed to D plasma with a fluence of 110 24 D/m 2 with or without nitrogen pre-implantation at 300 K and 500 K, respectively. Nuclear reaction analysis was applied for the determination of N content and D retention in the near surface. Optical microscopy was used to investigate the surface modification by blistering after implantation. It is shown that, the W:N layers formed during the N preimplantation play very different roles on D retention and blistering in the samples at different temperatures. At 500 K, the W:N layer seems to enhance D diffusion into the bulk by suppressing D loss from the surface, which results in a much higher D concentration in the bulk and larger blisters than without N pre-implantation. At 300 K, the effect of this layer is much less pronounced than that at 500 K.

Magnetron-sputtered tungsten nitride (WNx) films deposited on bulk tungsten (W) were used as model system to study the interaction of deuterium (D) plasmas with W walls in nuclear fusion devices during or after N-seeded discharges. D plasma implantation was performed at 300 K with ion energies below 215 eV. WNx composition and thickness was determined by Rutherford backscattering. The deuterium amount in the sample was analyzed by nuclear reaction analysis (NRA). The resolution for D depth profiling was improved compared to standard NRA by consecutive low-energy argon plasma sputtering of the D containing 100-nm-WNx film. It is shown that D is implanted only within the ion penetration range and does not diffuse into deeper layers at 300 K.

The surface morphology and deuterium retention were investigated of polycrystalline tungsten targets that were exposed to deuterium plasmas at widely varying conditions. By changing only one parameter at a time, the isolated effects of flux, time and pre-damaging on surface modifications and deuterium retention were studied. The sample exposed to low-flux plasma (10 20 m −2 s −1 ) is mostly smooth with only a few areas containing very large blisters(50-500 µ m). The samples exposed to high-flux plasmas (10 24 m −2 s −1 ) show large numbers of smaller blisters(1-10 µ m) and in addition even smaller protrusions (

Five tungsten (W) grades were simultaneously exposed to deuterium (D) plasma with 10 20 D/(m 2 s) of 38 eV/D up to 10 26 D/m 2 at 500 K specimen temperature. The D inventories and their depth profiles within the topmost 12 μm were determined by nuclear reaction analysis (D( 3 He, p)α). Morphological modifications at and below the surface were analysed by confocal laser scanning microscopy and scanning electron microscopy assisted by focused ion beam cross-sectioning. The observed variation of the D inventory by more than one order of magnitude (0.5–15 × 10 20 D/m 2 ) is attributed only to the different properties of each W grade. Spherical blisters and stepped flat-topped extrusions are observed depending on the W grade. These modifications are interpreted as an indication for hydrogen loading-induced damaging. The exposure conditions and W grades were chosen to allow a comparison between published data sets.

The intense influx of hydrogen plasma onto the tungsten wall of a nuclear fusion reactor causes severe microstructural damage in the near-surface layer. The evolution of hydrogen-induced damage is often promoted by local plastic flow. Since hydrogen solutes are known to lower the hardness of many metals, the question arises as to the extent to which the tungsten walls are softened by the implantation of hydrogen. In this study, we investigated the change in slip resistance of tungsten single crystals following deuterium implantation. To determine the inherent yield stress of tungsten, we performed nanoindentation on high-purity single crystals. The statistical distribution of pop-in stresses (yield stress upon a single slip) revealed a significant reduction in pop-in load upon hydrogen implantation. In addition, the average stress at pop-in was found to be a function of crystallographic orientation of the samples, with much larger pop-in loads observed on (100) and (110) surface planes than on (111) surfaces.

