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Improved heat and particle flux mitigation in high core confinement, baffled, alternative divertor configurations in the TCV tokamak
No full textNitrogen seeded detachment has been achieved in the tokamak a configuration variable (TCV) in alternative divertor configurations (ADCs), namely X-divertor and X-point target, with and without baffles in H-mode plasmas with high core confinement. Both ADCs show a remarkable reduction in the inter-ELM particle and heat fluxes to the target compared to the standard divertor configuration. 95%–98% of the inter-ELM peak heat flux to the target is mitigated as a synergetic effect of ADCs, baffling, and nitrogen seeded detachment. The effect of divertor geometry and baffles on core-divertor compatibility is investigated in detail. The power balance in these experiments is also investigated to explore the physics behind the observed reduction in heat fluxes in the ADCs
Structure-dependent adsorption and desorption of hydrogen on FCC and HCP cobalt surfaces
Full text linkThe interaction of hydrogen with cobalt surfaces is of fundamental interest for Fischer-Tropsch synthesis. In the present work, the adsorption and desorption of hydrogen was studied on various cobalt single crystal surfaces that together represent the surface structures exposed by FCC and HCP cobalt nanoparticles used in applied catalysis. Dissociative hydrogen adsorption is activated on flat Co(0001), especially for hydrogen coverages beyond 0.5 ML. A tungsten filament creates hydrogen atoms and hot hydrogen molecules that increase the dissociative sticking probability and make it possible to obtain hydrogen coverages above 0.5 ML. Hydrogen in excess of 0.5 ML binds more weakly and desorbs in a separate low temperature desorption peak, in line with theoretical predictions. A third desorption peak appears above 1 ML and is attributed to subsurface hydrogen, the formation of which is attributed to hydrogen atoms produced by the tungsten filament. Adsorbed hydrogen atoms form (islands) of an ordered (2 x 2)-2H honeycomb structure for coverages between 0.3 and 0.8 ML which points to a specific stability of this structure. Step and kink sites on vicinal close-packed surfaces provide a low energy path for both hydrogen adsorption and desorption which results in a much higher dissociative sticking probability and a lower desorption temperature. The hydrogen adsorption strength on various FCC and HCP cobalt surfaces varies between 30 and 45 kJ/mol H ad(sorption) and is strongest on threefold hollow sites on the close-packed terraces while it is significantly lower on fourfold hollow sites on FCC-(100) and on threefold hollow sites on various open HCP surfaces. Under reaction conditions, the structure-dependent adsorption energy translates to a two to three orders of magnitude variation of the equilibrium constant for hydrogen.</p
CO2 conversion via coupled plasma-electrolysis process
No full textSurplus renewable electricity used to convert CO2 into CO, the building block of liquid fuels, advances the energy transition by enabling large-scale, long-term energy storage and the synthesis of fuel for long-haul transportation. Among the various technologies developed, renewable electricity driven conversion of CO2 by high-temperature electrolysis and by plasmolysis offer a tantalising potential. High-temperature electrolysis is characterized by high-yield and energy-efficiency and the direct separation of the CO2 dissociation products CO and O2. However, the difficulty to break the carbon-oxygen double bond poses challenging requirements on electrode materials. CO2 plasmolysis on the other hand, offers a similar energy efficiency, does not employ scarce materials, is easy to upscale, but requires efficient gas separation and recuperation because the produced CO remains mixed with O2 and residual CO2. Here, we demonstrate that the coupling of the two processes leads to a renewable-electricity-driven route for producing CO from CO2, overcoming the main bottleneck of CO2 plasmolysis. A simulated CO2 plasmolysis gas mixture is supplied to a high-temperature electrolyser to separate the product gases electrochemically. Our results show that the product stream of the coupled-process contains 91% less oxygen and 138% more CO compared with the bare plasmolysis process. Apart from upgrading the produced gas mixture, this coupled approach benefits from material stability. Durability tests (~100 h) show better stability in coupled operation when compared with conventional CO2 electrolysis. Synergy between plasmolysis and electrolysis opens up a novel route to efficient CO2 conversion into valuable CO feedstock for the synthesis of long-chain hydrocarbons.</p
Systematic extraction of a control-oriented model from perturbative experiments and SOLPS-ITER for emission front control in TCV
