Publications

2015

• Distribution of energetic oxygen and hydrogen in the near-Earth plasma sheet
  
  • Kronberg E. A.
  • Grigorenko E. E.
  • Haaland S. E.
  • Daly P. W.
  • Delcourt Dominique C.
  • Luo H.
  • Kistler L. M.
  • Dandouras I.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015, 120 (5), pp.3415-3431. The spatial distributions of different ion species are useful indicators for plasma sheet dynamics. In this statistical study based on 7 years of Cluster observations, we establish the spatial distributions of oxygen ions and protons at energies from 274 to 955 keV, depending on geomagnetic and solar wind (SW) conditions. Compared with protons, the distribution of energetic oxygen has stronger dawn-dusk asymmetry in response to changes in the geomagnetic activity. When the interplanetary magnetic field (IMF) is directed southward, the oxygen ions show significant acceleration in the tail plasma sheet. Changes in the SW dynamic pressure (P<SUB>dyn</SUB>) affect the oxygen and proton intensities in the same way. The energetic protons show significant intensity increases at the near-Earth duskside during disturbed geomagnetic conditions, enhanced SW P<SUB>dyn</SUB>, and southward IMF, implying there location of effective inductive acceleration mechanisms and a strong duskward drift due to the increase of the magnetic field gradient in the near-Earth tail. Higher losses of energetic ions are observed in the dayside plasma sheet under disturbed geomagnetic conditions and enhanced SW P<SUB>dyn</SUB>. These observations are in agreement with theoretical models. (10.1002/2014JA020882)
  DOI : 10.1002/2014JA020882
• Predator-prey model for the self-organization of stochastic oscillators in dual populations.
  
  • Moradi S.
  • Anderson J.
  • Gürcan Özgür D.
  Physical Review E, American Physical Society (APS), 2015, 92, pp.06293. A predator-prey model of dual populations with stochastic oscillators is presented. A linear cross-coupling between the two populations is introduced following the coupling between the motions of a Wilberforce pendulum in two dimensions: one in the longitudinal and the other in torsional plain. Within each population a Kuramoto-type competition between the phases is assumed. Thus, the synchronization state of the whole system is controlled by these two types of competitions. The results of the numerical simulations show that by adding the linear cross-coupling interactions predator-prey oscillations between the two populations appear, which results in self-regulation of the system by a transfer of synchrony between the two populations. The model represents several important features of the dynamical interplay between the drift wave and zonal flow turbulence in magnetically confined plasmas, and a novel interpretation of the coupled dynamics of drift wave-zonal flow turbulence using synchronization of stochastic oscillator is discussed. (10.1103/PhysRevE.92.062930)
  DOI : 10.1103/PhysRevE.92.062930
• Direct identification of predator-prey dynamics in gyrokinetic simulations
  
  • Kobayashi Sumire
  • Gürcan Özgür D.
  • Diamond P.H.
  Physics of Plasmas, American Institute of Physics, 2015, 22 (9), pp.090702. The interaction between spontaneously formed zonal flows and small-scale turbulence in nonlinear gyrokinetic simulations is explored in a shearless closed field line geometry. It is found that when clear limit cycle oscillations prevail, the observed turbulent dynamics can be quantitatively captured by a simple Lotka-Volterra type predator-prey model. Fitting the time traces of full gyrokinetic simulations by such a reduced model allows extraction of the model coefficients. Scanning physical plasma parameters, such as collisionality and density gradient, it was observed that the effective growth rates of turbulence (i.e., the prey) remain roughly constant, in spite of the higher and varying level of primary mode linear growth rates. The effective growth rate that was extracted corresponds roughly to the zonal-flow-modified primary mode growth rate. It was also observed that the effective damping of zonal flows (i.e., the predator) in the parameter range, where clear predator-prey dynamics is observed, (i.e., near marginal stability) agrees with the collisional damping expected in these simulations. This implies that the Kelvin-Helmholtz-like instability may be negligible in this range. The results imply that when the tertiary instability plays a role, the dynamics becomes more complex than a simple Lotka-Volterra predator prey. (10.1063/1.4930127)
  DOI : 10.1063/1.4930127
• Turbulence elasticity: a key concept to a unified paradigm of L -> I -> H transition
  