Fundamental understanding of hydrogen–metal interactions is challenging due to a lack of knowledge on defect production and/or evolution upon hydrogen ingression, especially for metals undergoing hydrogen irradiation with ion energy below the displacement thresholds reported in literature. Here, applying a novel low-energy argon-sputter depth profiling method with significantly improved depth resolution for tungsten (W) surfaces exposed to deuterium (D) plasma at 300 K, we show the existence of a 10 nm thick D-supersaturated surface layer (DSSL) with an unexpectedly high D concentration of ~10 at.% after irradiation with ion energy of 215 eV. Electron back-scatter diffraction reveals that the W lattice within this DSSL is highly distorted, thus strongly blurring the Kikuchi pattern. We explain this strong damage by the synergistic interaction of energetic D ions and solute D atoms with the W lattice. Solute D atoms prevent the recombination of vacancies with interstitial W atoms, which are produced by collisions of energetic D ions with W lattice atoms (Frenkel pairs). This proposed damaging mechanism could also be active on other hydrogen-irradiated metal surfaces. The present work provides deep insight into hydrogen-induced lattice distortion at plasma–metal interfaces and sheds light on its modelling work.

Tungsten is a promising material for armouring the plasma-facing wall of future nuclear fusion experiments and power plants. It has a very high melting point, good thermal conductivity and is highly resistant against physical sputtering by energetic particles from the plasma. It also has a very low solubility for hydrogen isotopes. This is important both for safety and also for economic reasons, in particular with regard to the radioactive fusion fuel tritium. Due to this low solubility, the retention of hydrogen isotopes in tungsten materials after exposure to a plasma is dominated by the trapping of hydrogen isotopes at tungsten lattice defects. Therefore, a strong dependence of the hydrogen isotope retention on the microstructure of the tungsten is to be expected. This work describes a survey study of tungsten with different microstructures exposed to deuterium plasmas under a wide range of different plasma exposure conditions. The isotope deuterium was used because its natural abundance is much smaller than that of hydrogen (i.e., 1H). This allows detecting even very small amounts retained in the tungsten practically without background signal. Furthermore, the use of deuterium allows utilising the nuclear reaction 2D(3He,p)4He for depth-resolved quantification of the deuterium inventory up to depths of several microns. In order to standardise the specimens as far as possible, they were all cut from the same initial material from a single manufacturing batch. After a chemo-mechanical polishing procedure, which produces a well-defined surface, the specimens were annealed at either of four different temperatures in order to modify the grain structure and the dislocation density. These were then characterised by scanning electron microscopy and scanning transmission electron microscopy. The specimens were subsequently exposed in a fully characterised deuterium plasma source at different specimen temperatures, ion energies and deuterium fluences. In addition to highly reproducible exposure conditions, exposing all four types of specimens to the plasma simultaneously ensured that their retention behaviour could be directly compared. After two months of storage in a vacuum exsiccator, the long-term deuterium inventory of the specimens was analysed by depth-resolved nuclear reaction analysis and thermal desorption spectroscopy. Modifications of the specimen surfaces, in particular the formation of subsurface cracks and gas-filled subsurface cavities called blisters, were studied by a variety of optical and electron microscopy techniques, including in situ cross-sectioning with a focused ion beam. This extensive survey allowed finding a clear correlation between the initial dislocation density of the material and the observed deuterium retention, whereas the grain size, and therefore the density of grain boundaries, appeared to play a minor role. The binding energy of the dominant binding site, which is consequently assigned to dislocations, was determined by a detailed thermal desorption spectroscopy study of the specimen type with the highest dislocation density. It was also shown that defects capable of trapping significant amounts of deuterium can be produced during plasma exposure. Within the ion range of several nanometres, the tungsten surface is practically amorphised, which leads to observed local deuterium concentrations in the per cent range. Far beyond the ion range, blisters and cracks do not only retain deuterium