Table of contents

International Programme Committee

List of participants

In the past the role of tungsten as a fusion reactor plasma-facing material has been fairly limited. It has appeared sparingly in tokamaks, but usually only for experimental purposes. This is likely to change in the future. Tungsten has a very high threshold for sputtering as well as a high melting point and high thermal conductivity. Applications of tungsten in areas where the energy of the plasma particles can be kept below the sputtering threshold removes the plasma impurity problem often associated with the use of tungsten in fusion reactors. In the area of recycling and retention, tungsten is unlike carbon and beryllium in that hydrogen appears to stay in solution in the metal (at least at low concentrations) and diffuse somewhat classically. This paper presents a review of the hydrogen isotope retention and migration properties of tungsten as they relate to fusion applications. The review is begun with an examination of past experiments on the diffusivity, solubility, and permeability of hydrogen in tungsten. Fusion specific topics such as implantation and surface effects are then covered. Trapping is shown to be an important aspect of understanding hydrogen transport in this material. Blister and bubble formation are also addressed.

In order to investigate the difference of the hydrogen transport behavior of IDP (ion-driven permeation, i.e. permeation under impingement of the energetic hydrogen ion beam) and ADP (atom-driven permeation, i.e. permeation under exposure of thermal atom beam), an ADP experiment through Nb (niobium) was performed. The dependence of the ADP flux on the specimen temperature turned out to be different from the IDP, where the maximum was observed in the temperature range of 500-1000 K. Such a case has not been seen in the IDP or GDP (gas-driven permeation) case. The deuterium concentration at the downstream-side end bulk volume was estimated employing the phenomenological recombination rate coefficient. Computer calculation showed that the profile of the deuterium concentration becomes almost "flat" under the experimental conditions. Though the experiment was performed under UHV conditions, the assumption of Sieverts' law-like relation between P_U* (virtual pressure of atomic gas, or pressure equivalent to "atomic flow" in the upstream-side vacuum in terms of the molecular flow theory), bulk concentration and P_D (pressure of permeated molecules) explains well the strange temperature dependence of the permeation plot.

The work function is quite sensitive to the change of surface electronic properties of solid materials. Recently, the present authors have developed experimental systems which enable to measure the work function of solid materials under various circumstances. Pellets of lithium oxide ceramics, Li₄SiO₄, Li₂ZrO₃, Li₂TiO₃, Li₂O and LiAlO₂, were employed and installed in a "high temperature Kelvin probe" to measure the work function of these materials at elevated temperatures as well as under controlled chemical compositions of the surrounding gas phase. Further work is under way in which the effect of beam irradiation, either by H⁺ (H⁺₂) or He⁺ ions or by neutrons, on the work function is being investigated.

The design of a new instrument is described along with results demonstrating the capabilities of in situ preparation and analysis. The purpose of the ARTOSS experiment is the preparation and analysis of multi-component surface layers on materials – in particular those which are well-suited for the first wall of present and future fusion devices – and their behaviour with respect to the hydrogen inventory in such materials. Relevant preparation and analysis techniques are combined to achieve this aim. For preparation, surfaces can be cleaned in situ and layer preparation includes vapour deposition, ion beam bombardment at energies between 100 eV and 20 keV as well as admission of gases. Photoelectron spectroscopies analyse the chemical state of elements within the surface layers, ion beam techniques enable quantitative measurements, including hydrogen isotope analysis. For ion beam analysis, low-energy (up to 5 keV) as well as high-energy ions (3 MV tandem accelerator) are available in situ. Thermal desorption and re-emission measurements complete the capabilities for hydrogen detection in multi- component materials. Sample transfer into the ultra-high vacuum system is possible via a sample load-lock. Results are presented for carbon layers on beryllium and surface modifications of beryllium by C⁺ and CO⁺ ion bombardment to demonstrate the capabilities of the new experiment.

