Publications

2013

• Autocalibration Method for Anisotropic Magnetoresistive Sensors Using Offset Coils
  
  • Mohamadabadi K.
  • Jeandet Alexis
  • Hillion M.
  • Coillot Christophe
  IEEE Sensors Journal, Institute of Electrical and Electronics Engineers, 2013, 13 (2), pp.772-776. In this paper, we present a zero-cost indoor calibration method for anisotropic magnetoresistive (AMR) sensors. The implemented circuit is designed to calibrate AMR sensors using integrated coils. A microcontroller is used to generate an artificial three-dimensional magnetic field by injecting three separate currents into three offset coils. We show the similarity of the results for residual calibration norm by using this method compared with the calibration of the sensor in free Earth's magnetic field. Furthermore, this method does not need any other instruments such as Helmholtz coils or a platform for rotating the sensor. Here the sensor is placed inside a mu-metal box during calibration, and the calibration process is completely autonomous. (10.1109/JSEN.2012.2227595)
  DOI : 10.1109/JSEN.2012.2227595
• Spatio-temporal evolution of the H -> L back transition
  
  • Miki K.
  • Diamond P.H.
  • Schmitz L.
  • Mcdonald D. C.
  • Estrada T.
  • Gürcan Özgür D.
  • Tynan G.R.
  Physics of Plasmas, American Institute of Physics, 2013, 20 (6), pp.062304. Since ITER will operate close to threshold and with limited control, the H → L back transition is a topic important for machine operations as well as physics. Using a reduced mesoscale model [Miki et al., Phys. Plasmas 19, 092306 (2012)], we investigate ELM-free H → L back transition dynamics in order to isolate transport physics effects. Model studies indicate that turbulence spreading is the key process which triggers the back transition. The transition involves a feedback loop linking turbulence and profiles. The I-phase appears during the back transition following a slow power ramp down, while fast ramp-downs reveal a single burst of zonal flow during the back transition. The I-phase nucleates at the pedestal shoulder, as this is the site of the residual turbulence in H-mode. Hysteresis in the profile gradient scale length is characterized by the Nusselt number, where Nu = χi,turb/χi,neo. Relative hysteresis of temperature gradient vs density gradient is sensitive to the pedestal Prandtl number, where Prped = Dped/χi,neo. We expect the H-mode to be somewhat more resilient in density than in temperature. (10.1063/1.4812555)
  DOI : 10.1063/1.4812555
• Microsecond ramp compression of a metallic liner driven by a 5 MA current on the SPHINX machine using a dynamic load current multiplier pulse shaping
  
  • d'Almeida Thierry
  • Lassalle Francis
  • Morell Alain
  • Grunenwald Julien
  • Zucchini Frédéric
  • Loyen Arnaud
  • Maysonnave Thomas
  • Chuvatin Alexandre S.
  Physics of Plasmas, American Institute of Physics, 2013, 20 (09), pp.092512. SPHINX is a 6 MA, 1-μs Linear Transformer Driver (LTD) operated by the CEA Gramat (France) and primarily used for imploding Z-pinch loads for radiation effects studies. Among the options that are currently being evaluated to improve the generator performances are an upgrade to a 20 MA, 1-μs LTD machine and various power amplification schemes, including a compact Dynamic Load Current Multiplier (DLCM). A method for performing magnetic ramp compression experiments, without modifying the generator operation scheme, was developed using the DLCM to shape the initial current pulse in order to obtain the desired load current profile. In this paper, we discuss the overall configuration that was selected for these experiments, including the choice of a coaxial cylindrical geometry for the load and its return current electrode. We present both 3-D Magneto-hydrodynamic and 1D Lagrangian hydrodynamic simulations which helped guide the design of the experimental configuration. Initial results obtained over a set of experiments on an aluminium cylindrical liner, ramp-compressed to a peak pressure of 23 GPa, are presented and analyzed. Details of the electrical and laser Doppler interferometer setups used to monitor and diagnose the ramp compression experiments are provided. In particular, the configuration used to field both homodyne and heterodyne velocimetry diagnostics in the reduced access available within the liner's interior is described. Current profiles measured at various critical locations across the system, particularly the load current, enabled a comprehensive tracking of the current circulation and demonstrate adequate pulse shaping by the DLCM. The liner inner free surface velocity measurements obtained from the heterodyne velocimeter agree with the hydrocode results obtained using the measured load current as the input. An extensive hydrodynamic analysis is carried out to examine information such as pressure and particle velocity history profiles or magnetic diffusion across the liner. The potential of the technique in terms of applications and achievable ramp pressure levels lies in the prospects for improving the DLCM efficiency through the use of a closing switch (currently under development), reducing the load dimensions and optimizing the diagnostics. (10.1063/1.4823720)
  DOI : 10.1063/1.4823720
• Antisunward structure of thin current sheets in the Earth's magnetotail : Implications of quasi-adiabatic theory
  
