Browsing by Author "Loffhagen, Detlef"

Now showing 1 - 9 of 9
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    Derivation of Moment Equations for the Theoretical Description of Electrons in Nonthermal Plasmas
    (Irvine, Calif. : Scientific Research Publ., 2013) Becker, Markus M.; Loffhagen, Detlef
    The derivation of moment equations for the theoretical description of electrons is of interest for modelling of gas discharge plasmas and semiconductor devices. Usually, certain artificial closure assumptions are applied in order to derive a closed system of moment equations from the electron Boltzmann equation. Here, a novel four-moment model for the description of electrons in nonthermal plasmas is derived by an expansion of the electron velocity distribution function in Legendre polynomials. The proposed system of partial differential equations is consistently closed by definition of transport coefficients that are determined by solving the electron Boltzmann equation and are then used in the fluid calculations as function of the mean electron energy. It is shown that the four-moment model can be simplified to a new drift-diffusion approximation for electrons without loss of accuracy, if the characteristic frequency of the electric field alteration in the discharge is small in comparison with the momentum dissipation frequency of the electrons. Results obtained by the proposed fluid models are compared to those of a conventional drift-diffusion approximation as well as to kinetic results using the example of low pressure argon plasmas. It is shown that the results provided by the new approaches are in good agreement with kinetic results and strongly improve the accuracy of fluid descriptions of gas discharges.
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    Does the energy transfer from Ar(1s) atoms to N2 lead to dissociation?
    (Hoboken, NJ : Wiley Interscience, 2020) Klages, Claus‐Peter; Martinovs, Andris; Bröcker, Lars; Loffhagen, Detlef
    Dielectric-barrier discharges (DBDs) in Ar–N2 mixtures, with N2 fractions in 0.1–1% range, would be attractive alternatives to DBDs in pure N2 if energy-transfer reactions between Ar(1s) atoms and N2 molecules were an efficient source of N atoms. Attempts to functionalize polyolefins in flowing postdischarges fed by such DBDs, as well as the search for the First Positive System in the emission spectrum, however, failed. Evidently, the energy-transfer reactions do not produce N atoms. For Ar(1s3) and Ar(1s5) metastable states, this fact has already been reported in the literature. For Ar(1s2) and Ar(1s4) resonant states, a quantitative argument is derived in this paper: energy transfer from Ar(1s) atoms to N2 molecules is not an efficient source of N atoms.
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    Formation mechanisms of striations in a filamentary dielectric barrier discharge in atmospheric-pressure argon
    (Bristol : IOP Publ., 2023) Jovanović, Aleksandar P.; Hoder, Tomáš; Höft, Hans; Loffhagen, Detlef; Becker, Markus M.
    Formation mechanisms of striations along the discharge channel of a single-filament dielectric barrier discharge (DBD) in argon at atmospheric pressure are investigated by means of a time-dependent, spatially two-dimensional fluid-Poisson model. The model is applied to a one-sided DBD arrangement with a 1.5 mm gap using a sinusoidal high voltage at the powered metal electrode. The discharge conditions are chosen to mimic experimental conditions for which striations have been observed. It is found that the striations form in both half-periods during the transient glow phase, which follows the streamer breakdown phase. The modelling results show that the distinct striated structures feature local spatial maxima and minima in charged and excited particle densities, which are more pronounced during the positive polarity. Their formation is explained by a repetitive stepwise ionisation of metastable argon atoms and ionisation of excimers, causing a disturbance of the spatial distribution of charge carriers along the discharge channel. The results emphasise the importance of excited states and stepwise ionisation processes on the formation of repetitive ionisation waves, eventually leading to striations along the discharge channel.
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    Grundlegende Charakterisierung und Modellierung der VUV-Ausbeute von Xenon-Entladungen : Schlussbericht zum Teilvorhaben
    (Hannover : Technische Informationsbibliothek (TIB), 2005) Lange, Hartmut; Bussiahn, René; Gortchakov, Sergej; Holtz, Peter; Loffhagen, Detlef; Uhrlandt, Dirk
    [no abstract available]
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    Introduction and verification of FEDM, an open-source FEniCS-based discharge modelling code
    (Bristol : IOP Publ., 2023) Jovanović, Aleksandar P.; Loffhagen, Detlef; Becker, Markus M.