in the form of molecular gas, but also produce a high density of defects in the surrounding tungsten lattice, most likely dislocations emitted near crack tips. A one-dimensional rate equation model was adapted to include these defect production mechanisms by an empirical term. By that, it was able to reproduce the fluence dependence of the deuterium retention as well as the shift of the main desorption peak for varying heating rates during thermal desorption spectroscopy. Wolfram ist ein vielversprechendes Material, um die erste Wand in zukünftigen Kernfusionsexperimenten und –reaktoren auszurüsten. Es besitzt einen hohen Schmelzpunkt, gute Wärmeleitfähigkeit und ist widerstandsfähig gegen physikalische Zerstäubung durch energiereiche Teilchen aus dem Plasma. Ebenfalls besitzt es eine sehr geringe Löslichkeit für Wasserstoffisotope. Dies ist sowohl aus Gründen der Sicherheit als auch der Wirtschaftlichkeit, insbesondere in Bezug auf den radioaktiven Fusionsbrennstoff Tritium, wichtig. Aufgrund dieser geringen Löslichkeit ist die Rückhaltung von Wasserstoffisotopen in Wolframmaterialien nach Bestrahlung mit einem Plasma von der Bindung der Wasserstoffisotope an Gitterdefekte im Wolfram dominiert. Deshalb ist ein starker Einfluss der Mikrostruktur des Wolframs auf dessen Rückhaltevermögen für Wasserstoffisotope zu erwarten. Diese Arbeit beschreibt eine Studie von Wolfram mit verschiedenen Mikrostrukturen, das Deuteriumplasmen unter einer Vielzahl verschiedener Bedingungen ausgesetzt wurde. Das Isotop Deuterium wurde eingesetzt, da seine natürliche Häufigkeit sehr viel kleiner ist als die von Wasserstoff (d.h., 1H). Dadurch lassen sich selbst kleinste rückgehaltene Mengen nahezu untergrundfrei nachweisen. Zudem erlaubt die Verwendung von Deuterium den Einsatz der Kernreaktion 2D(3He,p)4He zur tiefenaufgelösten Quantifizierung des Deuteriuminventars bis zu Tiefen von einigen Mikrometern. Um die Wolframproben so weit wie möglich zu standardisieren, wurden alle aus dem gleichen Material aus einer einzigen Fertigungscharge hergestellt. Nach einem chemo-mechanischen Politurprozess, der eine wohldefinierte Oberfläche produziert, wurden die Proben bei vier verschiedenen Temperaturen wärmebehandelt, um die Kornstruktur und die Versetzungsdichte zu verändern. Diese wurden dann durch Rasterelektronenmikroskopie und Raster-Transmissionselektronenmikroskopie charakterisiert. Die Proben wurden anschließend in einer vollständig charakterisierten Deuterium-Plasmaquelle bei verschiedenen Probentemperaturen, Ionenenergien und Deuteriumfluenzen exponiert. Zusätzlich zur guten Reproduzierbarkeit der Plasmabedingungen wurden jeweils alle vier Probentypen gleichzeitig dem Plasma ausgesetzt. Dies stellte sicher, dass ihr Rückhalteverhalten für Deuterium stets direkt vergleichbar war. Nach zweimonatiger Lagerung in einem Vakuum-Exsikkator wurde das langfristige Deuteriuminventar der Proben mittels tiefenaufgelöster Kernreaktionsanalyse und anschließend mit thermischer Desorptionsspektroskopie untersucht. Veränderungen der Probenoberfläche, insbesondere die Bildung von oberflächennahen Rissen und gasgefüllten Hohlräumen, sogenannten "Blistern", wurde mit Licht- und Elektronenmikroskopie untersucht. Dies wurde durch in-situ-Präparation von Querschnitten mit Hilfe eines fokussierten Ionenstrahls unterstützt. Diese breit angelegte Studie erlaubte es, eine deutliche Korrelation zwischen der ursprünglichen Versetzungsdichte des jeweiligen Materials und der beobachteten Deuteriumrückhaltung nachzuweisen. Die Korngröße, und damit die Korngrenzendichte, schien dagegen eine untergeordnete Rolle zu spielen. Die Bindungsenergie des dominanten Bindungsplatzes, die entsprechend Versetzungen zugeordnet wird, wurde in einer ausführlichen Thermodesorptionsstudie des Probentyps mit der größten Versetzungsdichte bestimmt. Des Weiteren wurde gezeigt, dass während der Plasmaexposition Defekte produziert werden können, die signifikante Mengen an Deuterium binden können. Innerhalb der Ionenreichweite von einigen Nanometern wird die Wolframoberfläche praktisch amorphisiert, was zu beobachteten lokalen Deuteriumkonzentrationen im Prozentbereich führt. Weit jenseits der Ionenreichweite beinhalten Blister und Risse nicht nur Deuterium als molekulares Gas, sondern produzieren im umgebenden Wolframgitter auch eine hohe Dichte an Defekten, höchstwahrscheinlich Versetzungen, die an Rissspitzen emittiert werden. Ein eindimensionales Ratengleichungsmodell wurde so angepasst, dass es über einen empirischen Term diese Mechanismen der Defektproduktion berücksichtigt. Damit war es in der Lage, sowohl die beobachtete Fluenzabhängigkeit als auch die Verschiebung des Haupt-Desorptionsmaximums für unterschiedliche Heizraten während der thermischen Desorptionsspektroskopie zu reproduzieren.