  • Malova H. V.
  • Popov V. Y.
  • Delcourt Dominique C.
  • Petrukovich A. A.
  • Zelenyi L. M.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2013, 118. We developed a self-consistent kinetic model of thin current sheets (TCS), taking into account the inhomogeneity of TCS parameters in the antisunward direction. We show that the charged particle dynamics depending on the magnetic field distribution in the downtail direction completely determines the magnetotail equilibrium structure. We demonstrate that transient ions as well as electrons are the main current carriers in this system, but the first ones support mostly the background (1-D) structure of the current sheet. The influence of electrons and quasi-trapped ions is found to vary depending upon downtail distance along the sheet. Assuming the conservation of the so-called quasi-adiabatic invariant, we show that quasi-trapped particles are distributed along the current sheet in such a way that they concentrate in the region with large values of normal magnetic field component. As a result quasi-trapped ions can dominate near the earthward edge of TCS. In contrast, the electron current becomes stronger in the TCS tailward region where the normal magnetic field component becomes weaker, and field line curvature drifts are enhanced. Our quasi-adiabatic model predicts that thin current sheets in the Earth's magnetotail should have weakly 2-D configuration which, similar to its 1-D analog considered earlier, conserves the multiscale matreshka structure with multiple embedded layers. (10.1002/jgra.50390)
  DOI : 10.1002/jgra.50390
• Electron nongyrotropy in the context of collisionless magnetic reconnection
  
  • Aunai Nicolas
  • Hesse Michael
  • Kuznetsova M. M.
  Physics of Plasmas, American Institute of Physics, 2013, 20, pp.2903. Collisionless magnetized plasmas have the tendency to isotropize their velocity distribution function around the local magnetic field direction, i.e., to be gyrotropic, unless some spatial and/or temporal fluctuations develop at the particle gyroscales. Electron gyroscale inhomogeneities are well known to develop during the magnetic reconnection process. Nongyrotropic electron velocity distribution functions have been observed to play a key role in the dissipative process breaking the field line connectivity. In this paper, we present a new method to quantify the deviation of a particle population from gyrotropy. The method accounts for the full 3D shape of the distribution and its analytical formulation allows fast numerical computation. Regions associated with a significant degree of nongyrotropy are shown, as well as the kinetic origin of the nongyrotropy and the fluid signature it is associated with. Using the result of 2.5D Particle-In-Cell simulations of magnetic reconnection in symmetric and asymmetric configurations, it is found that neither the reconnection site nor the topological boundaries are generally associated with a maximized degree of nongyrotropy. Nongyrotropic regions do not correspond to a specific fluid behavior as equivalent nongyrotropy is found to extend over the electron dissipation region as well as in non-dissipative diamagnetic drift layers. The localization of highly nongyrotropic regions in numerical models and their correlation with other observable quantities can, however, improve the characterization of spatial structures explored by spacecraft missions. (10.1063/1.4820953)
  DOI : 10.1063/1.4820953
• Chlorine atom densities in the (3p<SUP>5</SUP>)<SUP>2</SUP> P<SUP>0</SUP><SUB>1/2</SUB> excited spin-orbit state measured by two-photon absorption laser-induced fluorescence in a chlorine inductively coupled plasma
  
