On phenomena in ionized gases
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- Modelling heat dominated electric breakdown in air with adaptivity to electron or ion timescales
- 1. Introduction
- 2. Methodology
- 4. Acknowledgements
- 5. References
- Complete and consistent set of electron-neutral scattering cross sections for carbon monoxide
- 2. Description of the cross section set
- 3. Results and discussion
- 4. References
- Structure at the top of premixed burner flame with the superposition of pulsed dielectric barrier discharge
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4. References [1] L. Marot et al, Fusion Eng. Des. 88 9-10 (2013) 1718-1721 [2] A. Kreter et al, Phys. Scr. T159 (2014) 014039 [3] K. Ouaras et al, J. Phys. Conf. Ser. 591 (2015) 012029
[4] E.A. Hodille et al, J. Nucl. Mater. 467 (2015) 424-43
[5] C. Sang et al, J. Nucl. Mater. 463 (2015) 367- 371
Fig. 1 SEM image of an aluminium target exposed to hydrogen plasma
14 203 XXXIII ICPIG, July 9-14, 2017, Estoril/Lisbon, Portugal
Modelling heat dominated electric breakdown in air with adaptivity to electron or ion timescales
A. Agnihotri 1 P , W. Hundsdorfer 1,2 P , U. Ebert 1,3 P
P 1 P
P
P
3 Department of Applied Physics, Eindhoven University of Technology, Eindhoven, The Netherlands
We simulate heat dominated electrical breakdown in air in a short planar gap by coupling the discharge dynamics with the air dynamics. The electric discharge model is of diffusion drift reaction type for electrons, positive and negative ions, including secondary electron emission from the cathode. The air dynamics is modelled with the Euler equations and an energy balance equation for the heat. To follow the discharge dynamics over sufficiently long times, we derived a reduced model on the ion timescale from the full model on the electron timescale, and we switch to the reduced model when appropriate. We discuss in detail the implementation of a time-adaptive numerical scheme. We use this scheme to simulate the short and long time dynamics. As the electric discharge develops the air temperature rises due to Ohmic heating. The heated air expands, and the transport and reaction coefficients of the discharge change accordingly. We observe electric breakdown in an initially undervolted gap through a sequence of ionization waves.
studied in the context of fast gas heating, plasma- assisted combustion, atmospheric lightning etc. The majority of simulation studies so far pertaining to gas heating in atmospheric pressure discharges have been performed in 0D or 1D approximation. The large difference in timescales between electron dynamics on the one hand and ion dynamics and gas movement on the other hand makes computations very expensive. Recently 2D axisymmetric simulations [1-3] have been published, but they are either only on the ion or only on the electron time scale. Here we overcome this limitation and study the long time dynamics. 2. Methodology We simulate the dynamics in a short planar gap in 2D (r-z coordinates) with secondary electron emission where ionization grows due to Ohmic heating rather than through space charge dominated streamer breakdown. This dynamics is challenging to simulate as in the initial stage the electron dynamics has to be followed, and later on the much slower ion dynamics has to be resolved, and both have to be coupled to the hydrodynamics of the medium. We present a model on the time scale of ion motion, that is a reduced version of the full discharge dynamics; it is a generalization of the reduced model introduced in [4] where it was applied to study the transition from Townsend to glow discharge. Our calculations are adaptive in time, i.e., we switch between the full model on the electron time scale and the reduced model on the ion time scale as required. We also discuss possible numerical switching criteria.
We developed and employed the 2D cylindrically symmetric model to simulate heat dominated electric breakdown in air. We observe a cyclic process whereby the positive ions hit the cathode, liberate electrons via secondary emission and these electrons feed the discharge channel by producing more electrons and ions via impact ionization. Ohmic heating causes the temperature of the gas to rise. Eventually, the heated gas expands resulting in electric breakdown near the discharge axis. Detailed results are submitted [5].
4. Acknowledgements A.A. acknowledges financial support under the Computational Sciences for Energy Research (CSER) initiative by Shell and FOM (Dutch Physics funding agency) under project number 12CSER058.