No full textSystematic extraction of locally valid dynamic models from experiments is necessary for controller design and the validation of high fidelity models. This paper describes the extraction of a dynamic model in the form of a transfer function, giving the dynamic response of the CIII (465.0 nm) emission front position to deuterium gas puffing in the TCV divertor during flattop, relevant for heat exhaust control. The model is extracted using frequency response data from both SOLPS-ITER simulations and perturbative experiments. We use the steady-state solutions of the model SOLPS-ITER to obtain an additional data point at the zero frequency, as the identifiable frequency range by perturbative experiments is lower bounded by discharge time. We specifically approach the problem from a control engineering point of view, aiming to develop control-oriented models for the systematic design of impurity emission front controllers. We find a transfer function structure based on a diffusive process to best describe the obtained frequency response data. The resulting transfer function model accurately reproduces the local dynamic response measured during experiments, so it can be used to assess new controllers offline for similar discharge scenarios.</p
The Super-Alfvenic Rotational Instability in accretion disks about black holes
No full textThe theory of instability of accretion disks about black holes, neutron stars or proto-planets, is revisited by means of the recent method of the Spectral Web. The cylindrical accretion disk differential equation is shown to be governed by the forward and backward Doppler-shifted continuous Alfvén spectra Ω±A≡mΩ±ωA, where ωA is the static Alfvén frequency. It is crucial to take non-axisymmetry (m≠0) and super-Alfvénic rotation of the Doppler frames (|mΩ|≫|ωA|) into account. The continua Ω+A and Ω−A then overlap, ejecting a plethora of Super-Alfvénic Rotational Instabilities (SARIs). In-depth analysis for small inhomogeneity shows that the two Alfvén singularities reduce the extent of the modes to sizes much smaller than the width of the accretion disk. Generalization for large inhomogeneity leads to the completely unprecedented result that, for mode numbers |k|≫|m|, any complex ω in a wide neighborhood of the real axis is an approximate `eigenvalue\u27. The difference with genuine eigenmodes is that the amount of complementary energy to excite the modes is tiny, |Wcom|≤c, with c the machine accuracy of the computation. This yields a multitude of two-dimensional continua of quasi-discrete modes: quasi-continuum SARIs. We conjecture that the onset of 3D turbulence in magnetized accretion disks is governed, not by the excitation of discrete axisymmetric Magneto-Rotational Instabilities, but by the excitation of modes from these two-dimensional continua of quasi-discrete non-axisymmetric Super-Alfvénic Rotational Instabilities
Challenges Of Big-science: A Matrix-based Interface Model To Manage Technical Integration Risks In Multi-organizational Engineering Projects
No full textDiscovery of aza-aromatic anolytes for aqueous redox flow batteries via high-throughput screening
Full text linkAza-aromatics have recently emerged as a propitious class of electroactive compounds for energy storage in aqueous redox flow batteries (ARFBs). Here, using high-throughput virtual screening (HTVS), we explored a focused chemical subspace of aza-aromatics to determine the top performing candidates as anolytes in ARFBs. First, we designed a virtual chemical library that contains 13,406 aza-aromatic redox pairs, which was populated through the chemical functionalization of alloxazine, phenazine, and indigo backbones with five different R-groups that are known to affect the key battery properties. Then, we predicted the redox potential, aqueous solubility, and the likelihood of decomposition due to the undesirable hydration and tautomerization reactions of the compounds. An analysis of the decomposition thermodynamics of the aza-aromatic subclasses revealed differing correlations between the redox properties and the chemical stability of the compounds, where the latter is found to strongly depend on the type and quantity of the functional groups. Consequently, a total of 516 anolyte candidates that have lower redox potential and higher solubility than a typical anolyte compound, alloxazine 7-carboxylic acid (ACA), have been identified. Additionally, an automated vendor search for the HTVS-predicted top-performing compounds yielded two molecules that are readily purchasable for experimental validation. Finally, an analysis of the quantitative structure-property relationships showed that the mid-sized aza-aromatics, which are not well-explored in experiments, achieved the largest property tunability windows. Based on the new findings, we also propose a molecular engineering strategy in a way to balance the inherent trade-offs among the redox, solubility and chemical stability features of the aza-aromatic anolytes for ARFBs.
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Impact of Self-Absorption and Cavity Effects on the Electroluminescence Spectra of Thin-Film Solar Cells
No full textPhotoelectrochemical properties of plasma-induced nanostructured tungsten oxide
Full text linkHelium (He)-induced nanostructured tungsten sheets were synthesized by He plasma irradiation under different plasma exposure durations. After calcination, nanostructured tungsten oxide samples were used as photoelectrodes to test photoelectrochemical (PEC) performance. The results showed that nanostructured WO3 photoanodes have higher PEC performance compared to the sample without nanostructures. The 15 min irradiated sample had the highest photocurrent density of 3.5 mA/cm2 under the thermodynamic potential of water oxidation (1.23 V vs. RHE). It was found that the oxide layer thickness and exposed crystal facet have a significant impact on PEC performance. The plasma synthesis technique has proved to be an effective method for preparing nanostructured WO3 photoelectrodes.</p