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- ItemCorrigendum: Dielectric barrier discharges: progress on plasma sources and on the understanding of regimes and single filaments (Plasma Sources Science and Technology (2017) 26 (053001) DOI: 10.1088/1361-6595/aa6426)(Bristol : IOP Publ., 2018) Brandenburg, RonnyThe paper Dielectric barrier discharges: progress on plasma sources and on the understanding of regimes and single filaments? [1] contains a serious misprint in the section ?Electrical behavior and characterization?. The correct equation (5) for the determination of the gap voltage Ug is as follows (Formula Presented). The second term is the voltage across the barrier (Ub) which is determined by the charge Q t and the capacity of the barrier(s) Cd. The parameter 1 Ccell as written in the original paper is not correct, and a misprint. The sum of gap voltage and barrier voltage equals the applied voltage V t . Detailed explanations about the formula can be found in the cited literature, in particular [2-4].
- ItemStability and excitation dynamics of an argon micro-scaled atmospheric pressure plasma jet(Bristol : IOP Publ., 2015) Dünnbier, M.; Becker, M.M.; Iseni, S.; Bansemer, R.; Loffhagen, D.; Reuter, S.; Weltmann, K.-D.A megahertz-driven plasma jet at atmospheric pressure—the so-called micro-scaled atmospheric pressure plasma jet (μAPPJ)—operating in pure argon has been investigated experimentally and by numerical modelling. To ignite the discharge in argon within the jet geometry, a self-made plasma tuning unit was designed, which additionally enables measurements of the dissipated power in the plasma itself. Discharges in the α-mode up to their transition to the γ-mode were studied experimentally for varying frequencies. It was found that the voltage at the α–γ transition behaves inversely proportional to the applied frequency f and that the corresponding power scales with an f  3/2law. Both these findings agree well with the results of time-dependent, spatially one-dimensional fluid modelling of the discharge behaviour, where the f  3/2 scaling of the α–γ transition power is additionally verified by the established concept of a critical plasma density for sheath breakdown. Furthermore, phase resolved spectroscopy of the optical emission at 750.39 nm as well as at 810.37 nm and 811.53 nm was applied to analyse the excitation dynamics of the discharge at 27 MHz for different applied powers. The increase of the power leads to an additional maximum in the excitation structure of the 750.39 nm line emission at the α–γ transition point, whereas the emission structure around 811 nm does not change qualitatively. According to the fluid modelling results, this differing behaviour originates from the different population mechanisms of the corresponding energy levels of argon.
- ItemAdvanced fluid modeling and PIC/MCC simulations of low-pressure ccrf discharges(Bristol : IOP Publ., 2017-3-2) Becker, M.M.; Kählert, H.; Sun, A.; Bonitz, M.; Loffhagen, D.Comparative studies of capacitively coupled radio-frequency discharges in helium and argon at pressures between 10 and 80 Pa are presented applying two different fluid modeling approaches as well as two independently developed particle-in-cell/Monte Carlo collision (PIC/MCC) codes. The focus is on the analysis of the range of applicability of a recently proposed fluid model including an improved drift-diffusion approximation for the electron component as well as its comparison with fluid modeling results using the classical drift-diffusion approximation and benchmark results obtained by PIC/MCC simulations. Main features of this time- and space-dependent fluid model are given. It is found that the novel approach shows generally quite good agreement with the macroscopic properties derived by the kinetic simulations and is largely able to characterize qualitatively and quantitatively the discharge behavior even at conditions when the classical fluid modeling approach fails. Furthermore, the excellent agreement between the two PIC/MCC simulation codes using the velocity Verlet method for the integration of the equations of motion verifies their accuracy and applicability.
- ItemDielectric barrier discharges: progress on plasma sources and on the understanding of regimes and single filaments(Bristol : IOP Publ., 2017-3-30) Brandenburg, RonnyDielectric barrier discharges (DBDs) are plasmas generated in configurations with an insulating (dielectric) material between the electrodes which is responsible for a self-pulsing operation. DBDs are a typical example of nonthermal atmospheric or normal pressure gas discharges. Initially used for the generation of ozone, they have opened up many other fields of application. Therefore DBDs are a relevant tool in current plasma technology as well as an object for fundamental studies. Another motivation for further research is the fact that so-called partial discharges in insulated high voltage systems are special types of DBDs. The breakdown processes, the formation of structures, and the role of surface processes are currently under investigation. This review is intended to give an update to the already existing literature on DBDs considering the research and development within the last two decades. The main principles and different modes of discharge generation are summarized. A collection of known as well as special electrode configurations and reactor designs will be presented. This shall demonstrate the different and broad possibilities, but also the similarities and common aspects of devices for different fields of applications explored within the last years. The main part is devoted to the progress on the investigation of different aspects of breakdown and plasma formation with the focus on single filaments or microdischarges. This includes a summary of the current knowledge on the electrical characterization of filamentary DBDs. In particular, the recent new insights on the elementary volume and surface memory mechanisms in these discharges will be discussed. An outlook for the forthcoming challenges on research and development will be given.