  • Sirse Nishant
  • Booth Jean-Paul
  • Chabert Pascal
  • Surzhykov A.
  • Indelicato P.
  Journal of Physics D: Applied Physics, IOP Publishing, 2013, 46 (29), pp.295203. Chlorine atom densities in the spinorbit excited state were measured by two-photon absorption laser-induced fluorescence (TALIF) in an inductively coupled plasma discharge in pure Cl2. The atoms were excited by two photons at 235.702 nm to the state and detected by fluorescence to the (4s) 4P5/2 state at 726 nm. The population of this state relative to that in the ground state, was determined from the relative TALIF signal intensity from the two states, combined with new calculations of the two-photon absorption cross-sections. was found to increase continuously with radio-frequency power (50500 W), whereas with Cl2 pressure (590 mTorr) it passes through a maximum at 10 mTorr, reaching ~30% at 500 W. This maximum corresponds to the maximum of electron density in the discharge. Combining this density ratio measurement with previous measurements of the absolute ground state chlorine atom density [1] allows the absolute spin-orbit excited state density to be estimated. A significant fraction of the total chlorine atom density is in this excited state which should be included in plasma chemistry models. (10.1088/0022-3727/46/29/295203)
  DOI : 10.1088/0022-3727/46/29/295203
• Effects of the surface conductivity and the IMF strength on the dynamics of planetary ions in Mercury's magnetosphere
  
  • Seki Kanako
  • Terada Naoki
  • Yagi Manabu
  • Delcourt Dominique C.
  • Leblanc François
  • Ogino Tatsuki
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2013, 118 (6), pp.3233-3242. To examine the effects of planetary surface conductivity and the southward IMF strength on ion dynamics, systematic trajectory tracings of Na⁺ ions were performed in the electric and magnetic field configurations obtained from magnetohydrodynamics (MHD) simulations of the solar wind-Mercury interaction. Comparison with a previous study, which used an analytical model that rescaled the Earth's magnetosphere and assumed the existence of the distant neutral line (DNL) in Mercury's magnetotail, shows a drastic change in the Na⁺ precipitation pattern onto due to the formation of the near-Mercury neutral line (NMNL) in MHD simulations. The Na⁺ precipitation band at approximately 30 degrees of latitude (LAT), which was obtained in the previous study, disappeared in the equivalent low-conductivity MHD case due to the NMNL formation, while the NMNL formation causes high-energy Na⁺ precipitation in the equatorial region. The change in strength of the southward IMF (sBz) alters the location of the NMNL and the Na⁺ precipitation pattern. In the low-conductivity sBz = 5 case, both the equatorial precipitation and the Na⁺ band at approximately LAT = 30 are formed. In the high-conductivity sBz = 5 case, magnetospheric convection through the polar regions is suppressed, which results in a region of dense Na⁺ near the planet. These results suggest that the precipitation pattern of planetary ions onto Mercury's surface changes significantly with the activity level of Mercury's magnetosphere. It is also suggested that observations of the magnetospheric convection, the distribution of Na⁺ ions around the planet, or the precipitation pattern of Na⁺ ions onto the planetary surface can provide us information about the surface conductivity. (10.1002/jgra.50181)
  DOI : 10.1002/jgra.50181
• CLUSTER observation of polar electron precipitation above the polar caps during periods of Northward IMF
  
  • Fontaine Dominique
  • Maggiolo R.
  AGU Fall Meeting Abstracts, AGU, 2013, 21, pp.07. The CLUSTER spacecraft revealed the presence of successive current sheets of opposite polarity above the polar caps during periods of northward or weak IMF. We first present the general electrodynamical context. At CLUSTER altitude (5-7 RE), the upward part of this current system consists of ion beams accelerated by quasi-static electric fields, associated with precipitating electrons. They are surrounded by low energy upflowing electron beams carrying a downward current. We then focus on the precipitating electrons above the polar cap which form acceleration structures at about 100 - 300 eV. This acceleration is interpreted as the effect of an electrostatic potential along magnetic field lines located above CLUSTER altitude, i.e. typically above 5-7 RE. We present statistics on the characteristics of these precipitating electron structures and we discuss the source regions and the mechanisms possibly at their origin.
• Physics of Stimulated L->H Transitions
  