Tungsten-doped amorphous carbon films with 0–9 at.% W concentration were produced by magnetron sputtering and eroded in oxygen plasmas applying different bias voltages and substrate temperatures. The partial C and W erosion rates were determined from the C and W areal density changes measured by Rutherford backscattering spectrometry (RBS). The initial C removal rate increases with increasing ion energy and temperature and decreases with increasing W concentration. For W-doped films the erosion rate decreases with increasing plasma exposure duration. At low bias voltages the erosion process stops after W accumulation at the surface, which protects the carbon underneath from further erosion. RBS and X-ray photoelectron spectroscopy suggest that the W-rich layer at the surface is carbon free and consists of porous WO3. Biasing to 200 V leads to removal of W by physical sputtering and, therefore, inhibits the formation of the protecting W oxide layer and the C erosion proceeds.

Development of methods to characterise the first wall in ITER and future fusion devices without removal of wall tiles is important to support safety assessments for tritium retention and dust production and to understand plasma wall processes in general. Laser based techniques are presently under investigation to provide these requirements, among which Laser Induced Desorption Spectroscopy (LIDS) is proposed to measure the deuterium and tritium load of the plasma facing surfaces by thermal desorption and spectroscopic detection of the desorbed fuel in the edge of the fusion plasma. The method relies on its capability to desorb the hydrogen isotopes in a laser heated spot. The application of LID on bulk tungsten targets exposed to a wide range of deuterium fluxes, fluences and impact energies under different surface temperatures is investigated in this paper. The results are compared with Thermal Desorption Spectrometry (TDS), Nuclear Reaction Analysis (NRA) and a diffusion model.

The extreme brittleness of tungsten (W) is one of the challenges of using W as first wall material. One attempt to alleviate this problem is to use W alloys with better mechanical properties. However these alloying elements must not degrade the favorable properties of W with respect to its application at the first wall of fusion devices: low sputter yield and hydrogen inventory. In this work we investigate the hydrogen retention in the recently proposed W/Ta alloys under deuterium ion bombardment. By directly comparing pure W and W/Ta alloys with 1% and 5% Ta content we found that the W/Ta alloys retain significantly more hydrogen than pure W under identical implantation conditions. Our finding of increased hydrogen retention together with the fact that the Ta alloying did not improve the brittleness makes W/Ta alloys an unacceptable choice for the first wall of fusion devices.

The microstructure of tungsten samples is systematically modified by recrystallization to investigate the structure dependence of deuterium (D) retention. After a thorough characterization by scanning and transmission electron microscopy, non-recrystallized samples and samples recrystallized by different degrees are loaded with deuterium in a low-temperature plasma device. The deuterium inventory is measured by nuclear reaction analysis and thermal desorption spectroscopy. The post implantation surface morphology is investigated by scanning electron, optical and atomic force microscopy. The modification by recrystallization allows a wide variation in the crystallite size and has a strong impact on the measured D retention. Both the total amount and the binding state of the retained D are changed.

