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Application of a dual-thermopile radical probe to expanding hydrogen plasmas
No full textWe compare the performance of a hydrogen radical probe to historic data determined via Two-Photon Absorption Laser Induced Fluorescence (TALIF) using a comparable cascaded arc source under similar operating conditions. This probe has dual heat flux sensors (DHFS) each coated with materials with different catalytic properties for hydrogen atoms. In the ideal situation, the hydrogen radical flux can be deduced based on the difference between the heat loads measured by these two sensors. The influence of DHFS temperature on the performance was also assessed. The experimental results showed measurement errors of <10% could be obtained regardless of the probe temperature during plasma exposures. To convert heat fluxes into atomic fluxes, we calibrated the difference of the recombination coefficients using a vacuum ultraviolet (VUV) absorption technique, which is more reliable than modeled values based on assumptions or scattered values reported in literature. As a result, we measured the hydrogen plasma and radical parameters at various settings using both a double Langmuir probe and the DHFS. The typical atom flux in the 1022 m-2s-1 range was in good agreement with those obtained using optical techniques. We also observed that the ion and atom fluxes are both sensitive to the background gas pressure. These findings validate application of the DHFS to the cascaded arc source, and could pave the way for optimization of the source performance in the plasma material processing experiments.
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Nonuniform plasma meniscus modelling based on backward calculation of negative ion beamlet
No full textThe shape of a plasma meniscus is a key factor to determine the beam focusing. The physics model of the meniscus formation for hydrogen negative ion sources has not been established yet. A backward trajectory calculation based on experimental observation is performed in order to derive the particle information at the meniscus. It is observed that the negative ion density is spatially nonuniform in the direction parallel to the magnets for suppression of co-extracted electrons. A nonuniformity of the negative ion density in the vicinity of the meniscus is taken into account in the forward trajectory calculation. It reveals that the nonuniform negative ion distribution leads to degradation of the beam focusing and the beam splitting in phase space. The importance of the spatial distribution of negative ions on meniscus modelling is discussed with a comparison to uniform extraction model.<br/
Observation and rationalization of nitrogen oxidation enabled only by coupled plasma and catalysts
No full textHeterogeneous catalysts coupled with non-thermal plasmas (NTP) are known to achieve reaction yields that exceed the contributions of the individual components. Rationalization of the enhancing potential of catalysts, however, remains challenging because the background contributions from NTP or catalysts are often non-negligible. Here, we first demonstrate platinum (Pt)-catalyzed nitrogen (N2) oxidation in a radio frequency plasma afterglow at conditions at which neither catalyst nor plasma alone produces significant concentrations of nitric oxide (NO). We then develop reactor models based on reduced NTP- and surface-microkinetic mechanisms to identify the features of each that lead to the synergy between NTP and Pt. At experimental conditions, NTP and thermal catalytic NO production are suppressed by radical reactions and high N2 dissociation barrier, respectively. Pt catalyzes NTP-generated radicals and vibrationally excited molecules to produce NO. The model construction further illustrates that the optimization of productivity and energy efficiency involves tuning of plasma species, catalysts properties, and the reactor configurations to couple plasma and catalysts. These results provide unambiguous evidence of synergism between plasma and catalyst, the origins of that synergy for N2 oxidation, and a modeling approach to guide material selection and system optimization.</p
Potassium hydride-intercalated graphite as an efficient heterogeneous catalyst for ammonia synthesis
No full textDue to the high energy needed to break the N ≡ N bond (945 kJ mol−1), a key step in ammonia production is the activation of dinitrogen, which in industry requires the use of transition metal catalysts such as iron (Fe) or ruthenium (Ru), in combination with high temperatures and pressures. Here we demonstrate a transition-metal-free catalyst—potassium hydride-intercalated graphite (KH 0.19 C 24)—that can activate dinitrogen at very moderate temperatures and pressures. The catalyst catalyses NH3 synthesis at atmospheric pressure and achieves NH3 productivity (µmolNH3 gcat−1 h−1) comparable to the classical noble metal catalyst Ru/MgO at temperatures of 250–400 °C and 1 MPa. Both experimental and computational calculation results demonstrate that nanoconfinement of potassium hydride between the graphene layers is crucial for the activation and conversion of dinitrogen. Hydride in the catalyst participates in the hydrogenation step to form NH3. This work shows the promise of light metal hydride materials in the catalysis of ammonia synthesis.</p
Salisbury screen with lossy nonconducting materials: Way to increase spectral selectivity of absorption
Full text linkWe present a modified Salisbury screen design in which the thin absorbing metal layer is replaced with a layer of nonconducting lossy material. The conditions for total absorption in such a structure are explained using a simplified analytical model and rigorous numerical calculations. For the proof-of-principle experiments, germanium/silicon bilayers deposited on aluminium substrates are designed and manufactured. The structures demonstrate nearly perfect absorption in the near-IR spectral range. Compared to conventional Salisbury screens, a lossy semiconductor top layer exhibits increased spectral selectivity of absorption. The wavelength of nearly perfect absorption remains tunable by optimizing the thicknesses of the lossy and transparent layers.</p
Vibrational excitement: mode-selective conversion of CO2
No full textEmbargo 1 year, pdf open access 14-6-202
Linear stability of the JET H-mode pedestal
No full textThe stability of microtearing (MT) in the JET H-mode pedestal is investigated by means of both linear gyrokinetic simulations using the GENE code and a theoretical calculation. In order to determined the role played by MT in tokamak pedestal and to evaluate the role played by physical parameters, on MT destabilization a reduced linear model has been presented and compared with gyrokinetic simulations. The analytical model allows a good prediction of the impact of the different physical parameters, like the collisionality in the pedestal
Latest results of EUROfusion Plasma-Facing Components research in the areas of power loading, material erosion and fuel retention
No full textThe interaction between the edge-plasma in a fusion reactor and the surrounding first-wall components is one of the main issues for the realisation of fusion energy power plants. The EUROfusion Work Package on Plasma-Facing Components addresses the key areas of plasma-surface interaction in view of ITER and DEMO operation, which are mostly related to material erosion, surface damage and fuel retention. These aspects are both investigated experimentally (in tokamaks, linear plasma devices and lab experiments) and by modelling. Here, selective results regarding the main research topics are presented: In the area of tungsten (W) surface modifications, the interplay between W fuzz formation and W fuzz erosion depends strongly on the local plasma and surface conditions, as demonstrated by tokamak experiments. Complementary, experimental findings on the dependence of erosion on the surface structure in lab-scale experiments have led to the successful implementation of surface structure effects in numerical modelling. The qualification of ITER-like monoblocks at high fluences of up to 1031 D/m² in linear plasma facilities has shown no visible damages at cold plasma conditions. However, experiments with simultaneous plasma and pulsed heat loading (edge-localized modes simulations) show that synergistic effects can lower the W damage thresholds. Additionally, fuel retention studies show that nitrogen as a plasma impurity increases the fuel retention in W, and that deuterium implanted in the surface of W is capable of stabilizing displacement damages caused by neutron damage. Finally, the implications of these results on ITER and DEMO operation are discussed and an outlook on follow-up experiments is given: The results indicate that there are possible impacts on the ITER divertor lifetime and tritium removal. Other areas like the divertor shaping and the erosion need additional investigations in the future to quantify the impact on ITER and DEMO operation