The depth profiles of D atoms and D₂ molecules in W single crystals and hot-rolled W implanted with 6 keV D ions at 300 and 650 K were determined by means of secondary ion mass spectrometry (SIMS) and residual gas analysis (RGA) measurements in the course of surface sputtering. Retention of deuterium and lattice damage in W single crystal irradiated with 10 keV D ions at 300 K were investigated by means of nuclear reaction analysis (NRA) and Rutherford backscattering spectrometry and ion channelling techniques (RBS/C). There are at least two types of ion-induced defects which are responsible for trapping of deuterium: (i) D₂-filled microvoids (deuterium bubbles) localised in the implantation zone and (ii) dislocations distributed from the surface to depths far beyond 1 µm which capture deuterium in the form of D atoms. Additionally, D atoms can be trapped by vacancies and adsorbed on bubble walls. At 650 K, deuterium is retained as D atoms only.

Basic facts are presented of the absorption of hydrogen gas by metals and the diffusion of hydrogen in metals. Specifically considered are crystalline metals without defects and lattice disorder (pure metals), low hydrogen concentrations and the possibility of high hydrogen gas pressures. The first introductory topic is a short presentation of typical phase diagrams of metal hydrogen systems. Then, hydrogen absorption is discussed and shown to be decisively determined by the enthalpy of solution, in particular by its sign which specifies whether absorption is exothermic or endothermic. The formation of high-pressure hydrogen gas bubbles in a metal, which can lead to blistering, is addressed. It is demonstrated that bubble formation will, under realistic conditions, only occur in strongly endothermically hydrogen absorbing metals. The chief aspects of hydrogen diffusion in metals are discussed, especially the large size of the diffusion coefficient and its dependence on lattice structure. It is shown that forces can act on hydrogen in metals, causing a directed hydrogen flux. Such forces arise, for instance, in the presence of stress and temperature gradients and can result in local hydrogen accumulation with potential material failure effects. The final aspect discussed is hydrogen permeation, where the absorption behavior of the hydrogen is found to be in general more decisive on the permeation rate than the value of the diffusion coefficient.

Several different methods can be and have been applied for measuring the amounts and depth profiles of the hydrogen isotopes in the surface layers of solids, such as the vessel walls of fusion experiments. Mostly MeV ion beam analysis techniques are applied. In addition low energy ion beam sputtering and detection of the sputtered positive or negative ions (SIMS and AMS), thermal desorption spectrometry as well as counting the T activity with an open detector are used. The detection limits for these measurements are best for SIMS, AMS, and counting the T activity being of the order of 10⁸ atoms/cm², which has to be calibrated, while the MeV ion beam analysis giving directly quantitative numbers is about two to three orders of magnitude and TDS about 8 orders of magnitude less sensitive.

A general model based on Statistical Mechanics and Random Walk is presented which allows to desribe the behavior of hydrogen in disordered systems, i.e. metallic glasses, amorphous silicon, nanocrystalline metals, deformed metals, disordered metallic solutions, and metallic multi layers. The various systems are specified by a lattice with an appropriate site energy disorder and a distribution of site transitions rates. Lattice sites are filled according to Fermi–Dirac Statistics because double occupancy is excluded. Thus the model is applicable to adsorption on heterogeneous surfaces or solutions of small particles in oxide glasses and polymers. With a given distribution of site energies a relationship between chemical potential (Fermi energy) of hydrogen and its concentration can be derived and compared with experimental results. It is a unique feature of hydrogen that its chemical potential and its diffusion coefficient can be determined rather easily by electrochemical techniques or by measuring partial pressures at moderate temperatures around 300 K. With increasing H-content the sites are usually filled from lower to higher energies. As a consequence Henry's Law is not fulfilled and the diffusion coefficient increases because at high concentrations low energy sites are saturated and additional H-atoms have to perform their random walk through sites of low occupancy or small time of residence, respectively. Some results for metallic glasses, nanocrystalline metals, deformed metals, and metallic multi layers are presented and compared with the model. Thus information on the interaction between defects (dislocations, grain boundries, distorted tetrahedral sites in glasses) and hydrogen are obtained. For extended defects the diffusion is strongly anisotropic, i.e. it differs in a Pd/Nb-multi layer by a factor of 10⁵ for diffusion in plane and out of plane.

