Publications

2014

• Evidence of magnetic reconnection from wave measurements at Saturn's magnetopause: Cassini RPWS observations
  
  • Retinò Alessandro
  • Masters A.
  • Vaivads A.
  • Khotyaintsev Y. V.
  • Fujimoto M.
  • Kasahara S.
  • Badman S. V.
  • Canu Patrick
  • Chust Thomas
  • Modolo Ronan
  • Gurnett D. A.
  • Dougherty M. K.
  , 2014, 9, pp.EPSC2014-802. Magnetic reconnection is a universal mechanism that is responsible for major energy conversion in planetary magnetospheres. Recent theoretical estimations suggest that reconnection is infrequent at Saturn's magnetopause and that it is not a major driver of the dynamics of the kronian magnetosphere. This scenario need however to be confirmed by in situ observations at the magnetopause current sheet. Evidence of reconnection in the form of accelerated plasma jets is difficult at Saturn's magnetopause due to limitations in the field of view of particle detectors. Here we show evidence of reconnection for one magnetopause event by using measurements of low frequency waves (lower-hybrid, whistlers, plasma/upper hybrid). We discuss how wave measurements can be used as evidence of reconnection in planetary magnetospheres.
• Characterization of predator-prey dynamics, using the evolution of free energy in plasma turbulence
  
  • Morel Pierre
  • Gürcan Özgür D.
  • Berionni Vincent
  Plasma Physics and Controlled Fusion, IOP Publishing, 2014, 56 (1), pp.015002. A simple dynamical cascade model for the evolution of free energy is considered in the context of gyrokinetic formalism. It is noted that the dynamics of free energy, that characterize plasma micro-turbulence in magnetic fusion devices, exhibit a predatorprey character. Various key features of predatorprey dynamics such as the time delay between turbulence and large-scale flow structures, or the intermittency of the dynamics are identified in the quasi-steady-state phase of the nonlinear gyrokinetic simulations. A novel prediction on the ratio of turbulence amplitudes in different parts of the wavenumber domain that follows from this simple predatorprey model is compared to a set of nonlinear simulation results and is observed to hold quite well in a large range of physical parameters. Detailed validation of the predatorprey hypothesis using nonlinear gyrokinetics provides a very important input for the effort to apprehend plasma micro-turbulence, since the predatorprey idea can be used as a very effective intuitive tool for understanding and designing efficient transport models. (10.1088/0741-3335/56/1/015002)
  DOI : 10.1088/0741-3335/56/1/015002
• Simulation of long term variation of the F2-layer critical frequency f0F2 at the northern tropical crest of ionization at Phu Thuy, Vietnam with the thermosphere-ionosphere-electrodynamics general circulation model (TIE-GCM)
  
  • Pham Thi Thu Hong
  • Amory-Mazaudier Christine
  • Le Huy Minh
  Vietnam Journal of Earth Sciences, Vietnam Academy of Science and Technology (VAST), 2014 (36), pp.470-479. In this work, the long-term variations of the simulated f0F2 by the NCAR thermosphere ionosphere-electrodynamics general circulation model (TIE-GCM) at the northern tropical crest of ionization at Phu Thuy-Vietnam (geographic latitudes 21.030N and longitude: 105.950E) during the period from 1962 to 2002 are examined to evaluate the ability of this model to reproduce the major features of the f0F2 as observed. The TIE-GCM simulates the influences of migrating and non-migrating diurnal and semidiurnal tides at the lower thermosphere, and changes of geomagnetic activity on the long-term variation of the f0F2. It reproduces well the diurnal and seasonal variations. We analyze the diurnal and seasonal variations of the observed f0F2 at Phu Thuy in approximately the same solar activity condition as in 1964, 1996 for the March and September equinoxes and June and December solstices. The local time and seasonal structures of these simulated results correspond well to those that are observed in 1964. On the contrary, the TIE-GCM model does not reproduce the amplitude observed at Thuy Phu in 1996. The TIE-GCM model with the chosen inputs does not yet allow us to explain well the long-term variations observed at Phu Thuy. We also try the different numerical simulations to understand how the long-term variation of the f0F2 is formed, how it relates to the current global system and its relationship with the thermosphere wind. The simulations show that the calculated NmF2 values are lower than the observed values. We find that the modeled contributions of the migrating and non-migrating diurnal and semidiurnal tides may cause them to play a major role in reducing the amplitude of the NmF2. The contributions of the integrated hemispheric power of auroral electrons and the cross polar cap potential seem to play an important role in increasing the amplitude of the NmF2. Keywords: F2 layer; Long-term trends; Ionosphere equatorial ionization anomaly; Ionosphere (ionospheric conductivities, ionospheric currents and electric field), thermospheric tides, Electrodynamics of the ionosphere (ionospheric dynamo).
• On the relationship between quadrupolar magnetic field and collisionless reconnection
  
