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Author: Methling, Ralf Ă— End date: 2024 Ă— Start date: 2020 Ă— DDC: 530 Ă— Type: Article Ă— WGL Institute: INP Ă—

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    Plasma-based VAD process for multiply doped glass powders and high-performance fiber preforms with outstanding homogeneity
    (Hoboken, NJ : Wiley Interscience, 2020) Trautvetter, Tom; Schäfer, Jan; Benzine, Omar; Methling, Ralf; Baierl, Hardy; Reichel, Volker; Dellith, Jan; Köpp, Daniel; Hempel, Frank; Stankov, Marjan; Baeva, Margarita; Foest, RĂ¼diger; Wondraczek, Lothar; Wondraczek, Katrin; Bartelt, Hartmut
    An innovative approach using the vapor axial deposition (VAD), for the preparation of silica-based high-power fiber laser preforms, is described in this study. The VAD uses a plasma deposition system operating at atmospheric pressure, fed by a single, chemically adapted solution containing precursors of laser-active dopants (e.g., Yb2O3), glass-modifier species (e.g., Al2O3), and the silica matrix. The approach enables simultaneous doping with multiple optically active species and overcomes some of the current technological limitations encountered with well-established fiber preform technologies in terms of dopant distribution, doping levels, and achievable active core diameter. The deposition of co-doped silica with outstanding homogeneity is proven by Raman spectroscopy and electron probe microanalysis. Yb2O3 concentrations are realized up to 0.3 mol% in SiO2, with simultaneous doping of 3 mol% of Al2O3.
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    Unified modelling of TIG microarcs with evaporation from copper anode
    (Praha : Czech Technical University in Prague, Faculty of Electrical Engineering, Department of Physics, 2021) Baeva, Margarita; Methling, Ralf; Uhrlandt, Dirk
    A previously developed unified model of a tungsten-inert gas (TIG) microarc has been extended to take into account the melting of the anode made of copper and the release of copper atoms due to its evaporation. The copper atoms enter the plasma to become excited and ionized. The presence of copper atoms and ions can strongly change the plasma parameters. The extended unified model further includes excited states of copper and collisional and radiative processes between them. Predictions of the parameters of the microarc plasma in the presence of copper species are presented and discussed.
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    Positive streamers: inception and propagation along mineral-oil/solid interfaces
    (Bristol ; Philadelphia, PA : IOP Publishing Ltd., 2020) Ariza, David; Hollertz, Rebecca; Methling, Ralf; Gortschakow, Sergey
    This paper presents an experimental characterization of the prebreakdown phenomena in liquid/solid interfaces. The characterization is devoted to the 2nd mode positive streamers initiated and propagated along interfaces of mineral-oil and solids with different chemical composition and physical properties. Polymers of low density polyethylene (LDPE), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) polyvinylidene fluoride (PVDF) and papers made of kraft paper and a kraft fibril paper (made from cellulosic micro and nano fibrils), lignin-free paper and paper with high lignin content (referred to as k107 kraft paper) are used as the solid to study their influence on the streamer inception and propagation. The streamers are initiated at the interface by applying steps of voltage to a point-plane electrode arrangement with a solid (dielectric barrier) into the gap. The solid is placed diagonal to the oil gap and near to the point electrode. Shadowgraphs, charge and light intensity recordings are obtained during the inception and propagation of the streamers. Thus, estimations of the streamer length, velocity, current and average charge, are also presented. A time delay has been observed before the initiation of the streamer. This delay is probably correlated to the initiation process and formation of the gaseous phase of the streamer near to the interface. The threshold propagation voltage of the 2nd mode streamers at mineral-oil/solid interfaces is shown to be independent of the interface. However, the inception voltage is highly influenced by the interface. Additionally, the observed characteristics of streamers propagation (e.g. current, length, velocity, etc) along the tested interfaces cannot be fully explained by a capacitive coupling effect (permittivity mismatch). This open a discussion for the possibility that properties of the solid such as chemical composition, wettability and surface roughness can influence the streamer propagation.