Publications

2011

• On efficient renormalization of the ITG mode
  
  • Gürcan Özgür D.
  , 2011 (Oral). http://perso.ens-lyon.fr/nicolas.plihon/HOUCHES
• Solar Wind turbulence, from MHD to electron scales
  
  • Sahraoui Fouad
  • Belmont Gérard
  • Goldstein M. L.
  • Huang S. Y.
  • Kiyani K. H.
  • Narita Y.
  , 2011.
• RF waveform tailoring to control film morphology during nanocrystalline silicon PECVD
  
  • Booth Jean-Paul
  • Johnson E.V.
  • Delattre Pierre-Alexandre
  • Verbeke T.
  • Vanel J-C
  , 2011.
• Simulation numérique de la reconnexion magnétique : mécanismes cinétiques sous-jacents à la description fluide des ions
  
  • Aunai Nicolas
  , 2011. La capacité à libérer l’énergie stockée dans le champ magnétique et à briser le théorème du gel font de la reconnexion magnétique un des phénomènes les plus importants de la physique des plasmas. Lorsqu’elle se produit dans un environnement non-collisionel comme la magnétosphère terrestre, une modélisation cinétique est à priori nécessaire. Cependant la plupart de notre compréhension du phénomène se base sur un interprétation fluide, plus intuitive. Dans quelle mesure ces deux interprétations d’un même phénomène sont-elles reliées ? C’est la problématique à laquelle cette thèse s’intéresse, dans le cas de la reconnexion antiparallèle et pour la population ionique du plasma. La première partie de ce travail s’intéresse à l’accélération fluide et cinétique des protons au sein de la région de reconnexion. Il est montré comment le mouvement individuel des particules joue un rôle du point de vue fluide via la force de pression, jusqu’alors négligée dans les modèles. Ces résultats ont également mené dans une seconde partie à des prédictions et vérifications observationnelles basées sur les données des satellites Cluster. Dans un troisième temps, nous montrons le rôle important joué par le flux d’énergie thermique dans le transfert d’énergie au cours du processus de reconnexion, dans le cas symétrique et asymétrique. Enfin la dernière partie de ce manuscrit propose une solution au problème fondamental consistant décrire une couche de courant tangentielle asymétrique dans un état d’équilibre cinétique
• Seeing Inside Plasma Etch Reactors: from diagnostics to sensors for control
  
  • Booth Jean-Paul
  • Chabert Pascal
  • Sarot Rodolphe
  • Zaka-Ul-Islam Mujahid
  , 2011.
• Space Weather : An introduction
  
  • Amory-Mazaudier Christine
  , 2011.
• Time variations of the ionosphere at the northern tropical crest of ionization at Phu Thuy, Vietnam
  
  • Pham Thi Thu Hong
  • Amory-Mazaudier Christine
  • Le Huy M.
  Annales Geophysicae, European Geosciences Union, 2011, pp.197-207. This study is the first which gives the climatology of the ionosphere at the northern tropical crest of ionization in the Asian sector. We use the data from Phu Thuy station, in Vietnam, through three solar cycles (20, 21 and 22), showing the complete morphology of ionosphere parameters by analyzing long term variation, solar cycle variation and geomagnetic activity effects, seasonal evolution and diurnal development. Ionospheric critical frequencies, foF2, foF1 and foE, evolve according to the 11-year sunspot cycle. Seasonal variations show that foF2 exhibits a semiannual pattern with maxima at equinox, and winter and equinoctial anomalies depending on the phases of the sunspot solar cycle. 1foF2 exhibits a semiannual variation during the minimum phase of the sunspot solar cycle 20 and the increasing and decreasing phases of solar cycle 20, 21 and 22. 1foF1 exhibits an annual variation during the maximum phase of solar cycles 20, 21 and 22. 1h0F2 shows a regular seasonal variation for the different solar cycles while 1h0F1 exhibits a large magnitude dispersion from one sunspot cycle to another. The long term variations consist in an increase of 1.0MHz for foF2 and of 0.36MHz for foF1. foE increases 0.53MHz from solar cycle 20 to solar cycle 21 and then decreases −0.23MHz during the decreasing phase of cycle 21. The diurnal variation of the critical frequency foF2 shows minima at 05:00 LT and maxima around 14:00 LT. foF1 and foE have a maximum around noon. The diurnal variation of h0F2 exhibits a maximum around noon. The main features of h0F1 are a minimum near noon and the maximum near midnight. Other minima and maxima occur in the morning, at about 04:00 or 05:00 LT and in the afternoon, at about 18:00 or 19:00 LT but they are markedly smaller. Only during the maximum phase of all sunspot solar cycles the maximum near 19:00 LT is more pronounced. (10.5194/angeo-29-197-2011)
  DOI : 10.5194/angeo-29-197-2011
• Space Weather : An Introduction
  
