Browsing by Author "Gans, T."

Now showing 1 - 10 of 10
  • Thumbnail Image
    Item
    Bridging the gap between global models and full fluid models: A fast 1D semi-analytical fluid model for electronegative plasmas
    (Bristol : IOP Publ., 2016) Hurlbatt, A.; O’Connell, D.; Gans, T.
    Analytical and numerical models allow investigation of complicated discharge phenomena and the interplay that makes plasmas such a complex environment. Global models are quick to implement and can have almost negligible computation cost, but provide only bulk or spatially averaged values. Full fluid models take longer to develop, and can take days to solve, but provide accurate spatio-temporal profiles of the whole plasma. The work presented here details a different type of model, analytically similar to fluid models, but computationally closer to a global model, and able to give spatially resolved solutions for the challenging environment of electronegative plasmas. Included are non-isothermal electrons, gas heating, and coupled neutral dynamics. Solutions are reached in seconds to minutes, and spatial profiles are given for densities, fluxes, and temperatures. This allows the semi-analytical model to fill the gap that exists between global and full fluid models, extending the tools available to researchers. The semi-analytical model can perform broad parameter sweeps that are not practical with more computationally expensive models, as well as exposing non-trivial trends that global models cannot capture. Examples are given for a low pressure oxygen CCP. Excellent agreement is shown with a full fluid model, and comparisons are drawn with the corresponding global model.
  • Thumbnail Image
    Item
    Capacitively coupled hydrogen plasmas sustained by tailored voltage waveforms: Excitation dynamics and ion flux asymmetry
    (Bristol : IOP Publ., 2016) Bruneau, B.; Diomede, P.; Economou, D.J.; Longo, S.; Gans, T.; O’Connell, D.; Greb, A.; Johnson, E.; Booth, J.-P.
    Parallel plate capacitively coupled plasmas in hydrogen at relatively high pressure (∼1 Torr) are excited with tailored voltage waveforms containing up to five frequencies. Predictions of a hybrid model combining a particle-in-cell simulation with Monte Carlo collisions and a fluid model are compared to phase resolved optical emission spectroscopy measurements, yielding information on the dynamics of the excitation rate in these discharges. When the discharge is excited with amplitude asymmetric waveforms, the discharge becomes electrically asymmetric, with different ion energies at each of the two electrodes. Unexpectedly, large differences in the H2+ fluxes to each of the two electrodes are caused by the different H3+ energies. When the discharge is excited with slope asymmetric waveforms, only weak electrical asymmetry of the discharge is observed. In this case, electron power absorption due to fast sheath expansion at one electrode is balanced by electron power absorption at the opposite electrode due to a strong electric field reversal.
  • Thumbnail Image
    Item
    Concepts and characteristics of the ‘COST Reference Microplasma Jet’
    (Bristol : IOP Publ., 2016) Golda, J.; Held, J.; Redeker, B.; Konkowski, M.; Beijer, P.; Sobota, A.; Kroesen, G.; Braithwaite, N.S.J.; Reuter, S.; Turner, M.M.; Gans, T.; O’Connell, D.; Schulz-von der Gathen, V.
    Biomedical applications of non-equilibrium atmospheric pressure plasmas have attracted intense interest in the past few years. Many plasma sources of diverse design have been proposed for these applications, but the relationship between source characteristics and application performance is not well-understood, and indeed many sources are poorly characterized. This circumstance is an impediment to progress in application development. A reference source with well-understood and highly reproducible characteristics may be an important tool in this context. Researchers around the world should be able to compare the characteristics of their own sources and also their results with this device. In this paper, we describe such a reference source, developed from the simple and robust micro-scaled atmospheric pressure plasma jet (μ-APPJ) concept. This development occurred under the auspices of COST Action MP1101 'Biomedical Applications of Atmospheric Pressure Plasmas'. Gas contamination and power measurement are shown to be major causes of irreproducible results in earlier source designs. These problems are resolved in the reference source by refinement of the mechanical and electrical design and by specifying an operating protocol. These measures are shown to be absolutely necessary for reproducible operation. They include the integration of current and voltage probes into the jet. The usual combination of matching unit and power supply is replaced by an integrated LC power coupling circuit and a 5 W single frequency generator. The design specification and operating protocol for the reference source are being made freely available.
  • Thumbnail Image
    Item
    Corrigendum: 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.