  • Miki K.
  • Diamond P.H.
  • Hahn S. -H.
  • Xiao W. W.
  • Gürcan Özgür D.
  • Tynan G.R.
  Physical Review Letters, American Physical Society, 2013, 110, pp.195002. We report on model studies of stimulated L&#8594;H transitions. These studies use a novel reduced mesoscale model. Studies reveal that L&#8594;H transitions can be triggered by particle injection into a subcritical state (i.e., P<PThresh). Particle injection alters the edge mean flow shear via changes of density and temperature gradients. The change of edge mean flow shear is critical to turbulence collapse and the subsequent stimulated transition. For low ambient heating, strong injection is predicted to trigger a transient turbulence collapse. We predict that repetitive injection can maintain the turbulence collapse and so sustain a driven H-mode-like state. The total number of particles required to induce a transition by either injection or gas puffing is estimated. Results indicate that the total number of injected particles required is much smaller than that required for inducing a transition by gas puffing. Thus, we show that internal injection is more efficient than gas puffing of comparable strength. We also observe that zonal flows do not play a critical role in stimulated transitions. (10.1103/PhysRevLett.110.195002)
  DOI : 10.1103/PhysRevLett.110.195002
• Nonlinear saturation of wave packets excited by low-energy electron horseshoe distributions
  
  • Krafft C.
  • Volokitin A.
  Physical Review E, American Physical Society (APS), 2013, 87, pp.053107. Horseshoe distributions are shell-like particle distributions that can arise in space and laboratory plasmas when particle beams propagate into increasing magnetic fields. The present paper studies the stability and the dynamics of wave packets interacting resonantly with electrons presenting low-energy horseshoe or shell-type velocity distributions in a magnetized plasma. The linear instability growth rates are determined as a function of the ratio of the plasma to the cyclotron frequencies, of the velocity and the opening angle of the horseshoe, and of the relative thickness of the shell. The nonlinear stage of the instability is investigated numerically using a symplectic code based on a three-dimensional Hamiltonian model. Simulation results show that the dynamics of the system is mainly governed by wave-particle interactions at Landau and normal cyclotron resonances and that the high-order normal cyclotron resonances play an essential role. Specific features of the dynamics of particles interacting simultaneously with two or more waves at resonances of different natures and orders are discussed, showing that such complex processes determine the main characteristics of the wave spectrum's evolution. Simulations with wave packets presenting quasicontinuous spectra provide a full picture of the relaxation of the horseshoe distribution, revealing two main phases of the evolution: an initial stage of wave energy growth, characterized by a fast filling of the shell, and a second phase of slow damping of the wave energy, accompanied by final adjustments of the electron distribution. The influence of the density inhomogeneity along the horseshoe on the wave-particle dynamics is also discussed. (10.1103/PhysRevE.87.053107)
  DOI : 10.1103/PhysRevE.87.053107
• On the origin of the quasi-perpendicular ion foreshock: Full-particle simulations
  
  • Savoini Philippe
  • Lembège Bertrand
  • Stienlet J.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2013, 118 (3), pp.1132-1145. Many space missions have already evidenced the existence of the ion foreshock region located upstream of the Earth's bow shock and populated by energetic backstreaming ions reflected by the shock front. In order to analyse this region, a curved shock is simulated with a 2-D particle-in-cell (PIC) code. The analysis is presently restricted to the quasi-perpendicular angular range defined by 45° ≤ θBn ≤ 90°. In agreement with experimental data, present results evidence two distinct ion populations back-streaming from the shock front along the interplanetary magnetic field: (i) the Field-Aligned Beam population (hereafter "FAB") and (ii) the gyrophase bunched population (hereafter "GPB") which differ from each other by their gyrotropic or non-gyrotropic behavior, respectively. Excluded by a simulation time which is too short, ion instabilities pitch-angle scattering cannot be the source of "GPB". Two new criteria are proposed to identify more precisely each population: their interaction time Δtint with the shock front and their downstream penetration depth. These criteria show that (i) the "FAB" population moves back and forth between the upstream edge of the shock front and the overshoot, and is characterized by a Δtint covering several upstream gyro-periods. (ii) In contrast, the "GPB" ions suffer a short interaction time (i.e. 1 < τci). We observe that the "FAB" ions may have different originsalthough all "GPB" ions seem to be produced by the electrostatic field built up at the shock and are emitted in a burst-like mode rather than in continuous way. (10.1002/jgra.50158)
  DOI : 10.1002/jgra.50158
• A spectroscopic study of ethylene destruction and by-product generation using a three-stage atmospheric packed-bed plasma reactor
  
