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Reducing tin droplet ejection from capillary porous structures under hydrogen plasma exposure in Magnum-PSI
No full textLiquid metal based divertors could be a more robust alternative to a solid tungsten design for DEMO. The liquid is confined in a sponge-like tungsten layer, called a capillary porous structure (CPS). It has been found previously that under certain conditions, many tin droplets eject from a CPS when it is brought into contact with a hydrogen plasma. These would present a contamination issue for the plasma core. Stability analysis suggests that droplet ejection can be suppressed by reduction of the pore size. To test this, stainless-steel CPS targets with pore size ranging from 0.5-100um filled with tin were exposed to identical loading conditions. This was done in the linear plasma device Magnum-PSI, capable of reaching divertor relevant plasma conditions. Furthermore, the influence of the CPS manufacturing techniques is considered by comparing the performance of a 3D printed, a mesh felts and a sintered CPS, all made from tungsten. Each target was surrounded by four witness plates, which were analysed post-mortem for Sn content by Rutherford backscattering. During plasma exposure, tin droplets were observed using a fast visible camera and plasma light emission via survey optical emission spectroscopy. The results imply that Sn erosion can be reduced by a factor of 50 when reducing the pore size. Moreover, it highlights the importance of avoiding overfilling of CPS targets with Sn
The CPS\u27s pre-heating effect on the capability to withstand extreme plasma loads
No full textThis work presents experimental studies of plasma surface interactions (PSIs) during QSPA plasma impacts on the Sn Capillary Porous Structures (CPSs) in conditions, simulating disruption and ELM-like loads. The 3D tungsten target filled by Sn was exposed to the plasma streams with an energy density of up to 3 MJ/m2 which resulted in tungsten melting and evaporation. The initial temperature of a sample between plasma pulses had two options: remained at a room value for one sample, and reached 300 C which is close to Sn melting point for another one. The number of plasma irradiations reached 100 pulses. It is shown that the particle ejection was observed during PSI for pre-heated as well as cold CPS targets. The difference in values of the number of particles ejected from the targets, their velocities, and the start-time during exposition is discussed. Particles leave the target surface during all expositional pulses for both CPS targets. The threshold character of the ejection is more pronounced for the cold target. After the 100 pulses reduced Sn pool is observed. Moreover, areas without Sn occur on the exposed targets. Nevertheless, the severe damage of the W base of the CPS targets is not revealed during high-cycle powerful plasma loads
Post-plasma Quenching to Improve Conversion and Energy Efficiency in a CO2 Microwave Plasma
No full textTransforming CO2 into value-added chemicals is crucial to realizing a carbon–neutral economy, and plasma-based conversion, a Power-2-X technology, offers a promising route to realizing an efficient and scalable process. This paper investigates the effects of post-plasma placement of a converging–diverging nozzle in a vortex-stabilized 2.45 GHz CO2 microwave plasma reactor to increase energy efficiency and conversion. The CDN leads to a 21% relative increase in energy efficiency (31%) and CO2 conversion (13%) at high flow rates and near-atmospheric conditions. The most significant performance improvement was seen at low flow rates and sub-atmospheric pressure (300 mbar), where energy efficiency was 23% and conversion was 28%, a 71% relative increase over conditions without the CDN. Using CFD simulations, we found that the CDN produces a change in the flow geometry, leading to a confined temperature profile at the height of the plasma, and forced extraction of CO to the post-CDN region
Anti-Ferromagnetic RuO2: A Stable and Robust OER Catalyst over a Large Range of Surface Terminations
No full textRutile RuO2 is a prime catalyst for the oxygen evolution reaction (OER) in water splitting. Whereas RuO2 is typically considered to be non-magnetic (NM), it has recently been established as being anti-ferromagnetic (AFM) at room temperature. The presence of magnetic moments on the Ru atoms signals an electronic configuration that is markedly different from what is commonly assumed, the effect of which on the OER is unknown. We use density functional theory (DFT) calculations within the DFT+U approach to model the OER process on NM and AFM RuO2(110) surfaces. In addition, we model the thermodynamic stability of possible O versus OH terminations of the RuO2(110) surface and their effect on the free energies of the OER steps. We find that the AFM RuO2(110) surface gives a consistently low overpotential in the range 0.4–0.5 V, irrespective of the O versus OH coverage, with the exception of a 100% OH-covered surface, which is, however, unlikely to be present under typical OER conditions. In contrast, the NM RuO2(110) surface gives a significantly higher overpotential of ∼0.7 V for mixed O/OH terminations. We conclude that the magnetic moment of RuO2 supplies an important contribution to obtaining a low overpotential and to its insensitivity to the exact O versus OH coverage of the (110) surface.</p
Turbulence mitigation in maximum-J stellarators with electron-density gradient
