Study of hydrogen isotopes behavior in tungsten by a multi trapping macroscopic rate equation model

International audienceDensity functional theory (DFT) studies show that in tungsten a mono vacancy can contain up to six hydrogen isotopes (HIs) at 300 K with detrapping energies varying with the number of HIs inthe vacancy. Using these predictions, a multi trapping rate equation model has been built and used to model thermal desorption spectrometry (TDS) experiments performed on single crystaltungsten after deuterium ions implantation. Detrapping energies obtained from the model to adjust temperature of TDS spectrum observed experimentally are in good agreement with DFTvalues within a deviation below 10%. The desorption spectrum as well as the diffusion of deuterium in the bulk are rationalized in light of the model results

Dynamic modelling of local fuel inventory and desorption in the whole tokamak vacuum vessel for auto-consistent plasma-wall interaction simulations

An extension of the SolEdge2D-EIRENE code package, named D-WEE, has been developed to add the dynamics of thermal desorption of hydrogen isotopes from the surface of plasma facing materials. To achieve this purpose, D-WEE models hydrogen isotopes implantation, transport and retention in those materials. Before launching auto-consistent simulation (with feedback of D-WEE on SolEdge2D-EIRENE), D-WEE has to be initialised to ensure a realistic wall behaviour in terms of dynamics (pumping or fuelling areas) and fuel content. A methodology based on modelling is introduced to perform such initialisation. A synthetic plasma pulse is built from consecutive SolEdge2D-EIRENE simulations. This synthetic pulse is used as a plasma background for the D-WEE module. A sequence of plasma pulses is simulated with D-WEE to model a tokamak operation. This simulation enables to extract at a desired time during a pulse the local fuel inventory and the local desorption flux density which could be used as initial condition for coupled plasma-wall simulations. To assess the relevance of the dynamic retention behaviour obtained in the simulation, a confrontation to post-pulse experimental pressure measurement is performed. Such confrontation reveals a qualitative agreement between the temporal pressure drop obtained in the simulation and the one observed experimentally. The simulated dynamic retention during the consecutive pulses is also studied.EURATOM 63305

Analytical model of hydrogen inventory saturation in the subsurface of the wall material and comparison to Reaction-Diffusion simulations

International audienceWe herein introduce an analytical model of hydrogen inventory saturation in the subsurface of materials (several micrometers depth) under plasma implantation. This model is valid for materials for which the desorption process is not limited by hydrogen recombination at the surface. It is based on a simplified assumption for the implantation of both hydrogen ions and atoms (point sources) and on a stationary approach. The model provides an approximation of the density profile of mobile/interstitial hydrogen and of the density profile of hydrogen trapped at materials defects in the subsurface layer. The analytical model shows good agreement with Reaction-Diffusion simulations of deuterium implantation in tungsten at different material temperatures. For the fusion relevant materials tungsten and beryllium, it is shown that most of the inventory is found in traps. The model gives the filling ratio of traps in the subsurface at steady-state f BULK stat,i. This simple parameter indicates how the total subsurface inventory builds up during plasma exposure and provides a simple way to understand the retention dynamics observed during non-linear Reaction-Diffusion simulations

Macroscopic rate equation modeling of trapping/detrapping of hydrogen isotopes in tungsten materials

International audienceCode development to solve numerically the model equations of diffusion and trapping of hydrogen in metals. Parametrization of the model trapping parameters (detrapping energies and density): fitting of experimental TDS spectrum. Confrontation model/experiment: evolution of retention with fluence and implantation temperature. Investigation of period of rest between implantation and TDS on retention and depth profile. a b s t r a c t Relevant parameters for trapping of Hydrogen Isotopes (HIs) in polycrystalline tungsten are determined with the MHIMS code (Migration of Hydrogen Isotopes in MaterialS) which is used to reproduce Thermal Desorption Spectrometry experiments. Three types of traps are found: two intrinsic traps (detrapping energy of 0.87 eV and 1.00 eV) and one extrinsic trap created by ion irradiation (detrapping energy of 1.50 eV). Then MHIMS is used to simulate HIs retention at different fluences and different implantation temperatures. Simulation results agree well with experimental data. It is shown that at 300 K the retention is limited by diffusion in the bulk. For implantation temperatures above 500 K, the retention is limited by trap creation processes. Above 600 K, the retention drops by two orders of magnitude as compared to the retention at 300 K. With the determined detrapping energies, HIs outgassing at room temperature is predicted. After ions implantation at 300 K, 45% of the initial retention is lost to vacuum in 300 000 s while during this time the remaining trapped HIs diffuse twice as deep into the bulk