  • Hubner Antoine
  • Guaitella Olivier
  • Rousseau Antoine
  • Roepcke J.
  Journal of Applied Physics, American Institute of Physics, 2013, 114, pp.033301. Using a three-stage dielectric packed-bed plasma reactor at atmospheric pressure, the destruction of ethylene, a typical volatile organic compound, and the generation of major by-products have been studied by means of Fourier Transform Infrared Spectroscopy. A test gas mixture air at a gas flow of 1 slm containing 0.12% humidity with 0.1% ethylene has been used. In addition to the fragmentation of the precursor gas, the evolution of the concentration of ten stable reaction products, CO, CO2, O3, NO2, N2O, HCN, H2O, HNO3, CH2O, and CH2O2 has been monitored. The concentrations of the by-products range between 5&#8201;ppm, in the case of NO2, and 1200&#8201;ppm, for H2O. By the application of three sequentially working discharge cells at a frequency of f&#8201;=&#8201;4 kHz and voltage values between 9 and 12&#8201;kV, a nearly complete decomposition of C2H4 could be achieved. Furthermore, the influence of the specific energy deposition (SED) on the destruction process has been studied and the maximum value of SED was about 900 J l&#8722;1. The value of the characteristic energy &#946;, characterizing the energy efficiency of the ethylene destruction in the reactor, was found to be 330 J l&#8722;1. It was proven that the application of three reactor stages suppresses essentially the production of harmful by-products as formaldehyde, formic acid, and NO2 compared to the use of only one or two stages. Based on the multi-component detection, the carbon balance of the plasma chemical conversion of ethylene has been analyzed. The dependence of the fragmentation efficiencies of ethylene (RF(C2H4)&#8201;=&#8201;5.5&#8201;×&#8201;1019 molecules J&#8722;1) and conversion efficiencies to the produced molecular species (RC&#8201;=&#8201;(0.13)&#8201;×&#8201;1016 molecules J&#8722;1) on the discharge conditions could be estimated in the multistage plasma reactor. (10.1063/1.4813409)
  DOI : 10.1063/1.4813409
• Anisotropy of radiation emitted from planar wire arrays
  
  • Kantsyrev Viktor L.
  • Chuvatin Alexandre S.
  • Esaulov A. A.
  • Safronova Alla S.
  • Rudakov Leonid I.
  • Velikovich A. L.
  • Williamson Kenneth M.
  • Osborne Glenn C.
  • Shrestha I. K.
  • Weller Michael E.
  • Shlyaptseva V. V.
  Physics of Plasmas, American Institute of Physics, 2013, 20 (07), pp.070702. The planar wire array (PWA) is a promising load for new multi-source inertial confinement fusion (ICF) hohlraums [B. Jones et al. Phys. Rev. Lett. 104 125001 (2010)]. The hohlraum radiation symmetry is an important issue for ICF. It was found that extreme ultraviolet and sub-keV photon emission from PWAs may have considerable anisotropy in the load azimuthal plane. This experimental result is obtained on the UNR 11.7 MA Zebra generator. The time-dependent anisotropy effect is detected. This feature is studied in 2D numerical simulations and can be explained by initial anisotropy of implosion of those non-cylindrical loads radiating essentially as surface sources in sub-keV quanta and also by radiation absorption in cold magnetized plasma tails forming in the direction of magnetic compression. (10.1063/1.4817023)
  DOI : 10.1063/1.4817023
• Various Coiled Magnetoimpedance Based on Differential Magnetic Permeability Variation
  
  • Moutoussamy Joël
  • Coillot C.
  • Chanteur Gérard
  • Alves Francisco
  Sensor letters, American Scientific Publishers, 2013, 11 (1), pp.40-43. The present work is focused on high sensitivity (5000 V/T) transverse coiled GMI transducers manufactured with various magnetic materials, magnetically excited at low frequencies (f < 50 kHz) by an insulated coil which is also used for the measurement of the DC and low frequency magnetic field. The impedance and the sensitivity of different types of ferromagnetic material as nanocrystalline ribbons (Finemet) annealed under longitudinal and transverse magnetic fields, as mumetal ribbons and Mn–Zn thin ferrite core are investigated with respect to the static magnetic field. From these results, the differential magnetic permeability is approximated in order to predict sensitivity behavior in relation to the anisotropy magnetic field and the magnitude and the frequency of current excitation. (10.1166/sl.2013.2800)
  DOI : 10.1166/sl.2013.2800
• Solar wind turbulence: the fight between the direct turbulent cascade and the (anisotropic) expansion
  
