• Energy Research

Pure Be, Be-O and Be-O-C thin coatings were deposited using high-power impulse magnetron sputtering (HiPIMS) with and without incorporation of deuterium. The coatings produced without deuterium were implanted afterwards with 15 keV 2H+ ion beams with a fluence limited to 2 × 1017 ion/cm2 in order to mitigate the damage imposed by ion irradiation and prevent a fast gas release. The as- deposited and as-implanted coatings were analysed by IBA techniques, namely by elastic and Rutherford backscattering spectrometries (EBS and RBS, respectively), nuclear reaction analysis (NRA) and by time-of-flight elastic recoil detection analysis (ToF-ERDA). Despite distinct deuterium depth profiles in the implanted samples, the results show that for the present ion implantation and deposition parameters, similar retained amounts are revealed in the films loaded by ion implantation or during the HiPIMS deposition, assuring ion implantation as a competitive and reliable method for fuel incorporation in thin Be-based films for retention studies in controlled conditions.

Pure Be, W and Be:W mixed coatings with nominal compositions of (5:5) and (1:9) were deposited on silicon plates and implanted at room temperature with 30 keV N+ ions with fluences up to 5e17 ions/cm2. Ion beam and X-ray diffraction analysis evidenced the formation of the α-Be3N2 and β-W2N nitrides. The identified tungsten nitride phase evolves from a BCC W lattice to a BCC W(N) solid solution after irradiating at a fluence of 1e17 N+/cm2 and to the compact FCC β-W2N structure at 5e17 N+/cm2. Thermal stability of β-W2N was investigated by annealing the coatings for 1 h up to 1073 K. The results point to the release of non-bonded nitrogen solute in β-W2N over the annealing range and to the thermal stability of the nitride phase up to 1073 K.

The use of lithium (Li) or tin (Sn) as a liquid metal plasma facing component is proposed as a solution to the high power load issue on the divertor region of nuclear fusion reactors. The possibility to use these materials depends on their compatibility with hydrogen plasmas. With the purpose of realizing deuterium retention studies, specimens of pure Sn (99.999% Sn) and Li–Sn alloy (30 at.% Li) were exposed in the ISTTOK edge plasma. Ex situ analysis of the samples was performed by means of ion beam diagnostics. Nuclear reaction analysis (NRA) technique was applied using the D(3He,p)4He reaction to quantify the fuel retention on the samples.In this work the deuterium retention is compared between pure Sn and Li–Sn alloy samples in both liquid and solid states. All the samples were found to have retention ratios smaller than 0.1 at.%. This low retention ratio is expected for pure tin given its high mass and the instability of tin hydrides. However the retention was unexpectedly low for the case of Li–Sn which was thought to be dominated by the lithium fraction in the alloy. These results suggest that tin has a role in the retention mechanism in this material. Keywords: Liquid metals, Plasma-surface interaction, Lithium, Tin, Deuterium retention, Tokamak ISTTOK

Liquid lithium-tin (Li-Sn) alloys are being considered in alternative concepts of plasma facing components for nuclear fusion applications. Nevertheless, the high Li content and high melting temperature of Li-rich intermetallics may hinder their use due to a possible excessive evaporation of Li to the vessel with evident issues in fuel retention. Their formation was inhibited in this study in homogeneous alloys with Li contents as higher as 5 at.% via a mechanical alloying route of pure Li and Sn sources. A 50 at.% Li-Sn master powder was initially prepared and diluted afterwards with pure Sn to achieve the final compositions. The structure, elemental composition and homogeneity of the alloys were assessed by scanning electron microscopy, ion beam analysis and X-ray diffraction. 5 at.% Li and 1 at.% Li alloys composed by phases with low melting temperatures, LiSn, Li2Sn5 and β-Sn, mainly, were produced with reduced oxygen and metallic impurity contents.

Tungsten and beryllium were foreseen to be the plasma facing materials (PFMs) for the first wall and divertor in ITER. Particles eroded from PFMs surfaces will be transported by the plasma and co-deposited in different locations of the vessel walls. Elemental mixing in the exposed surfaces enables the formation of distinct phases that may influence the stability of PFMs in a wide temperature range. In the present work, Be and W coatings were deposited on W and Be plates, respectively, and annealed in vacuum in the 673–1073 K temperature range. Additionally, Be75W25 coatings deposited on Be plates were annealed in the 873–1273 K range. The Be-W chemical reactivity, phase formation and coatings stability were followed by ion beam analysis, X-ray diffraction and electron microscopy. The reactivity is too weak up to 973 K. It becomes fast at 1073 K with a competitive formation of Be2W and Be12W. At 1173 K, a strong Be22W formation occurs, and Be22W is the only stable phase at 1273 K. The growth of BeO occurs at the coatings surface and increases with temperature. Despite the presence of distinct phase structures, the experiment evidences that the growth of tungsten beryllides enhances the mechanical stability of the coatings from 973 K to 1073 K. In opposition, Be22W formation at 1173 K and 1273 K induced fracture and split off from the Be plates.

The use of tin (Sn) as a liquid metal for plasma facing components has been recently proposed as a solution to the high heat load issue on the divertor target plates in nuclear fusion reactors. Due to its low vapor pressure, low reactivity with hydrogen and good resilience to neutron impact, tin is a good candidate as plasma facing component. However its high atomic number poses concerns about plasma contamination. In this paper two fundamental aspects have been investigated: deuterium retention and erosion fluxes from the Sn surface towards the plasma. The samples were exposed to plasma inside the linear machine GyM in magnetic cusp configuration. This setup permits to expose free liquid specimens without the need for the Capillary Porous System. Moreover it permits to lower the magnetic field in order to increase Sn Larmor radius and consequently limit Sn re-deposition in erosion experiments. Ex-situ analyses by ion beam diagnostics on solid samples exposed to deuterium plasma have proved that the amount of retained atomic deuterium is very low, approximately 0.18 at% estimated by Nuclear Reaction Analysis and 0.25 at% estimated by Elastic Recoil Detection Analysis. In the framework of erosion studies, the spectroscopic parameter S/XB was evaluated in Ar plasma for the SnI line at 380.1 nm by Optical Emission Spectroscopy and mass loss measurements in the 5-11 eV Te range, at a density ne ~ 1.5×1011 cm-3. An average value of 150 ± 23 was obtained.