  • Thumbnail Image
    Item
    Electron power absorption dynamics in capacitive radio frequency discharges driven by tailored voltage waveforms in CF4
    (Bristol : IOP Publ., 2016) Brandt, S.; Berger, B.; Schüngel, E.; Korolov, I.; Derzsi, A.; Bruneau, B.; Johnson, E.; Lafleur, T.; O’Connell, D.; Koepke, M.; Gans, T.; Booth, J.-P.; Donkó, Z.; Schulze, J.
    The power absorption dynamics of electrons and the electrical asymmetry effect in capacitive radio-frequency plasmas operated in CF4 and driven by tailored voltage waveforms are investigated experimentally in combination with kinetic simulations. The driving voltage waveforms are generated as a superposition of multiple consecutive harmonics of the fundamental frequency of 13.56 MHz. Peaks/valleys and sawtooth waveforms are used to study the effects of amplitude and slope asymmetries of the driving voltage waveform on the electron dynamics and the generation of a DC self-bias in an electronegative plasma at different pressures. Compared to electropositive discharges, we observe strongly different effects and unique power absorption dynamics. At high pressures and high electronegativities, the discharge is found to operate in the drift-ambipolar (DA) heating mode. A dominant excitation/ionization maximum is observed during sheath collapse at the edge of the sheath which collapses fastest. High negative-ion densities are observed inside this sheath region, while electrons are confined for part of the RF period in a potential well formed by the ambipolar electric field at this sheath edge and the collapsed (floating potential) sheath at the electrode. For specific driving voltage waveforms, the plasma becomes divided spatially into two different halves of strongly different electronegativity. This asymmetry can be reversed electrically by inverting the driving waveform. For sawtooth waveforms, the discharge asymmetry and the sign of the DC self-bias are found to reverse as the pressure is increased, due to a transition of the electron heating mode from the α-mode to the DA-mode. These effects are interpreted with the aid of the simulation results.
  • Thumbnail Image
    Item
    Experimental and computational investigations of electron dynamics in micro atmospheric pressure radio-frequency plasma jets operated in He/N2 mixtures
    (Bristol : IOP Publ., 2018) Bischoff, L.; Hübner, G.; Korolov, I.; Donkó, Z.; Hartmann, P.; Gans, T.; Held, J.; Schulz-von der Gathen, V.; Liu, Y; Mussenbrock, T.; Schulze, J.
    The electron power absorption dynamics in radio frequency driven micro atmospheric pressure capacitive plasma jets are studied based on experimental phase resolved optical emission spectroscopy and the computational particle in cell simulations with Monte Carlo treatment of collisions. The jet is operated at 13.56 MHz in He with different admixture concentrations of N 2 and at several driving voltage amplitudes. We find the spatio-temporal dynamics of the light emission of the plasma at various wavelengths to be markedly different. This is understood by revealing the population dynamics of the upper levels of selected emission lines/bands based on comparisons between experimental and simulation results. The populations of these excited states are sensitive to different parts of the electron energy distribution function and to contributions from other excited states. Mode transitions of the electron power absorption dynamics from the Ω- to the Penning-mode are found to be induced by changing the N 2 admixture concentration and the driving voltage amplitude. Our numerical simulations reveal details of this mode transition and provide novel insights into the operation details of the Penning-mode. The characteristic excitation/emission maximum at the time of maximum sheath voltage at each electrode is found to be based on two mechanisms: (i) a direct channel, i.e. excitation/emission caused by electrons generated by Penning ionization inside the sheaths and (ii) an indirect channel, i.e. secondary electrons emitted from the electrode due to the impact of positive ions generated by Penning ionization at the electrodes.
  • Thumbnail Image
    Item
    Experimental benchmark of kinetic simulations of capacitively coupled plasmas in molecular gases
    (Bristol : IOP Publ., 2018) Donkó, Z.; Derzsi, A.; Korolov, I.; Hartmann, P.; Brandt, S.; Schulze, J.; Berger, B.; Koepke, M.; Bruneau, B.; Johnson, E.; Lafleur, T.; Booth, J.-P.; Gibson, A. R.; O’Connell, D.; Gans, T.
    We discuss the origin of uncertainties in the results of numerical simulations of low-temperature plasma sources, focusing on capacitively coupled plasmas. These sources can be operated in various gases/gas mixtures, over a wide domain of excitation frequency, voltage, and gas pressure. At low pressures, the non-equilibrium character of the charged particle transport prevails and particle-based simulations become the primary tools for their numerical description. The particle-in-cell method, complemented with Monte Carlo type description of collision processes, is a well-established approach for this purpose. Codes based on this technique have been developed by several authors/groups, and have been benchmarked with each other in some cases. Such benchmarking demonstrates the correctness of the codes, but the underlying physical model remains unvalidated. This is a key point, as this model should ideally account for all important plasma chemical reactions as well as for the plasma-surface interaction via including specific surface reaction coefficients (electron yields, sticking coefficients, etc). In order to test the models rigorously, comparison with experimental 'benchmark data' is necessary. Examples will be given regarding the studies of electron power absorption modes in O2, and CF4-Ar discharges, as well as on the effect of modifications of the parameters of certain elementary processes on the computed discharge characteristics in O2 capacitively coupled plasmas.