  • Grappin Roland
  , 2013.
• Building small scales in MHD turbulence
  
  • Verdini Andrea
  • Grappin Roland
  • Pinto Rui
  • Velli Marco
  , 2013, 1539, pp.74-77. Magneto-hydrodynamic turbulence (MHD) with a mean large-scale field is known to produce an anisotropic cascade, with energy mostly in perpendicular scales. We use a shell-model version of the Reduced MHD equations to simulate turbulence in homogeneous periodic conditions, in coronal loops, and in the solar wind. We compare the perpendicular and parallel spectra and show that different regimes of weak turbulence develop in loops and in the solar wind. We briefly comment on the way their characteristic large-scale features influence the weak turbulence spectra and their transition to strong turbulence. (10.1063/1.4810993)
  DOI : 10.1063/1.4810993
• Goniopolarimetry with Coupled Electric and Magnetic Measurements
  
  • Cecconi B.
  • Gautier A.-L.
  • Bergman J.E.S
  • Chust Thomas
  • Marchaudon A.
  • Cavoit C.
  • Santolík O.
  , 2013.
• Plasma/surface interaction: example of air plasmas and plasma bullets
  
  • Guaitella Olivier
  , 2013.
• Cavitation in the vicinity of the high-voltage electrode as a key step of nanosecond breakdown in liquids
  
  • Marinov Ilya
  • Guaitella Olivier
  • Rousseau Antoine
  • Starikovskaia Svetlana
  Plasma Sources Science and Technology, IOP Publishing, 2013, 22, pp.042001. Fast shadowgraphy of nanosecond discharge in liquids with different dielectric permittivity, namely in water, ethanol and n-pentane, has been performed. Formation of a gas cavity at a nanosecond time scale was observed as a pre-breakdown phenomenon at amplitudes of the high-voltage pulse close to the breakdown threshold. This phenomenon is considered as a possible key step of high-voltage breakdown in polar liquids. (10.1088/0963-0252/22/4/042001)
  DOI : 10.1088/0963-0252/22/4/042001
• Negative ion extraction from hydrogen plasma bulk
  
  • Oudini N.
  • Taccogna F.
  • Minelli P.
  • Aanesland Ane
  • Raimbault Jean-Luc
  Physics of Plasmas, American Institute of Physics, 2013, 20 (10), pp.103506. A two-dimensional particle-in-cell/Monte Carlo collision model has been developed and used to study low electronegative magnetized hydrogen plasma. A configuration characterized by four electrodes is used: the left electrode is biased at Vl&#8201;=&#8201;&#8722;100&#8201;V, the right electrode is grounded, while the upper and lower transversal electrodes are biased at an intermediate voltage Vud between 0 and &#8722;100&#8201;V. A constant and homogeneous magnetic field is applied parallel to the lateral (left/right) electrodes. It is shown that in the magnetized case, the bulk plasma potential is close to the transversal electrodes bias inducing then a reversed sheath in front of the right electrode. The potential drop within the reversed sheath is controlled by the transversal electrodes bias allowing extraction of negative ions with a significant reduction of co-extracted electron current. Furthermore, introducing plasma electrodes, between the transversal electrodes and the right electrode, biased with a voltage just above the plasma bulk potential, increases the negative ion extracted current and decreases significantly the co-extracted electron current. The physical mechanism on basis of this phenomenon has been discussed. (10.1063/1.4825246)
  DOI : 10.1063/1.4825246
• Radio-frequency capacitively coupled plasmas excited by tailored voltage waveforms: comparison of experiment and particle-in-cell simulations
  