  • Guo Z. B.
  • Diamond P.H.
  • Kosuga Y.
  • Gürcan Özgür D.
  Nuclear Fusion, IOP Publishing, 2015, 55 (4), pp.043022. We present a theory of turbulence elasticity, which follows from delayed response of drift waves (DWs) to zonal flow (ZF) shears. It is shown that when |&#12296;V&#12297;'ZF|/&#916;&#969;k &#8805; 1, with |&#12296;V&#12297;'ZF| the ZF shearing rate and &#916;&#969;k the local turbulence decorrelation rate, the ZF evolution equation is converted from a diffusion equation to a telegraph equation. This insight provides a natural framework for understanding temporally periodic ZF structures, e.g., propagation of the ZF/turbulence intensity fronts. Furthermore, by incorporating the elastic property of the DWZF turbulence, we propose a unified paradigm of low-confinement-mode to intermediate-confinement-mode to high-confinement-mode (L &#8594; I &#8594; H) transitions. In particular, we predict the onset and termination conditions of the limit cycle oscillations, i.e. the I-mode. The transition from an unstable L-mode to I-mode is predicted to occur when &#916;&#969;k < |&#12296;V&#12297;'ZF|<&#12296;V&#12297;'cr, where &#12296;V&#12297;'cr is a critical flow shearing rate and is derived explicitly. If |&#12296;V&#12297;'E×B| > &#12296;V&#12297;'cr(&#12296;V&#12297;E×B is mean E × B shear flow driven by edge radial electrostatic field), the I-mode will terminate and spiral into the H-mode. (10.1088/0029-5515/55/4/043022)
  DOI : 10.1088/0029-5515/55/4/043022
• Strong Ionization Asymmetry in a Geometrically Symmetric Radio Frequency Capacitively Coupled Plasma Induced by Sawtooth Voltage Waveforms
  
  • Bruneau Bastien
  • Gans T.
  • O'Connell D.
  • Greb Arthur
  • Johnson E.V.
  • Booth Jean-Paul
  Physical Review Letters, American Physical Society, 2015, 114, pp.125002. The ionization dynamics in geometrically symmetric parallel plate capacitively coupled plasmas driven by radio frequency tailored voltage waveforms is investigated using phase resolved optical emission spectroscopy (PROES) and particle-in-cell (PIC) simulations. Temporally asymmetric waveforms induce spatial asymmetries and offer control of the spatiotemporal dynamics of electron heating and associated ionization structures. Sawtooth waveforms with different rise and fall rates are employed using truncated Fourier series approximations of an ideal sawtooth. Experimental PROES results obtained in argon plasmas are compared with PIC simulations, showing excellent agreement. With waveforms comprising a fast voltage drop followed by a slower rise, the faster sheath expansion in front of the powered electrode causes strongly enhanced ionization in this region. The complementary waveform causes an analogous effect in front of the grounded electrode. (10.1103/PhysRevLett.114.125002)
  DOI : 10.1103/PhysRevLett.114.125002
• Equatorial noise emissions with quasiperiodic modulation of wave intensity
  
  • Nemec F.
  • Santolík O.
  • Hrbackova Z.
  • Pickett J. S.
  • Cornilleau-Wehrlin Nicole
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015, 120, pp.2649-2661. Equatorial noise (EN) emissions are electromagnetic wave events at frequencies between the proton cyclotron frequency and the lower hybrid frequency observed in the equatorial region of the inner magnetosphere. They propagate nearly perpendicular to the ambient magnetic field, and they exhibit a harmonic line structure characteristic of the proton cyclotron frequency in the source region. However, they were generally believed to be continuous in time. We investigate more than 2000 EN events observed by the Spatio-Temporal Analysis of Field Fluctuations and Wide-Band Data Plasma Wave investigation instruments on board the Cluster spacecraft, and we show that this is not always the case. A clear quasiperiodic (QP) time modulation of the wave intensity is present in more than 5% of events. We perform a systematic analysis of these EN events with QP modulation of the wave intensity. Such events occur usually in the noon-to-dawn magnetic local time sector. Their occurrence seems to be related to the increased geomagnetic activity, and it is associated with the time intervals of enhanced solar wind flow speeds. The modulation period of these events is on the order of minutes. Compressional ULF magnetic field pulsations with periods about double the modulation periods of EN wave intensity and magnitudes on the order of a few tenths of nanotesla were identified in about 46% of events. We suggest that these compressional magnetic field pulsations might be responsible for the observed QP modulation of EN wave intensity, in analogy to formerly reported VLF whistler mode QP events. (10.1002/2014JA020816)
  DOI : 10.1002/2014JA020816
• Negative hydrogen ion production mechanisms
  
