DIFFER: Publications
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Application of multiple regression for sensitivity analysis of helium line emissions to the electron density and temperature in Magnum-PSI
Get PDFHelium line intensities have been utilized to measure the electron density, n e, and temperature, T e, by comparing measured line intensities to a collisional-radiative model (CRM). In this study, we use multiple regression analysis to train a model of the helium line intensities and n e/T e obtained from a Thomson scattering system in the linear plasma device Magnum-PSI; based on the trained model, we predict n e and T e from line intensities. We show that this method can also obtain radial profiles of n e and T e. We discuss appropriate selections of line pairs for the prediction based on the multiple regression analysis. A big advantage of this method against the standard technique using CRM is that modeling of atomic population distributions is not required, which sometimes needs to take into account various effects such as radiation trapping, transport of helium atoms in metastable states, etc.</p
Tin deposition on ruthenium and its influence on blistering in multi-layer mirrors
No full textAn atomistic description of tin deposition on ruthenium and its effect on blistering damage is of great interest in extreme ultraviolet (EUV) lithography. In EUV machines, tin debris from the EUV-emitting tin plasma may be deposited on the mirrors in the optical path. Tin facilitates the formation of hydrogen-filled blisters under the ruthenium top layer of the multi-layer mirrors. We have used Density Functional Theory (DFT) to show that tin deposition on a clean ruthenium surface exhibits a film-plus-islands (Stranski–Krastanov) growth mode, with the first atomic layer bonding strongly to the substrate. We find that a single tin layer allows hydrogen to reach the ruthenium surface and subsurface more easily than on clean ruthenium, but hydrogen penetration through the tin film becomes progressively more difficult when more layers are added. The results indicate that hydrogen penetration and blistering occur when only a thin layer of tin is present
Development of an 11-channel multi wavelength imaging diagnostic for divertor plasmas in MAST Upgrade
No full textDivertor detachment and alternative divertor magnetic geometries are predicted to be promising approaches to handle the power exhaust of future fusion devices. In order to understand the detachment process caused by volumetric losses in alternative divertor magnetic geometries, a Multi-Wavelength Imaging (MWI) diagnostic has recently been designed and built for the Mega Amp Spherical Tokamak Upgrade. The MWI diagnostic will simultaneously capture 11 spectrally filtered images of the visible light emitted from divertor plasmas and provide crucial knowledge for the interpretation of observations and modeling efforts. This paper presents the optical design, mechanical design, hardware, and test results of an 11-channel MWI system with a field of view of 40°. The optical design shows better than 5 mm FWHM spatial resolution at the plasma on all 11 channels across the whole field of view. The spread of angle of incidence on the surface of each filter is also analyzed to inform the bandwidth specification of the interference filters. The results of the initial laboratory tests demonstrate that a spatial resolution of better than 5 mm FWHM is achieved for all 11 channels, meeting the specifications required for accurate tomography.</p
Rational Design of Photoelectrodes for the Fully Integrated Polymer Electrode Membrane–Photoelectrochemical Water-Splitting System: A Case Study of Bismuth Vanadate
Get PDFPhotoelectrochemical (PEC) reactors based on polymer electrolyte membrane (PEM) electrolyzers are an attractive alternative to improve scalability compared to conventional monolithic devices. To introduce narrow band gap photoabsorbers such as BiVO4 in PEM-PEC system requires cost-effective and scalable deposition techniques beyond those previously demonstrated on monolithic FTO-coated glass substrates, followed by the preparation of membrane electrode assemblies. Herein, we address the significant challenges in coating narrow band gap metal-oxides on porous substrates as suitable photoelectrodes for the PEM-PEC configuration. In particular, we demonstrate the deposition and integration of W-doped BiVO4 on porous conductive substrates by a simple, cost-effective, and scalable deposition based on the SILAR (successive ionic layer adsorption and reaction) technique. The resultant W-doped BiVO4 photoanode exhibits a photocurrent density of 2.1 mA·cm–2, @1.23V vs RHE, the highest reported so far for the BiVO4 on any porous substrates. Furthermore, we integrated the BiVO4 on the PEM-PEC reactor to demonstrate the solar hydrogen production from ambient air with humidity as the only water source, retaining 1.55 mA·cm–2, @1.23V vs RHE. The concept provides insights into the features necessary for the successful development of materials suitable for the PEM-PEC tandem configuration reactors and the gas-phase operation of the reactor, which is a promising approach for low-cost, large-scale solar hydrogen production.</p
