Publications

2015

• Electron-less negative ion extraction from ion-ion plasmas
  
  • Rafalskyi D.V.
  • Aanesland Ane
  Applied Physics Letters, American Institute of Physics, 2015, 106 (10), pp.104101. This paper presents experimental results showing that continuous negative ion extraction, without co-extracted electrons, is possible from highly electronegative SF6 ion-ion plasma at low gas pressure (1 mTorr). The ratio between the negative ion and electron densities is more than 3000 in the vicinity of the two-grid extraction and acceleration system. The measurements are conducted by both magnetized and non-magnetized energy analyzers attached to the external grid. With these two analyzers, we show that the extracted negative ion flux is almost electron-free and has the same magnitude as the positive ion flux extracted and accelerated when the grids are biased oppositely. The results presented here can be used for validation of numerical and analytical models of ion extraction from ion-ion plasma. (10.1063/1.4914507)
  DOI : 10.1063/1.4914507
• Wave-number spectrum of dissipative drift waves and a transition scale
  
  • Ghantous K.
  • Gürcan Özgür D.
  Physical Review E, American Physical Society (APS), 2015, 92 (3), pp.033107. We study the steady state spectrum of the Hasegawa-Wakatani (HW) equations that describe drift wave turbulence. Beyond a critical scale kc, which appears as a balance between the nonlinear time and the parallel conduction time, the adiabatic electron response breaks down nonlinearly and an internal energy density spectrum of the form F(k⊥)∝k−3⊥, associated with the background gradient, is established. More generally a dual power law spectrum, approximately of the form F(k⊥)∝k−3⊥(k−2c k−2⊥) is obtained, which captures this transition. Using dimensional analysis, an expression of the form kc∝C/κ is derived for the transition scale, where C and κ are normalized parameters of the HW equations signifying the electron adiabaticity and the density gradient, respectively. The results are numerically confirmed using a shell model developed and used for the Hasegawa-Wakatani system. (10.1103/PhysRevE.92.033107)
  DOI : 10.1103/PhysRevE.92.033107
• Ignition of methane and n-butane containing mixtures at high pressures by pulsed nanosecond discharge
  
  • Boumehdi M.A.
  • Stepanyan S.A.
  • Vanhove Guillaume
  • Desgroux Pascale
  • Starikovskaia Svetlana
  Combustion and Flame, Elsevier, 2015, 162, pp.1336-1349. A novel experimental scheme to study the ignition of combustible mixtures at high pressures under the action of a high-voltage nanosecond discharge has been developed. The experiments were performed in the combustion chamber of a Rapid Compression Machine (RCM) with a specially designed system of electrodes. A nanosecond surface dielectric barrier discharge (SDBD) provided two-dimensional low-temperature non-equilibrium plasma in the vicinity of the end plate of the combustion chamber. Radially symmetric plasma channels triggered multi-point ignition of gas mixtures at controlled pressure and temperature. Ignition delay times and energies deposited in the gaseous mixtures by the discharge were measured for different parameters of high voltage pulse, for positive or negative high-voltage pulses. The propagation of the subsequent flame in the combustion chamber was recorded with the help of high repetition rate imaging. Preliminary numerical analysis of the ignition under the action of a pulsed nanosecond discharge has been made; it was shown that production of atomic oxygen by the discharge, will modify the ignition chemistry by perturbation of the radical pool. Experiments and calculations were performed in methaneoxygen and n-butaneoxygen mixtures with equivalence ratios between 0.3 and 1 diluted by 7076% of Ar or nitrogen for temperatures between 600 and 1000 K and pressures between 6 and 16 bar. (10.1016/j.combustflame.2014.11.006)
  DOI : 10.1016/j.combustflame.2014.11.006
• TALIF measurements of oxygen atom density in the afterglow of a capillary nanosecond discharge
  