  • Bacal M.
  • Wada M.
  Applied Physics Reviews, AIP Publishing, 2015, 2 (2), pp.021305. Negative hydrogen/deuterium ions can be formed by processes occurring in the plasma volume and on surfaces facing the plasma. The principal mechanisms leading to the formation of these negative ions are dissociative electron attachment to ro-vibrationally excited hydrogen/deuterium molecules when the reaction takes place in the plasma volume, and the direct electron transfer from the low work function metal surface to the hydrogen/deuterium atoms when formation occurs on the surface. The existing theoretical models and reported experimental results on these two mechanisms are summarized. Performance of the negative hydrogen/deuterium ion sources that emerged from studies of these mechanisms is reviewed. Contemporary negative ion sources do not have negative ion production electrodes of original surface type sources but are operated with caesium with their structures nearly identical to volume production type sources. Reasons for enhanced negative ion current due to caesium addition to these sources are discussed. (31 pages, 265 refs) (10.1063/1.4921298)
  DOI : 10.1063/1.4921298
• The Plasma E × B Staircase: Turbulence Self-Regulation through Spontaneous Flow Patterning
  
  • Dif-Pradalier Guilhem
  • Hornung G
  • Ghendrih Philippe
  • Clairet R
  • Diamond P H
  • Sarazin Yanick
  • Vermare L
  • Abiteboul J
  • Cartier-Michaud T
  • Ehrlacher C
  • Estève D
  • Garbet Xavier
  • Grandgirard Virginie
  • Gürcan Özgür D.
  • Hennequin P
  • Kosuga Y
  • Latu Guillaume
  • Morel Pierre
  • Norscini C
  • Sabot R
  , 2015. The E × B staircase [1, 2] is a spontaneously formed, turbulence-driven self-organising pattern of quasi-regular, long-lived and localised shear flow [3] and stress layers. These layers coincide with long-lived pressure corrugations and are interspersed between regions of turbulent avalanching. The typical spacing between these layers is mesoscale, noted ∆ ∼ 25 − 30ρ i [1, 4] —in-between the turbulence auto-correlation length c ∼ 5ρ i at micro scales and the profile macroscale L 100ρ i , see e.g. Fig.2 in [1]— and sets the outer scale of the turbulent avalanching. Here ρ i is ion Larmor radius. Whilst arresting, statistically, to mesoscales the detrimental avalanching these layers, located at the " steps of the staircase " are beneficial to confinement. The E×B staircase is thus best understood as a self-organising and dynamical set of weak or permeable transport barriers. Strong mean zonal flows are generated and endure at the steps of the staircase, resulting in localised deviations of the poloidal flow from its oft-assumed neoclassical prediction [5].
• Further details on the plasma E × B staircase
  
  • Dif-Pradalier Guilhem
  • Hornung G
  • Ghendrih Philippe
  • Sarazin Yanick
  • Clairet F
  • Vermare L
  • Diamond P H
  • Abiteboul J
  • Cartier-Michaud T
  • Ehrlacher C
  • Esteve D
  • Garbet Xavier
  • Grandgirard Virginie
  • Gürcan Özgür D.
  • Hennequin P
  • Kosuga A
  • Latu Guillaume
  • Morel Pierre
  • Norscini C
  • Sabot R
  • Storelli A.
  , 2015. Turbulence in hot magnetised plasmas spontaneously has been shown to spontaneously organise on global scales into the so-called " E × B staircase ". Further characterisation of this structure is detailed below.
• Gyrokinetic turbulence cascade via predator-prey interactions between different scales
  
  • Kobayashi Sumire
  • Gürcan Özgür D.
  Physics of Plasmas, American Institute of Physics, 2015, 22 (5), pp.050702. Gyrokinetic simulations in a closed fieldline geometry are presented to explore the physics of nonlinear transfer in plasma turbulence. As spontaneously formed zonal flows and small-scale turbulence demonstrate predator-prey dynamics, a particular cascade spectrum emerges. The electrostatic potential and the density spectra appear to be in good agreement with the simple theoretical prediction based on Charney-Hasegawa-Mima equation |||&#981;&#771;k|||2&#8764;||n&#771;k||2&#8733;k&#8722;3/(1 k2)2, with the spectra becoming anisotropic at small scales. The results indicate that the disparate scale interactions, in particular, the refraction and shearing of larger scale eddies by the self-consistent zonal flows, dominate over local interactions, and contrary to the common wisdom, the comprehensive scaling relation is created even within the energy injection region. (10.1063/1.4920965)
  DOI : 10.1063/1.4920965
• Kinetic scale turbulence and dissipation in the solar wind: key observational results and future outlook
  
