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Characterization of low-frequency inter-ELM modes of H-mode discharges at ASDEX Upgrade
No full textThe steep edge gradient region of tokamak plasmas in the high confinement regime is known to drive instabilities, which cause transport. Several diagnostics are used to allow for a high degree of characterization of low-frequency modes appearing in between type-I edge localizes modes (ELMs). These modes are dominantly observed in electron cyclotron emission (ECE) and ECE imaging measurements as a modulation of radiation temperature (delta T rad). In the radial magnetic field (B t) measurements, the frequency range of 4 kHz to 12 kHz is observed. The position of the mode is determined to be at the upper part of the steep gradient region, the poloidal mode velocity is changing from 1.5 ± 0.5 km s−1 to 2.5 ± 0.5 km s−1 and the toroidal mode number is 13 to 14. A comparison with the measured E x B velocity leads to the conclusion that the phase velocity of the mode is smaller than 3 km s−1 or zero. The poloidal structure of the modes is found to agree with the poloidal structure size associated with n = 13 as estimated from the equilibrium calculations. The modes are compared between two different heating phases during one discharge, and are found to differ in duration, velocity, frequency and toroidal mode number. The possibility of non-linear interaction between these modes and other, high frequency modes existing in the narrow pedestal, is assessed via bicoherence analysis. The presented analysis gives an unprecedented picture of the mode, its position, its structure and its velocity, calling for comparison with non-linear modelling.</p
Elastic versus alloying effects in Mg-based hydride films
Get PDFMagnesium thin films covered with a layer of Pd absorb hydrogen at much higher pressures than bulk Mg. Such an effect was originally explained as a consequence of elastic clamping on Mg by the capping Pd layer. An alternative interpretation later suggested that the pressure increase could originate from simple alloying between Mg and Pd. Here we resolve this controversy by measuring the hydrogenation and dehydrogenation isotherms of Mg-Pd thin film alloys over a wide range of compositions. Our results disentangle the effects of elastic clamping and alloying, and highlight the role of plastic deformations.</p
Proton conductive cationic nanoporous polymers based on smectic liquid crystal hydrogen-bonded heterodimers
No full textThe fabrication of a cationic nanoporous smectic liquid crystal network (LCN) based on hydrogen bonded heterodimers is presented. The method relies on a supramolecular complex made from a pyridyl bearing reactive mesogen hydrogen bonded to a non-reactive benzoic acid template. Upon addition of a cross-linker, a smectic liquid crystalline phase is obtained that can be fixed by photopolymerization. It was found that the lamellar structure was maintained after template removal when 25 wt% or more cross-linker was used, yielding a nanoporous LCN. After H3PO4 immobilization in the pores of the LCN, a cationic 2D nanoporous polymer is obtained showing high and anisotropic anhydrous proton conductivity.</p
Fast H isotope and impurity mixing in ion-temperature-gradient turbulence
No full textIn ion-temperature-gradient (ITG) driven turbulence, the resonance condition leads to ion particle turbulent transport coefficients significantly larger than electron particle turbulent transport coefficients. This is shown in nonlinear gyrokinetic simulations and explained by an analytical quasilinear model. It is then illustrated by JETTO-QuaLiKiz integrated modelling. Large ion particle transport coefficients implies that the ion density profiles are uncorrelated to the corresponding ion source, allowing peaked isotope density profiles even in the absence of core source. This also implies no strong core accumulation of He ash. Furthermore, the relaxation time of the individual ion profiles in a multi-species plasma can be significantly faster than the total density profile relaxation time which is constrained by the electrons. This leads to fast isotope mixing and fast impurity transport in FM regimes. In trapped-electron- mode (TEM) turbulence, in presence of electron heating about twice the ion heating, the situation is the inverse: ion particle turbulent transport coefficients are smaller than their electron counterpart
From nanometre to millimetre: A range of capabilities for plasma-enabled surface functionalization and nanostructuring
No full textDriven by the innate tendency of the system to attain a local energy minimum, self-organization enables the creation of complex systems out of relatively simple parts and elements. The ability to form hierarchical, multicomponent systems that may be difficult, or even impossible to fabricate using pre-set, template-enabled processes makes self-organisation very attractive for synthesis and assembly of advanced material systems across multiple length scales. Yet, driving and controlling such self-organisation processes is not a trivial task as they often arise from a complex interplay of physical and chemical processes. These in turn depend on the environment in which self-organisation takes place. In this topical review, we focus on one such environment and outline unique opportunities, salient characteristics and challenges presented by self-organization on surfaces exposed to partially ionised gases, i.e. plasmas. Using a select number of recent examples, we aim to show how salient features of plasma environments, particularly high fluxes of energy and matter from the plasma to the surface, enable functionalization and growth of complex nanostructures and metamaterials via self-organization on plasma-exposed surfaces. We will show how by controlling different physical and chemical parameters of the plasma environment and how it interacts with surfaces, it is possible to control self-organization processes at multiple length scales, making it a promising enabling platform for nanosynthesis. We will discuss examples starting from the self-driven growth of perfect crystalline lattices, such as nano-diamonds and graphenes at nanoscale, all the way to template- and pattern-free synthesis of large, highly or-dered arrays of nanostructures at millimetre and even centimetre scales. We will outline the key enabling features of plasmas that drive these processes at respective scales, focusing predominantly on plasma-induced electric fields at the surface or in the plasma-nanostructure sheath, as well as charge-related effects. The outlook section summarizes advantages of plasma-driven self-organization, and outlines principal challenges and opportunities for the development of this field