We present an electron-cyclotron-resonance plasma source with a biased sample holder that is well suited for the implantation of deuterium ions into tungsten. Achievable energies range from several electronvolts up to 200 eV per deuteron. The deuterium ion fluxes from this plasma source were thoroughly quantified with a retarding field analyser and a plasma monitor, i.e. an energy resolving mass spectrometer. We present the results of these measurements for different microwave input powers, D2 gas pressures and sample bias settings. Typical achievable deuteron fluxes are of the order of 1019 to 1020 D m−2 s−1 and are predominantly carried by ions. Additionally, an attempt to quantify the flux of neutral deuterium atoms to the sample surface is presented.

The use of nitrogen seeding to reduce the edge plasma temperature has recently been successfully applied in ASDEX Upgrade. While the plasma performance was significantly improved compared with other seeding species such as Ar or Ne, questions remained as to the interaction of nitrogen with a tungsten first wall. In particular the formation of thick tungsten nitride layers with reduced melting temperature and increased physical sputtering was a concern. Therefore dedicated laboratory experiments have been performed to investigate the interaction of W surfaces with N plasmas. Tungsten coated Si samples were exposed to N ions from plasma and ion gun sources at energies from 20 eV to 10 keV and W surface temperatures from 300 to 750 K. After exposure to the N plasma with fluences of up to several 1023 N m−2 the N content in the samples was measured by nuclear reaction analysis. The sputter erosion was determined by measuring the thickness change of the W layer by Rutherford backscattering. The formation of W-nitride phases was investigated in separate XPS experiments where the samples were implanted in situ with kiloelectronvolt N ions. It was found that only very small fractions of N are accumulated on the W surface and that N is bound in a nitride state. At temperatures above 600 K the nitrides are no longer stable which further reduces the N uptake into the W. Moreover the accumulation of N on the surface leads to a decrease in W physical sputtering due to the lower W concentration at the surface.

Hydrocarbon injection experiments with in situ calibration of spectroscopic signals have been carried out to determine the hydrocarbon flux and the chemical erosion yield under detached plasma conditions in ASDEX Upgrade. A plasma scenario was developed to detach the outer divertor in L-mode and to provide a cold (Te1–2 eV) and volume recombining deuterium target plasma. A significant decrease of carbon-related intrinsic photon fluxes, in particular of the CH Gero and of the C2 Swan band was observed in detachment. Moderate methane/ethene injections were performed through a single gas valve and an increase of carbon-related intrinsic photon fluxes was detected, which can be attributed to the amount of injected molecules. The experimentally determined effective inverse photon efficiencies are slightly larger than the ones deduced in attached plasmas. Nevertheless, the low intrinsic CH and C2 light observed under detached conditions can still be attributed to a reduction of the hydrocarbon flux of about one order of magnitude. The impact of detachment on the chemical erosion is discussed.

The present work reports the results of an experimental study of the depth distribution and fluence dependence of deuterium plasma-induced material modification of tungsten and tungsten-tantalum alloys. Plasma-induced damage was created by exposure to high-flux deuterium plasma in the plasma generator Pilot-PSI, followed by the degassing and subsequent decoration of created defects with deuterium by another plasma exposure. The depth distribution of deuterium from the decorating exposure reflects the distribution of plasma-induced defects. Depth profiling of this decorating deuterium, was performed by nuclear reaction analysis. It was found that plasma-induced material modification, which manifested itself as an increase of the deuterium concentration in the samples pre-exposed with high-flux plasma in comparison to the samples without such pre-exposure extends down to more than 5 mu m from the surface. This increase features a tendency to saturation with increasing fluence of the damaging high-flux plasma. Over the entire probing range, with the exception of the narrow surface region and the deep region beyond 5 mu m, the deuterium content is lower in pre-exposed W-Ta than in similarly pre-exposed W. Sub-surface features formed as a result of high-flux plasma exposure were studied with the help of focused ion beam cross-sectioning. W was found to contain plasma-induced cavities down to much larger depth than W-Ta.

