DIFFER: Publications
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Energy-resolved plasmonic chemistry in individual nanoreactors
No full textPlasmonic resonances can concentrate light into exceptionally small volumes, which approach the molecular scale. The extreme light confinement provides an advantageous pathway to probe molecules at the surface of plasmonic nanostructures with highly sensitive spectroscopies, such as surface-enhanced Raman scattering. Unavoidable energy losses associated with metals, which are usually seen as a nuisance, carry invaluable information on energy transfer to the adsorbed molecules through the resonance linewidth. We measured a thousand single nanocavities with sharp gap plasmon resonances spanning the red to near-infrared spectral range and used changes in their linewidth, peak energy and surface-enhanced Raman scattering spectra to monitor energy transfer and plasmon-driven chemical reactions at their surface. Using methylene blue as a model system, we measured shifts in the absorption spectrum of molecules following surface adsorption and revealed a rich plasmon-driven reactivity landscape that consists of distinct reaction pathways that occur in separate resonance energy windows
Study of detached plasma profile in the divertor simulation experimental module of tandem mirror GAMMA 10/PDX
No full textA divertor simulation experimental module (D-module) in the tandem mirror GAMMA 10/PDX was used for the study of plasma detachment. In previous studies, it was difficult to measure far-upstream plasma parameters in the D-module, and only electrostatic probes on the target plate were used to perform electron temperature and density measurements. To study the detached plasma structure, a Thomson scattering (TS) system and a microwave interferometer system have been installed to measure the inside plasma parameters of the D-module, and a movable electrostatic probe has been placed at the inlet of the D-module to measure the inlet plasma density and temperature. The TS system in the central cell observed the electron temperature and density of the core plasma simultaneously. These measurements revealed the entire density and temperature structure from the core plasma to the divertor plate. The line average electron density measured by the microwave interferometer showed a rollover behavior during detachment. The results indicated that the ionization region was located around the center of the D-module, and it appears to move upstream along the axis.
See for correction on original figures 1&5, Publisher\u27s note AIP Advances 12, 029902 (2022); DOI: https://doi.org/10.1063/5.0085087</p
Plasma Diagnostics
No full textFor proper operation of the tokamak, the active and simultaneous control of many plasma parameters is needed. This implies that new robust and failsafe diagnostic techniques need to be developed, for instance to control the temperature, density and current density profiles. Progress in plasma diagnostics is on the one hand dictated by the desire to understand the detailed physical processes occurring in the plasma and on the other hand by the wish to actively control many important plasma parameters. An additional driver for diagnostic innovation comes from the requirement for better machine protection systems.
https://fusenet.eu/book/fundamentals-of-magnetic-fusion-technology</p
Pushing the limits of solubility prediction via quality-oriented data selection
No full textAccurate prediction of the solubility of chemical substances in solvents remains a challenge. The sparsity of high-quality solubility data is recognized as the biggest hurdle in the development of robust data-driven methods for practical use. Nonetheless, the effects of the quality and quantity of data on aqueous solubility predictions have not yet been scrutinized. In this study, the roles of the size and the quality of data sets on the performances of the solubility prediction models are unraveled, and the concepts of actual and observed performances are introduced. In an effort to curtail the gap between actual and observed performances, a quality-oriented data selection method, which evaluates the quality of data and extracts the most accurate part of it through statistical validation, is designed. Applying this method on the largest publicly available solubility database and using a consensus machine learning approach, a top-performing solubility prediction model is achieved. Accompanying software codes are available at https://doi.org/10.24433/CO.3467849.v2</p
Multiple-isotope pellet cycles captured by turbulent transport modelling in the JET tokamak