[1] A. Agnihotri, W. Hundsdorfer and U. Ebert, Jap. J. Appl. Phys. 55 (2016) 07LD06. [2] F. Tholin and A. Bourdon: J. Phys. D, Appl. Phys. 46 (2013) 365205. [3] A. Komuro and R. Ono, J. Phys. D, Appl. Phys. 47 (2014) 155202. [4] D. D. Sijacic, U. Ebert and I. Rafatov, Physical Review E. 71 (2005) 066402. [5] A. Agnihotri, W. Hundsdorfer and U. Ebert, submitted to Plasma Sources Sci. Technol.
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XXXIII ICPIG, July 9-14, 2017, Estoril/Lisbon, Portugal
Complete and consistent set of electron-neutral scattering cross sections for carbon monoxide
Polina Ogloblina, Antonio Tejero-del-Caz, Vasco Guerra, Luís L. Alves P
This work proposes a complete and consistent set of cross sections for electron collisions with carbon monoxide (CO) molecules to be published in the IST-Lisbon database with LXCat. The set is validated by comparing swarm parameters, calculated using a two-term Boltzmann solver, with available experimental data. It is shown that for low values of the reduced electric field (E/N<2 Td) both rotational excitations and de-excitations mechanisms, as well as superelastic collisions with the first vibrational excited level, have to be taken into account in order to accurately predict the electron energy distribution function and the corresponding swarm parameters. The role in the calculations of the effective / elastic momentum-transfer cross section is also discussed.
Carbon monoxide (CO) is one of the main constituents of Venus and Mars atmospheres [1]; it is the most abundant molecule observed in the interstellar space after hydrogen; and it is relevant in laboratory gas discharges for the production of syngas and the reforming of CO 2 . The study of the electron kinetics is essential to understand how the energy gained by the electrons from the applied field is transferred to the different heavy-particles. This work presents a complete and consistent set of electron-neutral scattering cross sections for carbon monoxide, to be soon included in the IST-Lisbon database with LXCat.
The current set includes the elastic cross section, the cross sections for the excitation of 16 rotational states, 10 vibrational states and 7 electronic states, as well as the cross sections for dissociation, dissociative attachment and ionization. The cross sections are defined up to 1000 eV kinetic energy. The elastic cross section is built in two steps: the mid- and high-energy regions are taken from [2], with small modifications; the low-energy region was re-calculated from an effective cross section in order to ensure consistency when rotational excitations are explicitly accounted for. Special attention is given to rotational excitation and de-excitation mechanisms, which can be very important at low reduced electric fields. Rotational cross sections are taken from [3]. Vibrational and electronic excitation cross sections are essentially taken from [2,4]; vibrational excitation is currently under revision, based in [5]. Finally, the dissociation, dissociative attachment and ionization cross sections are the same as in [4].
3. Results and discussion The current set reproduces very well the available experimental swarm data. For example, figure 1 depicts the reduced Townsend ionisation coefficient. Further results (not shown) reveal that it is essential to consider rotational excitation and de- excitation mechanisms, as well as superelastic collisions with the first vibrational level, to correctly describe the low field region (E/N<2 Td) at gas temperatures T g ≤300 K.
100 200
300 400
10 -24
10 -23
10 -22
10 -21
Re du ce d To wn se nd c oe ffi
ci en t, m 2 Reduced electric field, Td T gas =300K
Bhalla et al 1961 Davies et al 1969
Parr et al 1971 Figure 1. Comparison between calculated and measured reduced Townsend coefficient.
4. References
[1] L. Campbell, M. Allan, M. J. Brunger. J. Geophys. Res. 116 (2011) A09321. [2] J. E. Land. J. Appl. Phys. 49 (1978) 5716. [3] R. D. Hake, A. V. Phelps. Phys. Rev. 158 (1967) 70. [4] Y. Itikawa. J. Phys. Chem. Ref. Data 44 (2015) 013105. [5] R. Celiberto et al. Plasma Sources Sci. Technol. 25 (2016) 033004.
