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Saturation and nonlinear electromagnetic stabilization of ITG turbulence
No full textEnergy transfer in ion-temperature-gradient-driven (ITG) turbulence and its role in modeling transport are examined for finite normalized plasma pressure β for a number of test cases and experimental discharges. The analysis shows that like the zero-β case, finite-β ITG turbulence saturates by nonlinear energy transfer to stable modes mediated by a zonal flow. Electromagnetic effects reliably increase stable mode amplitudes but affect heat fluxes only at the ≈5% level. The most important change with increased β is an increase in the correlation time of the triplet interaction of the unstable mode, stable mode, and zonal flow, thus providing a heightened nonlinear energy transfer efficiency, which allows the instability to saturate at lower amplitude. The heat flux is examined in connection with nonlinear electromagnetic stabilization, the phenomenon where the flux falloff with β is more pronounced than the falloff predicted by quasilinear transport models. The inclusion of the triplet correlation time in the quasilinear model captures most of the nonlinearly enhanced stabilization for the configurations studied her
Flux-driven integrated modelling of main ion pressure and trace tungsten transport in ASDEX Upgrade
Get PDFNeoclassical and turbulent heavy impurity transport in tokamak core plasmas are determined by main ion temperature, density and toroidal rotation profiles. Thus, in order to understand and prevent experimental behaviour of W accumulation, flux-driven integrated modelling of main ion heat and particle transport over multiple confinement times is a vital prerequisite. For the first time, the quasilinear gyrokinetic code QuaLiKiz is applied for successful predictions of core kinetic profiles in an ASDEX Upgrade H-mode discharge in the turbulence dominated region within the integrated modelling suite JETTO. Neoclassical contributions are calculated by NCLASS; auxiliary heat and particle deposition profiles due to NBI and ECRH are prescribed from previous analysis with TRANSP. Turbulent and neoclassical contributions are insufficient in explaining main ion heat and particle transport inside the q = 1 surface, necessitating the prescription of further transport coefficients to mimic the impact of MHD activity on central transport. The ion to electron temperature ratio at the simulation boundary at p tor=0.85 stabilizes ion scale modes while destabilizing ETG modes when significantly exceeding unity. Careful analysis of experimental measurements using Gaussian process regression techniques is carried out to explore reasonable uncertainties. In following trace W impurity transport simulations performed with additionally NEO, neoclassical transport under consideration of poloidal asymmetries alone is found to be insufficient to establish hollow central W density profiles. Reproduction of these conditions measured experimentally is found possible only when assuming the direct impact of a saturated (m, n) = (1, 1) MHD mode on heavy impurity transport.</p
Electrochemical water oxidation on WO3 surfaces: A density functional theory study
No full textDensity functional theory (DFT) calculations are performed to study the oxygen evolution reaction (OER) on tungsten oxide (WO3) surfaces. The free energies of the proton coupled electron transfer (PCET) steps are calculated and from these the OER overpotential is calculated as the characteristic parameter for the electrochemical activity. The effects of surface orientation, oxygen vacancies, and doping by Chromium (Cr) and Molybdenum (Mo), on the OER activities are analyzed. The difference in OER overpotential for the three surface orientations, (200), (002) and (020), is found to be very small (0.07 V). The presence of oxygen vacancies in the first WO3 sub-layer do not favor OER, while vacancies in the second sub-layer do reduce the OER overpotential. A volcano plot with all overpotentials calculated in this study shows that the OER is favored at the Mo doped (200) surface (reactions happen at the Mo site). This paper demonstrates that DFT calculations of the electrochemical activity are an efficient method to identify active surfaces and can therefore save a lot of experimental effort in designing more efficient photoelectrodes for water splitting. Open Access funded by VSN
Excitation and relaxation of the asymmetric stretch mode of CO2 in a pulsed glow discharge
No full textThe excitation and relaxation of the vibrations of CO2 as well as the reduction of CO2 to CO are studied in a pulsed glow discharge. Two diagnostics are employed: (1) time-resolved in situ Fourier transform infrared spectroscopy and (2) spatiotemporally resolved in situ rotational Raman spectroscopy. Experiments are conducted within a pressure range of 1.3–6.7 mbar and a current range of 10–50 mA. In the afterglow, the rate of exponential decay from the asymmetric stretch temperature (T 3) to the rotational temperature (T rot) is found to be only dependent on T rot, in the conditions under study. The decay rate rho T 3 - T rot follows the relation rho T 3-T rot=388 s-1 exp(T rot - 273 K / 154 K). Pressure and varying concentrations of CO and (presumably) atomic oxygen did not show to be of significant influence. In the active part of the discharge the excitation of T 3 showed to be positively related to current and negatively to pressure. However, the contribution of current to vibrational excitation is ambiguous: the conversion of CO2 and therefore the fraction of CO in the discharge, is found to be strongly dependent on the current, with a conversion factor of 0.05–0.18 for 10–50 mA, while CO can contribute to the excitation through near-resonant collisions. A clear relation between the elevation of T 3 and the dissociation of CO2 could not be confirmed, though conversion peaks are observed in the near afterglow, which motivate future experiments on vibrational ladder-climbing directly after termination of the discharge