  • Goldstein M. L.
  • Wicks R. T.
  • Perri S.
  • Sahraoui Fouad
  Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Royal Society, The, 2015, 373, pp.20140147. Turbulence is ubiquitous in the solar wind. Turbulence causes kinetic and magnetic energy to cascade to small scales where they are eventually dissipated, adding heat to the plasma. The details of how this occurs are not well understood. This article reviews the evidence for turbulent dissipation and examines various diagnostics for identifying solar wind regions where dissipation is occurring. We also discuss how future missions will further enhance our understanding of the importance of turbulence to solar wind dynamics. (10.1098/rsta.2014.0147)
  DOI : 10.1098/rsta.2014.0147
• What is the nature of magnetosheath FTEs?
  
  • Roux A.
  • Robert Patrick
  • Fontaine Dominique
  • Le Contel Olivier
  • Canu Patrick
  • Louarn P.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015, 120 (6), pp.4576-4595. Cluster multipoint measurements are used to study two successive magnetosheath flux transfer events (FTEs). Magnetic field lines in the leading region are found to be closed magnetospheric field lines. For event 1 these field lines are wounded up by a large current structure oriented eastward and moving poleward. Conversely, the trailing region corresponds to opened magnetic field lines. For both events the leading edge of the FTEs is a tangential discontinuity separating the magnetosheath from closed field lines. In the case of event 1 magnetosheath ions are accelerated through the FTE trailing edge via a rotational discontinuity and penetrate on closed field lines through a second discontinuity. Thus, the ion jet is accelerated equatorward of the spacecraft but the backtracking of the discontinuities and the lack of dispersion show that ion acceleration occurs at less than 2 R<SUB>E</SUB> from Cluster. On the other hand the extrapolation forward indicates that the FTE bulge steepens as in simulations of Dorelli and Bhattacharjee). Evidence is given for the penetration of magnetosheath ions inside the core of the FTE, on closed field lines. Magnetosheath electrons are accelerated in parallel and antiparallel directions on open and on closed field lines, thus breaking the frozen-in condition. Event 2 is also split in two distinct regions but no evidence is found for accelerated bidirectional magnetosheath electrons. For event 2 the two discontinuities at the trailing region are stacked together when they are crossed by the spacecraft, suggesting that the current splitting is a reconnection signature. (10.1002/2015JA020983)
  DOI : 10.1002/2015JA020983
• Kinetic simulations of secondary reconnection in the reconnection jet
  
  • Huang S. Y.
  • Zhou M.
  • Yuan Z. G.
  • Fu H.S.
  • He J. S.
  • Sahraoui Fouad
  • Aunai Nicolas
  • Deng X. H.
  • Fu S. Y.
  • Pang Y.
  • Wang D. D.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015, 120 (8), pp.61886198. Magnetic reconnection, as one important energy dissipation process in plasmas, has been extensively studied in the past several decades. Magnetic reconnection occurring in the downstream of a primary X line is referred to as secondary reconnection. In this paper, we used kinetic simulations to investigate the secondary reconnection in detail. We found that secondary reconnection is reversed by the compression caused by the outflowing jet originating from the primary reconnection site, which results in the erosion of the magnetic island between the two X lines within ~3&#8201;&#969;ci&#8722;1. We show the observational signatures expected in electromagnetic fields and plasma measurements in the Earth's magnetotail, associated with this mechanism. These simulation results could be applied to interpret the signatures associated with the evolution of earthward magnetic islands in the Earth's magnetotail. (10.1002/2014JA020969)
  DOI : 10.1002/2014JA020969
• 3D hybrid simulations of the interaction of a magnetic cloud with a bow shock
  