  • Smets Roch
  • Aunai Nicolas
  • Belmont Gérard
  • Boniface C.
  • Fuchs J. C.
  Physics of Plasmas, American Institute of Physics, 2014, 21 (6), pp.062111. Using hybrid simulations, we investigate the onset of fast reconnection between two cylindrical magnetic shells initially close to each other. This initial state mimics the plasma structure in High Energy Density Plasmas induced by a laser-target interaction and the associated self-generated magnetic field. We clearly observe that the classical quadrupolar structure of the out-of-plane magnetic field appears prior to the reconnection onset. Furthermore, a parametric study reveals that, with a non-coplanar initial magnetic topology, the reconnection onset is delayed and possibly suppressed. The relation between the out-of-plane magnetic field and the out-of-plane electric field is discussed. (10.1063/1.4885097)
  DOI : 10.1063/1.4885097
• Whistler mode waves at magnetotail dipolarization fronts
  
  • Viberg H.
  • Khotyaintsev Y. V.
  • Vaivads A.
  • André M.
  • Fu H.S.
  • Cornilleau-Wehrlin Nicole
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2014, 119, pp.2605-2611. We report the statistics of whistler mode waves observed in relation to dipolarization fronts (DFs) in Earth's magnetotail using data from the four Cluster spacecraft spanning a period of 9 years, 20012009. We show that whistler mode waves are common in a vicinity of DFs: between 30 and 60% of all DFs are associated with whistlers. Whistlers are about 7 times more likely to be observed near a DF than at any random location in the magnetotail. Therefore, whistlers are a characteristic signature of DFs. We find that whistlers are most often detected in the flux pileup region (FPR) following the DF, close to the center of the current sheet (Bx ∼ 0) and in association with anisotropic electron distributions (Tperp>T∥). This suggests that we typically observe emissions in the source region where they are generated by the anisotropic electrons produced by the betatron process inside the FPR. (10.1002/2014JA019892)
  DOI : 10.1002/2014JA019892
• Acetylene photocatalytic oxidation using continuous flow reactor: Gas phase and adsorbed phase investigation, assessment of the photocatalyst deactivation
  
  • Thevenet F.
  • Guillard C.
  • Rousseau A.
  Chemical Engineering Journal, Elsevier, 2014, 244, pp.50-58. Acetylene, used as a model volatile organic compound (VOC), is submitted to photocatalytic oxidation. This study is based on the simultaneous characterisation of: (i) the gas phase at the reactor outlet; (ii) the adsorbed phase on the photocatalytic media. Experiments are performed in a continuous flow reactor, and analytical procedures have been developed to identify and quantify the gaseous and adsorbed side-products. The photocatalytic media consists in P25 Degussa TiO2 nanoparticles deposited on glass fibres. First, the process efficiency is investigated through C2H2 conversion rate and mineralisation. The treatment performances tend to decrease with time regarding conversion and mineralisation. Then, the oxidation process is investigated through gaseous and adsorbed by-products. Three carboxylic acids have been quantified in the adsorbed phase during oxidative treatment; their temporal evolutions are determined. Their contribution to the carbon balance is discussed. It is evidenced that their adsorption on the photocatalytic media leads to a saturation of the surface. Formaldehyde, glyoxal and formic acid are quantified in the gas phase. Their cumulative temporal profiles and their contribution to the carbon balance confirm the hypothesis of a surface deactivation. The decrease of the process performances with time and the increase of gaseous by-products are correlated with the photocatalyst coverage by adsorbed acids. Carbon balances calculated for various treatment times, give an overview of the process evolution. The regeneration of the photocatalyst surface is possible under synthetic air; 80% of the adsorbed compounds are mineralised. The performances of a cyclic procedure (treatment/regeneration) are evaluated. This approach preserves the photocatalyst performances and ensures a mineralisation of 85% of the converted acetylene. (C) 2014 Elsevier B.V. All rights reserved. (10.1016/j.cej.2014.01.038)
  DOI : 10.1016/j.cej.2014.01.038
• Radiation in the neighbourhood of a double layer
  
