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Homogeneous CO2 conversion by microwave plasma: Wave propagation and diagnostics
Get PDFA suite of diagnostics is proposed to characterize microwave plasma dissociation of CO2: laser scattering, Fourier transform infrared spectroscopy, and passive emission imaging. It provides a comprehensive performance characterization as is illustrated on the basis of experiments in a 2.45 GHz, 1 kW microwave reactor with tangential gas injection. For example, two operating regimes are identified as function of pressure: the diffuse and constricted plasma mode. Their occurrence is explained by evaluation of microwave propagation, which changes with the electron-heavy particle collision frequency ve−h. In the diffuse mode, gas temperatures of 1500–3500 K are determined. The measured conversion degree, specific energy input, and temperature are summarized in a two-temperature thermal model, which is solved to obtain the gas temperature at the periphery of the reactor and the size of the hot zone. Solutions are found with edge temperatures of hundreds of K, and hot zone fractions which agree with the measured behavior. The agreement shows that non-thermal processes play only a marginal role in the measured parameter space of the diffuse discharge. In the constricted mode, the radial plasma size is independent of power. A skin depth equal to the plasma size corresponds to electron densities of 1018–1019 m−3. Temperatures in the central filament are in the range 3000–5000 K. Both discharge modes are up to 50% energy efficient in CO production. Rayleigh signals increase in the afterglow, hinting at rapid gas cooling assuming that the gas composition remains unchanged.</p
Overview of progress in European medium sized tokamaks towards an integrated plasma-edge/wall solution
No full textIntegrating the plasma core performance with an edge and scrape-off layer (SOL) that leads to tolerable heat and particle loads on the wall is a major challenge. The new European medium size tokamak task force (EU-MST) coordinates research on ASDEX Upgrade (AUG), MAST and TCV. This multi-machine approach within EU-MST, covering a wide parameter range, is instrumental to progress in the field, as ITER and DEMO core/pedestal and SOL parameters are not achievable simultaneously in present day devices. A two prong approach is adopted. On the one hand, scenarios with tolerable transient heat and particle loads, including active edge localised mode (ELM) control are developed. On the other hand, divertor solutions including advanced magnetic configurations are studied. Considerable progress has been made on both approaches, in particular in the fields of: ELM control with resonant magnetic perturbations (RMP), small ELM regimes, detachment onset and control, as well as filamentary scrape-off-layer transport. For example full ELM suppression has now been achieved on AUG at low collisionality with n = 2 RMP maintaining good confinement H H(98,y2) =approx 0.95. Advances have been made with respect to detachment onset and control. Studies in advanced divertor configurations (Snowflake, Super-X and X-point target divertor) shed new light on SOL physics. Cross field filamentary transport has been characterised in a wide parameter regime on AUG, MAST and TCV progressing the theoretical and experimental understanding crucial for predicting first wall loads in ITER and DEMO. Conditions in the SOL also play a crucial role for ELM stability and access to small ELM regimes
Terahertz diffraction enhanced transparency probed in the near field
Get PDFElectromagnetically induced transparency in metamaterials allows to engineer structures which transmit narrow spectral ranges of radiation while exhibiting a large group index. Implementation of this phenomenon frequently calls for strong near-field coupling of bright (dipolar) resonances to dark (multipolar) resonances in the metamolecules comprising the metamaterials. The sharpness and contrast of the resulting transparency windows thus depends strongly on how closely these metamolecules can be placed to one another, placing constraints on fabrication capabilities. In this manuscript, we demonstrate that the reliance on near-field interaction strength can be relaxed, and the magnitude of the electromagnetic-induced transparency enhanced, by exploiting the long-range coupling between metamolecules in periodic lattices. By placing dolmen structures resonant at THz frequencies in a periodic lattice, we show a significant increase of the transparency window when the in-plane diffraction is tuned to the resonant frequency of the metamolecules, as confirmed by direct mapping of the THz near-field amplitude across a lattice of dolmens. Through the direct interrogation of the dark resonance in the near field, we show the interplay of near-and far-field couplings in optimizing the response of planar dolmen arrays via diffraction-enhanced transparency.</p
Response of tungsten surfaces to helium and hydrogen plasma exposure under ITER relevant steady state and repetitive transient conditions