    This paper introduces the finite element discharge modelling (FEDM) code, which was developed using the open-source computing platform FEniCS (https://fenicsproject.org). Building on FEniCS, the FEDM code utilises the finite element method to solve partial differential equations. It extends FEniCS with features that allow the automated implementation and numerical solution of fully coupled fluid-Poisson models including an arbitrary number of particle balance equations. The code is verified using the method of exact solutions and benchmarking. The physically based examples of a time-of-flight experiment, a positive streamer discharge in atmospheric-pressure air and a low-pressure glow discharge in argon are used as rigorous test cases for the developed modelling code and to illustrate its capabilities. The performance of the code is compared to the commercial software package COMSOL Multiphysics® and a comparable parallel speed-up is obtained. It is shown that the iterative solver implemented by FEDM performs particularly well on high-performance compute clusters.
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    Modeling of Atmospheric-Pressure Dielectric Barrier Discharges in Argon with Small Admixtures of Tetramethylsilane
    (Dordrecht : Springer Science + Business Media B.V., 2021) Loffhagen, Detlef; Becker, Markus M.; Czerny, Andreas K.; Klages, Claus-Peter
    A time-dependent, spatially one-dimensional fluid-Poisson model is applied to analyze the impact of small amounts of tetramethylsilane (TMS) as precursor on the discharge characteristics of an atmospheric-pressure dielectric barrier discharge (DBD) in argon. Based on an established reaction kinetics for argon, it includes a plasma chemistry for TMS, which is validated by measurements of the ignition voltage at the frequency f=86.2kHz for TMS amounts of up to 200 ppm. Details of both a reduced Ar-TMS reaction kinetics scheme and an extended plasma-chemistry model involving about 60 species and 580 reactions related to TMS are given. It is found that good agreement between measured and calculated data can be obtained, when assuming that 25% of the reactions of TMS with excited argon atoms with a rate coefficient of 3.0×10−16m3/s lead to the production of electrons due to Penning ionization. Modeling results for an applied voltage Ua,0=4kV show that TMS is depleted during the residence time of the plasma in the DBD, where the percentage consumption of TMS decreases with increasing TMS fraction because only a finite number of excited argon species is available to dissociate and/or ionize the precursor via energy transfer. Main species resulting from that TMS depletion are presented and discussed. In particular, the analysis clearly indicates that trimethylsilyl cations can be considered to be mainly responsible for the film formation.
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    Plasma parameters of microarcs towards minuscule discharge gap
    (Weinheim : Wiley-VCH, 2020) Baeva, Margarita; Loffhagen, Detlef; Becker, Markus M.; Siewert, Erwan; Uhrlandt, Dirk
    This paper describes the behaviour of the plasma parameters of microarcs generated between a cooled copper anode and a ceriated tungsten cathode by means of a one-dimensional unified non-equilibrium model for gap lengths between 15 and 200 μm and current densities from 2 × 105 up to 106 A/m2. The results obtained show that the decrease of the gap length to a few tens of micrometres for a given current density results in a progressive shrinking of the quasi-neutral bulk in the microplasma and its complete disappearance. The decrease of the gap length further leads to an increase of the discharge voltage and the electron temperature and to slightly less heating of the gas. © 2020 The Authors. Contributions to Plasma Physics Published by Wiley-VCH Verlag GmbH & Co. KGaA
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    Schlussbericht des BMBF-Verbundprojekts PT-Grid - Plasma-Technologie-Grid : Berichtszeitraum: 01.05.2009 - 30.04.2012
    (Hannover : Technische Informationsbibliothek (TIB), 2012) Loffhagen, Detlef
    [no abstract available]
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    Streamer-surface interaction in an atmospheric pressure dielectric barrier discharge in argon
    (Bristol : IOP Publ., 2022) Jovanović, Aleksandar P.; Loffhagen, Detlef; Becker, Markus M.
    An atmospheric-pressure dielectric barrier discharge (DBD) in argon is investigated using a time-dependent and spatially two-dimensional fluid-Poisson model in axisymmetric geometry. The focus is on the streamer-surface interaction and the cathode-layer formation during the first discharge event in the single-filament DBD driven by sinusoidal voltage. A characteristic structure consisting of a volume streamer propagating just above the dielectric and simultaneous development of an additional surface discharge near the cathode is observed. The analysis of the electric field, electron production and loss rates, and surface charge density distribution shows that the radial deflection of the volume streamer is driven by free electrons remaining in the volume from the Townsend pre-phase and guided by the radial component of the electric field. The surface discharge occurring between the deflected volume streamer, which acts as virtual anode, and the dielectric surface is governed by ion-induced secondary electron emission and the surface charges accumulated on the dielectric.