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Resolving discharge parameters from atomic oxygen emission
Get PDFA method is proposed to spatially resolve discharge parameters from experimental measurements of emission intensity and 1D numerical simulations including an O atom collisional-radiative model. The method can be used for different plasmas and conditions. Here, contracted microwave discharges for CO2 conversion are studied at intermediate to high pressures (100 to 300 mbar). Radial profiles of electron density (n e) are used as input in the model and corrected to successfully simulate the measured Gaussian profiles of emission intensity of the 777 nm transition (I777). As a result, radially-resolved parameters inaccessible in experiments, such as ne, power density (P abs), electron temperature (T e), electric field and reaction rates, are numerically-obtained for several conditions. n e and P abs approximately follow Gaussian profiles that are broader than that of I777. For pressures below 150 mbar, the difference in full width at half maximum (FWHM) is typically a factor 1.6. This consists in a phenomenon of optical contraction, which is due to concave profiles of O molar fraction and T e. The implications of the simulated profiles on the study of plasmas for CO2 conversion are discussed and it is shown that these profiles allow to explain high reactor performances at low pressures.</p
Operando Attenuated Total Reflection Fourier-transform Infrared (ATR-FTIR) Spectroscopy for Water Splitting
No full textOperando attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy is discussed in this paper for water splitting application. The first part of the paper focuses on the discussion of the opportunities and challenges of this method for the characterization of the solid-liquid interface in water splitting. The second part of the paper focuses on recent results and future perspectives: We present stable and robust operando ATR-FTIR measurements using low temperature processing of hematite and a set-up where the functional thin film is integrated on the ATR crystal. We find increased absorbance as a function of applied potential at wavenumbers of 1000 cm-1 to 900 cm-1 and relate this to changes in the surface species during water oxidation. We argue that this approach has the potential to be developed to a routine characterization method for the characterization of interfaces in water splitting. Such ATR-FTIR data is of crucial importance for the validation of models in microkinetic modeling. We show some recent results of microkinetic modeling of the hematite-electrolyte interface and explain how a combination of operando ATR-FTIR measurements and microkinetic modeling enables the identification of the reaction mechanism in water splitting. We discuss how this combined approach will enable designing of tailored catalysts and accelerating their development in the future
Plasma Activated Electrochemical Ammonia Synthesis from Nitrogen and Water
Get PDFAmmonia is an important precursor of fertilizers, as well as a potential carbon-free energy carrier. Nowadays, ammonia is synthesized via the Haber–Bosch process, which is a capital- and energy-intensive process with an immense CO2 footprint. Thus, alternative processes for the sustainable and decentralized ammonia production from N2 and H2O using renewable electricity are required. The key challenges for the realization of such processes are the efficient activation of the N2 bond and selectivity toward NH3. In this contribution, we report an all-electric method for sustainable ammonia production from nitrogen and water using a plasma-activated proton conducting solid oxide electrolyzer. Hydrogen species produced by water oxidation over the anode are transported through the proton conducting membrane to the cathode where they react with the plasma-activated nitrogen toward ammonia. Ammonia production rates and Faradaic efficiencies up to of 26.8 nmol of NH3 s–1 cm–2 and 88%, respectively, were achieved.</p
A novel hydrogenic spectroscopic technique for inferring the role of plasma-molecule interaction on power and particle balance during detached conditions
No full textDetachment, an important mechanism for reducing target heat deposition, is achieved through reductions in power, particle and momentum; which are induced through plasma–atom and plasma–molecule interactions. Experimental research in how those reactions precisely contribute to detachment is limited. Both plasma–atom as well as plasma–molecule interactions can result in excited hydrogen atoms which emit atomic line emission. In this work, we investigate a new Balmer Spectroscopy technique for Plasma–Molecule Interaction—BaSPMI. This first disentangles the Balmer line emission from the various plasma–atom and plasma–molecule interactions and secondly quantifies their contributions to particle (ionisation and recombination) and power balance (radiative power losses). Its performance is verified using synthetic diagnostic techniques of both attached and detached TCV and MAST-U SOLPS-ITER simulations. We find that H2 plasma chemistry involving H2+ and/or H− can substantially elevate the Hα emission during detachment, which we show is an important precursor for Molecular Activated Recombination. An example illustration analysis of the full BaSPMI technique shows that the hydrogenic line series, even Lyα as well as the medium-n Balmer lines, can be significantly influenced by plasma–molecule interactions by tens ofpercent. That has important implications for using atomic hydrogen spectroscopy for diagnosing divertor plasmas.</p
A Quantitative Evaluation of Computational Methods to Accelerate the Study of Alloxazine-Derived Electroactive Compounds for Energy Storage
