• Energy Research
• 2021-2025close
• physical sciences close
• Nuclear Materials and Energy close

Superpermeation allows for hydrogen fluxes through metal foil membranes at rates orders of magnitude higher than pressure driven permeation. This process occurs only for hydrogen isotopes, meaning it is hydrogen-selective, and it can work against a pressure gradient, implying pumping capabilities. These characteristics allow for using superpermeation as the base process for a very efficient, selective separation of hydrogen from other gases. However, the efficacy of superpermeation needs further research both experimentally and theoretically. Its efficiency relies on a surface energetic barrier that hinders both adsorption of molecular hydrogen on the downstream side and desorption on the upstream side, while leaving atomic hydrogen absorption unaffected. Such a barrier can be created by a monolayer of non-metallic impurities (usually oxygen) that naturally develops at group 5 metal surfaces. The physics explaining why such a monolayer drastically affects atomic hydrogen reactions are being explored in this work via density functional theory (DFT) calculations for the implementation of which we use the Vienna ab-initio Simulations Package (VASP). By performing structural relaxations and saddle point-searching calculations deploying a dimer method using VASP, energy diagrams for atomic hydrogen absorption are obtained for two representative materials, namely niobium and vanadium. The differences in these diagrams are analyzed and compared in order to determine which material is optimal for superpermeation. To that end, slabs with (1 0 0) surface orientation are compared for the case with and without an O monolayer coverage. The characteristic energies involved according to the diagrams and the types of absorption sites will be key parameters to understand and, eventually, optimize for the emerging phenomena. It was found that the presence of an oxygen monolayer is necessary of superpermeation to occur, and that for the 100 orientation, the vanadium system provides better characteristics.

Since tungsten (W) was considered as the most promising plasma facing materials (PFMs) in fusion reactors, there has been extensive research on the physical performance of W-PFMs. It is found that under the extreme conditions in a fusion reactor, W-PFMs should be in a nonequilibrium state of high electronic temperature and low ionic temperature. This leads to the possibility of non-thermal phase transitions, where the crystal structure of the tungsten material may change from body-centered cubic (bcc) phase to hexagonal close-packed (hcp) phase or face-centered cubic (fcc) phase. Consequently, it is necessary to investigate the relevant physical properties of hcp-W and fcc-W under the electron-excited state. In this work, the fundamental physical properties, including atomic structures, electronic structures, elastic constants, and vacancy formation energies, of bcc-W, hcp-W and fcc-W, were theoretically calculated at various electronic temperatures. The mechanical stability of these three phases was also systematically analyzed under varying electronic temperatures. The results of this research are expected to provide a certain guidance in the optimization of W-PFMs in future fusion reactors.

The clean wall condition in tokamak devices can be achieved by utilizing the active chemical property of lithium (Li). This also exposes Li to air contamination even when protected by inert gas. To determine whether air contamination affects the retention of hydrogen isotopes in Li, the desorption performances of deuterium (D2) in liquid Li with D/Li ratios of 4.1 mol.%, 2.5 mol.%, and 0.6 mol.% were compared. At a D/Li ratio as low as 0.6 mol.%, the results revealed a significant change in the desorption temperature of D2 by Li contaminants. The ratio of contaminants to D2 in Li was discovered to be important. In this regard, the influence of N2, O2, and H2O (accounting for 15 mol.% of D2) on the desorption performance of D2 in liquid Li was discussed. Both H2O and N2 preabsorbed in liquid Li could react with D2, leading to the changed desorption temperature of D2. Notably, the H2O present in Li consumed all of D2 via reaction, whereas the same amount of N2 as H2O could only react with a part of D2. However, the effect of O2 on the desorption temperature and the effect of impurities on the desorption amount of D2 were not obvious. The impacts of the findings of this work on the retention and extraction of hydrogen isotopes in Li may be further explored to prompt the application of Li in fusion devices.

The understanding and the predictive capability for turbulence in the plasma edge and scrape-off layer (SOL) are crucial for the development of magnetic confinement fusion reactors. To this end, we characterise turbulent transport across the edge and SOL of the diverted ASDEX Upgrade tokamak in attached L-mode conditions by means of validated, global simulations. The collisionality is controlled by the divertor neutrals density, as their ionisation increases the plasma density and decreases the temperature. The radial E×B particle and heat transport, quantified by effective diffusivities, rises strongly with collisionality. The modest increase in fluctuation amplitudes is not a sufficient explanation. The reason is shown to be the destabilisation of resistive drift-ballooning modes, resulting in larger phase shifts between the pressure and electrostatic potential. The transport varies both radially and poloidally. Due to its ballooning nature, radial transport is close to zero on the inboard mid-plane. On the outboard mid-plane, significant transport is driven in the SOL by large filaments (blobs) with amplitudes of up to 250% of the mean, propagating ballistically from the separatrix to the wall. This non-local transport leads to large radial variations of diffusivities, as they do not necessarily correlate with the local gradient. Ion temperature fluctuations in the plasma edge are shown to be involved in blob seeding at the separatrix, and are the largest in the SOL. Radial diffusivities peak at the top and bottom of the device, since gradients are flatter there due to the flux expansion while the cross-field flow is sustained by streamers—a feature which should be considered in mean-field transport modelling. The increase of SOL E×B transport with collisionality is likely fostered by the simultaneously decreasing radial electric field, resulting from a flattened electron temperature profile. Large amplitude blobs are a hazard for plasma facing components of fusion reactors, but they could be restrained by control of SOL collisionality.