  • Klochko A.V.
  • Lemainque J.
  • Booth Jean-Paul
  • Starikovskaia Svetlana
  Plasma Sources Science and Technology, IOP Publishing, 2015, 24 (2), pp.025010. The atomic oxygen density has been measured in the afterglow of a capillary nanosecond discharge in 2430 mbar synthetic air (N 2  : O 2 = 4 : 1) by the two-photon absorption laser-induced fluorescence (TALIF) technique, combined with absolute calibration by comparison with xenon TALIF. The discharge was initiated by a train of 30 ns FWHM pulses of alternating positivenegativepositive polarity, separated by 250 ns, with a train repetition frequency of 10 Hz. The amplitude of the first pulse was 10 kV in the cable. A flow of synthetic air through the tube provided complete gas renewal between pulse trains. The O-atom density measurements were made over the time interval200 ns2 µ s after the initial pulse. The gas temperature was determined by analysis of the molecular nitrogen second positive system optical emission spectrum. The influence of the gas temperature on the atom density measurements, and the reactions producing O atoms, are discussed. (10.1088/0963-0252/24/2/025010)
  DOI : 10.1088/0963-0252/24/2/025010
• Gas temperature measurements in oxygen plasmas by high-resolution Two-Photon Absorption Laser-induced Fluorescence
  
  • Booth Jean-Paul
  • Marinov Daniil
  • Foucher Mickaël
  • Guaitella Olivier
  • Bresteau D.
  • Cabaret Louis
  • Drag Cyril
  Journal of Instrumentation, IOP Publishing, 2015, 10 (11), pp.C11003. One of the most important, and difficult to measure, parameters of laboratory discharges in molecular gases is the gas translational temperature. We propose a novel technique to measure directly, with excellent spatial and temporal resolution, the velocity distribution of ground-state atoms (oxygen atoms in this case) in plasmas from the Doppler broadening of their laser excitation spectra. The method is based on the well-known Two-Photon Laser-induced Fluorescence (TALIF) technique, but uses a specially-built pulsed tunable ultraviolet laser with very narrow bandwidth which allows the Doppler profiles to be measured with high precision. This laser consists of a pulsed Nd:YAG-pumped Ti:Sapphire ring cavity which is injection-seeded by a single-mode cw Ti:sapphire laser. The single-mode infrared output pulses are frequency quadrupled by two non-linear crystals to reach the necessary UV wavelength (226 nm, 0.2 mJ) for TALIF excitation. This technique should be applicable to a wide range of discharges, ranging from low-pressure RF plasmas for surface processing to atmospheric pressure plasmas. Results of preliminary tests on low-pressure O 2 DC discharges are presented. (10.1088/1748-0221/10/11/C11003)
  DOI : 10.1088/1748-0221/10/11/C11003
• Proof-of-concept demonstration of the PEGASES plasma thruster
  
  • Lafleur Trevor
  • Rafalskyi D.V.
  • Chabert Pascal
  • Aanesland Ane
  International Electric Propulsion Conference, 2015, pp.IEPC-2015-114/ISTS-2015-b-114. Here we experimentally demonstrate the working principle of a gridded plasma thrusterthat alternatively extracts and accelerates both positive and negative ions to generate thrust. The plasma is created in an inductively coupled plasma source, and negativeion formation is enhanced by cooling electrons using a magnetic filter, which creates an almost electron-free plasma region near the source exit. By then applying square voltage waveforms with frequencies between 20-950kHz, positive and negative ions are extractedand accelerated to high energies (100?s of eV). Downstream measurements show that at sufficiently large frequencies the ion beams can be well neutralized. The behaviour of the measured ion current with frequency is explained with an analytical model which extends the Child-Langmuir law to AC ion acceleration.
• Phase imaging microscopy for the diagnostics of plasma-cell interaction
  
  • Ohene Yolanda
  • Marinov Ilya
  • de Laulanié Lucie
  • Dupuy Corinne
  • Wattelier Benoit
  • Starikovskaia Svetlana
  Applied Physics Letters, American Institute of Physics, 2015, 106. Phase images of biological specimens were obtained by the method of Quadriwave Lateral Shearing Interferometry (QWLSI). The QWLSI technique produces, at high resolution, phase images of the cells having been exposed to a plasma treatment and enables the quantitative analysis of the changes in the surface area of the cells over time. Morphological changes in the HTori normal thyroid cells were demonstrated using this method. There was a comparison of the cell behaviour between control cells, cells treated by plasma of a nanosecond dielectric barrier discharge, including cells pre-treated by catalase, and cells treated with an equivalent amount of H<SUB>2</SUB>O<SUB>2</SUB>. The major changes in the cell membrane morphology were observed at only 5 min after the plasma treatment. The primary role of reactive oxygen species (ROS) in this degradation is suggested. Deformation and condensation of the cell nucleus were observed 2-3 h after the treatment and are supposedly related to apoptosis induction. The coupling of the phase QWLSI with immunofluorescence imaging would give a deeper insight into the mechanisms of plasma induced cell death. (10.1063/1.4922525)
  DOI : 10.1063/1.4922525
• Properties of Jupiter's Magnetospheric Turbulence Observed by the Galileo Spacecraft
  
  • Tao Chihiro
  • Sahraoui Fouad
  • Fontaine Dominique
  • de Patoul Judith
  • Chust Thomas
  • Kasahara S.
  • Retinò Alessandro
  , 2015.
• Advanced Ion Mass Spectrometer
  
  • Sittler E.C.
  • Cooper J.F.
  • Paschalidis N.
  • Jones S.
  • Rodriguez M.
  • Ali A.
  • Coplan M.A.
  • Chornay D.
  • Sturners S.J.
  • Brown S.
  • Bateman F.B.
  • Fontaine Dominique
  • Verdeil Christophe
  • André N.
  • Federov A.
  • Wurz Peter
  , 2015.
• 3-D PIC Numerical Investigations of a Novel Concept of Multistage Axial Vircator for Enhanced Microwave Generation
  
  • Champeaux Stéphanie
  • Gouard Philippe
  • Cousin Richard
  • Larour Jean
  IEEE Transactions on Plasma Science, Institute of Electrical and Electronics Engineers, 2015, 43 (11), pp.3841-3855. The enhancement of power conversion efficiency of a classical axial VIRtual CAthode oscillaTOR (vircator) by introducing one or more reflectors beyond the anode in the cylindrical waveguide is numerically investigated. The targeted microwave (MW) output frequency lies in the S-band at around 3 GHz for an operation in TM01 mode. Powered by a 511-kV voltage signal for a duration of 45 ns, the design under consideration operates with an injected electron beam of mean voltage and mean current of around 508 kV and 19 kA, respectively. Full-wave 3-D modeling is performed using well-tested electromagnetic particle-in-cell codes such as Computer Simulation Technology Particle Studio and Magic. Simple rules for designing and installing the reflectors are given. The number of reflectors required to maximize the efficiency is discussed. The power conversion efficiency is shown to be improved over a classical axial vircator design by a factor of 12.8. A maximum mean output power of about 1.26 GW is delivered off-axis in the S-band at around 3 GHz, with an efficiency of nearly 13%. Besides, it is also shown that increasing the number of reflectors allows switching the operation mode from TM01 to TE11 along with a shift of MWfrequency from the S- to the L-band. A five- or six-reflector configuration is predicted to generate MW at both 2.86 and 1.4 GHz with conversion efficiencies ranging from 3.5% to 6.6%. A vircator including seven reflectors is expected to operate in TE11 mode at 1.4 GHz with an efficiency of about 8%. (10.1109/TPS.2015.2477561)
  DOI : 10.1109/TPS.2015.2477561
• Formation of self-organized shear structures in thin current sheets
  