  • Delattre Pierre-Alexandre
  • Lafleur Trevor
  • Johnson Erik
  • Booth Jean-Paul
  Journal of Physics D: Applied Physics, IOP Publishing, 2013, 46, pp.235201. Using a range of different diagnostics we have performed a detailed experimental characterization of a capacitively coupled rf plasma discharge excited by tailored voltage waveforms in argon (3?13 Pa). The applied pulse-type tailored waveforms consist of between 1 and 5 harmonics (with a fundamental of 15 MHz), and are used to generate an electrically asymmetric plasma response, manifested by the formation of a strong dc bias in the geometrically symmetric reactor used. Experimental measurements of the dc bias, electron density, ion current density, ion-flux energy distributions at the electrodes and discharge current waveforms, are compared with a one-dimensional particle-in-cell simulation for the same operating conditions. The experimental and simulation results are found to be in good agreement over the range of parameters investigated, and demonstrate a number of unique features present with pulse-type tailored waveforms, including: increased plasma density and ion flux with the number of harmonics, and a broader control range of the ion bombarding energy. (10.1088/0022-3727/46/23/235201)
  DOI : 10.1088/0022-3727/46/23/235201
• Direct observation of ozone formation on SiO<SUB>2</SUB> surfaces in O<SUB>2</SUB> discharges
  
  • Marinov Daniil
  • Guaitella Olivier
  • Booth Jean-Paul
  • Rousseau Antoine
  Journal of Physics D: Applied Physics, IOP Publishing, 2013, 46, pp.032001. Ozone production is studied in a pulsed O2 discharge at pressures in the range 1.36.7 mbar. Time-resolved absolute concentrations of O3 and O are measured in the post-discharge using UV absorption spectroscopy and two-photon absorption laser-induced fluorescence. In a bare silica discharge tube ozone is formed mainly by three-body gas-phase recombination. When the tube surface is covered by a high specific surface silica catalyst heterogeneous formation becomes the main source of ozone. The efficiency of this surface process increases with O2 pressure and is favoured by the presence of OH groups and adsorbed H2O on the surface. At p = 6.7 mbar ozone production accounts for up to 25% of the atomic oxygen losses on the surface. (10.1088/0022-3727/46/3/032001)
  DOI : 10.1088/0022-3727/46/3/032001
• Isopropanol saturated TiO<SUB>2</SUB> surface regeneration by non-thermal plasma : Influence of air relative humidity
  
  • Sivachandiran Loganathan
  • Thévenet Frédéric
  • Gravejat Paul
  • Rousseau Antoine
  Chemical Engineering Journal, Elsevier, 2013, 214, pp.17-26. Environmental regulation on air quality requires the development of energetic efficient volatile organic compounds (VOCs) abatement techniques. Adsorption, photocatalysis, non-thermal plasma and their combinations have been widely studied for VOC treatment. Even if the plasma material (sorbent or catalyst) association appears as one of the most efficient configuration for VOC removal, it mainly consists in operating continuously the discharge on the material surface as long as the effluent flows across the reactor. This work aims at investigating another approach of plasma material association for VOC removal: in a first step, the material is used as a sorbent until the complete coverage of adsorption sites; in a second step, once VOC saturation is achieved, the discharge is ignited on the material surface. During both steps, the influence of air relative humidity (RH) is investigated in order to evaluate its impact on the process. The objectives of our approach are: (i) the reduction of energy consumption; (ii) the increase of sorbent life-times by efficient regeneration; (iii) the investigation of plasma interaction with VOC saturated materials; (iv) the investigation of air RH influence on such VOC treatment process. A packed bed reactor coated with TiO2 has been designed. IPA is used as a model VOC. First, injected power in the packed-bed reactor is characterized as a function of air RH. Complete coverage of TiO2 surface over 35% RH is suggested as a significant parameter. Then, adsorption of IPA on TiO2 was monitored until IPA breakthrough. The amount of IPA adsorbed per TiO2 surface unit is compared to values reported by other authors. The influence of air RH on reversibly and irreversibly adsorbed IPA fractions is investigated. Over 35% RH irreversible adsorption is favored, adsorption modes are discussed. Plasma regeneration of IPA saturated TiO2 surface leads simultaneously to IPA desorption and mineralization. Increasing air RH favors IPA mineralization and diminishes acetone production. Carbon balance obtained after 1 h plasma treatment reaches 91% in the presence of 50% RH. A thermal treatment is performed after each plasma treatment in order to evidence plasma insensitive adsorbed species and to restore TiO2 initial surface state. 97% of the carbon balance is collected under 50% RH after thermal treatment. During the thermal step, acetone and CO2 are mainly produced, their formation pathways are discussed. (10.1016/j.cej.2012.10.022)
  DOI : 10.1016/j.cej.2012.10.022
• ECH effects on toroidal rotation: KSTAR experiments, intrinsic torque modelling and gyrokinetic stability analyses
  