Hydrogen and oxygen on tungsten (110) surface: adsorption, absorption and desorption investigated by density functional theory

International audienceIn this work we investigated the adsorption of oxygen and the co-adsorption of oxygen and hydrogen on the (110) surface of tungsten by means of Density Functional calculations. The absorption, recombination and release mechanisms of hydrogen across the (110) surface with oxygen are further established at saturation and above saturation of the surface. It is found that hydrogen and oxygen both adsorb preferentially at three-fold sites. The saturation limit was determined to one monolayer in adsorbate. Oxygen is found to lower the binding energy of hydrogen on the surface and to lower the activation barrier for the recombination of molecular hydrogen. Finally, as on the clean surface, oversaturation in adsorbate is shown to lower both activation barriers for hydrogen absorption and for molecular hydrogen recombination on the (110) surface of tungsten

Comparison of dynamic deuterium retention in single-crystal and poly-crystals of tungsten: The role of natural defects

International audienceThe time evolution of fuel retention in materials relevant for future fusion reactors is compared for two different tungsten microstructures: single-crystal versus recrystallized poly-crystals. The initial retention of both types of sample is similar. It decays exponentially with a time constant of ~18 hours at 300 K (the so-called short-term retention). After 48 hours at room temperature, a constant deuterium retention is measured (long-term retention) with the single-crystal containing systematically less deuterium than poly-crystals. Macroscopic rate equations models are built with density functional theory inputs to reproduce deuterium desorption observables with the MHIMS-R code. We found that the native oxide layer could explain the desorption peak located at ~450 K as well as most of the short-term retention in the single-crystal. The native oxide together with, dislocations for single-crystal and grain boundaries for poly-crystals, are responsible for the long-term retention. Dislocations should explain the desorption peak located at ~815 K for mechanically polished samples. The dual role of most of tungsten defects is related to their multi-trapping properties with filling-level-dependent detrapping energies. Finally, the use of an effective diffusivity of deuterium through the native oxide layer, i.e. its diffusion barrier character, is evaluated

Simulations of atomic deuterium exposure in self-damaged tungsten

International audienceSimulations of deuterium (D) atom exposure in self-damaged polycrystalline tungsten at 500 K and 600 K are performed using an evolution of the MHIMS (migration of hydrogen isotopes in materials) code in which a model to describe the interaction of D with the surface is implemented. The surface-energy barriers for both temperatures are determined analytically with a steady-state analysis. The desorption energy per D atom from the surface is 0.69  ±  0.02 eV at 500 K and 0.87  ±  0.03 eV at 600 K. These values are in good agreement with ab initio calculations as well as experimental determination of desorption energies. The absorption energy (from the surface to the bulk) is 1.33  ±  0.04 eV at 500 K, 1.55  ±  0.02 eV at 600 K when assuming that the resurfacing energy (from the bulk to the surface) is 0.2 eV. Thermal-desorption spectrometry data after D atom exposure at 500 K and isothermal desorption at 600 K after D atom exposure at 600 K can be reproduced quantitatively with three bulk-detrapping energies, namely 1.65  ±  0.01 eV, 1.85  ±  0.03 eV and 2.06  ±  0.04 eV, in addition to the intrinsic detrapping energies known for undamaged tungsten (0.85 eV and 1.00 eV). Thanks to analyses of the amount of traps during annealing at different temperatures and ab initio calculations, the 1.65 eV detrapping energy is attributed to jogged dislocations and the 1.85 eV detrapping energy is attributed to dislocation loops. Finally, the 2.06 eV detrapping energy is attributed to D trapping in cavities based on literature reporting observations on the growth of cavities, even though this could also be understood as D desorbing from the C-D bond in the case of hydrocarbon contamination in the experimental sample