  • Amory-Mazaudier Christine
  , 2011.
• Modeling Some Calibrated Cracks for E-NDE by a Difference Method
  
  • Bettaieb Laroussi
  • Kokabi Hamid
  • Poloujadoff Michel
  • Coillot Christophe
  Journal of Nondestructive Evaluation, Springer Verlag, 2011, 30 (3), pp.117 - 121. Electromagnetic Non Destructive Evaluation (E-NDE) is often conducted by inducing eddy currents in the structure to be examined, if this is conducting. Existence of flaws is detected by difference between the response of defect-free structures and damaged ones. In the present paper, we model such processes in order to predict the feasibility of this evaluation and to facilitate the interpretation of the observation. An original method is to represent a crack by a current source producing a magnetic signal. We have applied it to the case of a defect with a standard shape. The experimental evidences for the validity of this method are given. (10.1007/s10921-011-0097-5)
  DOI : 10.1007/s10921-011-0097-5
• Emission and propagation of Saturn kilometric radiation: Magnetoionic modes, beaming pattern, and polarization state
  
  • Lamy L.
  • Cecconi B.
  • Zarka P.
  • Canu Patrick
  • Schippers P.
  • Kurth W. S.
  • Mutel R. L.
  • Gurnett D. A.
  • Menietti D.
  • Louarn P.
  , 2011. The Cassini mission crossed the source region of the Saturn kilometric radiation (SKR) on 17 October 2008. On this occasion, the Radio and Plasma Wave Science (RPWS) experiment detected both local and distant radio sources, while plasma parameters were measured in situ by the magnetometer and the Cassini Plasma Spectrometer. A goniopolarimetric inversion was applied to RPWS three-antenna electric measurements to determine the wave vector k and the complete state of polarization of detected waves. We identify broadband extraordinary (X) mode as well as narrowband ordinary (O) mode SKR at low frequencies. Within the source region, SKR is emitted just above the X mode cutoff frequency in a hot plasma, with a typical electron-to-wave energy conversion efficiency of ˜1% (2% peak). The knowledge of the k vector is then used to derive the locus of SKR sources in the kronian magnetosphere, which shows X and O components emanating from the same regions. We also compute the associated beaming angle at the source θ‧ = (k, -B) either from (1) in situ measurements or a model of the magnetic field vector (for local to distant sources) or (2) polarization measurements (for local sources). Obtained results, similar for both modes, suggest quasi-perpendicular emission for local sources, whereas the beaming pattern of distant sources appears as a hollow cone with a frequency-dependent constant aperture angle: θ‧ = 75° ± 15° below 300 kHz, decreasing at higher frequencies to reach θ‧ (1000 kHz) = 50° ± 25°. Finally, we investigate quantitatively the SKR polarization state, observed to be strongly elliptical at the source, and quasi-purely circular for sources located beyond approximately two kronian radii. We show that conditions of weak mode coupling are achieved along the raypath, under which the magnetoionic theory satisfactorily describes the evolution of the observed polarization. These results are analyzed comparatively with the auroral kilometric radiation at Earth.
• Sq field characteristics at Phu Thuy, Vietnam, during solar cycle 23: comparisons with Sq field in other longitude sectors
  