New results obtained during the study of D implanted W (001) single crystal are presented. The reemission curves at different temperatures, evolution of microstructure, in-depth profiling of radiation damage and implanted deuterium, retention of deuterium atoms and molecules are described. All the data are compared with the results on D implanted Be and BeO. The analysis includes consideration of the energetics of hydrogen isotopes solubility, diffusivity, trapping, and chemisorption for non-irradiated materials mentioned above.

The decay curves of D and up-take curves of H on the exchange of D implanted into Li₂TiO₃ for H in H₂O vapor caused by exposure to normal-air at room temperature have been measured as a function of exposure time at different implantation concentrations by means of the elastic recoil detection technique. The re-emission curves of D retained and H up-taken in a specimen by isochronal annealing for 10 min have been also measured. It is found that the thermal re-emission of D and H takes place uniformly over the whole specimen due to local molecular recombination in the bulk and that the re-emission rates of H and D in the near-surface layers are slower than those in the deeper layers. It is also found that the decay of D caused by the D–H exchange takes place rapidly in the beginning and the retained amount of D attains at a constant level and the retained fraction of D are higher as the initial implantation concentrations of D are lower. The decay curves of D and the up-take curves of H have been analysed using the mass balance equations, in which the elementary processes are taken into account according to the exchange model of one way diffusion from the surface into the bulk. It is shown that the solution of the mass balance equations reproduces well the experimental data. The rate constants of the elementary processes determined are discussed.

Among the presently available low-Z materials beryllium represents one of the most promising candidate materials to be used as protection of the first wall and as neutron multiplier in the blanket of a next-step fusion reactor. Both sintered-product blocks and pebbles have been considered, and research and evaluations associated with safety, tritium release, heat transfer, thermal-mechanical and irradiation stability are underway to study the characteristics of several material grades. This paper presents the results of a series of out-of-pile annealing tests up to 1000 °C aimed at investigating both tritium and helium release kinetics from the S-65C beryllium grade irradiated in the BR2 reactor at temperatures of 235, 485 and 600 °C, with a fast neutron fluence (E_n > 1 MeV) of about 2.1 × 10²⁵ m^-2 and with a damage dose of 2.45, 2.1 and 2.3 dpa, respectively. In agreement with previous studies, all the beryllium samples show a tritium release which starts to increase above about 600–650 °C and reaches a maximum when the specimens first reach about 1000 °C. Although tritium is released between 600 °C and 900 °C, no helium release is observed in that temperature range. However, after several minutes heating at 1000 °C the samples showed a burst release leading to the release of essentially all retained tritium. Correspondingly, a peak of helium release was observed. This unambiguous and concurrent release of tritium and helium leads to the conclusion that T and He partially reside in common bubbles in the irradiated material.

Utilizing simplified statistical thermodynamic treatments, analytical expressions for the pressure-composition (p–c) isotherms of hydrogen-metal systems can be derived. Fitting a set of experimental isotherms for a given hydrogen isotope, A (A = H, D or T), to the model-derived functions, two energy-related parameters can be evaluated. These parameters are the pairwise A–A nearest neighbors interaction, η_AA, and an effective A-lattice interaction parameter, ε_A^eff, which includes both, the non-vibrational (electronic and elastic) contributions and the vibrational contribution. Comparing ε_A^eff for two hydrogen isotope systems (e.g. H and D) it is possible to estimate the corresponding zero-point vibrational energies of the isotopes in the hydride. This type of analysis has been performed for the Pd–H₂(D₂) system as well as for a series of the Laves phase TiCr_2-XMn_X (X = 0, 0.5 and 1) hydrides. For each system, experimental p–c isotherms of the two hydrogen isotopes (i.e. H and D) were obtained over a wide temperature and pressure range. This included the super-critical range (i.e. above the critical temperatures of the systems) for which the model-derived p–c isotherms better represent the experimental ones. A set of microscopic energy-related parameters were evaluated for these systems. A procedure which enabled the estimation of the average zero-point vibrational energies of the isotopes in the corresponding hydride is outlined. A comparison was made between the results obtained by this procedure and corresponding reported vibrational energies measured by some vibrational spectroscopy methods. The accuracy of the model-calculated results seems to be of the order of 15–50 %. A relation was obtained between the zero-point vibrational energies and the temperature where a transition occurs from a "positive" to a "negative" isotope effect.

