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Experimental evidence of enhanced recombination of a hydrogen plasma induced by nitrogen seeding in linear device Magnum-PSI
No full textIn this work we investigate the effects induced by the presence of nitrogen in a detached-like hydrogen plasmas in linear plasma machine Magnum-PSI. Detachment has been achieved by increasing the background neutral pressure in the target chamber by means of H 2 /N 2 puffing and two cases of study have been set up, i.e. at 2 and 4 Pa. Achieved n e are ITER-relevant i.e. above 10 20 m −3 and electron temperatures are in the range 0.8–2 eV. A scan among five different N 2 /H 2 +N 2 flux ratios seeded have been carried out, at values of 0, 5, 10, 15 and 20%. A n e decrease while increasing the fraction of N 2 has been observed for both background pressures, resulting in a plasma pressure drop of ̴ 30%. T e remains constant among all scans. The peak intensity of NH*(A 3 ∏->X 3 ∑ − , ∆v = 0) at 336 nm measured with optical emission spectroscopy increases linearly with the N 2 content, together with the NH 3 signal in the RGA. A further dedicated experiment has been carried out by puffing separately H 2 /N 2 and H 2 /He mixtures, being helium a poorly-reactive atomic species, hence excluding a priori nitrogen-induced molecular assisted recombination. Interestingly, plasma pressure and heat loads to the surface are enhanced when increasing the content of He in the injected gas mixture. In the case of N 2 , we observe an opposite behavior, indicating that N–H species actively contribute to convert ions to neutrals. Recombination is enhanced by the presence of nitrogen. Numerical simulations with two different codes, a global plasma-chemical model and a spatially-resolved Monte Carlo code, address the role of NH x species behaving as electron donor in the ion conversion with H + by means of what we define here to be N-MAR i.e. NH x + H + → NH x + + H, followed by NH x + + e − → NH x- 1 + H. Considering the experimental findings and the qualitative results obtained by modelling, N-MAR process is considered to be a possible plasma-chemical mechanism responsible for the observed plasma pressure drop and heat flux reduction. Further studies with a coupled code B2.5-Eunomia are currently ongoing and may provide quantitative insights on the scenarios examined in this paper
A fresh look on waves in ion-electron plasmas
No full textExploiting the general dispersion relation describing all waves in an ideal ion-electron fluid, we revisit established treatments on wave families in a cold ion-electron plasma. These contain the magnetohydrodynamic Alfvén and fast waves at low frequencies, long wavelengths, but are enriched by short wavelength resonance behaviors, electrostatic and electromagnetic mode types, and cut-off frequencies distinguishing propagating from evanescent waves. Our theoretical treatment exploits purely polynomial expressions, which for the cold ion-electron case only depend on 2 parameters: the ratio of masses over charges u and the ratio E of the electron gyro frequency to the combined ion-electron plasma frequency. We provide a complete description of all waves, which stresses the intricate variation of all five branches of eigenfrequencies w(k, @) depending on wavenumber k and angle @ between wavevector and magnetic field B. Corresponding 5-mode phase and group diagrams provide insight on wave transformations and energy transport. Special cases, like the high frequency modes in magneto-ionic theory following from Appleton-Hartree dispersion relations, are naturally recovered and critically discussed. Faraday rotation for electromagnetic waves is extended to all propagation angles @. The discussion covers all cold ion-electron plasma waves, up into the relativistic regime
Micromechanical and microstructural properties of tungsten fibers in the as-produced and annealed state: Assessment of the potassium doping effect
No full textDue to its high strength and low temperature ductility, tungsten fibers (W f ) have been widely used as reinforcement elements in metallic, ceramic and glass matrix composites to improve the strength, toughness and creep resistance. Materials designed for future fusion reactors also utilize the option of W f reinforcement, i.a. with a copper (W f /Cu) or tungsten (W f /W) matrix. W f /W composites are being intensively studied as risk-mitigation materials to replace bulk tungsten which is susceptible to embrittlement induced by neutrons resulting from fusion reaction. Operation of W f /W in high temperatures (up to 1300 °C and even higher) fusion environment implies a risk of recrystallization and grain growth, which dimishes the attractive properties of tungsten fibers. In this work, we assess this modification of micro-mechanical and microstructural