Browsing by Author "Donkó, Z."

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    The 2022 Plasma Roadmap: low temperature plasma science and technology
    (Bristol : IOP Publ., 2022) Adamovich, I.; Agarwal, S.; Ahedo, E.; Alves, L.L.; Baalrud, S.; Babaeva, N.; Bogaerts, A.; Bourdon, A.; Bruggeman, P.J.; Canal, C.; Choi, E.H.; Coulombe, S.; Donkó, Z.; Graves, D.B.; Hamaguchi, S.; Hegemann, D.; Hori, M.; Kim, H.-H.; Kroesen, G.M.W.; Kushner, M.J.; Laricchiuta, A.; Li, X.; Magin, T.E.; Mededovic Thagard, S.; Miller, V.; Murphy, A.B.; Oehrlein, G.S.; Puac, N.; Sankaran, R.M.; Samukawa, S.; Shiratani, M.; Šimek, M.; Tarasenko, N.; Terashima, K.; Thomas Jr, E.; Trieschmann, J.; Tsikata, S.; Turner, M.M.; Van Der Walt, I.J.; Van De Sanden, M.C.M.; Von Woedtke, T.
    The 2022 Roadmap is the next update in the series of Plasma Roadmaps published by Journal of Physics D with the intent to identify important outstanding challenges in the field of low-temperature plasma (LTP) physics and technology. The format of the Roadmap is the same as the previous Roadmaps representing the visions of 41 leading experts representing 21 countries and five continents in the various sub-fields of LTP science and technology. In recognition of the evolution in the field, several new topics have been introduced or given more prominence. These new topics and emphasis highlight increased interests in plasma-enabled additive manufacturing, soft materials, electrification of chemical conversions, plasma propulsion, extreme plasma regimes, plasmas in hypersonics, data-driven plasma science and technology and the contribution of LTP to combat COVID-19. In the last few decades, LTP science and technology has made a tremendously positive impact on our society. It is our hope that this roadmap will help continue this excellent track record over the next 5-10 years.
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    A computationally assisted technique to measure material-specific surface coefficients in capacitively coupled plasmas based on characteristics of the ion flux-energy distribution function
    (Bristol : IOP Publ., 2022) Schulze, C.; Donkó, Z.; Benedikt, J.
    We present a new method for the determination of surface coefficients, more specifically the effective ion-induced secondary electron yield, γ eff, and the effective elastic electron reflection coefficient, r eff, by means of a synergistic combination of energy-selective mass spectrometry measurements and numerical particle-in-cell/Monte Carlo collisions simulations of the ion flux-energy distribution function (IEDF) in a symmetric capacitively coupled plasma (CCP). In particular, we analyze the bimodal peak structure of the IEDF, which is caused by ions crossing the sheath without collisions. The position and width of this structure on the energy scale are defined by the time-averaged sheath potential and the ion transit time through the sheath, respectively. We find that both characteristics are differently influenced by γ eff and r eff. The ion-induced secondary electrons are accelerated in the large sheath potential and mainly influence the plasma density, sheath width and, consequently, the ion transit time and in this way the bimodal peak separation. Electron reflection from the electrodes acts mainly at times of sheath collapse, where low energy electrons can reach the surfaces. Their contribution to the plasma density increase is small, however, their longer residence time in the vicinity of the electrodes modifies the space charge density and the potential gradient. Additionally, the charge balance at the electrode requires an incident electron flux that is correlated to the flux of emitted ion induced secondary electrons and reflected electrons, which is realized by a change of the electron repelling sheath voltage. As a consequence, the electron reflection coefficient mainly influences the sheath potential and, hence, the position of the bimodal peak structure. These effects allow the simultaneous in situ determination of both surface parameters. The parameter values determined for stainless steel and Al2O3 surfaces are in good agreement with literature data. Our method opens a straightforward way of obtaining γ eff and r eff under realistic plasma conditions.
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    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.
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    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.
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    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.
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    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.