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LIBS study of ITER relevant tungsten–oxygen coatings exposed to deuterium plasma in Magnum-PSI
Full text linkWe discuss the applicability of laser induced breakdown spectroscopy (LIBS) for deuterium retention analysis in compact and porous tungsten-oxide (W-O) coatings. Deuterium loading was performed by exposing the coatings to deuterium plasma in Magnum-PSI linear plasma device. The deuterium signals obtained by ex-situ LIBS had sufficiently good signal-to-noise ratio for reliable separation of essentially broadened hydrogen and deuterium lines as well as for comparison of the lateral and depth distributions of deuterium in the coatings. Strong deuterium signal was obtained for the first laser shot which corresponded to the surface layer of the W-O coatings whereas deeper in the coating the signal decreased to noise level. In addition, the deuterium signal was highest in the central region of compact W-O coating. For both coatings, depth profiles of elements obtained by LIBS match with those recorded by secondary ion mass spectroscopy (SIMS) in the lateral direction along the sample surface. The results of LIBS and SIMS results were supported by Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) data which showed that the exposure to deuterium plasma resulted in remarkable changes in the surface morphology along the sample surface. The study demonstrates the LIBS potential in deuterium retention measurements in plasma facing components.</p
The EU strategy for solving the DEMO exhaust problem
Full text linkExhaust of power and particles is crucial for the DEMO device and the EU has developed a strategy to address the challenges. This strategy consists of a conventional approach based on extrapolation of the ITER solution (detached lower single null divertor) as well as the development of alternatives as risk mitigation. These comprise alternative magnetic divertor geometry, liquid metal targets and intrinsically ELM-free operational scenarios. On the experimental side, the EUROfusion programme has initiated both upgrades to existing linear and toroidal devices as well as plans to engage in new devices presently under construction in the EU. In parallel, the theory and modelling efforts are ramped up in a targeted effort to obtain the necessary understanding for safe extrapolation to DEMO. This is especially important for the alternatives, which cannot be tested in ITER.</p
Modelling of charge-exchange induced NBI losses in the COMPASS Upgrade tokamak
No full textThe COMPASS Upgrade tokamak [1] will be a tokamak of major radius R0 = 0.894m with the possibility to reach high field (Bt ~ 5 T) and high current (Ip ~ 2 MA). The machine should see its first plasma in 2023 and H-mode plasma will be obtained from 2025. The main auxiliary heating system used to access H-mode will be 4MW of Neutral Beam Injection (NBI) power. The NBI will have a nominal injection energy of 80 keV, a maximum injection radius Rtan = 0.65m and will create a population of well-confined energetic D ions. In this contribution, our modelling studies the NBI deposition and losses when a significant edge background density of neutrals is assumed. We follow the fast particles in the 3D field generated by the 16 toroidal field (TF) coils using the upgraded EBdyna orbit solver. We have implemented a Coulomb collision operator similar to that of NUBEAM and a charge-exchange operator that follows neutrals and allows for multiple re-ionizations. Detailed integrated modelling with the METIS code yields the pressure and current profiles for various sets of achievable engineering parameters. The FIESTA code calculates the equilibrium and a Biot-Savart solver is used to calculate the intensity of the perturbation induced by the TF coils. Initial distributions of the NBI born fast ions are obtained from the newly developed NUR code, based on [S. Suzuki et al. 1998 Plasma Phys. Control. Fusion 40 2097]. We evolve the NBI ions during the complete thermalization process and we calculate the amount of NBI ions loss in the edge region due to neutralizations. Results indicate the NBI losses for various injection geometries, various engineering parameters and various assumptions on the magnitude of the background neutral density. [1] R. Panek et al. Fusion Engineering and Design 123 (2017) 11–1
Electrochemical Activation of Atomic Layer-Deposited Cobalt Phosphate Electrocatalysts for Water Oxidation