  • Pham Thi Thu Hong
  • Amory-Mazaudier Christine
  • Le Huy M.
  Annales Geophysicae, European Geosciences Union, 2011, pp.1-17. Quiet days variations in the Earth's magnetic field (the Sq current system) are compared and contrasted for the Asian, African and American sectors using a new dataset from Vietnam. This is the first presentation of the variation of the Earth's magnetic field (Sq), during the solar cycle 23, at Phu Thuy, Vietnam (geographic latitudes 21.03 N and longitude: 105.95 E). Phu Thuy observatory is located below the crest of the equatorial fountain in the Asian longitude sector of the Northern Hemisphere. The morphology of the Sq daily variation is presented as a function of solar cycle and seasons. The diurnal variation of Phu Thuy is compared to those obtained in different magnetic observatories over the world to highlight the characteristics of the Phu Thuy observations. In other longitude sectors we find different patterns. At Phu Thuy the solar cycle variation of the amplitude of the daily variation of the X component is correlated to the F.10.7 cm solar radiation ( 0.74). This correlation factor is greater than the correlation factor obtained in two observatories located at the same magnetic latitudes in other longitude sectors: at Tamanrasset in the African sector ( 0.42, geographic latitude 22.79) and San Juan in the American sector ( 0.03, geographic latitude 18.38). At Phu Thuy, the Sq field exhibits an equinoctial and a diurnal asymmetry: - The seasonal variation of the monthly mean of X component exhibits the well known semiannual pattern with 2 equinox maxima, but the X component is larger in spring than in autumn. Depending of the phase of the sunspot cycle, the maximum amplitude of the X component varies in spring from 30 nT to 75 nT and in autumn from 20 nT to 60 nT. The maximum amplitude of the X component exhibits roughly the same variation in both solstices, varying from about 20 nT to 50 nT, depending on the position into the solar cycle. - In all seasons, the mean equinoctial diurnal Y component has a morning maximum Larger than the afternoon minimum i.e. the equivalent current flow over a day is more southward than northward. During winter, the asymmetry is maximum, it erases the afternoon minimum. At the Gnangara observatory, in Asian Southern Hemisphere, the diurnal Y pattern is opposite and the current flow is more northward. It seems that in the Asian sector, the northern and southern Sq current cells both contribute strongly to the equatorial electrojet. The pattern is different in the African and American sectors where the northern Sq current cell contribution to the equatorial electrojet is smaller than the southern one. These observations can explain the unexpected maximum of amplitude of the equatorial electrojet observed in the Asian sector where the internal field is very large. During winter the Y component flow presents an anomaly, it is always southward during the whole day and there is no afternoon northward circulation. (10.5194/angeo-29-1-2011)
  DOI : 10.5194/angeo-29-1-2011
• Modelling of the beam-plasma interaction in a strongly inhomogeneous plasma
  
  • Volokitin A.
  • Krasnoselskikh V.
  • Krafft C.
  • Kuznetsov Evgenii
  AIP Conference Proceedings, American Institute of Physics, 2013, 1539, pp.78-81. The interaction of a beam propagating in a inhomogeneous solar wind plasma is considered. The properties of the plasma waves are described by the one dimensional Zakharov's equation, the beam is modelled by means of particles moving in the electric fields of the Langmuir waves. We take into account the presence of high level density fluctuations that are known to be present in the solar wind. It is shown that when the level of density fluctuations is low, δn/n0≪3k2λ2d, the regime of beam relaxation is very similar to that occurring in a homogeneous plasma and can be described by the quasilinear (QL) equations. In this case, the relaxation length is very short and corresponds to that obtained using the QL approximation. On the contrary, when the level of density fluctuations overcomes some limit, i.e. δn/n0≥3k2λ2d, where δn/n0≪1, the plasma inhomogeneities crucially influence the process of relaxation. First, the linear wave growth becomes localized and clearly identifiable wave packets/clumps dominate the wave spectrum; this is associated with the kinematic properties of the waves' propagation and the wave-particle resonant interactions. Most of the wave packets grow in the regions of density gradients. The second important feature revealed by the beam relaxation consists in the generation of a tail of accelerated electrons with velocities V > Vb exceeding the beam drift. The beam widens in both directions, toward lower velocities as well as higher velocities; the density of the accelerated electrons can reach more than 10 ÷ 20 % of the beam density and the energy flux carried by this population can become as large as 40% of the initial energy flux of the beam. (10.1063/1.4810994)
  DOI : 10.1063/1.4810994
• A global model of micro-hollow cathode discharges in the stationary regime
  
  • Lazzaroni Claudia
  • Chabert Pascal
  Journal of Physics D: Applied Physics, IOP Publishing, 2011, 44, pp.445202. This paper presents a global model of micro-hollow cathode discharges working in argon gas. Inspired by experiments and two-dimensional numerical simulations, the discharge is decomposed into two main regions, labelled the cathodic region and the positive column region. The first is composed of a cathode sheath (surrounding the cathode) and a plasma in the centre, while the positive column region is essentially filled with plasma. A cathode sheath model is developed to calculate the sheath size and the power dissipated by ions and electrons in the sheath. Charged-particle transport equations are solved in one-dimensional cylindrical coordinates, in the quasi-neutral plasma region, in order to determine the radial density profiles and the electron temperature in the microhole. A global power balance is then performed in the two distinct regions in order to determine the absolute electron density. We found that the electron density is one order of magnitude higher in the cathodic region than in the positive column, showing that the power dissipation is mainly located in the cathode sheath. The calculated electron density in the cathodic region is in reasonable agreement with experiments. (10.1088/0022-3727/44/44/445202)
  DOI : 10.1088/0022-3727/44/44/445202
• Small-scale filamentary structures recorded in the auroral regions connected to the plasma sheet boundary layer
  