Tritium depth profiling measurements by accelerator mass spectrometry have been performed at a facility installed at the Rossendorf 3 MV Tandetron. In order to achieve an uniform erosion at the target surface inside of a commercial Cs ion sputtering source and to avoid edge effects, the samples were mechanically scanned inside of a commercial Cs sputter ion source. The sputtered negative ions were mass analysed by the injection magnet of the Tandetron. The tritium ions are counted after the acceleration with semiconductor detectors. Depth profiles have been measured for carbon samples which had been exposed to the plasma at the first wall of the Garching fusion experiment ASDEX-Upgrade and from the European fusion experiment JET, Culham/UK. A dedicated AMS facility with an air-insulated 100 kV tandem accelerator for depth profiling measurements at samples with high tritium concentration is under construction. First results of test operation are presented.

First measurements on laser induced desorption of deuterium incorporated in a boron layer formed by plasma chemical vapor deposition on tungsten and graphite limiter surfaces have been performed. A ruby laser (λ = 694 nm) with maximum energy of 50 J and a pulse length of about 0.5 ms was used as a heat source. The desorbed deuterium was detected by mass spectroscopy and total pressure analysis in the residual gas. The amount of desorbed deuterium is about 10¹⁷ D atoms cm^-2. The majority of the deuterium is released during the first laser pulse. The limiter heads were investigated post-mortem by means of ion beam analysis to determine the spatial distribution of boron and deuterium and to investigate the effect of the laser pulse on the release of deuterium and sublimation of boron in the laser spot. All the deuterium has been released by the laser pulse. The boron is sublimated partly from the graphite and removed nearly completely from the tungsten surface.

Retention and thermal release of D atoms implanted into W and Mo containing carbon or oxygen in the surface layers were studied by ion beam analysis techniques. Ion implanted D atoms accumulated within the surface oxide layer on Mo and W crystals to a high concentration at room temperature, although the D distribution in the substrate crystal was not strongly affected by the large retention in the surface. When the oxide layer was filled with a high density of D atoms, it was reduced by subsequent D ion implantation at room temperature and by heat treatment, especially for Mo crystals. A carbon-enriched layer on the Mo crystal retained a large amount of D atoms and also significantly enhanced the D trapping in the substrate crystal at room temperature. D atoms were weakly trapped in the substrate crystal as well as in the interface where strain fields seemed to be created simultaneously with D retention.

Many hydrogenated pure metals and industrial alloys with FCC lattices show a distinct internal friction peak above room temperature caused by the thermal desorption of hydrogen. Based on these observations, a new method for studying the hydrogen diffusivity and the interaction of hydrogen with lattice defects is proposed for the temperature range of 250–500 K. The desorption origin of the peak was studied in detail in hydrogen-charged Inconel 600 alloys with different carbon contents. The peak was shown to possess transient and non-relaxation nature. In some cases, a fine structure of the peak caused by hydrogen traps can be observed. A theoretical model of the thermal desorption effects on the losses of mechanical energy, i.e., internal friction, in metals is developed.

Deuterium gas driven permeation through martensitic steel F82H was investigated experimentally and by computer modeling in the range of 573–873 K. Surface contamination was demonstrated to lead to increase of the time-lag and decrease of the permeation rate. The values of the diffusivity and solubility obtained by using the diffusion-limited approach are underestimated if surface contamination reduces the rates of dissociation and recombination. Surface effects give an uncertainty in determination of D and S values. To reduce this uncertainty, the advantage of using two series of experiments with different surface conditions was demonstrated. Calculations with surface effects taken into account gave a very good agreement with the two sets of experiments with the same combination of D and S values but with different values of surface parameters.