  • Turc Lucile
  • Fontaine Dominique
  • Savoini Philippe
  • Modolo Ronan
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015, 120 (8), pp.6133-6151. In this paper, we investigate the interaction of a magnetic cloud (MC) with a planetary bow shock using hybrid simulations. It is the first time to our knowledge that this interaction is studied using kinetic simulations which include self-consistently both the ion foreshock and the shock wave dynamics. We show that when the shock is in a quasi-perpendicular configuration, the MC's magnetic structure in the magnetosheath remains similar to that in the solar wind, whereas it is strongly altered downstream of a quasi-parallel shock. The latter can result in a reversal of the magnetic field north-south component in some parts of the magnetosheath. We also investigate how the MC affects in turn the outer parts of the planetary environment, i.e., from the foreshock to the magnetopause. We find the following: (i) The decrease of the Alfvén Mach number at the MC's arrival causes an attenuation of the foreshock region because of the weakening of the bow shock. (ii) The foreshock moves along the bow shock's surface, following the rotation of the MC's magnetic field. (iii) Owing to the low plasma beta, asymmetric flows arise inside the magnetosheath, due to the magnetic tension force which accelerates the particles in some parts of the magnetosheath and slows them down in others. (iv) The quasi-parallel region forms a depression in the shock's surface. Other deformations of the magnetopause and the bow shock are also highlighted. All these effects can contribute to significantly modify the solar wind/magnetosphere coupling during MC events. (10.1002/2015JA021318)
  DOI : 10.1002/2015JA021318
• Polarization analysis of CuXX-lines emitted from X-pinch
  
  • Baronova E O
  • Larour Jean
  • Rosmej F B
  • Khattak F y
  Journal of Physics: Conference Series, IOP Science, 2015, 653, pp.012145. Soft x-ray emission from CuXX L-shell lines emitted by a dense X-pinch plasma have been investigated with high-resolution curved Bragg crystals at different angles of orientation. Single shot time integrated spectra show clear evidences of polarization for the Ne-like spectral lines 2s22p6 1S0 → 2s22p53s 1P1 (λ = 12.570 Å), 2s22p6 1S0 → 2s22p53s 3P1 (λ = 12.8277 Å). The variation of the intensity ratio of these two well-separated L-shell lines is discussed in view of its application for suprathermal electron characterization under real experimental conditions of pinch plasmas. We demonstrated that the simultaneous use of two different polarization spectrometers (means 4 Bragg crystals) permitted a high level of confidence for the analysis of the variation of the line ratios due to polarization. (10.1088/1742-6596/653/1/012145)
  DOI : 10.1088/1742-6596/653/1/012145
• Matched dipole probe for magnetized low electron density laboratory plasma diagnostics
  
  • Rafalskyi D.V.
  • Aanesland Ane
  Physics of Plasmas, American Institute of Physics, 2015, 22 (7), pp.073504. In this paper, a diagnostic method for magnetized and unmagnetized laboratory plasma is proposed, based on impedance measurements of a short matched dipole. The range of the measured electron densities is limited to low density plasmas (10121015&#8201;m&#8722;3), where other diagnostic methods have strong limitations on the magnetic field strength and topology, plasma dimensions, and boundary conditions. The method is designed for use in both large- and small-dimension plasma (<10&#8201;cm) without or with strong non-homogeneous magnetic field, which can be undefined within the probe size. The design of a matched dipole probe allows to suppress the sheath resonance effects and to reach high sensitivity at relatively small probe dimensions. Validation experiments are conducted in both magnetized (B&#8201;&#8764;&#8201;170&#8201;G) and unmagnetized (B&#8201;=&#8201;0) low density (7&#8201;×&#8201;1012&#8201;m&#8722;37&#8201;×&#8201;1013&#8201;m&#8722;3) low pressure (1 mTorr) 10&#8201;cm scale plasmas. The experimentally measured data show very good agreement with an analytical theory both for a non-magnetized and a magnetized case. The electron density measured by the matched dipole and Langmuir probes in the range of 7&#8201;×&#8201;1012&#8201;m&#8722;37&#8201;×&#8201;1013&#8201;m&#8722;3 show less than 30% difference. An experimentally measured tolerance/uncertainty of the dipole probe method is estimated to ±1% for plasma densities above 2&#8201;×&#8201;1013&#8201;m&#8722;3. A spatial resolution is estimated from the experiments to be about 3d, where d is the dipole diameter. The diagnostic method is also validated by comparing the measured plasma impedance curves with results of analytical modelling. (10.1063/1.4926447)
  DOI : 10.1063/1.4926447
• The impingement of a kHz helium atmospheric pressure plasma jet on a dielectric surface
  
  • Guaitella Olivier
  • Sobota Ana
  Journal of Physics D: Applied Physics, IOP Publishing, 2015, 48 (25), pp.255202. A parametric study of the impingement of a helium kHz atmospheric pressure plasma jet on a flat glass surface was performed by means of time-resolved intensified charge-coupled device imaging. The development of the plasma on the target is linked to the plasma evolution in the source and governed by the power supply. The glass surface takes part in the elongation of the plasma jet by the virtue of two mechanisms: the local enhancement of the electric field and the supply of pre-deposited charge. The evidence for the pre-deposited charge is the formation of a sheath on the glass surface, and the faint discharge formed on the glass surface during the negative voltage slope starting at the maximum of the negative current peak. The influence of the gas flow dynamics taking into account various gas flows, incident angles and distances is more important for the behaviour of the discharge on the surface than the voltage amplitude or the geometry of the source. The capacitance of the target strongly modifies the interaction with the plasma jet and increases the deposited surface charge density, featuring a streamer-like propagation mechanism in the case of high electric field enhancement at the surface. (10.1088/0022-3727/48/25/255202)
  DOI : 10.1088/0022-3727/48/25/255202
• Isopropanol removal using Mn<SUB>X</SUB>O<SUB>Y</SUB> packed bed non-thermal plasma reactor: Comparison between continuous treatment and sequential sorption/regeneration
  
  • Sivachandiran Loganathan
  • Thévenet Frédéric
  • Rousseau Antoine
  Chemical Engineering Journal, Elsevier, 2015, 270, pp.327-335. MnXOY coated glass beads packed bed non-thermal plasma (NTP) reactor has been designed and operated for isopropanol (IPA) removal close to indoor air conditions. The IPA removal efficiency of continuous NTP treatment is compared with the sequential approach, i.e. adsorption of IPA on MnXOY and subsequent regeneration of the saturated MnXOY surface by non-thermal plasma. The comparison between both approaches has been achieved with the same packed bed reactor and model VOC under equivalent indoor air conditions. Firstly, based on carbon mass balance calculations, the continuous treatment has shown better performances from an IPA abatement point of view, as well as from a mineralization point of view. However, the characterization of ppb level side-products evidenced that the continuous treatment leads to a more significant release of organic side products which may impact indoor air quality. Secondly, both processes have been compared in terms of energetic costs regarding (i) IPA removal, and (ii) CO2 forma- tion. Interestingly, it is evidenced that, to treat the same amount of IPA, the sequential approach requires 14.5 times less energy than the continuous NTP treatment process. Similarly, to produce the same amount of CO2, the sequential approach consumes 10 times less energy. This comparison evidences the interest of adsorption combined with subsequent non-thermal plasma regeneration for indoor air effluent treatment. (10.1016/j.cej.2015.01.055)
  DOI : 10.1016/j.cej.2015.01.055
• Nature of the MHD and Kinetic Scale Turbulence in the Magnetosheath of Saturn: Cassini Observations
  
  • Hadid Lina
  • Sahraoui Fouad
  • Kiyani K. H.
  • Retinò Alessandro
  • Modolo Ronan
  • Canu Patrick
  • Masters Adam
  • Dougherty Michele K.
  The Astrophysical Journal Letters, Bristol : IOP Publishing, 2015, 813 (2), pp.L29. Low-frequency turbulence in Saturn's magnetosheath is investigated using in situ measurements of the Cassini spacecraft. Focus is put on the magnetic energy spectra. A set of 42 time intervals in the magnetosheath were analyzed, and three main results that contrast with known features of solar wind turbulence are reported. (10.1088/2041-8205/813/2/L29)
  DOI : 10.1088/2041-8205/813/2/L29
• WEST Physics Basis
  
  • Bourdelle C.
  • Artaud J.F.
  • Basiuk Vincent
  • Bécoulet M.
  • Brémond S.
  • Bucalossi J.
  • Bufferand H.
  • Ciraolo G.
  • Colas L.
  • Corre Y.
  • Courtois X.
  • Decker J.
  • Delpech L.
  • Devynck P.
  • Dif-Pradalier Guilhem
  • Doerner R.P.
  • Douai D.
  • Dumont Rémi
  • Ekedahl A.
  • Fedorczak N.
  • Fenzi C.
  • Firdaouss M.
  • Garcia J.
  • Ghendrih Philippe
  • Gil C.
  • Giruzzi G.
  • Goniche M.
  • Grisolia C.
  • Grosman A.
  • Guilhem D.
  • Guirlet R.
  • Gunn J.
  • Hennequin Pascale
  • Hillairet J.
  • Hoang T.
  • Imbeaux Frédéric
  • Ivanova-Stanik Irena
  • Joffrin E.
  • Kallenbach A.
  • Linke J.
  • Loarer T.
  • Lotte P.
  • Maget P.
  • Marandet Yannick
  • Mayoral M.L.
  • Meyer O.
  • Missirlian M.
  • Mollard P.
  • Monier-Garbet P.
  • Moreau P.
  • Nardon Eric
  • Pégourié B.
  • Peysson Y.
  • Sabot R.
  • Saint-Laurent F.
  • Schneider M.
  • Travère J. M.
  • Tsitrone E.
  • Vartanian S.
  • Vermare Laure
  • Yoshida M.
  • Zagorski R.
  • Jet Contributors
  Nuclear Fusion, IOP Publishing, 2015, 55 (6), pp.063017. With WEST (Tungsten Environment in Steady State Tokamak) (Bucalossi et al 2014 Fusion Eng. Des. 89 [http://dx.doi.org/10.1016/j.fusengdes.2014.01.062] 907?12 ), the Tore Supra facility and team expertise (Dumont et al 2014 Plasma Phys. Control. Fusion 56 [http://dx.doi.org/10.1088/0741-3335/56/7/075020] 075020 ) is used to pave the way towards ITER divertor procurement and operation. It consists in implementing a divertor configuration and installing ITER-like actively cooled tungsten monoblocks in the Tore Supra tokamak, taking full benefit of its unique long-pulse capability. WEST is a user facility platform, open to all ITER partners. This paper describes the physics basis of WEST: the estimated heat flux on the divertor target, the planned heating schemes, the expected behaviour of the L?H threshold and of the pedestal and the potential W sources. A series of operating scenarios has been modelled, showing that ITER-relevant heat fluxes on the divertor can be achieved in WEST long pulse H-mode plasmas. (10.1088/0029-5515/55/6/063017)
  DOI : 10.1088/0029-5515/55/6/063017
• Reply to Comment on "A review on ion-ion plasmas created in weakly magnetized electronegative plasmas
  
  • Aanesland Ane
  • Bredin Jérôme
  • Chabert Pascal
  Plasma Sources Science and Technology, IOP Publishing, 2015, 24 (3), pp.038002. This is a reply to the comments made by Bogdanov et al on our paper entitled ?A review on ion?ion plasmas created in weakly magnetized electronegative plasmas?. We have clarified some issues regarding the electronegative plasma stratification and the definition of an ion?ion plasma, and we discuss the issue of extracting negative ions from a steady-state ion?ion plasma formed at the periphery or downstream a magnetized electronegative plasma. (10.1088/0963-0252/24/3/038002)
  DOI : 10.1088/0963-0252/24/3/038002
• Numerical experiment to estimate the validity of negative ion diagnostic using photo-detachment combined with Langmuir probing
  
  • Oudini N.
  • Sirse Nishant
  • Benallal R.
  • Taccogna F.
  • Aanesland Ane
  • Bendib A.
  • Ellingboe A. R.
  Physics of Plasmas, American Institute of Physics, 2015, 22 (7), pp.073509. This paper presents a critical assessment of the theory of photo-detachment diagnostic method used to probe the negative ion density and electronegativity &#945;&#8201;=&#8201;n-/ne. In this method, a laser pulse is used to photo-detach all negative ions located within the electropositive channel (laser spot region). The negative ion density is estimated based on the assumption that the increase of the current collected by an electrostatic probe biased positively to the plasma is a result of only the creation of photo-detached electrons. In parallel, the background electron density and temperature are considered as constants during this diagnostics. While the numerical experiments performed here show that the background electron density and temperature increase due to the formation of an electrostatic potential barrier around the electropositive channel. The time scale of potential barrier rise is about 2&#8201;ns, which is comparable to the time required to completely photo-detach the negative ions in the electropositive channel (&#8764;3&#8201;ns). We find that neglecting the effect of the potential barrier on the background plasma leads to an erroneous determination of the negative ion density. Moreover, the background electron velocity distribution function within the electropositive channel is not Maxwellian. This is due to the acceleration of these electrons through the electrostatic potential barrier. In this work, the validity of the photo-detachment diagnostic assumptions is questioned and our results illustrate the weakness of these assumptions. (10.1063/1.4926826)
  DOI : 10.1063/1.4926826
• Plasma acceleration using a radio frequency self-bias effect
  
  • Rafalskyi D.V.
  • Aanesland Ane
  Physics of Plasmas, American Institute of Physics, 2015, 22 (6), pp.063502. In this work plasma acceleration using a RF self-bias effect is experimentally studied. The experiments are conducted using a novel plasma accelerator system, called Neptune, consisting of an inductively coupled plasma source and a RF-biased set of grids. The plasma accelerator can operate in a steady state mode, producing a plasma flow with separately controlled plasma flux and velocity without any magnetic configuration. The operating pressure at the source output is as low as 0.2&#8201;mTorr and can further be decreased. The ion and electron flows are investigated by measuring the ion and electron energy distribution functions both space resolved and with different orientations with respect to the flow direction. It is found that the flow of electrons from the source is highly anisotropic and directed along the ion flow and this global flow of accelerated plasma is well localized in the plasma transport chamber. The maximum flux is about 7.5·1015 ions s&#8722;1 m&#8722;2 (at standard conditions) on the axis and decreasing to almost zero at a radial distances of more than 15&#8201;cm from the flow axis. Varying the RF acceleration voltage in the range 20350&#8201;V, the plasma flow velocity can be changed between 10 and 35&#8201;km/s. The system is prospective for different technology such as space propulsion and surface modification and also interesting for fundamental studies for space-related plasma simulations and investigation of the dynamo effect using accelerated rotating plasmas. I. INTRODUCT (10.1063/1.4922065)
  DOI : 10.1063/1.4922065
• Solar illumination control of ionospheric outflow above polar cap arcs
  
  • Maes Lukas
  • Maggiolo R.
  • de Keyser J.
  • Dandouras I.
  • Fear R. C.
  • Fontaine Dominique
  • Haaland S.
  Geophysical Research Letters, American Geophysical Union, 2015, 42 (5), pp.1304-1311. We measure the flux density, composition, and energy of outflowing ions above the polar cap, accelerated by quasi-static electric fields parallel to the magnetic field and associated with polar cap arcs, using Cluster. Mapping the spacecraft position to its ionospheric foot point, we analyze the dependence of these parameters on the solar zenith angle (SZA). We find a clear transition at SZA between 94° and 107°, with the O<SUP> </SUP> flux higher above the sunlit ionosphere. This dependence on the illumination of the local ionosphere indicates that significant O<SUP> </SUP> upflow occurs locally above the polar ionosphere. The same is found for H<SUP> </SUP>, but to a lesser extent. This effect can result in a seasonal variation of the total ion upflow from the polar ionosphere. Furthermore, we show that low-magnitude field-aligned potential drops are preferentially observed above the sunlit ionosphere, suggesting a feedback effect of ionospheric conductivity. (10.1002/2014GL062972)
  DOI : 10.1002/2014GL062972
• Geodesic acoustic modes in a fluid model of tokamak plasma: the effects of finite beta and collisionality
  
  • Singh Rameswar
  • Storelli A.
  • Gürcan Özgür D.
  • Hennequin Pascale
  • Vermare Laure
  • Morel Pierre
  • Singh R.
  Plasma Physics and Controlled Fusion, IOP Publishing, 2015, 57, pp.125002. Starting from the Braginskii equations, relevant for the tokamak edge region, a complete set of nonlinear equations for the geodesic acoustic modes (GAM) has been derived which includes collisionality, plasma beta and external sources of particle, momentum and heat. Local linear analysis shows that the GAM frequency increases with collisionality at low radial wave number and decreases at high . GAM frequency also decreases with plasma beta. Radial profiles of GAM frequency for two Tore Supra shots, which were part of a collisionality scan, are compared with these calculations. A discrepancy between experiment and theory is observed, which seems to be explained by a finite for the GAM when flux surface averaged density \langle n\rangle and temperature are assumed to vanish. It is shown that this agreement is incidental and self-consistent inclusion of and responses enhances the disagreement more with at high . So the discrepancy between the linear GAM calculation and experiment, (which also persist for more 'complete' linear models such as gyrokinetics) can probably not be resolved by simply adding a finite . (10.1088/0741-3335/57/12/125002)
  DOI : 10.1088/0741-3335/57/12/125002