DIFFER: Publications
Not a member yet
3526 research outputs found
Sort by
Mechanistic insight into carbon-carbon bond formation on cobalt under simulated Fischer-Tropsch synthesis conditions
No full textFacile C-C bond formation is essential to the formation of long hydrocarbon chains in Fischer-Tropsch synthesis. Various chain growth mechanisms have been proposed previously, but spectroscopic identification of surface intermediates involved in C-C bond formation is scarce. We here show that the high CO coverage typical of Fischer-Tropsch synthesis affects the reaction pathways of C2Hx adsorbates on a Co(0001) model catalyst and promote C-C bond formation. In-situ high resolution x-ray photoelectron spectroscopy shows that a high CO coverage promotes transformation of C2Hx adsorbates into the ethylidyne form, which subsequently dimerizes to 2-butyne. The observed reaction sequence provides a mechanistic explanation for CO-induced ethylene dimerization on supported cobalt catalysts. For Fischer-Tropsch synthesis we propose that C-C bond formation on the close-packed terraces of a cobalt nanoparticle occurs via methylidyne (CH) insertion into long chain alkylidyne intermediates, the latter being stabilized by the high surface coverage under reaction conditions
Distinguishing Among All Possible Activation Mechanisms of a Plasmon-Driven Chemical Reaction
No full textLocalized surface plasmon resonances (LSPRs) in metal nanoparticles can drive chemical reactions at their surface, but it is often challenging to disentangle the exact activation mechanism. The decay of LSPRs can lead to photothermal heating, electromagnetic hot spots, and the ejection of nonthermalized charge carriers, but all of these processes typically occur simultaneously and on ultrafast time scales. Here, we develop a plasmon-assisted Au@Ag core@shell nanorod synthesis in which each plasmon-decay mechanism can be independently assessed. Using different illumination wavelengths combined with extinction spectroscopy, transmission electron microscopy, thermal characterization, and finite-difference time-domain simulations, we unequivocally identify the transfer of interband holes to ascorbic acid as the rate-limiting step in the silver shell growth reaction. Our conclusion is corroborated by single-particle studies of gold nanospheres that display isotropic reactivity, consistent with interband hole-driven nanoparticle syntheses. Our strategy for distinguishing among plasmon-activation mechanisms can be extended to a variety of light-driven processes, including photocatalysis, nanoparticle syntheses, and drug delivery.</p
Smart Tungsten-based Alloys for a First Wall of DEMO
Get PDFDuring an accident with loss-of-coolant and air ingress in DEMO, the temperature of tungsten first wall cladding may exceed 1000 °C and remain for months leading to tungsten oxidation. The radioactive tungsten oxide can be mobilized to the environment at rates of 10–150 kg per hour. Smart tungsten-based alloys are under development to address this issue. Alloys are aimed to function as pure tungsten during regular plasma operation of DEMO. During an accident, alloying elements will create a protective layer, suppressing release of W oxide. Bulk smart alloys were developed by using mechanical alloying and field-assisted sintering technology. The mechanical alloying process was optimized leading to an increased powder production by at least 40 %. Smart alloys and tungsten were tested under a variety of DEMO-relevant plasma conditions. Both materials demonstrated similar sputtering resistance to deuterium plasma. Under accident conditions, alloys feature a 40-fold reduction of W release compared to that of pure tungsten.</p
Precise Control of Phase Separation Enables 12% Efficiency in All Small Molecule Solar Cells
No full textMomentum-space-resolved measurements using oblique electron cyclotron emission for the validation of the quasi-linear theory of electron cyclotron current drive
No full textElectron cyclotron resonance heating (ECRH) can drive large current densities through electron cyclotron current drive (ECCD). ECCD is expected to be crucial for high-performance plasmas in future fusion reactors like ITER and DEMO, making the current drive efficiency of ECCD a critical design parameter for future reactors. In present-day devices, good agreement between measured and predicted current drive efficiency has been found. However, to ensure the reliability in future machines, a direct validation of the electron momentum distribution function is needed. As a first step towards this goal, we present in this paper oblique ECE measurements of a low-density plasma in the ASDEX Upgrade tokamak. Two oblique ECE diagnostics are used to allow the simultaneous measurements of electrons streaming co- and counter-directionally with the plasma current. Predictions for the distribution function are computed with the bounce-averaged Fokker-Planck code RELAX (E. Westerhof et al., Rijnhuizen report,1992). To allow direct comparison with the measurements, synthetic radiation temperatures are computed with the code ECRad (S. Denk et al., Computer Physics Communications, p. 107175, 2020). Good agreement is found if radial transport occurring predominantly at low electron energies is included. We demonstrate that the oblique ECE diagnostics measure the electron distribution function directly at the ECRH deposition site in phase space. Furthermore, they are sensitive to the abundance of pitch-angle scattered electrons that reduce the ECCD efficiency. Limitations and uncertainties of the measurements and the modeling are discussed.</p
Mode resolved heating dynamics in pulsed microwave CO2 plasma from laser Raman scattering
No full textEfficient CO2 reduction is predicted for CO2 microwave plasma by virtue of predominant excitation of the asymmetric stretch vibration. Although interpretation of ongoing research is generally based on this mechanism, direct measurement of the power partitioning to support the assumed preferential vibrational excitation in CO2 microwave plasma is currently lacking. Here, such measurements are performed on a 100 µs pulsed microwave CO2 discharge. The <1% duty cycle ensures low gas temperature conditions at the discharge onset. Raman and Rayleigh scattering are employed to reveal vibrational, rotational, and gas temperatures in a spatially and temporally resolved manner. A novelty in the approach is that asymmetric stretch excitation is determined from the bending – symmetric stretch Raman spectrum. During the first 40 μs a significant inter-vibrational non-equilibrium is observed with the symmetric stretch and bending temperature reaching 750 K and the asymmetric stretch temperature reaching 1150 K. A maximum rotational-vibrational non-equilibrium occurs after 60 μs when the rotational temperature is half of the 1150 K vibrational temperature. Rotational and translational modes are measured to be in equilibrium at all times. The power partitioning is analyzed to estimate the power consumed by vibrational excitation, which is used to estimate the reduced electric field in the discharge. This work confirms strong vibrational excitation in CO2 microwave plasma albeit less predominant than often assumed