  • Malova H. V.
  • Mingalev O. V.
  • Grigorenko E. E.
  • Mingalev I. V.
  • Melnik M. N.
  • Popov V. Y.
  • Delcourt Dominique C.
  • Petrukovich A. A.
  • Shen C.
  • Rong Z. J.
  • Zelenyi L. M.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015, 120, pp.4802-4824. Self-consistent kinetic (particle-in-cell) model of magnetotail thin current sheet (TCS) is used to understand the formation of self-consistent sheared magnetic structures. It is shown that shear configurations appear in TCS as a result of self-consistent evolution of some initial magnetic perturbation at the current sheet center. Two general shapes of shear TCS components are found as a function of the transverse coordinate: symmetric and antisymmetric. We show that TCS formation goes together with the emergence of field-aligned currents in the center of the current sheet, as a result of north-south asymmetry of quasi-adiabatic ion motions. Ion drift currents can also contribute to the magnetic shear evolution, but their role is much less significant, their contribution depending upon the normal component B<SUB>z</SUB> and the amplitude of the initial perturbation in TCS. Parametric maps illustrating different types of TCS equilibria are presented that show a higher probability of formation of symmetric shear TCS configuration at lower values of the normal magnetic component. (10.1002/2014JA020974)
  DOI : 10.1002/2014JA020974
• Plasma Sources in Planetary Magnetospheres: Mercury
  
  • Raines J. M.
  • Dibraccio G. A.
  • Cassidy T. A.
  • Delcourt Dominique C.
  • Fujimoto M.
  • Jia X.
  • Mangano V.
  • Milillo A.
  • Sarantos M.
  • Slavin J. A.
  • Wurz Peter
  Space Science Reviews, Springer Verlag, 2015, 192, pp.91-144. Not Available (10.1007/s11214-015-0193-4)
  DOI : 10.1007/s11214-015-0193-4
• Magnetopause orientation: Comparison between generic residue analysis and BV method,
  
  • Dorville Nicolas
  • Haaland S.
  • Anekallu C.
  • Belmont Gérard
  • Rezeau Laurence
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015 (120). Determining the direction normal to the magnetopause layer is a key step for any study of this boundary. Various techniques have been developed for this purpose. We focus here on generic residue analysis (GRA) methods, which are based on conservation laws, and the new iterative BV method, where B represents the magnetic field and V refers to the ion velocity. This method relies on a fit of the magnetic field hodogram against a modeled geometrical shape and on the way this hodogram is described in time. These two methods have different underlying model assumptions and validity ranges. We compare here magnetopause normals predicted by BV and GRA methods to better understand the sensitivity of each method on small departures from its own physical hypotheses. This comparison is carried out first on artificial data with magnetopause-like noise. Then a statistical study is carried out using a list of 149 flank and dayside magnetopause crossings from Cluster data where the BV method is applicable, i.e., where the magnetopause involves a single-layer current sheet, with a crudely C-shaped magnetic hodogram. These two comparisons validate the quality of the BV method for all these cases where it is applicable. The method provides quite reliable normal directions in all these cases, even when the boundary is moving with a varying velocity, which distorts noticeably the results of most of the other methods. (10.1002/2014JA020806)
  DOI : 10.1002/2014JA020806
• TEC variations along an East Euro-African chain during 5th april 2010 geomagnetic storm
  
  • Shimeis Amira
  • Borries Claudia
  • Amory-Mazaudier Christine
  • Fleury Rolland
  • Mahrous Ayman Mohamed
  • Hassan A.F.
  • Nawar Samir
  Advances in Space Research, Elsevier, 2015, 55 (9), pp.2239-2247. In this paper, we analyzed the variations of TEC along a latitudinal East Euro-African chain, during the storm of April 5, 2010. We observed a large asymmetry between the two hemispheres. We detected the presence of a TID in the Northern hemisphere on April 5. The propagation time of the TID from high to low latitudes and the speed of the TID was determined. On April 5, 6 and 7, we observed a decrease of the TEC and changes of the NO+ in the Northern hemisphere. This depletion is caused by the large-scale thermospheric wind disturbances due to Joule heating dissipation in the auroral zone. (10.1016/j.asr.2015.01.005)
  DOI : 10.1016/j.asr.2015.01.005
• Bandwidths and amplitudes of chorus-like banded emissions measured by the TC-1 Double Star spacecraft
  
  • Macusova E.
  • Santolík O.
  • Cornilleau-Wehrlin Nicole
  • Yearby K. H.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015, 120, pp.1057-1071. Characteristics of banded whistler-mode emissions are derived from a database of chorus-like events obtained from the complete data set of the wave measurements provided by the Spatio-Temporal Analysis of Field Fluctuation-Digital Wave Processing (STAFF-DWP) wave instrument on board the TC-1 Double Star spacecraft. Our study covers the full operational period of this spacecraft (almost 4 years). Our entire data set has been collected within 30° of geomagnetic latitude at L shells between 2 and 12 and below 4 kHz. All events have been processed automatically to accurately determine their power spectral density (PSD), bandwidth, and amplitude. We found most cases of chorus-like banded emissions at L<=10 on the dawnside and dayside. The upper band emissions (above one half of the equatorial electron cyclotron frequency) occur almost 20 times less often than the lower band, and their average amplitude is almost 3 times smaller than for the lower band. Intense upper band emissions cover smaller L shell, magnetic local time (MLT), and magnetic latitudes regions than intense lower band emissions. The intense nightside and dawnside chorus-like banded emissions were observed at low magnetic latitudes, while the intense dayside and duskside emissions were mostly found at higher magnetic latitudes. The amplitudes of dayside lower band waves slightly increase as they propagate away from the geomagnetic equator and are smaller than chorus amplitudes on nightside and dawnside. The PSD, the amplitude of the lower band, its frequency bandwidth, and its occurrence rate significantly increase with increasing geomagnetic activity, while all these parameters for the upper band are not so strongly dependent on the geomagnetic activity. (10.1002/2014JA020440)
  DOI : 10.1002/2014JA020440
• Nanosecond barrier discharge in a krypton /helium mixture containing mercury dibromide: Optical emission and plasma parameters
  
  • Malinina A. A.
  • Starikovskaia Svetlana
  • Malinin A. N.
  Optics and Spectroscopy, MAIK Nauka/Interperiodica, 2015, 118 (1), pp.26-36. Spectral and electrical characteristics of atmospheric-pressure nanosecond barrier discharge plasma in a HgBr2/Kr/He mixture have been investigated. The discharge was initiated by positive 10-kV voltage pulses with a rise time of 4 ns and a half-amplitude duration of 28 ns. Emission from exciplex HgBr (B2&#931;12/ &#8722; X2&#931;12/ ) and KrBr (B2&#931;12/ &#8722; X2&#931;12/ , C3/2&#8722;A&#928;1/2, D1/2&#8722;A&#928;1/2) molecules have been studied. From the time evolution of the B-X transition spectra of the HgBr molecule (502 nm) and KrBr molecule (207 nm), a mechanism of the formation of the exciplex molecules in the nanosecond discharge has been deduced. The distributions of the energies and rates of the processes responsible for emission from HgBr and KrBr molecules have been analyzed by numerically solving the Boltzmann equation for the electron distribution function. Experiments have confirmed the possibility of optimizing the voltage supply pulse for maximizing the efficiency of simultaneous emission in the UV and visible (green) spectral ranges from atmospheric-pressure discharge in the HgBr2/Kr/He mixture. (10.1134/S0030400X14120157)
  DOI : 10.1134/S0030400X14120157
• The science program of the TCV tokamak: exploring fusion reactor and power plant concepts
  
  • Coda S.
  • Tcv Team
  Nuclear Fusion, IOP Publishing, 2015, 55 (10), pp.104004. TCV is acquiring a new 1 MW neutral beam and 2 MW additional third-harmonic electron cyclotron resonance heating (ECRH) to expand its operational range. Its existing shaping and ECRH launching versatility was amply exploited in an eclectic 2013 campaign. A new sub-ms real-time equilibrium reconstruction code was used in ECRH control of NTMs and in a prototype shape controller. The detection of visible light from the plasma boundary was also successfully used in a position-control algorithm. A new bang-bang controller improved stability against vertical displacements. The RAPTOR real-time transport simulator was employed to control the current density profile using electron cyclotron current drive. Shot-by-shot internal inductance optimization was demonstrated by iterative learning control of the current reference trace. Systematic studies of suprathermal electrons and ions in the presence of ECRH were performed. The L?H threshold power was measured to be ?50?75% higher in both H and He than D, to increase with the length of the outer separatrix, and to be independent of the current ramp rate. Core turbulence was found to decrease from positive to negative edge triangularity deep into the core. The geodesic acoustic mode was studied with multiple diagnostics, and its axisymmetry was confirmed by a full toroidal mapping of its magnetic component. A new theory predicting a toroidal rotation component at the plasma edge, driven by inhomogeneous transport and geodesic curvature, was tested successfully. A new high-confinement mode (IN-mode) was found with an edge barrier in density but not in temperature. The edge gradients were found to govern the scaling of confinement with current, power, density and triangularity. The dynamical interplay of confinement and magnetohydrodynamic modes leading to the density limit in TCV was documented. The heat flux profile decay lengths and heat load profile on the wall were documented in limited plasmas. In the snowflake (SF) divertor configuration the heat flux profiles were documented on all four strike points. SF simulations with the EMC3-EIRENE code, including the physics of the secondary separatrix, underestimate the flux to the secondary strike points, possibly resulting from steady-state E × B drifts. With neon injection, radiation in a SF was 15% higher than in a conventional divertor. The novel triple-null and X-divertor configurations were also achieved in TCV. (10.1088/0029-5515/55/10/104004)
  DOI : 10.1088/0029-5515/55/10/104004
• Zonal flows and pattern formation
  
  • Gürcan Özgür D.
  • Diamond P.H.
  Journal of Physics A: Mathematical and Theoretical, IOP Publishing, 2015, 48 (29), pp.293001. The general aspects of zonal flow physics, their formation, damping and interplay with quasi two dimensional turbulence are explained in the context of magnetized plasmas and quasi-geostrophic fluids with an emphasis on formation and selection of spatial patterns. General features of zonal flows as they appear in planetary atmospheres, rotating convection experiments and fusion plasmas are reviewed. Detailed mechanisms for excitation and damping of zonal flows, and their effect on turbulence via shear decorrelation is discussed. Recent results on nonlocality and staircase formation are outlined. (10.1088/1751-8113/48/29/293001)
  DOI : 10.1088/1751-8113/48/29/293001
• Energy balance in surface nanosecond dielectric barrier discharge. Plasma-assisted ignition of heavy hydrocarbons at high pressures
  
  • Shcherbanev S.A.
  • Stepanyan S.A.
  • Boumehdi Mohamed
  • Vanhove Guillaume
  • Desgroux Pascale
  • Starikovskaia Svetlana
  , 2015, 29 (9), pp.6118 - 6125. The paper presents experimental study of nanosecond surface dielectric barrier discharge (nSDBD) in air and application of nSDBD for initiation of the two-stage ignition of n-heptane in mixture with air. The emission spectroscopy study of rotational and vibra-tional structure of 2 + system of N2 molecules has been performed for a classical air flow control SDBD configuration. The energy deposition into the discharge has been measured. Strongly non-equilibrium distribution of rotational population is observed in the spectra at the leading and trailing edges of the high-voltage pulse. The time resolved spatial distribution of the rotational temperature along the dielectric surface has been measured. The second part of the paper concerns the ignition of stoichiometric mixture of C7H16/O2/N2 with nanosecond SDBD in Rapid Compression Machine (RCM). Different regimes of ignition initiated by discharge are discussed, including cool flame and fast ignition. (10.2514/6.2015-0668)
  DOI : 10.2514/6.2015-0668
• Auroral Processes at the Giant Planets: Energy Deposition, Emission Mechanisms, Morphology and Spectra
  
  • Badman Sarah V.
  • Branduardi-Raymont Graziella
  • Galand Marina
  • Hess Sebastien
  • Krupp Norbert
  • Lamy Laurent
  • Melin Henrik
  • Tao Chihiro
  Space Science Reviews, Springer Verlag, 2015, 187 (1-4), pp.99-179. The ionospheric response to auroral precipitation at the giant planets is reviewed, using models and observations. The emission processes for aurorae at radio, infrared, visible, ultraviolet, and X-ray wavelengths are described, and exemplified using ground- and space-based observations. Comparisons between the emissions at different wavelengths are made, where possible, and interpreted in terms of precipitating particle characteristics or atmospheric conditions. Finally, the spatial distributions and dynamics of the various components of the aurorae (moon footprints, low-latitude, main oval, polar) are related to magnetospheric processes and boundaries, using theory, in situ, and remote observations, with the aim of distinguishing between those related to internally-driven dynamics, and those related to the solar wind interaction. (10.1007/s11214-014-0042-x)
  DOI : 10.1007/s11214-014-0042-x
• Investigation of the Chirikov resonance overlap criteria for equatorial magnetosonic waves
  
  • Walker S. N.
  • Balikhin M. A.
  • Canu Patrick
  • Cornilleau-Wehrlin Nicole
  • Moiseenko I.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015, 120, pp.8774-8781. Observations of equatorial magnetosonic waves made during the Cluster Inner Magnetospheric Campaign clearly show discrete spectra consisting of emissions around harmonics of the proton gyrofrequency. Equatorial magnetosonic waves are important because of their ability to efficiently scatter electrons in energy and pitch angle. This wave-particle interaction is numerically modeled through the use of diffusion coefficients, calculated based on a continuous spectrum such as that observed by spectrum analyzers. Using the Chirikov overlap resonance criterion, the calculation of the diffusion coefficient will be assessed to determine whether they should be calculated based on the discrete spectral features as opposed to a continuous spectrum. For the period studied, it is determined that the discrete nature of the waves does fulfill the Chirikov overlap criterion and so the use of quasi-linear theory with the assumption of a continuous frequency spectrum is valid for the calculation of diffusion coefficients. (10.1002/2015JA021718)
  DOI : 10.1002/2015JA021718
• Fourier spectrum and phases for a signal in a finite interval
  
  • Belmont Gérard
  • Dorville Nicolas
  • Sahraoui Fouad
  • Rezeau Laurence
  , 2015, 17, pp.5320. When investigating the physics of turbulent media, as the solar wind or the magnetosheath plasmas, obtaining accurate Fourier spectra and phases is a crucial issue. For the different fields, the spectra allow in particular verifying whether one or several power laws can be determined in different frequency ranges. Accurate phases are necessary as well for all the "higher order statistics" studies in Fourier space, the coherence ones and for the polarization studies. Unfortunately, the Fourier analysis is not unique for a finite time interval of duration T: the frequencies lower than 1/T have a large influence on the result, which can hardly be controlled. This unknown "trend" has in particular the effect of introducing jumps at the edges of the interval, for the function under study itself, as well as for all its derivatives. The Fourier transform obtained directly by FFT (Fast Fourier Transform) is generally much influenced by these effects and cannot be used without care for wide band signals. The interference between the jumps and the signal itself provide in particular characteristic "hairs" on the spectrum, which are clearly visible on it with df&#8776;1/T. These fluctuations are usually eliminated by smoothing the spectrum, or by averaging several successive spectra. Nevertheless, such treatments introduce uncertainties on the spectral laws (the phases being anyway completely lost). Windowing is also a method currently used to suppress or decrease the jumps, but it modifies the signal (the windowed trend has a spectrum, which is convolved with the searched one) and the phases are generally much altered. Here, we present a new data processing technique to circumvent these difficulties. It takes advantage of the fact that the signal is generally not unknown out of the interval under study: the complete signal is tapered to this interval of interest thanks to a new window function, sharp but not square. This window function is chosen such that the spectrum obtained can be deconvolved almost exactly, through a minimization procedure based on the -weak- hypothesis that it is smooth at the scale of a few successive spectral points. Then, a later step allows reconstructing the phases. Tests with synthetic data and first applications to Cluster data are presented, which demonstrate the capability of the method to better estimate the Fourier spectra.
• Edge-to-center density ratios in low-temperature plasmas
  
  • Lafleur Trevor
  • Chabert Pascal
  Plasma Sources Science and Technology, IOP Publishing, 2015, 24 (2), pp.025017. The ion flux leaving a plasma at a boundary can be given by: &#915; i = h L n 0 u B , where n 0 is the maximum central plasma density, u B is the Bohm velocity, and h L is the sheath edge-to-center plasma density ratio. Such h L factors have become synonymous with global modeling of plasma discharges, where they play a vital role in the prediction of plasma losses to bounding surfaces. By performing one-dimensional (1D) particle-in-cell simulations of inductively and capacitively coupled plasmas (ICPs and CCPs) over a wide pressure range, we explicitly test the validity of standard heuristic formulae commonly used to estimate h L . The ICP simulation results are found to be in very good agreement, while a large discrepancy is present for the CCP results at high pressures. The onset of this discrepancy is found to be correlated with the bulk-to-sheath edge ionization transition that occurs in CCPs at high pressures. Consequently, global models will strongly underestimate plasma losses in this regime. (10.1088/0963-0252/24/2/025017)
  DOI : 10.1088/0963-0252/24/2/025017
• Sub-nanosecond delays of light emitted by streamer in atmospheric pressure air: Analysis of N<SUB>2</SUB>(C<SUP>3</SUP>Pi<SUB>u</SUB>) and N<SUB>2</SUB><SUP>+</SUP>(B<SUP>2</SUP>Sigma<SUB>u</SUB><SUP>+</SUP>) emissions and fundamental streamer structure
  
  • Hoder Tomas
  • Bonaventura Zdenek
  • Bourdon Anne
  • Simek Milan
  Journal of Applied Physics, American Institute of Physics, 2015, 117, pp.073302. Theoretical analysis of ultra-short phenomena occurring during the positive streamer propagation in atmospheric pressure air is presented. Motivated by experimental results obtained with tens-of picoseconds and tens-of-microns precision, it is shown that when the streamer head passes a spatial coordinate, emission maxima from N2 and N2 radiative states follow with different delays. Thesedifferent delays are caused by differences in the dynamics of populating the radiative states, due to different excitation and quenching rates. Associating the position of the streamer head with the maximum value of the self-enhanced electric field, a delay of 160 ps was experimentally found for the peak emission of the first negative system of N2 . A delay dilatation was observed experimentally on early-stage streamers and the general mechanism of this phenomenon is clarified theoretically. In the case of the second positive system of N2, the delay can reach as much as 400 ps. In contrast to the highly nonlinear behavior of streamer events, it is shown theoretically that emission maximum delays linearly depend on the ratio of the streamer radius and its velocity. This is found to be one of the fundamental streamer features and its use in streamer head diagnostics is proposed. Moreover,radially resolved spectra are synthesized for selected subsequent picosecond moments in order to visualize spectrometric fingerprints of radial structures of N2(C3Piu) and N2 (B2Sigma u) populations created by streamer-head electrons (10.1063/1.4913215)
  DOI : 10.1063/1.4913215
• The Earth: Plasma Sources, Losses, and Transport Processes
  
  • Welling D. T.
  • André M.
  • Dandouras Iannis
  • Delcourt Dominique C.
  • Fazakerley A.
  • Fontaine Dominique
  • Foster John
  • Ilie R.
  • Kistler L. M.
  • Lee J. H.
  • Liemohn M. W.
  • Slavin J. A.
  • Wang Chih-Ping
  • Wiltberger M.
  • Yau Andrew
  Space Science Reviews, Springer Verlag, 2015, 192 (1-4), pp.145-208. This paper reviews the state of knowledge concerning the source of magnetospheric plasma at Earth. Source of plasma, its acceleration and transport throughout the system, its consequences on system dynamics, and its loss are all discussed. Both observational and modeling advances since the last time this subject was covered in detail (Hultqvist et al., Magnetospheric Plasma Sources and Losses, 1999) are addressed. (10.1007/s11214-015-0187-2)
  DOI : 10.1007/s11214-015-0187-2