Get PDFFor the first time the pellet cycle of a multiple-isotope plasma is successfully reproduced with reduced turbulent transport modelling, within an integrated simulation framework. Future nuclear fusion reactors are likely to be fuelled by cryogenic pellet injection, due to higher penetration and faster response times. Accurate pellet cycle modelling is crucial to assess fuelling efficiency and burn control. In recent JET tokamak experiments, deuterium pellets with reactor-relevant deposition characteristics were injected into a pure hydrogen plasma. Measurements of the isotope ratio profile inferred a Deuterium penetration time comparable to the energy confinement time. The modelling successfully reproduces the plasma thermodynamic profiles and the fast deuterium penetration timescale. The predictions of the reduced turbulence model QuaLiKiz in the presence of a negative density gradient following pellet deposition are compared with GENE linear and nonlinear higher fidelity modelling. The results are encouraging with regard to reactor fuelling capability and burn control.</p
Angular dependence measurements of Magnum-PSI plasmas using MAST-U angled-tip Langmuir probes
No full textMeasurements made using flush-mounted Langmuir probes in tokamaks are difficult to interpret when operating at grazing angles of magnetic field incidence due to the effects of sheath expansion on the probe collection area. The Super-X divertor on the upgraded Mega Amp Spherical Tokamak (MAST-U) can have very shallow angles of magnetic field incidence to plasma facing components (1–10°), making the use of conventional flush-mounted probes problematic. A novel probe tip geometry, based on the angled-tip design used successfully on JET and DIII-D, has therefore been used in MAST-U to mitigate sheath expansion effects by increasing the projected probe extent. To verify whether the new design of probe tip allows temperature (Te) and density (ne) measurements to be performed accurately at low angles of incidence, a 4-probe array based on this design was used on Magnum-PSI. Parameter scans were made on a range of hydrogen plasmas in conditions comparable to those expected in MAST-U. Plasma parameters were extracted from the measured IV characteristics – using a novel fitting approach which minimised a goodness-of-fit parameter (TeδTe) – and compared to Magnum-PSI Thomson scattering measurements. The measured plasma parameters show that the MAST-U angled-tip design seems to successfully mitigate the effects of sheath expansion at low angles of magnetic field incidence. The standard MAST-U Langmuir probe also shows an apparent upper operational limit of θ=8°. This covers the vast majority of expected plasma configurations in MAST-U when running both conventional and Super-X configurations.</p
Oxygen evolution reaction (OER) mechanism under alkaline and acidic conditions
No full textDensity functional theory (DFT) simulations of the oxygen evolution reaction (OER) are considered essential for understanding the limitations of water splitting. Most DFT calculations of the OER use an acidic reaction mechanism and the standard hydrogen electrode (SHE) as reference electrode. However, experimental studies are usually carried out under alkaline conditions using the reversible hydrogen electrode (RHE) as reference electrode. The difference between the conditions in experiment and calculations is then usually taken into account by applying a pH-dependent correction factor to the latter. As, however, the OER reaction mechanisms under acidic and under alkaline conditions are quite different, it is not clear a priori whether a simple correction factor can account for this difference. We derive in this paper step by step the theory to simulate the OER based on the alkaline reaction mechanism and explain the OER process with this mechanism and the RHE as reference electrode. We compare the mechanisms for alkaline and acidic OER catalysis and highlight the roles of the RHE and the SHE. Our detailed analysis validates current OER simulations in the literature and explains the differences in OER calculations with acidic and alkaline mechanisms
Predicting the critical gradient of ITG turbulence in fusion plasmas
Get PDFThe quasilinear mixing-length approach to efficient prediction of transport in fusion devices is improved to account for the ``Dimits\u27\u27 upshift between linear and nonlinear critical pressure gradients in zonal-flow-saturated turbulence regimes. This modification uses the frequency mismatch between modes interacting turbulently to track changes in saturation efficiency. Near criticality, energy is transfered exclusively to stable eigenmodes, rapidly increasing the efficacy of the nonlinearity. The modified quasilinear model is able to predict below-threshold turbulent ion-temperature-gradient-driven transport accurately and also yields significantly improved predictions for trapped-electron-mode transport.</p
Revisiting spontaneous Raman scattering for direct oxygen atom quantification
Get PDFIn this letter the counterintuitive and largely unknown Raman activity of oxygen atoms is evaluated for its capacity to determine absolute densities in gases. The study involves CO₂ microwave plasma to generate a self-calibrating mixture and establish accurate cross-sections for the ³P₂↔³P₁ and ³P₂↔³P₀ transitions. The approach requires conservation of stoichiometry, which is confirmed within experimental uncertainty by a 1D fluid model. The measurements yield σ(J=2→1)=5.27±(sys: 0.42)(rand: 0.16)×10¯³¹ cm²/sr and σ(J=2→0)=2.11±(sys: 0.16)(rand: 0.06)×10¯³¹ cm²/sr and the detection limit is estimated to be 1×10¹⁵ cm¯³ for systems without other scattering species.</p