No full textThe effect of helium (He) plasma exposure, and associated surface modifications, on the thermal shock resistance of tungsten (W) under ITER relevant steady state and transient heat and particle loads was studied. W samples were exposed to steady state and pulsed He plasmas at surface base temperatures from 670 to 1170 K. The same exposures were repeated in hydrogen (H) to allow a direct comparison of the role of the ion species on the thermal shock resistance. Exposure to He plasma pulses caused the formation of fine cracking network on W samples which occurred at a higher density and smaller depths compared to H pulsed plasma irradiation. The peak temperature reached during an ELM-like plasma pulse increased by a factor 1.45 over the 100 s of He plasma exposure, indicating a deterioration of the thermal properties. Transient loading experiments were also performed using a high power pulsed laser during He plasma exposure, showing a significant modification of the target thermal response caused by the surface damage. The effect of He-induced morphology changes on the thermal response modification was found to be very small compared to that of transient-induced damage
Atomic layer deposition of highly dispersed Pt nanoparticles on a high surface area electrode backbone for electrochemical promotion of catalysis
Get PDFA novel catalyst design for electrochemical promotion of catalysis (EPOC) is proposed which overcomes the main bottlenecks that limit EPOC commercialization, i.e., the low dispersion and small surface area of metal catalysts. We have increased the surface area by using a porous composite electrode backbone made of (La0.8Sr0.2)0.95MnO3-δ/Ce0.9Gd0.1O1.95 (LSM/GDC). Highly dispersed Pt nanoparticles with an average diameter of 6.5nm have been deposited on LSM/GDC by atomic layer deposition (ALD). This novel design offers, for the first time, a controllable and reproducible method for the fabrication of EPOC catalysts. The bare electrode backbone shows negligible activity for propane oxidation, while in the presence of Pt nanoparticles a high catalytic activity is obtained above 200°C. The performance of the Pt-loaded LSM/GDC catalyst was significantly improved by application of small currents (I<500μΑ), leading to a 27–33% increase as a function of the open circuit catalytic rate, with apparent Faradaic efficiency values ranging from 1000 to 3860% at 300°C. Our results demonstrate that EPOC is a valid approach for enhancing the catalytic activity of nano-structured catalysts.</p
Remobilization of tungsten dust from castellated plasma-facing components
No full textStudies of tungsten dust remobilization from castellated plasma-facing components can shed light to whether gaps constitute a dust accumulation site with important implications for monitoring but also removal. Castellated structures of ITER relevant geometry that contained pre-adhered tungsten dust of controlled deposition profile have been exposed in the Pilot-PSI linear device. The experiments were performed under steady state and transient plasma conditions, as well as varying magnetic field topologies. The results suggest that dust remobilization from the plasma-facing monoblock surface can enhance dust trapping in the gaps and that tungsten dust is efficiently trapped inside the gaps
Mitigated blistering and deuterium retention in tungsten exposed to high-flux deuterium–neon mixed plasmas
Get PDFSurface morphology and deuterium retention in tungsten exposed at surface temperature of 550 K to mixed deuterium–neon plasmas of different neon concentrations are investigated. It is found that the addition of neon up to 20% mitigates blistering on the surface. Cross-section view of the surface shows the formation of pores near the surface in the depth less than 100 nm. Deuterium depth profile is featured by an enhanced deuterium concentration within a depth of 16 nm but a mitigated penetration in depth larger than 1 µ m. Deuterium retention is reduced by up to a factor of four. It is suggested the open pores formed in the surface serves as an escaping channel, mitigates deuterium penetration towards bulk and retention in the bulk.</p
Energy Level Tuning of Poly(phenylene-alt-dithienobenzothiadiazole)s for Low Photon Energy Loss Solar Cells
No full textSix poly(phenylene-alt-dithienobenzothiadiazole)-based polymers have been synthesized for application in polymer–fullerene solar cells. Hydrogen, fluorine, or nitrile substitution on benzothiadiazole and alkoxy or ester substitution on the phenylene moiety are investigated to reduce the energy loss per converted photon. Power conversion efficiencies (PCEs) up to 6.6% have been obtained. The best performance is found for the polymer–fullerene combination with distinct phase separation and crystalline domains. This improves the maximum external quantum efficiency for charge formation and collection to 66%. The resulting higher photocurrent compensates for the relatively large energy loss per photon (E loss = 0.97 eV) in achieving a high PCE. By contrast, the polymer that provides a reduced energy loss (E loss = 0.49 eV) gives a lower photocurrent and a reduced PCE of 1.8% because the external quantum efficiency of 17% is limited by a suboptimal morphology and a reduced driving force for charge transfer. (Figure presented.). © 2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei