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On microinstabilities and turbulence in steep-gradient regions of fusion devices
No full textHigher excitation states along the background field of plasma instabilities dominate over standard, ground-state eigenmodes once the gradient drive becomes sufficiently strong. At this point, mode parity can no longer be used as the sole identifier of microtearing activity. Not only ion- and electron-temperature-gradient-driven modes occur in higher states, but even trapped electron modes, where decorrelation mechanisms enable odd-parity mode structures without a vanishing bounce average. Nonlinearly, higher-order Hermite states imprint their structures on the turbulence. Even at considerably strong drive, however, quasilinear models can recover nonlinear fluxes, as long as all subdominantly unstable eigenmodes are included. Due to flux contributions from such modes, gradient scalings of fluxes can be stronger than linear expectations
Plasma Catalysis: Distinguishing between Thermal and Chemical Effects
No full textThe goal of this study is to develop a method to distinguish between plasma chemistry and thermal effects in a Dielectric Barrier Discharge nonequilibrium plasma containing a packed bed of porous particles. Decomposition of CaCO3 in Ar plasma is used as a model reaction and CaCO3 samples were prepared with different external surface area, via the particle size, as well as with different internal surface area, via pore morphology. Also, the effect of the CO2 in gas phase on the formation of products during plasma enhanced decomposition is measured. The internal surface area is not exposed to plasma and relates to thermal effect only, whereas both plasma and thermal effects occur at the external surface area. Decomposition rates were in our case found to be influenced by internal surface changes only and thermal decomposition is concluded to dominate. This is further supported by the slow response in the CO2 concentration at a timescale of typically 1 minute upon changes in discharge power. The thermal effect is estimated based on the kinetics of the CaCO3 decomposition, resulting in a temperature increase within 80 °C for plasma power from 0 to 6 W. In contrast, CO2 dissociation to CO and O2 is controlled by plasma chemistry as this reaction is thermodynamically impossible without plasma, in agreement with fast response within a few seconds of the CO concentration when changing plasma power. CO forms exclusively via consecutive dissociation of CO2 in the gas phase and not directly from CaCO3. In ongoing work, this methodology is used to distinguish between thermal effects and plasma–chemical effects in more reactive plasma, containing, e.g., H2
Solar Hydrogen Generation from Ambient Humidity Using Functionalized Porous Photoanodes
Get PDFSolar hydrogen is a promising sustainable energy vector, and steady progress has been made in the development of photoelectrochemical (PEC) cells. Most research in this field has focused on using acidic or alkaline liquid electrolytes for ionic transfer. However, the performance is limited by (i) scattering of light and blocking of catalytic sites by gas bubbles and (ii) mass transport limitations. An attractive alternative to a liquid water feedstock is to use the water vapor present as humidity in ambient air, which has been demonstrated to mitigate the above problems and can expand the geographical range where these devices can be utilized. Here, we show how the functionalization of porous TiO2 and WO3 photoanodes with solid electrolytes—proton conducting Aquivion and Nafion ionomers—enables the capture of water from ambient air and allows subsequent PEC hydrogen production. The optimization strategy of photoanode functionalization was examined through testing the effect of ionomer loading and the ionomer composition. Optimized functionalized photoanodes operating at 60% relative humidity (RH) and Tcell = 30–70 °C were able to recover up to 90% of the performance obtained at 1.23 V versus reverse hydrogen electrode (RHE) when water is introduced in the liquid phase (i.e., conventional PEC operation). Full performance recovery is achieved at a higher applied potential. In addition, long-term experiments have shown remarkable stability at 60% RH for 64 h of cycling (8 h continuous illumination–8 h dark), demonstrating that the concept can be applicable outdoors.</p
Irradiation hardening induced by blistering in tungsten due to low-energy high flux hydrogen plasma exposure
No full textIn this work, the microstructure evolution in the near-surface of tungsten under hydrogen (H)plasma exposure conditions was observed by means of scanning electron microscopy (SEM), plasma focused ion beam (FIB)and transmission electron microscopy (TEM)techniques. Blisters, with existing dislocations distributed around obviously, were observed beneath the tungsten surface when the exposure temperature was 573 K, which was rarely reported in previous studies. However, H bombardment at 1273 K did not lead to the formation of blister-like microstructures. Correspondingly, irradiation hardening occurred after low temperature exposure, but not after high temperature exposure, according to the Berkovich nano-indentation experiments. In order to characterize the indentation size effect and irradiation hardening behavior of plasma-exposed materials, a mechanistic model was proposed for the hardness-depth relationship. A good agreement between the experimental indentation data and theoretical results revealed that plasma-induced dislocations play a dominant role in determining the increase of hardness for H plasma-exposed tungsten
Effect of Charge-Transfer State Energy on Charge Generation Efficiency via Singlet Fission in Pentacene-Fullerene Solar Cells
No full textSinglet fission in pentacene creates two triplet excitons per absorbed photon. In a solar cell, each triplet can generate an electron-hole pair, and hence, external quantum efficiencies exceeding 100% have been reported for pentacene-fullerene solar cells. The energetics of this process are intriguing because the minimum photon enerfy loss, defined as the energy difference between the (triplet) exciton state and the open-circuit voltage, is less than 0.5 eV and distinctively smaller than that in most organic donor-acceptor solar cells. To investigate the energetics of this process, we analyze the effect of the energy of the lowest unoccupied molecular orbital (LUMO) for different fullerene derivatives. With the LUMO energy becoming less negative, the open-circuit voltage increases and charge generation decreases. For all but one of the fullerenes tested, the charge-transfer state energy is distinctively higher than the pentacene triplet energy, revealing that charge generation via singlet fission is actually endergonic. An elementary Marcus model for the rate of electron transfer provides a qualitative description of the experimental trends, in accordance with an endergonic charge transfer. Considering that charge generation from triplet states is endergonic, involvement of pentacene singlet states, either from direct photoexcitation or via triplet-triplet annihilation, cannot be excluded
Scenario development for D–T operation at JET
No full textThe JET exploitation plan foresees D–T operations in 2020 (DTE2). With respect to the first D–T campaign in 1997 (DTE1), when JET was equipped with a carbon wall, the experiments will be conducted in presence of a beryllium–tungsten ITER-like wall and will benefit from an extended and improved set of diagnostics and higher additional heating power (32 MW neutral beam injection + 8 MW ion cyclotron resonance heating). There are several challenges presented by operations with the new wall: a general deterioration of the pedestal confinement; the risk of heavy impurity accumulation in the core, which, if not controlled, can cause the radiative collapse of the discharge; the requirement to protect the divertor from excessive heat loads, which may damage it permanently. Therefore, an intense activity of scenario development has been undertaken at JET during the last three years to overcome these difficulties and prepare the plasmas needed to demonstrate stationary high fusion performance and clear alpha particle effects. The paper describes the status and main achievements of this scenario development activity, both from an operational and plasma physics point of view
Plasma activated electrolysis for cogeneration of nitric oxide and hydrogen from water and nitrogen
Get PDFWith increasing global interest in renewable energy technology given the backdrop of climate change, storage of electrical energy has become particularly relevant. Most sustainable technologies (e.g., wind and solar) produce electricity intermittently. Thus, converting electrical energy and base molecules (i.e., H2O, N2) into energy-rich ones (e.g., H2, NH3) or chemical feedstock (e.g., NO) is of paramount importance. While H2O splitting is compatible with renewable electricity, N2 fixation is currently dominated by thermally activated processes. In this work, we demonstrate an all-electric route for simultaneous NO and H2 production. In our approach, H2O is reduced to H2 in the cathode of a solid oxide electrolyzer while NO is produced in the anode by the reaction of O2– species (transported via the electrolyte) and plasma-activated N2 species. High faradaic efficiencies up to 93% are achieved for NO production at 650 °C, and NO concentration is >1000 times greater than the equilibrium concentration at the same temperature and pressure.</p
Application of Ion Sensitive Probe to High Density Plasmas in Magnum-PSI
No full textFeasibility of an ion sensitive probe was evaluated in high electron density (> 5 × 1019 m−3) helium plasmas produced in the Magnum-PSI device. The ion sensitive probe showed that the ion temperature was ∼ 1 eV and almost equal to the electron temperature. Increasing the neutral pressure to efficiently lead to the electron-ion recombination processes, the electron currents were collected at the ion collector. A secondary electron emission in the guard electrode might have an effect on the ion curren
Role of Electron–Ion Dissociative Recombination in CH4 Microwave Plasma on Basis of Simulations and Measurements of Electron Energy
No full textC–H bond activation was studied in low pressure microwave plasma discharges in methane. Electron energy loss channels were analyzed in view of promoting vibrational excitation. The molecular dissociative recombination (DR) channel is concluded to play multiple roles in the hydrocarbon plasma chemistry. DR increases the electron temperature with input power density and simultaneously breaks the hydrocarbon chains. Depending on the ionic species considered, plasma density n e in the range of 1017-1019 m−3 (10 −6 to 10 −4 ionization degree) and the electron mean energy in the range of 2−4 eV were estimated on basis of a Boltzman solver. from 2−3 eV measured with Thomson scattering anchored the microwave discharges in a preferential regime for vibrational excitation. The best agreement with experiments was obtained when C+ is the dominant ion in the CH4 microwave plasma, formed through successive DR and charge exchange reactions from molecular ions