Temperature programmed thermal desorption spectrometry (TPD) is used to study kinetics of molecular hydrogen release after sorption of hydrogen atoms. A sharp decrease of hydrogen release rate is observed if linear heating of the sample is interrupted at any intermediate temperature. A similar behaviour of release curves is observed after irradiation of the samples with hydrogen ions. A full set of tests supported with computer simulation indicates that atomic hydrogen is sorbed under the surface.

The study of interaction of atomic hydrogen with quasi-monocrystalline pyrolytic graphite (QMPG) by means of temperature programmed thermal desorption (TPD), scanning tunnel microscopy (STM) and atomic force microscopy (AFM) revealed pronounced effects of atomic hydrogen sorption on the surface morphology. The initially flat graphite surface with well-ordered atomic structure was changed after sorption of hydrogen atoms into a rough one with "bumps" up to 5 nm in height and 100 nm in diameter. After thermal release of hydrogen the surface became again atomically flat with etch-pits 1–2 layers in depth. It is suggested that the sorbed atomic hydrogen is stored between graphene layers as H₂ molecules.

For the Accelerator Production of Tritium Project (APT), spallation neutrons will be moderated and then absorbed in ³He gas to produce tritium. The spallation neutrons will be generated by the interaction of high energy (~1 GeV) protons with solid tungsten rods or cylinders. A byproduct of the spallation reactions is large amounts of helium and hydrogen gas generated in the rods and other structural materials. The release kinetics of these gases during various proposed off-normal scenarios involving loss of coolant and afterheat-induced rises in temperature is of particular interest to the APT Project. In addition, however, this data is of interest for fusion reactors where tungsten used in a tokamak divertor will also be exposed to neutrons. In this case, the generated protium will be accompanied by deuterium and tritium diffusing in from the plasma-facing surface. Tungsten rods irradiated with 800 MeV protons in the Los Alamos Neutron Science Center (LANCE) to high exposures have been sectioned to produce small specimens suitable for measurement of both hydrogen and helium. Hydrogen evolution was measured by subjecting the specimens to a simulated temperature ramp from ~200 to ~1200°C, similar to that expected due to a loss of coolant and subsequent afterheat. The release measurements were conducted using mass spectrometric techniques. Four release peaks at temperatures of approximately 550, 850, 1100 and 1200°C were observed, initially suggesting a number of trapping sites with different binding energies. Subsequent analysis, however, showed that the observed peaks were artifacts of the temperature heating profile, and that the release curve could be duplicated using a single trap energy of 1.4 eV.

Carbon erosion and deposition were investigated on the surface of a flat target covered with an a-C:H film and exposed for 197 s in the SOL of TEXTOR-94. The target was declined by 20° with respect to the toroidal direction and partly protected by an aluminum (3 mm) plate which created an 8 mm wide local shadow. Thickness changes were measured by colorimetry after each plasma discharge. Carbon is eroded from surface areas near the plasma edge (LCFS +1 cm) and transported into the local shadow regions. Accumulation rates up to ≈ 7 nm/s were found. The erosion in the local shadow regions (about -0.1 nm/s) is due to charge exchange neutrals. The observations are confirmed by ion beam analyses and by preliminary calculations with the B2-EIRENE and ERO-TEXTOR code.

The 5th International Workshop, "Hydrogen Isotopes in Solids", was held in May 17-19, 2000, in Stockholm, Sweden. The previous workshops took place 1992 in Livermore, 1994 in Nagoya, 1996 at JRC-Ispra, and 1998 in Santa Fe. The workshop was organized by the Royal Institute of Technology (KTH), Stockholm. Next workshop is planned to be held in Japan, at the University of Tokyo.

At present, the most advanced magnetic configuration is the tokamak, which is the reference configuration retained for the Next Step. Research, development and evaluation of other magnetic confinement configurations (such as stellarator, the reversed-field pinch and compact toroidal devices), as well as materials-related research, are being pursued in Europe and in the world.

The fusion fuels deuterium and lithium are non-radioactive and abundantly available all over the world. Lithium would used on the reactor site to generate tritium to be burnt with deuterium in order to produce fusion power. Annually around 200 kg of deuterium and 2-3 tonnes of lithium would be sufficient for fuelling a large power station (1000 MW). Regarding availability of other materials which would be required to construct the tritium breeding blanket of a commercial fusion power plant, appropriate reactor designs could ensure that only sufficiently abundant materials would be used.

To produce fusion energy it is necessary to reach plasma temperatures of millions of degrees, a high density and a particle confinement time of the order of 5-10 seconds. In present days of experiments the triple-produce nTτ (density, temperature, confinement time) has reached a value which is just about one order of magnitude from the value needed for a reactor.

The plasma particles are lost by diffusion and drift from the plasma. They mostly impinge onto the sides of limiters and onto divertor plates at fluxes of up to 10²¹ per m²s with energies in the range of 0.1 to several keV. These energetic neutrals are created by recombination and by charge exchange in the plasma. At the vessel walls a fraction of the energetic hydrogen atoms, i.e. of the order of 10 to 50% are backscattered at the nuclei of the wall materials, predominantly as neutral atoms. These backscattered hydrogen atoms are directly recycled into the plasma, while atoms not backscattered are implanted into the wall materials up to depths of the order of 0.1 to a few µm. These implanted atoms and ions may diffuse back to the side of implantation and be released into the plasma predominantly as molecules, or diffuse into the bulk. They may accumulate at trapping sites or diffuse to the other surface and be released, depending on the material and its temperature. These atoms are lost from the plasma, i.e. clean vessel walls "pump". The wall pumping allows to control the plasma density just by hydrogen fuelling, such as by gas puffing, by pellet injection and by neutral beam injection.

A critical and so far not yet resolved problem is related to the materials, in particular the plasma facing materials. By using low Z materials in recent experiments, fusion has advanced much, but the conclusion is that we are lacking full, and necessary, control of the hydrogen behaviour in materials.

Understanding of hydrogen in materials has attracted much interest from many scientists in various fields. In our particular field the problems are related to materials behaviour like embrittlement and blistering and change of materials properties, as well as the problem related to tritium. The high neutron flux to the materials may contribute to these problems but this research is still far from an understanding of the full impact of neutron irradiation. Materials behave in different ways when exposed to hydrogen isotopes. One of the most severe problems in fusion will be the possible build-up of tritium retention in plasma facing materials, which then can contribute to a severe radioactive source. An aim of this workshop was to collect scientific knowledge of hydrogen in materials for the understanding of these problems, although we realized that it would be impossible to fully cover this object in just one meeting.

Experience of hydrogen in materials exists within many disciplines, not only within the fusion community, but due to the extreme demands, understanding hydrogen in materials represents a major part of this research.

In organizing this workshop we wanted to continue work that had been presented in earlier workshops within this series, but we also wanted to turn to a wider science community for their complementary view on hydrogen isotopes in materials. As a result we succeeded to attract a number of experts outside fusion to publish their results on hydrogen in materials together with scientists from the fusion activities.

We are grateful to the sponsors of this workshop whith were the Royal Swedish Academy of Sciences through its Nobel Institute for Physics, the Swedish Natural Science Research Council, the Royal Institute of Technology, the Manne Siegbahn Laboratory, and last but not least the City of Stockholm which offered the participants a wonderful buffet in the City Hall.

We are indebted to the contributors who made this workshop possible by submitting important papers for publications. We also acknowledge the support from the journal Physica Scripta which accepted us as guest editors.

Many thanks to all who helped us to organize and solve all practical problems in organizing this workshop.