  • Thumbnail Image
    Item
    Investigation of the radially resolved oxygen dissociation degree and local mean electron energy in oxygen plasmas in contact with different surface materials
    (Melville, NY : American Inst. of Physics, 2017) Tsutsumi, T.; Greb, A.; Gibson, A. R.; Hori, M.; O'Connell, D.; Gans, T.
    Energy Resolved Actinometry is applied to simultaneously measure the radially resolved oxygen dissociation degree and local mean electron energy in a low-pressure capacitively coupled radio-frequency oxygen plasma with an argon tracer gas admixture. For this purpose, the excitation dynamics of three excited states, namely, Ar(2p1), O(3p3P), and O(3p5P), were determined from their optical emission at 750.46 nm, 777.4 nm, and 844.6 nm using Phase Resolved Optical Emission Spectroscopy (PROES). Both copper and silicon dioxide surfaces are studied with respect to their influence on the oxygen dissociation degree, local mean electron energy, and the radial distributions of both quantities and the variation of the two quantities with discharge pressure and driving voltage are detailed. The differences in the measured dissociation degree between different materials are related back to atomic oxygen surface recombination probabilities.
  • Thumbnail Image
    Item
    Reproducibility of 'COST reference microplasma jets'
    (Bristol : IOP Publ., 2020) Riedel, F.; Golda, J.; Held, J.; Davies, H.L.; van der Woude, M.W.; Bredin, J.; Niemi, K.; Gans, T.; Schulz-von der Gathen, V.; O’Connell, D.
    Atmospheric pressure plasmas have been ground-breaking for plasma science and technologies, due to their significant application potential in many fields, including medicinal, biological, and environmental applications. This is predominantly due to their efficient production and delivery of chemically reactive species under ambient conditions. One of the challenges in progressing the field is comparing plasma sources and results across the community and the literature. To address this a reference plasma source was established during the 'biomedical applications of atmospheric pressure plasmas' EU COST Action MP1101. It is crucial that reference sources are reproducible. Here, we present the reproducibility and variance across multiple sources through examining various characteristics, including: Absolute atomic oxygen densities, absolute ozone densities, electrical characteristics, optical emission spectroscopy, temperature measurements, and bactericidal activity. The measurements demonstrate that the tested COST jets are mainly reproducible within the intrinsic uncertainty of each measurement technique.
  • Thumbnail Image
    Item
    Tailored voltage waveform capacitively coupled plasmas in electronegative gases: Frequency dependence of asymmetry effects
    (Bristol : IOP Publ., 2016) Schüngel, E.; Korolov, I.; Bruneau, B.; Derzsi, A.; Johnson, E.; O’Connell, D.; Gans, T.; Booth, J.-P.; Donkó, Z.; Schulze, J.
    Capacitively coupled radio frequency plasmas operated in an electronegative gas (CF4) and driven by voltage waveforms composed of four consecutive harmonics are investigated for different fundamental driving frequencies using PIC/MCC simulations and an analytical model. As has been observed previously for electropositive gases, the application of peak-shaped waveforms (that are characterized by a strong amplitude asymmetry) results in the development of a DC self-bias due to the electrical asymmetry effect (EAE), which increases the energy of ions arriving at the powered electrode. In contrast to the electropositive case (Korolov et al 2012 J. Phys. D: Appl. Phys. 45 465202) the absolute value of the DC self-bias is found to increase as the fundamental frequency is reduced in this electronegative discharge, providing an increased range over which the DC self-bias can be controlled. The analytical model reveals that this increased DC self-bias is caused by changes in the spatial profile and the mean value of the net charge density in the grounded electrode sheath. The spatio-temporally resolved simulation data show that as the frequency is reduced the grounded electrode sheath region becomes electronegative. The presence of negative ions in this sheath leads to very different dynamics of the power absorption of electrons, which in turn enhances the local electronegativity and plasma density via ionization and attachment processes. The ion flux to the grounded electrode (where the ion energy is lowest) can be up to twice that to the powered electrode. At the same time, while the mean ion energies at both electrodes are quite different, their ratio remains approximately constant for all base frequencies studied here.