On the binding of nanometric hydrogen–helium clusters in tungsten

In this work we developed an embedded atom method potential for large scale atomistic simulations in the ternary tungsten–hydrogen–helium (W–H–He) system, focusing on applications in the fusion research domain. Following available ab initio data, the potential reproduces key interactions between H, He and point defects in W and utilizes the most recent potential for matrix W. The potential is applied to assess the thermal stability of various H–He complexes of sizes too large for ab initio techniques. The results show that the dissociation of H–He clusters stabilized by vacancies will occur primarily by emission of hydrogen atoms and then by break-up of V–He complexes, indicating that H–He interaction does influence the release of hydroge

Hybrid quantum/classical study of hydrogen-decorated screw dislocations in tungsten : ultrafast pipe diffusion, core reconstruction, and effects on glide mechanism

The interaction of hydrogen (H) with dislocations in tungsten (W) must be understood in order to model the mechanical response of future plasma-facing materials for fusion applications. Here, hybrid quantum mechanics/molecular mechanics (QM/MM) simulations are employed to study the ⟨111⟩ screw dislocation glide in W in the presence of H, using the virtual work principle to obtain energy barriers for dislocation glide, H segregation, and pipe diffusion. We provide a convincing validation of the QM/MM approach against full DFT energy-based methods. This is possible because the compact core and relatively weak elastic fields of ⟨111⟩ screw dislocations allow them to be contained in periodic DFT supercells. We also show that H segregation stabilizes the split-core structure while leaving the Peierls barrier almost unchanged. Furthermore, we find an energy barrier of less than 0.05 eV for pipe diffusion of H along dislocation cores. Our quantum-accurate calculations provide important reference data for the construction of larger-scale material models

Evaluación de la retención de elementos del plasma en tungsteno bajo exposición a plasma de altos flujos : un enfoque de modelización multi-escala

Tesis de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física Atómica, Molecular y Nuclear, leída el 27-04-2017Nuclear fusion can be regarded as a potentially clean, secure and virtually unlimited source of energy for the future. Currently, the most promising reactor concept – the so-called "tokamak" device employs magnetic confinement of fusion plasma. One of the most ambitious energy-related projects today is the construction of the world’s largest tokamak, also known as ITER, “The Way” in Latin. The experimental campaigns planned at ITER aim at testing integrated technologies, materials and physical regimes necessary for commercial production of fusion-based electricity. In other words, ITER aims at bridging the gap between today’s smaller fusion devices and the demonstrational power plant of the future, the DEMO reactor...La fusión nuclear puede ser considerada como potencialmente limpia, segura y virtualmente una fuente ilimitada de energía para el futuro. Actualmente, el concepto mas prometedor – el llamado “tokamak” utiliza el confinamiento magnético del plasma de fusión. Uno de los proyectos mas ambiciosos relacionados con la energía es hoy en día la construcción del tokamak mas grande del mundo, también conocido como ITER, El Camino en Latin. La campaña experimental planeada en ITER tiene como objetivo probar las tecnologías, los materiales y los regímenes físicos necesarios para la producción comercial de electricidad basada en la fusión nuclear. En otras palabras, el objetivo de ITER es de tender un puente entre el dispositivo de fusión mas pequeño que existe hoy y la planta de fusión nuclear del futuro, el reactor DEMO...Fac. de Ciencias FísicasTRUEunpu

Numerical analysis of TDS spectra under high and low flux plasma exposure conditions

A recently developed numerical model, based on the dislocation-driven nucleation of gas bubbles, is used to analyse experimental results on deuterium retention in tungsten under ITER relevant plasma exposure conditions. Focus is placed on understanding the relation between exposure temperature and flux on primary features of thermal desorption spectra: peak positions and intensities of the desorption flux. The model allows one to relate the peak positions with the size of plasma induced deuterium bubbles and envisage exposure conditions (temperature and flux) for their formation. Based on the performed analysis, dedicated experimental conditions to validate the model are proposed

Efficient and transferable machine learning potentials for the simulation of crystal defects in bcc Fe and W

Data-driven, or machine learning (ML), approaches have become viable alternatives to semiempirical methods to construct interatomic potentials, due to their capacity to accurately interpolate and extrapolate from first-principles simulations if the training database and descriptor representation of atomic structures are carefully chosen. Here, we present highly accurate interatomic potentials suitable for the study of dislocations, point defects, and their clusters in bcc iron and tungsten, constructed using a linear or quadratic input-output mapping from descriptor space. The proposed quadratic formulation, called quadratic noise ML, differs from previous approaches, being strongly preconditioned by the linear solution. The developed potentials are compared to a wide range of existing ML and semiempirical potentials, and are shown to have sufficient accuracy to distinguish changes in the exchange-correlation functional or pseudopotential in the underlying reference data, while retaining excellent transferability. The flexibility of the underlying approach is able to target properties almost unattainable by traditional methods, such as the negative divacancy binding energy in W or the shape and the magnitude of the Peierls barrier of the 1 2 ⟨ 111 ⟩ screw dislocation in both metals. We also show how the developed potentials can be used to target important observables that require large time-and-space scales unattainable with first-principles methods, though we emphasize the importance of thoughtful database design and degrees of nonlinearity of the descriptor space to achieve the appropriate passage of information to large-scale calculations. As a demonstration, we perform direct atomistic calculations of the relative stability of 1 2 ⟨ 111 ⟩ dislocations loops and three-dimensional C15 clusters in Fe and find the crossover between the formation energies of the two classes of interstitial defects occurs at around 40 self-interstitial atoms. We also compute the kink-pair formation energy of the 1 2 ⟨ 111 ⟩ screw dislocation in Fe and W, finding good agreement with density functional theory informed line tension models that indirectly measure those quantities. Finally, we exploit the excellent finite-temperature properties to compute vacancy formation free energies with full anharmonicity in thermal vibrations. The presented potentials thus open up many avenues for systematic investigation of free-energy landscape of defects with ab initio accuracy

matscipy : materials science at the atomic scale with Python

Behaviour of materials is governed by physical phenomena that occur at an extreme range of length and time scales. Computational modelling requires multiscale approaches. Simulation techniques operating on the atomic scale serve as a foundation for such approaches, providing necessary parameters for upper-scale models. The physical models employed for atomic simulations can vary from electronic structure calculations to empirical force fields. However, construction, manipulation and analysis of atomic systems are independent of the given physical model but dependent on the specific application. matscipy implements such tools for applications in materials science, including fracture, plasticity, tribology and electrochemistry

Assessment of retention of plasma components in tungsten under high flux plasma exposure : multi-scale modelling approach

Nuclear fusion can be regarded as a potentially clean, secure and virtually unlimited source of energy for the future. Currently, the most promising reactor concept – the so-called "tokamak" device employs magnetic confinement of fusion plasma. One of the most ambitious energy-related projects today is the construction of the world’s largest tokamak, also known as ITER, “The Way” in Latin. The experimental campaigns planned at ITER aim at testing integrated technologies, materials and physical regimes necessary for commercial production of fusion-based electricity. In other words, ITER aims at bridging the gap between today’s smaller fusion devices and the demonstrational power plant of the future, the DEMO reactor..