Site-specific reactivity of molecules with surface defects - the case of H2 dissociation on Pt
Get PDFThe classic system that describes weakly activated dissociation in heterogeneous catalysis has been explained by two dynamical models that are fundamentally at odds. Whereas one model for hydrogen dissociation on platinum(111) invokes a preequilibrium and diffusion toward defects, the other is based on direct and local reaction. We resolve this dispute by quantifying site-specific reactivity using a curved platinum single-crystal surface. Reactivity is step-type dependent and varies linearly with step density. Only the model that relies on localized dissociation is consistent with our results. Our approach provides absolute, site-specific reaction cross sections.</p
Relating Frontier Orbital Energies from Voltammetry and Photoelectron Spectroscopy to the Open-Circuit Voltage of Organic Solar Cells
No full textEfficient Thick-Film Polymer Solar Cells with Enhanced Fill Factors via Increased Fullerene Loading
No full textApplication of Gaussian process regression to plasma turbulent transport model validation via integrated modelling
Get PDFThis paper outlines an approach towards improved rigour in tokamak turbulence transport model validation within integrated modelling. Gaussian process regression (GPR) techniques were applied for profile fitting during the preparation of integrated modelling simulations allowing for rigourous sensitivity tests of prescribed initial and boundary conditions as both fit and derivative uncertainties are provided. This was demonstrated by a JETTO integrated modelling simulation of the JET ITER-like-wall H-mode baseline discharge #92436 with the QuaLiKiz quasilinear turbulent transport model, which is the subject of extrapolation towards a deuterium–tritium plasma. The simulation simultaneously evaluates the time evolution of heat, particle, and momentum fluxes over ~10 confinement times, with a simulation boundary condition at rho tor=0.85. Routine inclusion of momentum transport prediction in multi-channel flux-driven transport modelling is not standard and is facilitated here by recent developments within the QuaLiKiz model. Excellent agreement was achieved between the fitted and simulated profiles for n e , T e , T i , and omega tor within 2x, but the simulation underpredicts the mid-radius T i and overpredicts the core n e and T e profiles for this discharge. Despite this, it was shown that this approach is capable of deriving reasonable inputs, including derivative quantities, to tokamak models from experimental data. Furthermore, multiple figures-of-merit were defined to quantitatively assess the agreement of integrated modelling predictions to experimental data within the GPR profile fitting framework.</p
Haptic Assistance Improves Tele-manipulation With Two Asymmetric Slaves
No full textTele-manipulation of heavy loads typically requires the simultaneous use of two asymmetric slaves: a crane for vertical weight support; and a robot for accurate lateral positioning. The industrial standard prescribes a pair of operators for such tasks (one operator to control each slave), although in principle one operator might control both slaves with a single, hybrid interface. Accurate and safe co-operative handling of the expensive and fragile heavy components is difficult, presumably due to problems in the coordination of the subtasks and the lack of mutual awareness between the two operators. This study proposes a novel haptic assistance system to improve subtask coordination and task performance. Its novelty consists of haptically linking operators/interfaces through the joint task environment. The system\u27s efficacy is evaluated with fifteen pairs of co-operators and fifteen individual uni-manual operators who manoeuvred a heavy load through a bounded path in Virtual Reality. Haptic assistance improves task completion time for both groups. It also reduces control activity and self-reported workload without affecting a number of critical errors made by the operators. Moreover, without haptic assistance, uni-manual operators perform worse than co-operators, but this difference between the interfaces disappears with haptic assistance
Atmospheric-pressure silica-like thin film deposition using 200 kHz/13.56 MHz dual frequency excitation
Get PDFAtmospheric pressure plasma enhanced chemical vapour deposition (AP-PECVD) was used to synthesize silica-like thin films on polyethylene 2, 6 naphthalate (PEN) substrate with hexamethyldisiloxane (HMDSO) as the precursor and Ar/O2/N2 mixture as the working gas. A dual frequency (DF) excitation consisting of 200 kHz and 13.56 MHz frequencies was employed as the plasma source. The results have shown that compared to the single LF discharges, the DF excitation helps to improve plasma uniformity with less filaments. This could help to reduce the macro-defects and therefore to improve the permeation performance of the barriers. Besides, due to the increased electron density and gas temperature, the DF excitation demonstrates a more efficient breaking of Si-CH3 bonds and therefore more oxidized structures of the deposited silica-like thin films.</p