No full textThe development of efficient and stable earth-abundant water oxidation catalysts is vital for economically feasible water-splitting systems. Cobalt phosphate (CoPi)-based catalysts belong to the relevant class of nonprecious electrocatalysts studied for the oxygen evolution reaction (OER). In this work, an in-depth investigation of the electrochemical activation of CoPi-based electrocatalysts by cyclic voltammetry (CV) is presented. Atomic layer deposition (ALD) is adopted because it enables the synthesis of CoPi films with cobalt-to-phosphorous ratios between 1.4 and 1.9. It is shown that the pristine chemical composition of the CoPi film strongly influences its OER activity in the early stages of the activation process as well as after prolonged exposure to the electrolyte. The best performing CoPi catalyst, displaying a current density of 3.9 mA cm-2 at 1.8 V versus reversible hydrogen electrode and a Tafel slope of 155 mV/dec at pH 8.0, is selected for an in-depth study of the evolution of its electrochemical properties, chemical composition, and electrochemical active surface area (ECSA) during the activation process. Upon the increase of the number of CV cycles, the OER performance increases, in parallel with the development of a noncatalytic wave in the CV scan, which points out to the reversible oxidation of Co2+ species to Co3+ species. X-ray photoelectron spectroscopy and Rutherford backscattering measurements indicate that phosphorous progressively leaches out the CoPi film bulk upon prolonged exposure to the electrolyte. In parallel, the ECSA of the films increases by up to a factor of 40, depending on the initial stoichiometry. The ECSA of the activated CoPi films shows a universal linear correlation with the OER activity for the whole range of CoPi chemical composition. It can be concluded that the adoption of ALD in CoPi-based electrocatalysis enables, next to the well-established control over film growth and properties, to disclose the mechanisms behind the CoPi electrocatalyst activation
Characterisation of the influence of vanadium and tantalum on yttrium-based nano-oxides in ODS Eurofer steel
No full textOxide dispersion strengthened (ODS) steels are leading candidates for structural materials in nuclear fission and fusion power plants. Understanding the nature of nano-oxide particles in ODS steels is vital for a better control of the microstructure and mechanical properties to further their applications. In this study, electron microscopy and atom probe tomography (APT) have been used to investigate the nanocluster features in ODS Eurofer steel. With the addition of V and Ta in ODS Eurofer, the nanoclusters exhibit a higher number density with a decreased average diameter, indicating that V and Ta are beneficial for the formation of small clusters. Irrespective of the composition of the base material, the smaller particles have a variable stoichiometry while the larger particles are likely to have Y2O3 stoichiometry. The nanoclusters were found to have a core/shell structure, where Y, O and Ta are enriched in the core and Cr and V are predominant in the shell. The formation of the complex structure is possibly the result of a competing effect between Ta, Y, V and Cr binding with O. It is deduced that Ta tends to combine with O in the core (Y2O3) of the clusters due to a higher affinity, and pushes V and Cr to the surrounding shell during the formation of nanoclusters
Empowering hydrogen storage properties of haeckelite monolayers via metal atom functionalization
No full textUsing hydrogen as an energy carrier requires new technological solutions for its onboard storage. The exploration of two-dimensional (2D) materials for hydrogen storage technologies has been motivated by their open structures, which facilitates fast hydrogen kinetics. Herein, the hydrogen storage properties of lightweight metal functionalized r57 haeckelite sheets are studied using density functional theory (DFT) calculations. H2 molecules are adsorbed on pristine r57 via physisorption. The hydrogen storage capacity of r57 is improved by decorating it with alkali and alkaline-earth metals. In addition, the in-plane substitution of r57 carbons with boron atoms (B@r57) both prevents the clustering of metals on the surface of 2D material and increases the hydrogen storage capacity by improving the adsorption thermodynamics of hydrogen molecules. Among the studied compounds, B@r57-Li4, with its 10.0 wt% H2 content and 0.16 eV/H2 hydrogen binding energy, is a promising candidate for hydrogen storage applications. A further investigation, as based on the calculated electron localization functions, atomic charges, and electronic density of states, confirm the electrostatic nature of interactions between the H2 molecules and the protruding metal atoms on 2D haeckelite sheets. All in all, this work contributes to a better understanding of pure carbon and B-doped haeckelites for hydrogen storage.</p
A Closed-Form Solution to Estimate Spatially Varying Parameters in Heat and Mass Transport