Oxide dispersion strengthened tungsten (ODS-W) is a potential candidate for plasma-facing materials (PFMs) in future fusion reactors. In this work, deuterium (D) retention and surface blistering in W-1 wt% La2O3 (W-La2O3) have been investigated after exposure to low-energy (40 eV) D plasma with various exposure temperatures (400–600 K) and fluences (3.6 × 1024–1.4 × 1025 D/m2). Surface blistering and D retention exhibit a strong dependence on the exposure temperature and fluence. The most pronounced effect is found at 500 K. The blister-induced defects including dislocations and vacancies are considered to dominate the D retention. At 400 K and 600 K, the D retained in W-La2O3 is governed by unique intrinsic defects including interfaces, micro-pores, and unoxidized La particles. Regarding the exposure fluence, as expected, surface blistering and D retention are positively correlated with it, in which two dominant stages of nucleation and growth for blistering are identified from the changes in area density and size of blisters. Based on the results obtained from W-La2O3, comparisons with W are performed with the exposure condition (500 K, 1.4 × 1025 D/m2) where the blistering and D retention is most pronounced. Although the area density of blisters is similar between the two materials, the average size of blisters is larger in W-La2O3. Notably, an additional high-temperature D desorption shoulder appears in the release spectra of W-La2O3, which is probably due to the particular defects such as interfaces, micro-pores and La particles, and finally resulting in a higher D retention in W-La2O3 than that in W.

Plasma facing materials (PFMs) are subjected to long-duration high-energy particle streams and radiation in tokamak devices. The PFMs of EAST have been upgraded several times and Titanium-Zirconium-Molybdenum (TZM) tiles were installed into EAST as its first wall since 2011. However, with the gradually increasing of plasma parameters, several unexpected TZM melting phenomena were found at the high field side by post mortem inspection after each EAST plasma experimental campaign since 2017. The resolidified melted surface is general in wave shape with unobvious motion of melting layer. Three different grain shapes, i.e., columnar grain, isometric crystal and original rolled crystal from surface to deep region are found by means of metallurgical analysis, in which the superficial layer columnar grain is very thin with a thickness of 100 ∼ 200 μm and the thickness of intermediate isometric crystal is also small only about 300 ∼ 400 μm, strongly indicating there was a large temperature gradient near surface when melting occurred. Combined with plasma operation parameters and temperature evolution, the melting of TZM tiles were concluded to be induced by the transient heat flux during plasma disruption. These results imply the transient heat flux during plasma disruption in EAST can severely destruct the metal PFMs and should not be ignored, suggesting the active mitigation of plasma disruption is necessary for future long pulse and high parameters operation.

Infrared (IR) thermography system is a key diagnostic in fusion devices to monitor the Plasma Facing Components. Nevertheless, both qualitative and quantitative analysis (i.e. hot spot detection and surface temperature measurement) are challenging due to the presence of disturbance phenomena like variable emissivity and multiple reflections in fully metallic environment. Through the comparison with the experimental IR measurements, simulation is an essential tool for anticipating, quantifying and analysing the effects of the various errors involved in the interpretation of IR images. This paper goes a step further for achieving IR quantitative thermography in developing inverse methods to retrieve the real surface temperature, by taking into account variable emissivity and filtering reflections. Two approaches are studied: (1) using gradients methods through a reduced photonic model (2) using machine learning techniques based on simulated dataset. Applied on WEST-like tokamak numerical prototype, the temperatures are estimated, with these two approaches, with an accuracy better than 6%, which is a clear improvement compared to usual methods (i.e. assuming blackbody object).

Simultaneous control of transient heat load induced by large-amplitude edge-localized modes (ELMs) and steady-state heat load on divertor targets under metal wall environment is crucial for steady-state operation of future tokamak fusion reactors, such as ITER and the China Fusion Engineering Test Reactor (CFETR). In the recent experiments, sustained partial energy detachment without confinement degradation has been achieved in the Experimental Advanced Superconducting Tokamak (EAST) in high-performance grassy-ELM H-mode with q95 ~ 5.9 by a newly developed detachment feedback control scheme, in which we first used electron temperature (Tet) measured by divertor Langmuir probes to identify the onset of energy detachment, and then the system switched to the feedback control of total radiation power measured by absolute extreme ultraviolet (AXUV) system. Tet around the upper outer strike point was successfully maintained less than 8 eV with seeding of 80% Ne and 20% D2 mixture from upper outer divertor, and the total radiation power was maintained ~1.4 MW, around 52% of injected power. There was no significant decrease of the plasma stored energy and H98,y2 factor (~1) over the entire detachment feedback control process. These experiment results demonstrate good compatibility of the high-performance grassy-ELM regime with radiative divertor. In order to confirm the compatibility in a wider range, stable partial energy detachment in grassy-ELM H-mode with relatively lower q95 (~5.4) was also achieved in EAST through the newly developed integrated-feedback-control technique. The new detachment feedback control without confinement degradation in grassy-ELM H-mode provides a candidate mode for EAST long-pulse operation in the future with well control of ELM-induced transient and steady heat fluxes on the divertor target.