  • Pottelette Raymond
  • Berthomier Matthieu
  • Pickett J. S.
  Annales Geophysicae, European Geosciences Union, 2014, 32, pp.677-687. In the auroral kilometric radiation (AKR) source region, acceleration layers narrow in altitude and associated with parallel field-aligned potential drops of several kV can be identified by using both particles and wave-field high time-resolution measurements from the Fast Auroral SnapshoT explorer spacecraft (FAST). These so-called double layers (DLs) are recorded around density enhancements in the auroral cavity, where the enhancement can be at the edge of the cavity or even within the cavity at a small scale. Once immersed in the plasma, DLs necessarily accelerate particles along the magnetic field lines, thereby generating locally strong turbulent processes leading to the formation of nonlinear phase space holes. The FAST data reveal the asymmetric character of the turbulence: the regions located on the high-potential side of the DLs are characterized by the presence of electron holes, while on the low-potential side, ion holes are recorded. The existence of these nonlinear phase space holes may affect the AKR radiation pattern in the neighbourhood of a DL where the electron distribution function is drastically different from a horseshoe shape. We present some observations which illustrate the systematic generation of elementary radiation events occurring significantly above the local electron gyrofrequency in the presence of electron holes. These fine-scale AKR radiators are associated with a local electron distribution which presents a pronounced beam-like shape. (10.5194/angeo-32-677-2014)
  DOI : 10.5194/angeo-32-677-2014
• Experimental validation of the dual positive and negative ion beam acceleration in the plasma propulsion with electronegative gases thruster
  
  • Rafalskyi D.V.
  • Popelier Lara
  • Aanesland Ane
  Journal of Applied Physics, American Institute of Physics, 2014, 115 (5), pp.053301. The PEGASES (Plasma Propulsion with Electronegative Gases) thruster is a gridded ion thruster, where both positive and negative ions are accelerated to generate thrust. In this way, additional downstream neutralization by electrons is redundant. To achieve this, the thruster accelerates alternately positive and negative ions from an ion-ion plasma where the electron density is three orders of magnitude lower than the ion densities. This paper presents a first experimental study of the alternate acceleration in PEGASES, where SF6 is used as the working gas. Various electrostatic probes are used to investigate the source plasma potential and the energy, composition, and current of the extracted beams. We show here that the plasma potential control in such system is key parameter defining success of ion extraction and is sensitive to both parasitic electron current paths in the source region and deposition of sulphur containing dielectric films on the grids. In addition, large oscillations in the ion-ion plasma potential are found in the negative ion extraction phase. The oscillation occurs when the primary plasma approaches the grounded parts of the main core via sub-millimetres technological inputs. By controlling and suppressing the various undesired effects, we achieve perfect ion-ion plasma potential control with stable oscillation-free operation in the range of the available acceleration voltages (±350 V). The measured positive and negative ion currents in the beam are about 10 mA for each component at RF power of 100 W and non-optimized extraction system. Two different energy analyzers with and without magnetic electron suppression system are used to measure and compare the negative and positive ion and electron fluxes formed by the thruster. It is found that at alternate ion-ion extraction the positive and negative ion energy peaks are similar in areas and symmetrical in position with /− ion energy corresponding to the amplitude of the applied acceleration voltage. (10.1063/1.4863876)
  DOI : 10.1063/1.4863876
• Helicon plasma thruster discharge model
  
  • Lafleur Trevor
  Physics of Plasmas, American Institute of Physics, 2014, 21, pp.043507. By considering particle, momentum, and energy balance equations, we develop a semi-empirical quasi one-dimensional analytical discharge model of radio-frequency and helicon plasma thrusters. The model, which includes both the upstream plasma source region as well as the downstream diverging magnetic nozzle region, is compared with experimental measurements and confirms current performance levels. Analysis of the discharge model identifies plasma power losses on the radial and back wall of the thruster as the major performance reduction factors. These losses serve as sinks for the input power which do not contribute to the thrust, and which reduce the maximum plasma density and hence propellant utilization. With significant radial plasma losses eliminated, the discharge model (with argon) predicts specific impulses in excess of 3000 s, propellant utilizations above 90%, and thruster efficiencies of about 30%. (10.1063/1.4871727)
  DOI : 10.1063/1.4871727
• An electric field in nanosecond surface dielectric barrier discharge at different polarities of the high voltage pulse: spectroscopy measurements and numerical modeling
  
  • Stepanyan S.A.
  • Soloviev Victor
  • Starikovskaia Svetlana
  Journal of Physics D: Applied Physics, IOP Publishing, 2014, 47, pp.485201. The ratio of emission intensities of the second positive N2(C3Πu, v' = 0) → N2(B3Πg, v = 0), 337.1 nm and first negative, 391.4 nm systems of nitrogen have been measured in a nanosecond surface dielectric barrier discharge (SDBD). The measurements were carried out in synthetic air for a pressure range 13 bar for different polarities of the high-voltage (HV) pulse. For all the investigated conditions, the ratio of emission intensities at the wavelengthes 391.4 and 337.1 nm, measured experimentally, is systematically higher for the positive polarity of HV electrodes. To analyze the spatial distribution of N2(C3Πu) and emissions, comprehensive two-dimensional numerical modeling for P = 1 bar has been performed. The details of the formation of a narrow gap between the dielectric surface and the streamer channel in the case of positive polarity of HV electrodes are discussed. The ratio of integrated over space calculated emission intensities, , has been analyzed and compared with obtained experimental data. A good agreement was obtained for a negative polarity SDBD. For a positive polarity discharge, for all the considered conditions. Explanation for the observed effect is suggested. (10.1088/0022-3727/47/48/485201)
  DOI : 10.1088/0022-3727/47/48/485201
• Investigation of capillary nanosecond discharges in air at moderate pressure: comparison of experiments and 2D numerical modeling
  
  • Klochko A.V.
  • Starikovskaia Svetlana
  • Xiong Z.
  • Kushner M.J.
  Journal of Physics D: Applied Physics, IOP Publishing, 2014, 47, pp.365202. Nanosecond electrical discharges in the form of ionization waves are of interest for rapidly ionizing and exciting complex gas mixtures to initiate chemical reactions. Operating with a small discharge tube diameter can significantly increase the specific energy deposition and so enable optimization of the initiation process. Analysis of the uniformity of energy release in small diameter capillary tubes will aid in this optimization. In this paper, results for the experimentally derived characteristics of nanosecond capillary discharges in air at moderate pressure are presented and compared with results from a two-dimensional model. The quartz capillary tube, having inner and outer diameters of 1.5 and 3.4 mm, is about 80 mm long and filled with synthetic dry air at 27 mbar. The capillary tube with two electrodes at the ends is inserted into a break of the central wire of a long coaxial cable. A metal screen around the tube is connected to the cable ground shield. The discharge is driven by a 19 kV 35 ns voltage pulse applied to the powered electrode. The experimental measurements are conducted primarily by using a calibrated capacitive probe and back current shunts. The numerical modelling focuses on the fast ionization wave (FIW) and the plasma properties in the immediate afterglow after the conductive plasma channel has been established between the two electrodes. The FIW produces a highly focused region of electric field on the tube axis that sustains the ionization wave that eventually bridges the electrode gap. Results from the model predict FIW propagation speed and current rise time that agree with the experiment. (10.1088/0022-3727/47/36/365202)
  DOI : 10.1088/0022-3727/47/36/365202
• Electric field in nanosecond surface dielectric barrier discharge at different polarities of the high voltage pulse: spectroscopy measurements and numerical modeling.
  
  • Stepanyan S.A.
  • Victor Soloviev
  • Starikovskaia Svetlana
  Journal of Physics D: Applied Physics, IOP Publishing, 2014. Has been just accepted
• Regeneration of isopropyl alcohol saturated Mn<SUB>X</SUB>O<SUB>Y</SUB> surface: Comparison of thermal, ozonolysis and non-thermal plasma treatments
  
  • Sivachandiran Loganathan
  • Thévenet Frédéric
  • Rousseau Antoine
  Chemical Engineering Journal, Elsevier, 2014, 246, pp.184-195. IPA saturated MnXOY surface regeneration has been investigated under dry air. MnXOY coated glass beads packed-bed reactor has been designed and used for IPA storage under gas-flowing condition at 296 K. The coated MnXOY material is characterized by BrunauerEmmettTeller (BET), non destructive Optical Pro- filometer and X-ray diffraction (XRD) techniques. Atmospheric pressure gas phase Fourier Transform Infrared Spectroscopy (FTIR) and online Thermal Desorption coupled with Gas Phase Chromatography and Mass Spectrometry (TDGCMS) have been respectively used to quantify and to identify the gas phase species produced during the regeneration processes. This study mainly aims at investigating three different methods to regenerate the IPA saturated MnXOY surface. In this framework, methods have been investigated for IPA saturated MnXOY surface regeneration namely (i) direct thermal treatment (DTT), (ii) ozonolysis and (iii) In-Situ Non Thermal Plasma Treatment (NTP). Among the employed methods, In-Situ NTP treatment has shown better regeneration efficiency, and twice more CO2 selectivity. Notably, dry air In-Situ NTP treatment prior to thermal treatment has significantly improved the mineralization. The order of mineralization efficiency and/or COx selectivity can be written as follows: In-Situ NTP > dry air ozonolysis > dry air DTT. (10.1016/j.cej.2014.02.058)
  DOI : 10.1016/j.cej.2014.02.058
• Pure airplasma bullets propagating inside microcapillaries and in ambient air
  
  • Lacoste D.A.
  • Bourdon Anne
  • Kuribara Koichi
  • Urabe Keiichiro
  • Stauss Sven
  • Terashima Kazuo
  Plasma Sources Science and Technology, IOP Publishing, 2014, 23, pp.062006. This paper reports on the characterization of airplasma bullets in microcapillary tubes and in ambient air, obtained without the use of inert or noble gases. The bullets were produced by nanosecond repetitively pulsed discharges, applied in a dielectric barrier discharge configuration. The anode was a tungsten wire with a diameter of 50&#956;m, centered in the microcapillary, while the cathode was a silver ring, fixed on the outer surface of the fused silica tube. The effects of the applied voltage and the inner diameter of the microcapillary tube on the plasma behavior were investigated. Inside the tubes, while the topology of the bullets seems to be strongly dependent on the diameter, their velocity is only a function of the amplitude of the applied voltage. In ambient air, the propagation of air bullets with a velocity of about 1.25 × 105 ms&#8722;1 is observed. (10.1088/0963-0252/23/6/062006)
  DOI : 10.1088/0963-0252/23/6/062006
• Radiation from Ag high energy density Z-pinch plasmas and applications to lasing
  
  • Weller Michael E.
  • Safronova Alla S.
  • Kantsyrev Viktor L.
  • Esaulov A. A.
  • Shrestha Ishor
  • Apruzese J. P.
  • Giuliani J. L.
  • Chuvatin Alexandre S.
  • Stafford A.
  • Keim S. F.
  • Shlyaptseva V. V.
  • Osborne Glenn C.
  • Petkov E. E.
  Physics of Plasmas, American Institute of Physics, 2014, 21 (03), pp.031206. Silver (Ag) wire arrays were recently introduced as efficient x-ray radiators and have been shown to create L-shell plasmas that have the highest electron temperature (>1.8&#8201;keV) observed on the Zebra generator so far and upwards of 30&#8201;kJ of energy output. In this paper, results of single planar wire arrays and double planar wire arrays of Ag and mixed Ag and Al that were tested on the UNR Zebra generator are presented and compared. To further understand how L-shell Ag plasma evolves in time, a time-gated x-ray spectrometer was designed and fielded, which has a spectral range of approximately 3.55.0&#8201;Å. With this, L-shell Ag as well as cold L&#945; and L&#946; Ag lines was captured and analyzed along with photoconducting diode (PCD) signals (>0.8&#8201;keV). Along with PCD signals, other signals, such as filtered XRD (>0.2&#8201;keV) and Si-diodes (SiD) (>9&#8201;keV), are analyzed covering a broad range of energies from a few eV to greater than 53&#8201;keV. The observation and analysis of cold L&#945; and L&#946; lines show possible correlations with electron beams and SiD signals. Recently, an interesting issue regarding these Ag plasmas is whether lasing occurs in the Ne-like soft x-ray range, and if so, at what gains? To help answer this question, a non-local thermodynamic equilibrium (LTE) kinetic model was utilized to calculate theoretical lasing gains. It is shown that the Ag L-shell plasma conditions produced on the Zebra generator at 1.7 maximum current may be adequate to produce gains as high as 6&#8201;cm&#8722;1 for various 3p&#8201;&#8594;&#8201;3s transitions. Other potential lasing transitions, including higher Rydberg states, are also included in detail. The overall importance of Ag wire arrays and plasmas is discussed. (10.1063/1.4865368)
  DOI : 10.1063/1.4865368
• Waveforms of Langmuir turbulence in inhomogeneous solar wind plasmas
  