The role of target closure in detachment in Magnum-PSI
Get PDFA cylindrical target with a high degree of closure was exposed to ITER divertor-relevant plasmas with typical electron temperatures of 2 eV, electron densities of 5⋅1020 m−3, and heat fluxes up to 20 MWm−2 in the linear device Magnum-PSI. By terminating the plasma in an unpumped closed volume, neutral pressures were enhanced from about 0.5 to 20 Pa without any increase in the neutral flux returning to the plasma. Such pressures were sustained largely by the pressure exerted by the incoming plasma. By means of hydrogen gas injection, internal neutral pressures of up to 40 Pa were reached during plasma exposure. We find that at these high neutral pressures, a < 1 eV recombination front forms and expands from the back of the cylinder, so that downstream density drops dramatically. Furthermore, in these scenarios, heat deposition to the back plate vanishes and is redirected to the upstream part of the cylinder and to hot neutrals, which can carry 50% of the plasma input power. A power balance analysis reveals that even without additional gas puffing, only about 10% of the incoming heat load reaches the back plate for the 20 MWm−2 plasma. These results demonstrate the important role of closed target configurations and local gas puffing in mitigating plasma heat loads and indicate that the gained experience should be taken into account in next-generation divertor designs.</p
Evaluation of Computational Chemistry Methods for Predicting Redox Potentials of Quinone-Based Cathodes for Li-Ion Batteries
No full textHigh-throughput computational screening (HTCS) is an effective tool to accelerate the discovery of active materials for Li-ion batteries. For the evaluation of organic cathode materials, the effectiveness of HTCS depends on the accuracy of the employed chemical descriptors and their computing cost. This work was focused on evaluating the performance of computational chemistry methods, including semi-empirical quantum mechanics (SEQM), density-functional tight-binding (DFTB), and density functional theory (DFT), for the prediction of the redox potentials of quinone-based cathode materials for Li-ion batteries. In addition, we evaluated the accuracy of three energy-related descriptors: (1) the redox reaction energy, (2) the lowest unoccupied molecular orbital (LUMO) energy of reactant molecules, and (3) the highest occupied molecular orbital (HOMO) energy of lithiated product molecules. Among them, the LUMO energy of the reactant compounds, regardless of the level of theory used for its calculation, showed the best performance as a descriptor for the prediction of experimental redox potentials. This finding contrasts with our earlier results on the calculation of quinone redox potentials in aqueous media for redox flow batteries, for which the redox reaction energy was the best descriptor. Furthermore, the combination of geometry optimization using low-level methods (e.g., SEQM or DFTB) followed by energy calculation with DFT yielded accuracy as good as the full optimization of geometry using the DFT calculations. Thus, the proposed calculation scheme is useful for both the optimum use of computational resources and the systematic generation of robust calculation data on quinone-based cathode compounds for the training of data-driven material discovery models
Threshold Heat-Flux Reduction by Near-Resonant Energy Transfer
Get PDFNear-resonant energy transfer to large-scale stable modes is shown to reduce transport above the linear critical gradient, contributing to the onset of transport at higher gradients. This is demonstrated for a threshold fluid theory of ion temperature gradient turbulence based on zonal-flow-catalyzed transfer. The heat flux is suppressed above the critical gradient by resonance in the triplet correlation time, a condition enforced by the wave numbers of the interaction of the unstable mode, zonal flow, and stable mode.</p