On the triggerless onset of 2/1 neoclassical tearing modes in TCV
No full textTriggerless 2/1 neoclassical tearing modes (NTMs), i.e. 2/1 NTMs that originate from unstable safety factor profiles (with positive classical stability index at zero island width, i.e. ^\u270 >0) and saturate neoclassically under the effect of perturbed bootstrap current, have been observed reproducibly in TCV discharges with a strong near-axis electron cyclotron current drive (ECCD). An unexpected density dependence of the onset of these NTMs is newly observed based on the statistics of many TCV discharges, suggesting a certain range of density within which the NTMs can occur. The range is found to broaden with increasing near-axis ECCD power. Based on a different set of experiments and corresponding interpretative simulations with the modified Rutherford equation (MRE), a simple analytical model for ^\u270 been developed. This model proves to explain well the observed density dependence of mode onset, resulting from the density dependence of the stability of ohmic plasmas and of the ECCD efficiency. The simulations have also quantified various effects and reproduced self-consistently the entire island width evolution of the triggerless NTM, from the onset as a tearing mode at zero island width to the neoclassical saturation as an NTM. The standard terms of the MRE model used in the paper are relevant for NTMs that are seeded by other mechanisms
Scanning electron microscopy analyses of an ITER plasma-facing unit mockup exposed to extreme ion fluences in Magnum-PSI
No full textA small-scale tungsten monoblock mockup was exposed to continuous Magnum-PSI plasma beams under six different conditions, including pure hydrogen, deuterium and helium (He) plasmas as well as mixed D/He plasmas with low electron temperature (1.5–5 eV), with maximum surface temperatures up to 1600 °C, and with ion fluences up to 10 30 m−2. The recrystallization in the centre of the hottest exposure spots, the absence of cracking on all monoblocks, the formation of a nanostructured fuzz layer several microns thick by pure He exposure, and the presence of various impurities from the exposure in Magnum-PSI on the surface were observed and analysed with a dedicated scanning electron microscope able to handle the entire mockup without any prior cutting
Axisymmetric simulations of vertical displacement events in tokamaks: A benchmark of M3D-C1, NIMROD and JOREK
Get PDFA benchmark exercise for the modeling of vertical displacement events(VDEs) is presented and applied to the 3D nonlinear magneto-hydrodynamic codesM3D-C1, JOREK and NIMROD. The simulations are based on a vertically unstableNSTX equilibrium enclosed by an axisymmetric resistive wall with rectangular crosssection. A linear dependence of the linear VDE growth rates on the resistivity ofthe wall is recovered for sufficiently large wall conductivity and small temperatures inthe open field line region. The benchmark results show good agreement between theVDE growth rates obtained from linear NIMROD and M3D-C1simulations as wellas from the linear phase of axisymmetric nonlinear JOREK, NIMROD and M3D-C1simulations. Axisymmetric nonlinear simulations of a full VDE performed with thethree codes are compared and excellent agreement is found regarding plasma locationand plasma currents as well as eddy and halo currents in the wall.</p
Predictive multi-channel flux-driven modelling to optimise ICRH tungsten control and fusion performance in JET
No full textThe evolution of the JET high performance hybrid scenario, including central accumulation of the tungsten (W) impurity, is reproduced with predictive multi-channel integrated modelling over multiple confinement times using first-principle based core transport models. Eight transport channels (Ti,Te,j,nD,nBe,nNi,nW, omega) are modelled predictively, with self-consistent sources, radiation and magnetic equilibrium, yielding a system with multiple non-linearities: This system can reproduce the observed radiative temperature collapse after several confinement times. W is transported inward by neoclassical convection driven by the main ion density gradients and enhanced by poloidal asymmetries due to centrifugal acceleration. The slow evolution of the bulk density profile sets the timescale for W accumulation. Modelling this phenomenon requires a turbulent transport model capable of accurately predicting particle and momentum transport (QuaLiKiz) and a neoclassical transport model including the effects of poloidal asymmetries (NEO) coupled to an integrated plasma simulator (JINTRAC). The modelling capability is applied to optimise the available actuators to prevent W accumulation, and to extrapolate in power and pulse length. Central NBI heating is preferred for high performance, but gives central deposition of particles and torque which increase the risk of W accumulation by increasing density peaking and poloidal asymmetry. The primary mechanism for ICRH to control W in JET is via its impact through turbulence in reducing main ion density peaking (which drives inward neoclassical convection), increased temperature screening and turbulent W diffusion. The anisotropy from ICRH also reduces poloidal asymmetry, but this effect is negligible in high rotation JET discharges. High power ICRH near the axis can sensitively mitigate against W accumulation, and dominant ion heating (e.g. He-3 minority) is predicted to provide more resilience to W accumulation than dominant electron heating (e.g. H minority) in the JET hybrid scenario. Extrapolation to DT plasmas finds 17.5 MW of fusion power and improved confinement compared to DD, due to reduced ion-electron energy exchange, and increased Ti/Te stabilisation of ITG instabilities. The turbulence reduction in DT increases density peaking and accelerates the arrival of W on axis; this may be mitigated by reducing the penetration of the beam particle source with an increased pedestal density