DIFFER: Publications
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Wave modes in a cold pair plasma: the complete phase and group diagram point-of-view
Get PDFWe present a complete analysis of all wave modes in a cold pair plasma, significantly extending standard textbook treatments. Instead of identifying the maximal number of two propagating waves at fixed frequency w we introduce a unique labelling of all 5 mode pairs described by the general dispersion relation w(k), starting from their natural ordering at small wavenumber k. There, the 5 pairs start off as Alfvén (A), fast magnetosonic (F), modified electrostatic (M) and electromagnetic O and X branches, and each w(k) branch smoothly connects to large wavenumber resonances or limits. For cold pair plasmas, these 5 branches show avoided crossings, which become true crossings at exactly parallel or perpendicular orientation. Only for those orientations, we find a changed connectivity between small and large wavenumber behaviour. Analysing phase and group diagrams for all 5 wave modes, distinctly different from the Clemmow–Mullaly–Allis representation, reveals the true anisotropy of the A, M and O branches.</p
Limits to Strong Coupling of Excitons in Multilayer WS2 with Collective Plasmonic Resonances
No full textWe demonstrate the strong coupling of direct transition excitons in tungsten disulfide (WS2) with collective plasmonic resonances at room temperature. We use open plasmonic cavities formed by periodic arrays of metallic nanoparticles. We show clear anti-crossings with monolayer, bilayer, and thicker multilayer WS2 on top of the nanoparticle array. The Rabi energy of such hybrid system varies from 50 to 100 meV from monolayers to 16 layers, respectively, while it does not scale with the square root of the number of layers as expected for collective strong coupling. We prove that out-of-plane coupling components can be disregarded because the normal field is screened due to the high refractive index contrast of the dielectric layers. Even though the in-plane dipole moments of the excitons decrease beyond monolayers, the strong in-plane field distributed in the flake can still enhance the coupling strength with multilayers. The achieved coherent coupling of TMD multilayers with open cavities could be exploited for manipulating the dynamics and transport of excitons in 2D semiconductors and developing ultrafast spin-valley tronic devices.</p
Power handling and vapor shielding of pre-filled lithium divertor targets in Magnum-PSI
Get PDFTo develop realistic liquid lithium divertors for future fusion reactors, this paper aims to improve the understanding of their power handling capabilities. A liquid lithium divertor target prototype, designed to facilitate liquid metal experiments in tokamaks, was tested in Magnum-PSI. The target has an internal reservoir pre-filled with lithium and aims to passively re-supply the textured plasma facing surface during operation. To assess the power handling capability the target was exposed to helium plasmas with increasing power flux density in the linear plasma device Magnum-PSI. The temperature response of lithium targets was recorded via an infrared camera, and compared to finite element method modeling taking into account dissipation via lithium in the plasma. It was found that the target works as intended and can take up to 9 1 MW m−2 for 10 s before the mesh layer was damaged, and could continue operating at higher power densities even after being damaged. The total lifetime of the targets was up to 100 s. Overall the targets are found suitable for use in tokamak experiments. Additionally, a central surface temperature evolution indicative of vapor shielding was observed on intact targets. Predicting the target temperature (and consequently the evaporation rates and thermal stresses) is considered very relevant for the design of lithium divertor targets for DEMO. The observed temperature response could indeed be replicated through modeling, which showed that a significant power fraction was dissipated by the lithium in the plasma.</p
Overpotential analysis of alkaline and acidic alcohol electrolysers and optimized membrane-electrode assemblies
Get PDFAlcohol electrolysis using polymeric membrane electrolytes is a promising route for storing excess renewable energy in hydrogen, alternative to the thermodynamically limited water electrolysis. By properly choosing the ionic agent (i.e. H+ or OH) and the catalyst support, and by tuning the catalyst structure, we developed membrane-electrode-assemblies which are suitable for cost-effective and efficient alcohol electrolysis. Novel porous electrodes were prepared by Atomic Layer Deposition (ALD) of Pt on a TiO2-Ti web of microfibers and were interfaced to polymeric membranes with either H+ or OH conductivity. Our results suggest that alcohol electrolysis is more efficient using OH conducting membranes under appropriate operation conditions (high pH in anolyte solution). ALD enables better catalyst utilization while it appears that the TiO2-Ti substrate is an ideal alternative to the conventional carbon-based diffusion layers, due to its open structure. Overall, by using our developmental anodes instead of commercial porous electrodes, the performance of the alcohol electrolyser (normalized per mass of Pt) can be increased up to ~30 times.</p
Effects of transition metal dopants on the calcination of CaCO3 under Ar, H2O and H2