No full textAlloxazines are a promising class of organic electroactive compounds for application in aqueous redox flow batteries (ARFBs), whose redox properties need to be tuned further for higher performance. High-throughput computational screening (HTCS) enables rational and time-efficient study of energy storage compounds. We compared the performance of computational chemistry methods, including the force field based molecular mechanics, semi-empirical quantum mechanics, density functional tight binding, and density functional theory, on the basis of their accuracy and computational cost in predicting the redox potentials of alloxazines. Various energy-based descriptors, including the redox reaction energies and the frontier orbital energies of the reactant and product molecules, were considered. We found that the lowest unoccupied molecular orbital (LUMO) energy of the reactant molecules is the best performing chemical descriptor for alloxazines, which is in contrast to other classes of energy storage compounds, such as quinones that we reported earlier. Notably, we present a flexible in silico approach to accelerate both the singly and the HTCS studies, therewithal considering the level of accuracy versus measured electrochemical data, which is readily applicable for the discovery of alloxazine-derived organic compounds for energy storage in ARFBs.</p
The JOREK non-linear extended MHD code and applications to large-scale instabilities and their control in magnetically confined fusion plasmas
No full textJOREK is a massively parallel fully implicit non-linear extended MHD code for realistic tokamak X-point plasmas. It has become a widely used extremely versatile simulation code for studying large-scale plasma instabilities and their control and is continuously developed in an international community with strong involvements in the European fusion research program and ITER organization. This article gives a comprehensive overview of the physics models implemented, numerical methods applied for solving the equations and physics studies performed with the code. A dedicated section highlights some of the verification work done for the code. A hierarchy of different physics models is available including a free boundary and resistive wall extension and hybrid kinetic-fluid models. The code allows for flux-surface aligned iso-parametric finite element grids in single and double X-point plasmas which can be extended to the true physical walls and uses a robust fully implicit time stepping. Particular focus is laid on plasma edge and scrape-off layer (SOL) physics as well as disruption related phenomena. Among the key results obtained with JOREK regarding plasma edge and SOL, are deep insights into the dynamics of edge localized modes (ELMs), ELM cycles, and ELM control by resonant magnetic perturbations, pellet injection, as well as by vertical magnetic kicks. Also ELM free regimes, detachment physics, the generation and transport of impurities during an ELM, and electrostatic turbulence in the pedestal region are investigated. Regarding disruptions, the focus is on the dynamics of the thermal quench and current quench triggered by massive gas injection (MGI) and shattered pellet injection (SPI), runaway electron (RE) dynamics as well as the RE interaction with MHD modes, and vertical displacement events (VDEs). Also the seeding and suppression of tearing modes (TMs), the dynamics of naturally occurring thermal quenches triggered by locked modes, and radiative collapses are being studied
Rapid optimization of stationary tokamak plasmas in RAPTOR: demonstration for the ITER hybrid scenario with neural network surrogate transport model QLKNN
Get PDFThis work presents a fast and robust method for optimizing the stationary radial distribution of temperature, density and parallel current density in a tokamak plasma and its application to first-principle-based modeling of the ITER hybrid scenario. A new solver is implemented in the RAPTOR transport code, enabling direct evaluation of the stationary solution to which the radial plasma profiles evolve. Coupled to a neural network emulation of the quasi-linear gyrokinetic QuaLiKiz transport model (QLKNN-hyper-10D), a first-principle-based estimate of the stationary state of the core plasma can be found at unprecedented computational speed (typically a few seconds on standard hardware). The stationary state solver is then embedded in a numerical optimization scheme, allowing the optimization of tokamak plasma scenarios in only a few minutes. The proposed method is applied to investigate the performance of ITER hybrid scenarios at different values of total plasma current, plasma density and pedestal height and for different power contributions in a heating mix consisting of electron cyclotron and neutral beam heating. Optimizing the radial distribution of electron cyclotron current drive (ECCD) deposition, the q profile is tailored to maximize the fusion gain Q, by maximizing the energy confinement predicted through the first-principles-based transport model, while satisfying q > 1, avoiding sawtooth oscillations. It is found that optimal use of ECCD in ITER hybrid scenarios is to deposit power as close to the core as possible, while maintaining sufficient off-axis current drive to keep q above 1. Upper limits for the fusion gain Q are shown to be constrained either by minimum power requirements for the separatrix power flow to maintain H-mode or by minimum current drive requirements for q profile tailoring. Finally, it is shown that the ITER hybrid scenario operating window is significantly extended by an upgrade of the electron cyclotron power to 40 MW.</p
Redefining the Microwave Plasma-Mediated CO2 Reduction Efficiency Limit: The Role of O-CO2 Association
Get PDFReduction of CO2 to CO for sustainable fuel production is studied in high-temperature (>3500 K) microwave plasma. Raman scattering and chemical kinetics modeling reveal chemistry rates with spatial resolution that explain previously reported peak energy efficiency values of 50%. The necessary product quenching is established by fast transport in the core, at frequencies of 105 s–1, facilitating rapid mass and energy transfer between products and feedstock CO2. Moreover, the resulting chemical nonequilibrium yields additional CO2 dissociation in O–CO2 association, a reaction responsible for up to 45% of CO production. Three different thermal chemistry sets are invoked to qualitatively confirm this picture. It is shown how these lack predictive accuracy in the high gas temperature regime studied, which indicates that new CO2 chemistry rate coefficients are highly desirable. Improving reactor design with the identified enhancement mechanisms in mind can increase efficiency up to the newly defined thermal limit of 70%.</p