Atmospheric pressure plasma enhanced chemical vapour deposition (AP-PECVD) of silica: Understanding the role of the local deposition rate in the surface and film morphology
No full textPlasma assisted nitrogen oxide production from air: Using pulsed powered gliding arc reactor for a containerized plant
Get PDFThe production of NOx from air and air + O2 is investigated in a pulsed powered milli-scale gliding arc (GA) reactor, aiming at a containerized process for fertilizer production. Influence of feed mixture, flow rate, temperature, and Ar and O2 content are investigated at varying specific energy input. The findings are correlated with high-speed imaging of the GA dynamics. An O2 content of 40–48% was optimum, with an enhancement of 11% in NOx production. Addition of Ar and preheating of the feed resulted in lower NOx production. Lower flow rates produced higher NOx concentrations due to longer residence time in the GA. The volume covered by GA depends strongly on the gas flow rate, emphasizing that the gas flow rate has a major impact on the GA dynamics and the reaction kinetics. For 0.5 L/min, 1.4 vol % of NOx concentration was realized, which is promising for a containerized process plant to produce fertilizer in remote locations. © 2017 American Institute of Chemical Engineers AIChE J</p
Effects of nitrogen seeding on core ion thermal transport in JET ILW L-mode plasmas
Get PDFA set of experiments was carried out in JET ILW (Joint European Torus with ITER-Like Wall) L-mode plasmas in order to study the effects of light impurities on core ion thermal transport. N was puffed into some discharges and its profile was measured by active Charge Exchange diagnostics, while ICRH power was deposited on- and off-axis in (3 He) - D minority scheme in order to have a scan of local heat flux at constant total power with and without N injection. Experimentally, the ion temperature profiles are more peaked for similar heat fluxes when N is injected in the plasma. Gyro-kinetic simulations using the GENE code indicate that a stabilization of Ion Temperature Gradient driven turbulent transport due to main ion dilution and to changes in T_e /T_i and s / q is responsible of the enhanced peaking. The quasi-linear models TGLF and QuaLiKiz are tested against the experimental and the gyro-kinetic results.</p
A rotational Raman study under non-thermal conditions in a pulsed CO2 glow discharge
Get PDFThe implementation of \u27in situ\u27 rotational Raman spectroscopy is realized for a pulsed glow discharge in CO2 in the mbar range and is used to study the rotational temperature and molecular number densities of CO2, CO, and O2. The polarizability anisotropy of these molecules is required for extracting number densities from the recorded spectra and is determined for incident photons of 532 nm. The spatiotemporally-resolved measurements are performed in the same reactor and at equal discharge conditions (5-10 ms on-off cycle, 50 mA plasma current, 6.7 mbar pressure) as in recently published work employing \u27in situ\u27 Fourier transform infrared (FTIR) spectroscopy. The rotational temperature ranges from 394 K to 809 K from start to end of the discharge pulse and is constant over the length of the reactor. The discharge is demonstrated to be spatially uniform in gas composition, with a CO2 conversion factor of 0.15 ± 0.02. Rotational temperatures and molecular composition agree well with the FTIR results, while the spatial uniformity confirms the assumption made for the FTIR analysis of a homogeneous medium over the line-of-sight of absorption. Furthermore, the rotational Raman spectra of CO2 are related to vibrational temperatures through the vibrationally averaged nuclear spin degeneracy, which is expressed in the intensity ratio between even and odd numbered Raman peaks. The elevation of the odd averaged degeneracy above thermal conditions agrees well with the elevation of vibrational temperatures of CO2, acquired in the FTIR study
Liquid metals as a divertor plasma facing material explored using the Pilot-PSI and Magnum-PSI linear devices
Get PDFAbstract For DEMO and beyond liquid metal plasma facing components are considered due to their resilience to erosion through flowed replacement, potential for cooling beyond conduction and inherent immunity to many of the issues of neutron loading compared to solid materials. The development curve of liquid metals is behind that of e.g. tungsten however and tokamak-based research is currently somewhat limited in scope. Therefore investigation in linear plasma devices can provide faster progress under controlled and well-diagnosed conditions in assessing many of the issues surrounding the use of liquid metals. The linear plasma devices Magnum-PSI and Pilot-PSI are capable of producing DEMO relevant plasma fluxes which well replicate expected divertor conditions, and the exploration of physics issues for tin (Sn) and lithium (Li) such as vapour-shielding, erosion under high particle flux loading and overall power handing are reviewed here. A deeper understanding of erosion and deposition through this work indicates that stannane formation may play an important role in enhancing Sn erosion, while on the other hand the strong hydrogen isotope affinity reduces the evaporation rate and sputtering yields for Li. In combination with the strong re-deposition rates which have been observed under this type of high density plasma this implies an increase in the operational temperature range, implying a power handling range of 20-25 MW m -2 for Sn and up to 12.5 MW m -2 for Li could be achieved. Vapour shielding may be expected to act as a self-protection mechanism in reducing the heat load to the substrate for off-normal events in the case of Sn, but may potentially be a continual mode of operation for Li.</p