No full textIn fusion devices, the geometry of the confining magnetic field has a significant impact on the instabilities that drive turbulent heat loss. This is especially true of stellarators, where the density-gradient-driven branch of the "trapped electron mode" (TEM) is predicted to be linearly stable if the magnetic field has the maximum-J property, as is very approximately the case in certain magnetic configurations of the Wendelstein 7-X experiment (W7-X). Here we show, using both analytical theory and simulations, that the benefits of the optimisation of W7-X also serve to mitigate ion-temperature-gradient (ITG) modes as long as an electron density gradient is present. We find that the effect indeed carries over to nonlinear numerical simulations, where W7-X has low TEM-driven transport, and reduced ITG turbulence in the presence of a density gradient, giving theoretical support for the existence of enhanced confinement regimes, in the presence of strong density gradients (e.g. hydrogen pellet or neutral beam injection)
Data-driven discovery of small electroactive molecules for energy storage in aqueous redox flow batteries
No full textA high-throughput virtual screening (HTVS)-guided experimental study is applied for the large-scale exploration of quinone-like anolytes for aqueous redox flow batteries (ARFBs). This includes the design of a focused virtual chemical library inspired by stable molecules, quantum chemical prediction of redox properties, machine learning prediction of aqueous solubility, automated search for commercial availability on vendor databases, and electrochemical characterization of the promising compounds. Screening in a chemical space of 3,257 redox pairs led to 205 predicted candidates with higher solubility and lower redox potential than that of anthraquinone-2,7-disulfonic acid (AQDS) anolyte used in ARFBs. Through electrochemical studies on commercially available compounds, the molecules that show good performance in a practical ARFB setup are identified. Among them, indigo trisulfonate [Indigo-3(SO3H)] showed higher solubility, capacity retention, and coulombic efficiency than AQDS and its predecessors. The data-driven material design approach is therefore highly effective for the future explorations of electrochemical energy storage compounds
LPMLEn - A frequency domain method to estimate vertical streambed fluxes and sediment thermal properties in semi-infinite and bounded domains
No full textThis paper presents the LPMLEn, a new method to estimate vertical flux and thermal diffusivity from streambed temperature time-series using the frequency domain. The main advantages of this new method are: (1) the use of multiple frequencies and multiple sensors for the parameter estimation; (2) noise/uncertainty handling in an optimal way; (3) the possibility to estimate the parameters with both semi-infinite and bounded domain models; and (4) the compensation for temperature drifts in the data known as transients. The capabilities of the LPMLEn are demonstrated using both synthetic and field data, highlighting the advantages of the bounded domain model over the semi-infinite domain model in the parameter estimation process.
(LPMLEn - A code for estimating heat transport parameters in 1D, HydroShare (43.0 MB), http://www.hydroshare.org/resource/3b13760174174c31988120baeb84e2e8<br/
Insights into the limitations to vibrational excitation of CO2: validation of a kinetic model with pulsed glow discharge experiments
No full textVibrational excitation represents an efficient channel to drive the dissociation of CO2 in a non-thermal plasma. Its viability is investigated in low-pressure pulsed discharges, with the intention of selectively exciting the asymmetric stretching mode, leading to stepwise excitation up to the dissociation limit of the molecule. Gas heating is crucial for the attainability of this process, since the efficiency of vibration–translation (V–T) relaxation strongly depends on temperature, creating a feedback mechanism that can ultimately thermalize the discharge. Indeed, recent experiments demonstrated that the timeframe of V–T non-equilibrium is limited to a few milliseconds at ca. 6 mbar, and shrinks to the μs-scale at 100 mbar. With the aim of backtracking the origin of gas heating in pure CO2 plasma, we perform a kinetic study to describe the energy transfers under typical non-thermal plasma conditions. The validation of our kinetic scheme with pulsed glow discharge experiments enables to depict the gas heating dynamics. In particular, we pinpoint the role of vibration–vibration–translation relaxation in redistributing the energy from asymmetric to symmetric levels of CO2, and the importance of collisional quenching of CO2 electronic states in triggering the heating feedback mechanism in the sub-millisecond scale. This latter finding represents a novelty for the modelling of low-pressure pulsed discharges and we suggest that more attention should be paid to it in future studies. Additionally, O atoms convert vibrational energy into heat, speeding up the feedback loop. The efficiency of these heating pathways, even at relatively low gas temperature and pressure, underpins the lifetime of V–T non-equilibrium and suggests a redefinition of the optimal conditions to exploit the \u27ladder-climbing\u27 mechanism in CO2 discharges.</p
High-throughput computational screening of organic molecules for organic ion battery cathodes
No full textOpen access embargo untill 9-5-202