properties of tungsten fibers by means of nanoindentation, scanning electron microscopy, electron back-scattering diffraction analysis and corelate it with the ultimate tensile strength and fracture modes observed in the tensile tests. Both pure W and pottasium doped wires in the as-fabricated and annealed states are investigated and the results are compared with bulk tungsten, also exposed to several annealing temperatures. The results highlight the postive impact of potassium doping which shifts the threshold temperature for the grain growth by about 600 °C compared to pure tungsten wire. The results of the nanoindentation revealed systematic linear correlation with the ultimate tensile strength, which therefore offers a complimenatary way of micro-mechanical testing linking it with macro-scale properties of the wires
Investigation of recombination front region in detached plasmas in a linear divertor plasma simulator
No full textWe have investigated detailed plasma structure around the recombination front region (RFR), where volume electron-ion recombination (EIR) strongly occurs in detached plasmas, by using two-dimensional (2D) movable probe system as well as laser Thomson scattering diagnostics. The 2D distribution of electron density, electron temperature, and plasma potential around the RFR, has been revealed for the first time. The spatial distribution of the particle loss rate due to EIR estimated from electron temperature and electron density profiles indicated that the three-dimensional structure of the RFR adopts a cone structure. Furthermore, in the peripheral region of the RFR, the radial profile of the electron density was widened locally with increasing cross-field transport caused by plasma fluctuation. From the heat transport analysis of electrons, it was found that the thermal relaxation process between electrons and ions is important for the cooling of electrons, and that the convection of electrons regulates electron heat transport near the RFR
Thomson scattering near the high-fluence target surface of the Magnum-PSI linear plasma generator
No full textIn the quest to long-term operation of high-power magnetically confined fusion devices, it is crucial to control the particle and heat loads on the wall. In order to predict these loads, understanding of the plasma-wall interaction is important. Near the wall surface, the plasma is accelerated towards the Debye sheath edge. In plasma conditions with high density and low temperature, the interaction between the incoming plasma and recycled neutrals can become important. In this paper, we present incoherent Thomson Scattering (TS) measurements in the near-surface region of the Magnum-PSI linear plasma generator. To enable TS measurements close to the plasma target of Magnum-PSI, a stray light suppression up to a factor 104 was achieved, while retaining high transmission. By incrementally moving the target along the magnetic field, this adapted system was used down to 1.9 mm from the target. In the last 10–15 mm in front of the surface, the electron density as well as temperature were observed to decrease significantly. Under the assumption of constant particle flux in this region, the density drop indicates plasma acceleration. In that case, the measurements can be interpreted to show the plasma presheath, and its lengthscale: ~ 1 cm. The electron cooling indicates an energy loss channel for the electrons near the wall. A reduced electron temperature near the sheath entrance leads to lower estimates of particle and energy flux, as well as ion impact energy, on the target surface.</p
The importance of thermal dissociation in CO2 microwave discharges investigated by power pulsing and rotational Raman scattering
Get PDFThe input power of a CO2 microwave plasma is modulated at kHz rate in scans of duty cycle at constant average power to investigate gas heating dynamics and its relation to dissociation efficiency. Rotational temperature profiles obtained from rotational Raman scattering reveal peak temperatures of up to 3000 K, while the edge temperature remains cold (500 K). During the plasma \u27OFF\u27-period, the gas cools down convectively, but remains overall too hot to allow for strong overpopulation of vibrational modes (2200 K in the core). Fast optical imaging monitors plasma volume variations and shows that power density scales with peak power. As dissociation scales with observed peak rotational temperature, it is concluded that thermal processes dominate. A simple 0D model is constructed which explains how higher power density favors dissociation over radial energy transport. Thermal decomposition is reviewed in relation to quenching oxygen radicals with vibrationally excited CO2, to reflect on earlier reported record efficiencies of 90%.</p