- ItemSearching for order in atmospheric pressure plasma jets(Bristol : IOP Publ., 2017-11-10) Schäfer, Jan; Sigeneger, Florian; Å perka, JirÌŒÃ; Rodenburg, Cornelia; Foest, RüdigerThe self-organized discharge behaviour occurring in a non-thermal radio-frequency plasma jet in rare gases at atmospheric pressure was investigated. The frequency of the azimuthal rotation of filaments in the active plasma volume and their inclination were measured along with the gas temperature under varying discharge conditions. The gas flow and heating were described theoretically by a three-dimensional hydrodynamic model. The rotation frequencies obtained by both methods qualitatively agree. The results demonstrate that the plasma filaments forming an inclination angle α with the axial gas velocity uz are forced to a transversal movement with the velocity uφ=tan(α)*uz, which is oriented in the inclination direction. Variations of ${u}_{\phi }$ in the model reveal that the observed dynamics minimizes the energy loss due to convective heat transfer by the gas flow. The control of the self-organization regime motivates the application of the plasma jet for precise and reproducible material processing.
- ItemSpatio-temporal characterization of the multiple current pulse regime of diffuse barrier discharges in helium with nitrogen admixtures(Bristol : IOP Publ., 2017-09-20) Bogaczyk, Marc; Tschiersch, Robert; Nemschokmichal, Sebastian; Meichsner, JürgenThis work reports on the spatio-temporal characterization of the multiple current pulse regime of diffuse barrier discharges driven by sine-wave feeding voltage at a frequency of 2 kHz in helium with small nitrogen admixtures. The discharge gap of 3 mm is bounded by glass plates on both plane electrodes. Priority is given to the lateral discharge inhomogeneities, underlying volume- and surface-memory effects, and the breakdown mechanism. Therefore, relevant processes in the discharge volume and on the dielectric surfaces were investigated by ICCD camera imaging and optical emission spectroscopy in combination with electrical measurements and surface charge diagnostics using the electro-optic Pockels effect of a bismuth silicon oxide crystal. The number of current pulses per half-cycle of the sine-wave voltage rises with increasing nitrogen admixture to helium due to the predominant role of the Penning ionization. Here, the transition from the first glow-like breakdown to the last Townsend-like breakdown is favored by residual species from the former breakdowns which enhance the secondary electron emission during the pre-phase of the later breakdowns. Moreover, the surface charge measurements reveal that the consecutive breakdowns occur alternately at central and peripheral regions on the electrode surface. These spatial inhomogeneities are conserved by the surface charge memory effect as pointed out by the recalculated spatio-temporal development of the gap voltage.
- ItemSurface charge measurements on different dielectrics in diffuse and filamentary barrier discharges(Bristol : IOP Publ., 2017-2-10) Tschiersch, R.; Nemschokmichal, S.; Bogaczyk, M.; Meichsner, J.Previously, we reported on the measurement of surface charges during the operation of barrier discharges (BDs) using the electro-optic Pockels effect of a bismuth silicon oxide (BSO) crystal. With the present work, the next milestone is achieved by making this powerful method accessible to various dielectrics which are typically used in BD configurations. The dynamics and spatial distribution of positive and negative surface charges were determined on optically transparent borosilicate glass, mono-crystalline alumina and magnesia, respectively, covering the BSO crystal. By variation of the nitrogen admixture to helium and the pressure between 500 mbar and 1 bar, both the diffuse glow-like BD and the self-stabilized discharge filaments were operated inside of a gas gap of 3 mm. The characteristics of the discharge and, especially, the influence of the different dielectrics on its development were studied by surface charge diagnostics, electrical measurements and ICCD camera imaging. Regarding the glow-like BD, the breakdown voltage changes significantly by variation of the cathodic dielectric, due to the different effective secondary electron emission (SEE) coefficients. These material-specific SEE yields were estimated using Townsend's criterion in combination with analytical calculations of the effective ionization coefficient in helium with air impurities. Moreover, the importance of the surface charge memory effect for the self-stabilization of discharge filaments was quantified by the recalculated spatio-temporal behavior of the gap voltage.