Acknowledgements: This work was partially supported by the Portuguese FCT, under Projects UID/FIS/50010/2013 and PTDC/FIS- PLA/1420/2014 (PREMiERE), and grant
PD/BD/114398/2016 (PD-F APPLAuSE). Topic number 1 205
XXXIII ICPIG, July 9-14, 2017, Estoril/Lisbon, Portugal
Human Stratum Corneum Epidermidis modification by means of atmospheric-pressure cold plasma treatment
D. Athanasopoulos, P. Svarnas University of Patras, Electrical & Computer Eng. Dept., High Voltage Lab., 26504 Rion, Patras, Greece
In the frame of the emerging field of plasma medicine, the present work is devoted to human stratum corneum epidermidis modification by means of a sinusoidal-driven helium “plasma jet” (train of “guided streamers”). Stratum Corneum is the outermost layer of the epidermis and thus it plays the role of a barrier to protect the underlying tissues. Accordingly, and due to its composition, it exhibits highly hydrophobic nature and any drug delivery through skin is subjected to this barrier. Hereby, it is clearly demonstrated the possibility of increasing human stratum corneum epidermidis wettability as a function of the exposure time to plasma-induced reactive species.
1. Introduction Plasma medicine is emerging world-wide as a new field of medical research, with special applications to dermatology. Towards this direction, human stratum corneum epidermidis (SCE) is here subjected to atmospheric-pressure cold plasma (APCP) treatment for increasing wettability. SCE has the architecture of dead keratin filled cells in a lipid matrix [1]. Its thickness lies typically from 10 to 20 μm, and it functions to protect the underlying tissues. The wettability of human skin varies significantly depending on the anatomical site, the pre-conditioning (e.g. soap washing, water rinsing etc) [2] etc. However, clean human skin is hydrophobic, since it exhibits water contact angles (WCAs) up to 125 deg [2]. In this work, the wettability of SCE is reduced using a He APCP, sinusoidially driven at 10 kHz.
2. Experimental setup and specimens The reactor used for APCP production and the plasma interpretation in respect to biological applications, have been presented elsewhere [3]. The SCE samples are from the breast of a 63-years old Caucasian female (BMI 27) and supplied from Biopredic International. Briefly, the samples are de- frozen, immersed in phosphate buffered saline, pH 7.4, rinsed gently by ultrapure water, and their surface is dried carefully under weak flow of Ar gas. The wettability of the samples is evaluated with WCAs (drops of 3 μl) due to a motorized drop shape analysis system (Krüss GmbH; EasyDrop). The experiments are realized in triplicates, providing mean values and standard deviations.
3. Results and Discussion Fig. 1 depicts the SCE wettability evolution for increasing plasma treatment. 0 30
90 120
150 50 60 70 80 90 100 W at er C on ta ct A ng le ( de g) Plasma Treatment Time (s) 10 kHz sinusoidal; 10 kV pp; 2 slm; reactor nozzle-sample distance 25 mm.
Fig. 1: Human Stratum Corneum Epidermidis wettability versus the exposure time to APCP.
Following XPS analysis (not shown here) and plasma probing [3], the WCA decrease down to 56 deg is attributed to surface functionalization induced by plasma-generated RONS.
4. Conclusions APCP was here proposed as an efficient medium for SCE wettability increase, which in turns could be useful for therapeutic gel applications.
5. References [1] C. L. Silva et al., Biochim. Biophys. Acta 1768 (2007) 2647-2659. [2] M. E. Ginn et al., J. Colloid Interface Sci. 26 (1968) 146-151. [3] P. Svarnas et al., Appl. Phys. Lett. 101 (2012) 264103.
Acknowledgments D.A. acknowledge I.K.Y. (State Scholarships Foundation; NSRF 2014-2020) for financial support. Topic 17
150 s
206 XXXIII ICPIG, July 9-14, 2017, Estoril/Lisbon, Portugal
Controlling Atmospheric-Pressure Plasma Reactive Species Densities by means of Modulated Sinusoidal High Voltage
P. Svarnas 1 , M. Mitronika 1 , D. Athanasopoulos 1 , E. Mitronikas, 2 K. Gazeli 1,3
1 University of Patras, Electrical & Computer Eng. Dept., High Voltage Lab., 26504 Rion, Patras, Greece
2
26504 Rion, Patras, Greece 3 current address: Université Paris-Sud & Université Paris-Saclay, CNRS, LPGP, 91450 Orsay Cedex
Audio-frequency sinusoidal high voltage is here modulated by square-wave signals, by means of a microprocessor-based power supply. The modulated voltage drives a single-electrode microplasma reactor using helium at atmospheric pressure as feedstock gas. The emissive species induced downstream the reactor nozzle are identified with UV-visible optical emission spectroscopy, and the influence of two modulating parameters (i.e. period and duty cycle) on the relative density of excited probe molecules is studied independently. It is clearly demonstrated that, under the present experimental conditions, both parameters have a profound effect on reactive species densities, and may thus control and enhance the plasma chemistry with an engineerable manner.