Dehydration of supercritical carbon dioxide using dense polymeric membranes: A techno-economical evaluation
No full textSupercritical CO 2 (scCO 2 ), used in the food industry as a water extraction agent, requires dehydration units for regeneration. The present study assesses the economics of membrane-based dehydration of scCO 2 . In contrast to earlier studies, the contribution, next to the membrane, also the contributions of the mass transfer resistances of the feed and permeate boundary are included, which have a dominant effect on the final process design and economics. In addition, our work also extrapolates the process to industrial scale evaluating different configurations and process conditions. Specifically, the contribution of the membrane and membrane unit costs is discussed in more detail. Including the mass transfer resistances of feed and permeate boundary layer reduces the water flux across the membrane up to a factor 150, implying a larger required membrane surface area for a given water removal rate, and thus higher costs. Using a SPEEK-based membrane, the total drying costs, normalized for the amount of water removed, minimize around a skin layer thickness of 1 μm, i.e., not too thin to permeate and thus spill too much CO 2 and not too thick to hamper the H 2 O flux. Because the feed boundary layer dominates water transport, conditions that minimize its thickness reduce total costs. A reduction of the feed boundary layer dominance can be achieved by adjusting channel height, cross-flow velocity and the density and viscosity of scCO 2 , the latter two by increasing the operational temperature from 45 to 65 °C (at 130 bar). Compared to the benchmark zeolite process currently available, the membrane-based process for drying scCO 2 outlined and optimized in the present study results in a 50% saving of total drying costs. These savings can be achieved by using a dense polymeric membrane with a H 2 O permeability of at least 10,000 Barrer and a CO 2 permeability of at most 10 Barrer.</p
Effect of annealing on microstructure, texture and hardness of ITER-specification tungsten analyzed by EBSD, vickers micro-hardness and nano-indentation techniques
No full textTungsten and its alloys have been extensively studied to be used as materials for plasma facing components (PFCs) in future nuclear fusion reactors. Under fusion operating conditions, the surface of PFCs will experience high heat flux exposure which will affect some microstructural features of the material. In the present work, the effect of annealing temperature on microstructure, texture and mechanical behavior of ITER-specification commercial pure tungsten was investigated by combining Electron Backscatter Diffraction (EBSD), Vickers micro-hardness and nano-indentation techniques. The samples were analyzed in the as-received and in the annealed conditions. Annealing was done for 1 h at three different temperatures: 1300 °C, 1500 °C and 1800 °C. Three stages in microstructural evolution were observed: recovery at 1300 °C, recrystallization at 1500 °C and grain growth at 1800 °C. As the annealing temperature increases and recrystallization takes place, α-fiber texture components are strengthened. Simultaneously, a reduction in both Vickers and nano-indentation hardness is found
Impact of polymorphism on the optoelectronic properties of a low-bandgap semiconducting polymer
No full textBoosting the Performance of WO3/n‐Si Heterostructures for Photoelectrochemical Water Splitting: from the Role of Si to Interface Engineering
Get PDFMetal oxide/Si heterostructures make up an exciting design route to high‐performance electrodes for photoelectrochemical (PEC) water splitting. By monochromatic light sources, contributions of the individual layers in WO3/n‐Si heterostructures are untangled. It shows that band bending near the WO3/n‐Si interface is instrumental in charge separation and transport, and in generating a photovoltage that drives the PEC process. A thin metal layer inserted at the WO3/n‐Si interface helps in establishing the relation among the band bending depth, the photovoltage, and the PEC activity. This discovery breaks with the dominant Z‐scheme design idea, which focuses on increasing the conductivity of an interface layer to facilitate charge transport, but ignores the potential profile around the interface. Based on the analysis, a high‐work‐function metal is predicted to provide the best interface layer in WO3/n‐Si heterojunctions. Indeed, the fabricated WO3/Pt/n‐Si photoelectrodes exhibit a 2 times higher photocurrent density at 1.23 V versus reversible hydrogen electrode (RHE) and a 10 times enhancement at 1.6 V versus RHE compared to WO3/n‐Si. Here, it is essential that the native SiO2 layer at the interface between Si and the metal is kept in order to prevent Fermi level pinning in the Schottky contact between the Si and the metal.</p