  • Pottelette Raymond
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2011, 116. We use high time resolution data from the FAST spacecraft during a moderate substorm and concentrate on the auroral regions that are magnetically linked to the plasma sheet boundary layer (PSBL). The crossing of these regions is characterized by the presence of a pair of oppositely directed, field-aligned current sheets. The more poleward of the two current sheets is directed earthward. Strong turbulent fluctuations are detected in association with the field-aligned currents. The low-frequency component (1 Hz) of the electric field fluctuations consists of localized structures whose characteristics are not entirely consistent with the presence of Alfvén waves. They may represent a possible example of the coupling of Alfvén waves with electron acoustic waves on small length scales. In the high-frequency range (1-10 kHz), the turbulent fluctuations are dominated by large amplitude (500 mV/m peak to peak) bipolar electric field structures whose polarity depends on the direction of the field-aligned currents. These latter structures are moving earthward in the upward current region and antiearthward in the downward current region. The generation of both low- and high-frequency filamentary nonlinear structures appears as a natural consequence of the disturbances imposed during substorms on the auroral regions connected to the PSBL when hot and cold plasmas interact. (10.1029/2010JA016192)
  DOI : 10.1029/2010JA016192
• Typical properties of rising and falling tone chorus waves
  
  • Li W.
  • Thorne R. M.
  • Bortnik J.
  • Shprits Y. Y.
  • Nishimura Y.
  • Angelopoulos V.
  • Chaston C.
  • Le Contel Olivier
  • Bonnell J. W.
  Geophysical Research Letters, American Geophysical Union, 2011, 38, pp.14103. Chorus waves, which have received intense attention recently due to their significant role in radiation belt electron dynamics, frequently exhibit rising and falling tones. Lower-band chorus waves, observed using THEMIS wave burst data, are analyzed to obtain the typical properties of either class of chorus emissions. Our results show that rising tones are more likely to be quasi field-aligned, whereas falling tones are typically very oblique, close to the resonance cone. Furthermore, rising tones occur significantly more often than falling tones, and magnetic amplitudes of rising tones are generally much larger than those of falling tones. We also show the preferential regions of rising and falling tones dependent on MLT and magnetic latitude. Our new findings suggest that two separate mechanisms may be responsible for the generation and nonlinear evolution of rising and falling tone chorus. (10.1029/2011GL047925)
  DOI : 10.1029/2011GL047925
• Suprathermal electron acceleration during reconnection onset in the magnetotail
  
  • Vaivads A.
  • Retinò Alessandro
  • Khotyaintsev Y. V.
  • André M.
  Annales Geophysicae, European Geosciences Union, 2011, 29, pp.1917-1925. We study one event of reconnection onset associated to a small substorm on 27 September 2006 by using Cluster observations at inter-spacecraft separation of about 10 000 km. We focus on the acceleration of suprathermal electrons during different stages of reconnection. We show that several distinct stages of acceleration occur: (1) moderate acceleration during reconnection of pre-existing plasma sheet flux tubes, (2) stronger acceleration during reconnection of lobe flux tubes, (3) production of the most energetic electrons within dipolarization fronts (magnetic pile-up regions). The strongest acceleration is reached at the location of B<SUB>z</SUB> maxima inside the magnetic pile-up region where the reconnection jet stops. Very strong localized dawn-dusk electric field are observed within the magnetic pile-up regions and are associated to most of the magnetic flux transport. (10.5194/angeo-29-1917-2011)
  DOI : 10.5194/angeo-29-1917-2011
• Multievent study of the correlation between pulsating aurora and whistler mode chorus emissions
  