  • Shi Y. J.
  • Ko W. H.
  • Kwon J.M.
  • Diamond P.H.
  • Lee S. G.
  • Ko S. H.
  • Wang L.
  • Yi Sukyoung
  • Ida K.
  • Terzolo L.
  • Yoon S. W.
  • Lee K. D.
  • Lee J. H.
  • Nam U. N.
  • Bae Y. S.
  • Oh Y. K.
  • Kwak J. G.
  • Bitter M.
  • Hill K.
  • Gürcan Özgür D.
  • Hahm T.S.
  Nuclear Fusion, IOP Publishing, 2013, 53 (11). Toroidal rotation profiles have been investigated in KSTAR H-mode plasma using combined auxiliary heating by co-neutral beam injection (NBI) and electron cyclotron resonance heating (ECH). The ion temperature and toroidal rotation are measured with x-ray imaging crystal spectroscopy and charge exchange recombination spectroscopy. H-mode plasma is achieved using co-current 1.3MW NBI, and a 0.35MW ECH pulse is added to the flat-top of H-mode. The core rotation profiles, which are centrally peaked in the pure NBI heating phase, flatten when ECH is injected, while the edge pedestal is unchanged. Dramatic decreases in the core toroidal rotation values (Delta V-tor/V-tor similar to -30%) are observed when on-axis ECH is added to H-mode. The experimental data show that the decrease of core rotation velocity and its gradient are correlated with the increase of core electron temperature and its gradient, and also with the likely steepening of the density gradient. We thus explore the viability of a hypothesized ITG (ITG ion temperature gradient instability) -> TEM (trapped electron mode instability) transition as the explanation of the observed counter-current flow induced by ECH. However, the results of linear microstability analyses using inferred profiles suggest that the TEM is excited only in the deep core, so the viability of the hypothesized explanation is not yet clear. (10.1088/0029-5515/53/11/113031)
  DOI : 10.1088/0029-5515/53/11/113031
• Dynamics of tilted eddies in a transversal flow at the edge of tokamak plasmas and the consequences for L-H transition
  
  • Fedorczak N.
  • Ghendrih Philippe
  • Hennequin Pascale
  • Tynan G.R.
  • Diamond P.H.
  • Manz P.
  Plasma Physics and Controlled Fusion, IOP Publishing, 2013, 55 (12), pp.124024. The dynamical interaction between eddies and shear flow is investigated through a simplified model of vorticity conservation with tilted eddies. Energy is transferred either to the flow or to eddies, depending on the eddy tilt with respect to the flow shear. When eddies are tilted in the shear direction, the system is favorable to shear increase: tilt instability (TI) or the negative viscosity phenomenon. When eddies are tilted in the opposite direction, the shear flow is damped via a Kelvin-Helmholtz (KH) process. The TI generally dominates the interaction on the largest radial scale, but a fraction of the energy cascades to smaller radial scales through the alternation of tilting and KH dynamics. Within this eddy description, we show that the symmetry breaking required to generate a net residual stress is set by the intrinsic eddy tilt. We recall that magnetic shear can provide an intrinsic tilt to ballooning modes at the edge of tokamak plasmas, with an orientation which depends on flux surface geometry. In L-mode weak shear regimes, this residual stress can dominate the Reynolds stress. Coupled to momentum sources acting in the scrape-off layer, it can induce a significant difference of the edge radial electric field between lower single null and upper single null geometries. A comparison with experimental profiles measured across the edge of Tore Supra L-mode plasmas is discussed. (10.1088/0741-3335/55/12/124024)
  DOI : 10.1088/0741-3335/55/12/124024