No full textThis letter presents a closed-form solution to estimate space-dependent transport parameters of a linear one dimensional diffusion-transport-reaction equation. The infinite dimensional problem is approximated by a finite dimensional model by 1) taking a frequency domain approach, 2) linear parameterization of the unknown parameters, and 3) using a semi-discretization. Assuming full state knowledge, the commonly used output error criterion is rewritten as the equation error criterion such that the problem results in linear least squares. The optimum is then given by a closed-form solution, avoiding computational expensive optimization methods. Functioning of the proposed method is illustrated by means of simulation.</p
Low-cost wire-electrospun sulfonated poly(ether ether ketone)/poly(vinylidene fluoride) blend membranes for hydrogen-bromine flow batteries
No full textComparison of local and global gyrokinetic calculations of collisionless zonal flow damping in quasi-symmetric stellarators
No full textThe linear collisionless damping of zonal flows is calculated for quasi-symmetric stellarator equilibria in flux-tube, flux-surface, and fullvolume geometry. Equilibria are studied from the quasi-helical symmetry configuration of the Helically Symmetric eXperiment (HSX), a broken symmetry configuration of HSX, and the quasi-axial symmetry geometry of the National Compact Stellarator eXperiment (NCSX).
Zonal flow oscillations and long-time damping affect the zonal flow evolution, and the zonal flow residual goes to zero for small radial wavenumber. The oscillation frequency and damping rate depend on the bounce-averaged radial particle drift in accordance with theory. While each flux tube on a flux surface is unique, several different flux tubes in HSX or NCSX can reproduce the zonal flow damping from a flux-surface calculation given an adequate parallel extent. The flux-surface or flux-tube calculations can accurately reproduce the full-volume long-time residual for moderate k x, but the oscillation and damping time scales are longer in local representations, particularly for small k x approaching the system size
Bubble formation in liquid Sn under different plasma loading conditions leading to droplet ejection
No full textLiquid metals have been proposed as potential divertor materials for future fusion reactors, and surface stability is a vital requirement for such liquid metal divertors (LMDs). Capillary porous structures (CPSs) have been applied to the design of liquid metal targets as they can avoid MHD instability by surface tension and provide a stable liquid surface. However, our previous work has found that liquid Sn surfaces can be very unstable in hydrogen plasma even in cases without magnetic fields. To increase our understanding of the interaction of liquid Sn surfaces with plasmas, in this work we systematically investigated the surface behaviors of liquid Sn in different plasma exposures in linear plasma devices, either in Nano-PSI at low flux and without magnetic field, or in Magnum-PSI with strong magnetic field strength. Surface instability leading to droplet ejection has been observed and recorded in the experiments. The ejection of droplets is not dependent on magnetic fields and plasma currents, and is found to be dependent on the plasma species and plasma flux and surface temperature. The CPS meshes applied in the experiments cannot completely avoid droplet ejection but can decrease droplet size and lower droplet production rate. In H plasma, droplets were observed once Sn melted even at low fluxes. For the case of N plasma, the appearance of droplets started at a temperature marginally higher than tin–nitride decomposition temperature. Only at high fluxes (~1023–24 m−2 s−1) and high temperatures (900–1000 °C) were a few droplets observed in Ar or He plasma. For all cases, the ejection velocities of most droplets were around 1–5 m s−1. Bubble formation, growth and bursting in the plasma-species-supersaturated liquid Sn is proposed as the primary mechanism for the ejection of droplets. Plasma-enhanced solubility is responsible for the achievement of H/N-supersaturated liquid Sn, while high plasma flux implantation is responsible for Ar/He-supersaturated liquid Sn. Once the concentration of plasma species in liquid Sn reaches a certain supersaturation level, nucleation and growth of bubbles occur due to the desorption of dissolved plasma species from the liquid Sn. The formation and bursting of bubbles have been directly observed in the experiment. The sizes of most bubbles were estimated in the range of 40–400 μm or even smaller. A bubble growth model based on Sievert\u27s and Henry\u27s laws is invoked to describe bubble growth in liquid Sn.</p