1. Introduction For atmospheric-pressure plasma applications, numerous reports consider the role of the reactor design, driving voltage features (a.c. or pulsed d.c.), gas composition, and gas flow rate, in optimizing the density of various species. Differently, sinusoidal high voltage is here modulated by square- wave signals in respect to excited species densities.
2. Plasma and Diagnostic Setups The setups employed here are extensively presented elsewhere [1]. Here, the distance between the single-electrode tip and the capillary tube exit is 30 mm and a novel power supply (commercialized by our group) is used (see Fig. 1 and its caption). a)
b) c)
Figure 1: Representative oscillograms: a) square-wave modulating signal (TTL), b) modulated sinusoidal high voltage (9 kVpp; except the overshoot of each first cycle at 9.1 kVpp), and c) pattern of wavelength-integrated light impulses during “plasma bullet”-“guided streamer” propagation [1] (a.u.). The light is spatially-integrated over the first 5 mm in front of the reactor nozzle.
3. Results and Discussion Fig. 2 shows how the relative density of an excited probe molecule – in terms of optical emission spectroscopy intensity (resolution 0.01 nm; integration time 2 s) – is affected by the modulating parameters. The linear increase vs. duty cycle is probably due to the increasing mean power (ionization/excitation) of the plasma. But, the sharp increase vs. frequency is still under investigation. 0 10 20 30 40 50 60 70 80 90 100 0.0 0.2
0.4 0.6
0.8 1.0
Square-Wave Frequency (Hz) Duty Cycle Sweep (Frequency 100 Hz) ( error bars from 2 different sweeps)
N
m al iz ed O E S I nt en si ty ( I / I m ax ) Square-Wave Duty Cycle (%) 2 slm He
9 kVpp (see Fig. 1) 10 0
1 10 2 10 3 10 4 Frequency Sweep (Duty Cycle 50%) ( error bars from 2 different sweeps)
Figure 2: N 2 * (C 3 Π u -B 3 Π g ; v'-v'': 0-0) relative density.
Appropriately modulated sinusoidal high voltage is potentially a new way for controlling the chemistry of atmospheric-pressure plasmas.
5. References [1] K. Gazeli et al., J. Appl. Phys. 114 (2013) 103304.
Acknowledgments D.A. acknowledge I.K.Y. (State Scholarships Foundation; NSRF 2014-2020) for financial support. Topic 10 207
XXXIII ICPIG, July 9-14, 2017, Estoril/Lisbon, Portugal
Structure at the top of premixed burner flame with the superposition of pulsed dielectric barrier discharge
K. Sasaki and K. Zaima P
Division of Quantum Science and Engineering, Hokkaido University, Sapporo, Japan
We investigated the transient phenomena in a premixed burner flame with the superposition of a pulsed dielectric barrier discharge (DBD). The length of the flame was shortened by the superposition of DBD, indicating the activation of combustion chemical reactions with the help of the plasma. We observed the modulation of the top position of the flame and the formations of local minimums in the axial distribution of the optical emission intensity of OH. These experimental results reveal the oscillation of the rates of combustion chemical reactions as a response to the activation by pulsed DBD. The cycle of the oscillation was 0.18-0.2 ms, which could be understood as the eigenfrequency of the plasma-assisted combustion reaction system.
We have shown that the burning velocity is enhanced by superposing a dielectric barrier discharge (DBD) onto the bottom part of a steady-state premixed burner flame. The increase in the burning velocity is understood by the shortening of the flame length. However, it has been also observed that the flame length was not stationary. In this work, we report the transient change in the shape of premixed burner flame with the superposition of pulsed DBD. Download 9.74 Mb. Do'stlaringiz bilan baham: 
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