  • Krafft C
  • Volokitin A.
  • Krasnoselskikh V.V.
  • Dudok de Wit Thierry
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2014, 119, pp.9369–9382. Modulated Langmuir waveforms have been observed by several spacecraft in various regions of the heliosphere, such as the solar wind, the electron foreshock, the magnetotail, or the auroral ionosphere. Many observations revealed the bursty nature of these waves, which appear to be highly modulated, localized, and clumped into spikes with peak amplitudes typically 3 orders of magnitude above the mean. The paper presents Langmuir waveforms calculated using a Hamiltonian model describing self-consistently the resonant interaction of an electron beam with Langmuir wave packets in a plasma with random density fluctuations. These waveforms, obtained for different profiles of density fluctuations and ranges of parameters relevant to solar type III electron beams and plasmas measured at 1 AU, are presented in the form they would appear if recorded by a satellite moving in the solar wind. Comparison with recent measurements by the STEREO and WIND satellites shows that their characteristic features are very similar to the observations. (10.1002/2014JA020329)
  DOI : 10.1002/2014JA020329
• Ionospheric disturbance dynamo associated to a coronal hole: Case study of 5-10 April 2010
  
  • Fathy Ibrahim
  • Amory-Mazaudier Christine
  • Fathy A.
  • Mahrous A. M.
  • Yumoto K.
  • Ghamry E.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2014, 119 (5), pp.4120–4133. In this paper we study the planetary magnetic disturbance during the magnetic storm occurring on 5 April 2010 associated with high-speed solar wind stream due to a coronal hole following a coronal mass ejection. We separate the magnetic disturbance associated to the ionospheric disturbance dynamo (Ddyn) from the magnetic disturbance associated to the prompt penetration of magnetospheric electric field (DP2). This event exhibits different responses of ionospheric disturbance dynamo in the different longitude sectors (European-African, Asian, and American). The strongest effect is observed in the European-African sector. The Ddyn disturbance reduces the amplitude of the daytime H component at low latitudes during four consecutive days in agreement with the Blanc and Richmond's model of ionospheric disturbance dynamo. The amplitude of Ddyn decreased with time during the 4 days. We discuss its diverse worldwide effects. The observed signature of magnetic disturbance process in specific longitude sector is strongly dependent on which Earth's side faces the magnetic storms (i.e., there is a different response depending on which longitude sector is at noon when the SSC hits). Finally, we determined an average period of 22 h for Ddyn using wavelet analysis. (10.1002/2013JA019510)
  DOI : 10.1002/2013JA019510
• Electron heating in capacitively coupled plasmas revisited
  
  • Lafleur Trevor
  • Chabert Pascal
  • Booth Jean-Paul
  Plasma Sources Science and Technology, IOP Publishing, 2014, 23 (3), pp.035010. We revisit the problem of electron heating in capacitively coupled plasmas (CCPs), and propose a method for quantifying the level of collisionless and collisional heating in plasma simulations. The proposed procedure, based on the electron mechanical energy conservation equation, is demonstrated with particle-in-cell simulations of a number of single and multi-frequency CCPs operated in regimes of research and industrial interest. In almost all cases tested, the total electron heating is comprised of collisional (ohmic) and pressure heating parts. This latter collisionless component is in qualitative agreement with the mechanism of electron heating predicted from the recent re-evaluation of theoretical models. Finally, in very electrically asymmetric plasmas produced in multi-frequency discharges, we observe an additional collisionless heating mechanism associated with electron inertia. (10.1088/0963-0252/23/3/035010)
  DOI : 10.1088/0963-0252/23/3/035010
• A radio-frequency sheath model for complex waveforms
  