- ItemTemperature and pressure profiles of an ablation-controlled arc plasma in air(Bristol : IOP Publ., 2019) Becerra, Marley; Pettersson, Jonas; Franke, Steffen; Gortschakow, SergeyExperimental measurements of the spatial distribution of temperature and composition of ablation-controlled arc plasmas are a key to validate the predictions of metal evaporation and polymer ablation models. Thus, high-speed photography and space-resolved spectroscopic measurements have been performed to characterize a stable air arc plasma jet controlled by ablation of a polymer nozzle made of Polyoxymethylene copolymer (POM-C) or polyamide (PA6). The spectroscopic analysis is performed along a plane perpendicular to the arc jet axis for a current of 1.8 kA, corresponding to an estimated current density of ~65 A mm-2. Temperature and partial pressure profiles of the plasma for copper, hydrogen and carbon in the gas mixture are estimated as an inverse optimization problem by using measured side-on radiance spectra and radiative transfer spectral simulations. It is shown that the generated ablation-controlled arc has a complicated, non-uniform gas composition. Thus, the generated arc jet has a thin metallic core with a lower almost constant hydrogen pressure, surrounded by a thicker hydrogen and carbon mantle at partial pressures slightly lower than atmospheric pressure. The separation of hydrogen and carbon in the core is a consequence of demixing of the polymer vapour in the plasma. It is found that the overall shape of the temperature and pressure profiles obtained for the arc plasmas with the POM-C and PA6 nozzles are similar although differ in peak values and width. © 2019 IOP Publishing Ltd.
- ItemCorrigendum: Concepts and characteristics of the 'COST Reference Microplasma Jet' (Journal of Physics D: Applied Physics (2016) 49 (084003) DOI: 10.1088/0022-3727/49/8/084003)(Bristol : IOP Publ., 2019) Golda, J.; Held, J.; Redeker, B.; Konkowski, M.; Beijer, P.; Sobota, A.; Kroesen, G.; Braithwaite, N.St.J.; Reuter, S.; Turner, M.M.; Gans, T.; O’Connell, D.; Schulz-von der Gathen, V.There is an incorrect representation of the expression for resistances in parallel in equation (1) in section 4.1 'Voltage probe calibration' on page 6. The numerator and denominator in the equation are reversed and should read: I = Uc Rm + Rt/RmRt. Rm is the measuring resistor, Rt the terminating resistor at the oscilloscope and Uc is the voltage drop across Rm induced by the current I. None of the calculations and conclusions of the paper are affected. The authors apologise for any confusion that this transcription error may have caused. © 2018 IOP Publishing Ltd.
- ItemA collisional-radiative model of iron vapour in a thermal arc plasma(Bristol : IOP Publ., 2017-05-15) Baeva, M.; Uhrlandt, D.; Murphy, A.B.A collisional-radiative model for the ground state and fifty effective excited levels of atomic iron, and one level for singly-ionized iron, is set up for technological plasmas. Attention is focused on the population of excited states of atomic iron as a result of excitation, de-excitation, ionization, recombination and spontaneous emission. Effective rate coefficients for ionization and recombination, required in non-equilibrium plasma transport models, are also obtained. The collisional-radiative model is applied to a thermal arc plasma. Input parameters for the collisional-radiative model are provided by a magnetohydrodynamic simulation of a gas-metal welding arc, in which local thermodynamic equilibrium is assumed and the treatment of the transport of metal vapour is based on combined diffusion coefficients. The results clearly identify the conditions in the arc, under which the atomic state distribution satisfies the Boltzmann distribution, with an excitation temperature equal to the plasma temperature. These conditions are met in the central part of the arc, even though a local temperature minimum occurs here. This provides assurance that diagnostic methods based on local thermodynamic equilibrium, in particular those of optical emission spectroscopy, are reliable here. In contrast, deviations from the equilibrium atomic-state distribution are obtained in the near-electrode and arc fringe regions. As a consequence, the temperatures determined from the ratio of line intensities and number densities obtained from the emission coefficient in these regions are questionable. In this situation, the collisional-radiative model can be used as a diagnostic tool to assist in the interpretation of spectroscopic measurements.