Get PDFThis study compares thermal decomposition of pure and transition metal-doped (Fe, Co, Ni, Cu, and Zn) calcium carbonate samples under ˜0.1 mbar of Ar, H2O, and H2 in order to evaluate the effects of doping on CO2 release and conversion. All samples were synthesized via precipitation methods at room temperature from calcium chloride and sodium carbonate precursors, with additional doping of the relevant transition metal chloride. Structural and compositional analysis of the as-prepared and calcined materials is presented. TM-doping results in an earlier onset of CO2 release as compared to Pure CaCO3 irrespective of calcination gas. Cu-doping induced the largest temperature reduction. Calcination in H2O produces an additional lowering of the release temperature, as compared with calcination in Ar, with the Zn-doped sample exhibiting the largest enhancement. During calcination in H2, the Ni-, Co- and Fe-doped samples produce a significant enhancement of CO2 to CO conversion, whereas the overall conversion by the Cu- and Zn-doped samples remained comparable to that of Pure CaCO3. The Ni-doped samples, which produced the highest CO2 conversion, showed the largest comparative enhancement when the calcination gas was changed to H2.</p
Monolayer Nitrides Doped with Transition Metals as Efficient Catalysts for Water Oxidation: The Singular Role of Nickel
Get PDFExploration of precious-metal-free catalysts for water splitting is of great importance in developing renewable energy conversion and storage technologies. In this paper, on the basis of density functional theory calculations, we reveal the link between the oxygen evolution reaction (OER) activities and the electronic properties of pure and first-row transition-metal (TM)-doped AlN and GaN two-dimensional monolayers. We find that Ni-doped layers are singularly appealing because they lead to a low overpotential (0.4 V). Early TM dopants are not suited for the OER because they bind the intermediate species OH or O too strongly, which leads to very large overpotentials, or no OER activity at all. The late TM dopants Cu and Zn show less or no OER activity as they bind the intermediate species too weakly. Although in many cases the overpotential can be traced back to an OOH intermediate species being adsorbed too weakly compared to an OH species, the Ni dopant breaks this rule by stabilizing the OOH adsorbant. The stabilization can be correlated with a switch from a high-spin to a low-spin state of the dopant atom. This ability to change spin states offers an exciting ingredient for the design of OER catalysts.</p
Improvements in physics models of AFSI-ASCOT-based synthetic neutron diagnostics at JET
No full textNew development steps of AFSI-ASCOT based synthetic neutron diagnostics and validation at JET are reported in this contribution. Synthetic neutron diagnostics are important not only in existing tokamaks, where they are used to interpret experimental data, but also in the design of future reactors including DEMO and beyond, where neutron detectors are one of the few diagnostics available. Thus, development and validation of realistic synthetic diagnostics is necessary for increasing confidence in existing models and future diagnostic designs.
Recent development in AFSI includes physical corrections such as implementation of plasma rotation and reduction of the fast particle contribution in thermal reactant distribution. The rotation typically changes the beam-thermal reaction rates by 1–5%, while accounting for the fast particle density consistently reduces the neutron deficit (widely known inequality of the measured and calculated neutron rates) by up to 15% depending on the discharge.
Further developments include implementation of angular dependence of DD differential fusion cross sections and accounting for finite Larmor radius effect, which is important for high-energy particles such as ICRH. Additionally, the role of data based analysis in synthetic diagnostics development with the help of JETPEAK database is discussed
Studying divertor relevant plasmas in linear devices: experiments and transport code modelling
No full textEnhanced Quality Factors of Surface Lattice Resonances in Plasmonic Arrays of Nanoparticles
No full textKey in the application of plasmonics is the realization of low loss or high quality (Q) factor resonances. Nanoparticle arrays are systems capable of sustaining remarkably high Q‐factor resonances through the hybridization of plasmonic and photonic modes, known as surface lattice resonances (SLRs). SLRs result from the coupling of localized surface plasmon resonances (LSPRs) to in‐plane orders of diffraction known as Rayleigh anomalies (RAs). To date, the highest Q‐factors have been achieved with the (±1, 0) diffraction orders. However, these Q‐factors are highly sensitive to the angle of excitation. Here, a strategy is presented to generate high Q‐factor SLRs with low dispersion by coupling LSPRs to the (0, ±1) diffraction orders. 2D arrays of silver nanoparticles are investigated experimentally and numerically, and it is shown that the Q‐factor of SLRs critically depends on the quality of the metal film, the detuning between RAs and LSPRs, and the absorption of adhesive layer used between the substrate and the metallic nanoparticles. These silver nanoparticle arrays can achieve Q‐factors higher than 330 in the visible range. These extraordinarily high Q‐factors could be increased to values above 1500 if no adhesive layer is used, which could significantly improve sensors and enhance nonlinearities in plasmonic systems