  • Turner M.M.
  • Chabert Pascal
  Applied Physics Letters, American Institute of Physics, 2014, 104 (16), pp.164102. Plasma sheaths driven by radio-frequency voltages occur in contexts ranging from plasma processing to magnetically confined fusion experiments. An analytical understanding of such sheaths is therefore important, both intrinsically and as an element in more elaborate theoretical structures. Radio-frequency sheaths are commonly excited by highly anharmonic waveforms, but no analytical model exists for this general case. We present a mathematically simple sheath model that is in good agreement with earlier models for single frequency excitation, yet can be solved for arbitrary excitation waveforms. As examples, we discuss dual-frequency and pulse-like waveforms. The model employs the ansatz that the time-averaged electron density is a constant fraction of the ion density. In the cases we discuss, the error introduced by this approximation is small, and in general it can be quantified through an internal consistency condition of the model. This simple and accurate model is likely to have wide application. (10.1063/1.4872172)
  DOI : 10.1063/1.4872172
• Oxidation of isopropanol and acetone adsorbed on TiO<SUB>2</SUB> under plasma generated ozone flow: Gas phase and adsorbed species monitoring
  
  • Barakat Christelle
  • Gravejat Paul
  • Guaitella Olivier
  • Thévenet Frédéric
  • Rousseau Antoine
  Applied Catalysis B: Environmental, Elsevier, 2014, 147, pp.302-313. The regeneration of isopropanol (IPA) and/or acetone saturated TiO2 surface by ozone is investigated. TiO2 catalyst is placed downstream a dielectric barrier discharge and is subsequently exposed to ozone considered as the main oxidative species generated by non-thermal plasma and able to interact with the material surface at room temperature. The oxidation of isopropanol and/or acetone is monitored using two parallel and complementary infrared diagnostics: (1) Fourier Transform Infrared Spectroscopy for the analysis of the gas phase composition; and (2) Diffuse Reflectance Infrared Fourier Transform Spectroscopy for the in situ analysis of the adsorbent/catalyst surface. In this study, the pollutant is first adsorbed on the TiO2 surface, the plasma being switched off. The irreversibly adsorbed amounts of isopropanol and acetone have been respectively quantified as 5.3 &#956;mol/m2 and 1.9 &#956;mol/m2. In a second step, the plasma is switched on to regenerate the surface by mineralization of the adsorbed organic species. A 70-min plasma phase, with approximately 20 ppm of ozone constantly flowing through the adsorbent bed yields 8.5 nmol and 8.9 nmol of CO2 per injected joule of energy for isopropanol and acetone saturated surfaces, respectively. Acetone has been evidenced as the main oxidation intermediate of isopropanol on TiO2 surface. It has been proven that the complete oxidation of isopropanol and acetone is mainly limited by the acetone oxidation rate. Competitive adsorption on the surface of the catalyst between both compounds has been studied. Results obtained are compared with those observed in the photocatalytic oxidation of the same species. (10.1016/j.apcatb.2013.09.008)
  DOI : 10.1016/j.apcatb.2013.09.008
• Surface deactivation of vibrationally excited N<SUB>2</SUB> studied using infrared titration combined with quantum cascade laser absorption spectroscopy
  
  • Marinov Daniil
  • Lopatik D.
  • Guaitella Olivier
  • Ionikh Y.
  • Röpcke J.
  • Rousseau Antoine
  Journal of Physics D: Applied Physics, IOP Publishing, 2014, 47 (1), pp.015203. The wall de-excitation probability of vibrationally excited nitrogen molecules was determined using infrared (IR) titration with CO, CO 2 and N 2 O. Gas mixtures of N 2 with 0.05?0.5% of CO (CO 2 or N 2 O) were excited by a pulsed dc discharge at p = 133 Pa in a cylindrical discharge tube. During the afterglow, the vibrational relaxation of titrating molecules was monitored in situ with quantum cascade laser absorption spectroscopy. The value of was deduced from measured vibrational relaxation times using a model of vibrational kinetics in N 2 . It was found that adsorption of IR tracers on the surface may increase the value of by a factor up to two, depending on the molecule and the surface material. It was demonstrated that N 2 O is the most inert and reliable tracer and it was used for the determination of on silica, Pyrex, TiO 2 , Al 2 O 3 and anodized aluminum. Pretreatment of the silica surface by low-pressure plasma was found to have a strong effect on the vibrational de-excitation. Values of measured after O 2 , Ar and N 2 plasma pretreatment of the same silica discharge tube were 5.7 × 10 ?4 , 8.2 × 10 ?4 and 11 × 10 ?4 , respectively. This study clearly demonstrates that the presence of adsorbed atoms and molecules on the surface may significantly alter the value of . (10.1088/0022-3727/47/1/015203)
  DOI : 10.1088/0022-3727/47/1/015203
• A nanosecond surface dieletric barrier discharge in air at high pressure and different polarities of applied pulses. Transition to filamentary mode
  