Deuterium retention in toughened, fine-grained, recrystallized tungsten-based materials doped either with titanium carbide or tantalum carbide was investigated. Samples were either irradiated by 200 eV D−1 ions to fluences in the range 1 × 1022–1 × 1024 D m−2 at a temperature of about 310 K or by 38 eV D−1 ions to fluences in the range 6 × 1022–6 × 1024 D m−2 at temperatures of 300 and 600 K. The deuterium inventory in the samples was examined by nuclear reaction analysis and thermal desorption spectroscopy (TDS). At low irradiation temperatures, deuterium retention in both doped materials was comparable to retention in pure tungsten (both recrystallized and as-received), while the shapes of TDS spectra and their evolution with the fluence were different. Deuterium retention in both doped materials at 600 K and at the highest fluence was higher than that in both recrystallized and as-received pure tungsten, and this can be attributed to the carbide precipitates.

Understanding the influence of the microstructure of tungsten on hydrogen transport is crucial for the use of tungsten as first-wall material in fusion reactors. Here, we report the results of molecular dynamics and transition state studies on the influence of grain boundaries in tungsten on the transport of hydrogen. An exhaustive mapping of possible minimum activation energy migration trajectories for hydrogen as the trace impurity reveals a strongly modified activation energy distribution in the neighborhood of grain boundaries together with an altered connectivity matrix. The results indicate that grain boundaries in polycrystalline tungsten may provide an important transport channel, especially for neutron-damaged tungsten.

Tungsten is a promising candidate for plasma-facing materials in fusion reactors. In this work, two types of W materials were investigated: (i) sintered and forged tungsten (ITER-grade Japan, grain sizes 2–100 μm, elongated normal to the surface) and (ii) the same W grade, but after additional re-crystallization (at 2073 K, grain size ~50 μm). The samples were exposed to deuterium with an ion energy of 38 eV D−1, a fluence of 1027 D m−2 and a flux of ~1022 D m−2 s−1 in a plasma generator at elevated temperatures (320–700 K). The D retention (determined by thermal desorption spectroscopy and nuclear reaction analysis) of both sample types is compared. The samples were analysed with scanning electron microscopy combined with a focused ion beam for iterative cross-sectioning to obtain three-dimensional (3D) data of the sub-surface. Electron backscattered diffraction was applied to determine the grain orientation and deformation. First nano-secondary ion mass spectroscopy investigations were performed on a D-loaded sample to analyse the lateral accumulation of H/D on the surface.

During temperature-programmed desorption (TPD) of stress-relieved polycrystalline tungsten samples exposed to a deuterium plasma, short, intense bursts of D2 were observed on the low-temperature flank of the main desorption peak. These bursts are attributed to the rupturing of blisters filled with high-pressure D2 gas. A statistical analysis of the size distribution and temporal correlation of the bursts is presented. The influence of different measurement intervals and TPD heating rates on the observed bursts is simulated based on these statistics and compared to the experimental results. The contribution of bursts to the total D inventory in the sample is also estimated.

Up to now, analyzing the production of dislocation-type defects in the subsurface region of plasma or ion-exposed tungsten samples has been hampered by the challenging production of suitable cross-section samples for transmission electron microscopy. We present two reliable methods based on precision electropolishing to prepare cross-sections of tungsten that allow direct imaging of dislocation-type defects by scanning as well as by transmission electron microscopy. Using these methods, we are able to demonstrate a clear enhancement of the dislocation density in the caps of blisters on tungsten exposed to H isotope plasma, i.e., of surface morphologies that are correlated to subsurface cavities. As a benchmark, we also show a cross-section of tungsten irradiated by 20 MeV W6+ ions. Keywords: Tungsten, Plasma, Blistering, Cross-section, Electropolishing, Dislocations

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.