  • Stepanyan S.A.
  • Starikovskiy a Yu
  • Popov N.A.
  • Starikovskaia Svetlana
  Plasma Sources Science and Technology, IOP Publishing, 2014, 23 (4), pp.045003. The development of a nanosecond surface dielectric barrier discharge in air at pressures 16 bar is studied. At atmospheric pressure, the discharge develops as a set of streamers starting synchronously from the high-voltage electrode and propagating along the dielectric layer. Streamers cover the dielectric surface creating a 'quasi-uniform' plasma layer. At high pressures and high voltage amplitudes on the cathode, filamentation of the discharge is observed a few nanoseconds after the discharge starts. Parameters of the observed 'streamers-to-filaments' transition are measured; physics of transition is discussed on the basis of theoretical estimates and numerical modeling. Ionization-heating instability on the boundary of the cathode layer is suggested as a mechanism of filamentation. (10.1088/0963-0252/23/4/045003)
  DOI : 10.1088/0963-0252/23/4/045003
• Ambipolar and non-ambipolar diffusion in an rf plasma source containing a magnetic filter
  
  • Lafleur Trevor
  • Aanesland Ane
  Physics of Plasmas, American Institute of Physics, 2014, 21, pp.063510. By placing a magnetic filter across a rectangular plasma source (closed at one end with a ceramic plate and an rf antenna, and terminated at the opposite end by a grounded grid), we experimentally investigate the effect of conducting and insulating source walls on the nature of the plasma diffusion phenomena. The use of a magnetic filter creates a unique plasma, characterized by a high upstream electron temperature (Teu&#8764;5 eV) near the rf antenna and a low downstream electron temperature (Ted&#8764;1 eV) near the grid, which more clearly demonstrates the role of the source wall materials. For conducting walls a net ion current to ground is measured on the grid, and the plasma potential is determined by a mean electron temperature within the source. For insulating walls the plasma potential is determined by the downstream electron temperature (i.e., Vp&#8764;5.2Ted in argon), and the net current to the grid is exactly zero. Furthermore, by inserting a small additional upstream conductor (that can be made floating or grounded through an external circuit switch), we demonstrate that the plasma potential can be controlled and set to a low (Vp&#8764;5.2Ted), or high (Vp&#8764;5.2Teu) value. (10.1063/1.4885109)
  DOI : 10.1063/1.4885109
• Pulsed discharges in a wide density range: plasma development and media excitation
  
  • Starikovskaia Svetlana
  • Starikovskii A.Yu.
  , 2014. The Chapter demonstrates the peculiarities of pulsed nonequilibrium plasma generation in different media by nano- and picosecond high-voltage discharges. Fast ionization waves in long tubes at low pressures, streamers and dielectric barrier discharges at moderate and atmospheric pressures, surface dielectric barrier discharges at elevated pressures are considered. Special attention is given to start of picoseconds and nanosecond discharges in liquid media. The data used were consciously restricted to ICCD fast imaging of picoseconds and nanosecond discharges, to underline the dynamics of pulsed discharge development and the importance of spatial distribution of the discharge energy.
• Propagation of lower-band whistler-mode waves in the outer Van Allen belt: Systematic analysis of 11 years of multi-component data from the Cluster spacecraft
  
  • Santolík O.
  • Macusova E.
  • Kolmasova Ivana
  • Cornilleau-Wehrlin Nicole
  • Conchy Y.
  Geophysical Research Letters, American Geophysical Union, 2014, 41, pp.2729-2737. Lower-band whistler-mode emissions can influence the dynamics of the outer Van Allen radiation belts. We use 11&#8201;years of measurements of the STAFF-SA instruments onboard the four Cluster spacecraft to systematically build maps of wave propagation parameters as a function of position. We determine probability distributions of wave vector angle weighted by the wave intensity. The results show that wave vector directions of intense waves are close to a Gaussian-shaped peak centered on the local magnetic field line. The width of this peak is between 10 and 20 degrees. The cumulative percentage of oblique waves is below 1015%. This result is especially significant for an important class of whistler-mode emissions of lower-band chorus at higher latitudes, well outside their source region, where a simple ray tracing model fails and another mechanism is necessary to keep the wave vectors close to the field-aligned direction. (10.1002/2014GL059815)
  DOI : 10.1002/2014GL059815