  • Nishimura Y.
  • Bortnik J.
  • Li W.
  • Thorne R. M.
  • Chen Lin
  • Lyons L.R.
  • Angelopoulos V.
  • Mende S. B.
  • Bonnell J. W.
  • Le Contel Olivier
  • Cully C. M.
  • Ergun R.
  • Auster U.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2011, 116, pp.11221. A multievent study was performed using conjugate measurements of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft and an all-sky imager during periods of intense lower-band chorus waves. The thirteen identified cases support our previous finding, based on two events, that the intensity modulation of lower-band chorus near the magnetic equator is highly correlated with quasiperiodic pulsating auroral emissions near the spacecraft's magnetic footprint, indicating that lower-band chorus is the driver of the pulsating aurora. Furthermore, we identified a fortuitous measurement made simultaneously by two THEMIS spacecraft with small spatial separation. The two spacecraft were found to be located in a single pulsating chorus patch and the spacecraft footprints were in the same pulsating auroral patch when intense chorus bursts were measured simultaneously, whereas only one of the spacecraft's footprints was in a patch when the other spacecraft did not detect intense chorus. On the basis of this event, we can estimate the pulsating chorus patch size by mapping the pulsating auroral patches from the ionosphere toward the magnetic equator, giving a roughly circular region of ~5000 km diameter for corresponding azimuthally elongated patches with ~100 km size in the ionosphere. Using a ray-tracing-based calculation of the divergence of chorus raypaths from a point source, together with the corresponding resonant energies, we found that the chorus patch size is most probably not a result of ray divergence but a property of the wave excitation region. (10.1029/2011JA016876)
  DOI : 10.1029/2011JA016876
• Turbulence intensity pulse propagation with self-consistent nonlinear noise
  
  • Wang Z.H.
  • Diamond P.H.
  • Gürcan Özgür D.
  • Garbet X.
  • Wang X.G.
  Physics of Plasmas, American Institute of Physics, 2011, 18, pp.032306. A model of turbulence intensity spreading with self-consistent nonlinear noise is derived systematically for the simple dynamical model of resistivity gradient driven turbulence. Local effective drive, thermal conduction damping, nonlinear coupling, and spatial scattering effects are included. As a consequence of nonlinear mode coupling processes (i.e., triad mode interactions), turbulence energy can be spatially scattered, leading to turbulence propagation and spreading. However, the range of any nonlinear mode interactions of the background with a test mode is restricted to within a few mode scale widths from the test mode rational surface. The speed of a turbulent spreading front is calculated. This front speed is effectively constant on macroscopic scales. We show that the effect of self-consistent nonlinear noise on the intensity front speed is modest, as a consequence of the ordering &#916;c<Lf, where &#916;c is the turbulence correlation length and Lf is the scale length of the fronts leading edge. The implications of these results for turbulence spreading models and the important differences between self-consistent mode coupling noise and ad hoc external noise are discussed. The broader implications of these results for turbulence front propagation are identified and explained. (10.1063/1.3567142)
  DOI : 10.1063/1.3567142
• On plasma- and UV-induced VOC oxidation on TiO<SUB>2</SUB> surface pre-treated by plasma exposure monitored by QCLAS
  
  • Marinov Daniil
  • Lopatik D.
  • Guaitella Olivier
  • Hübner M.
  • Rousseau Antoine
  • Roepcke J.
  , 2011.
• Electron-cylotron maser radiation from electron holes: upward current region
  
  • Treumann R. A.
  • Baumjohann W.
  • Pottelette R.
  Annales Geophysicae, European Geosciences Union, 2011, 29, pp.1885-1904. Electron holes are suggested to be an important source for generation of electron-cyclotron maser radiation. We demonstrate that electron holes generated in a ring-horseshoe distribution in the auroral-kilometric radiation source region have the capacity to emit band-limited radiation. The radiation is calculated in the proper frame of a circular model hole and shown to be strictly perpendicular in this frame. Its bandwidth under auroral conditions is of the order of ~1 kHz, which is a reasonable value. It is also shown that much of the drift of fine structure in the radiation can be interpreted as Doppler shift. Estimates based on data are in good agreement with theory. Growth and absorption rates have been obtained for the emitted radiation. However, the growth rate of a single hole obtained under conservative conditions is small, too small for reproducing the observed fine structure flux. Trapping of radiation inside the hole for the hole's lifetime helps amplifying the radiation additionally but introduces other problems. This entire set of questions is discussed at length and compared to radiation from the global horseshoe distribution. The interior of the hole produces a weak absorption at slightly higher frequency than emission. The absorptivity is roughly two orders of magnitude below the growth rate of the radiation thus being weak even when the emission and absorption bands overlap. Transforming to the stationary observer's frame it is found that the radiation becomes oblique against the magnetic field. For approaching holes the radiated frequencies may even exceed the local electron cyclotron frequency. (10.5194/angeo-29-1885-2011)
  DOI : 10.5194/angeo-29-1885-2011
• Collective Light Scattering for the Study of Fluctuations in Magnetized Plasmas: The Hall Thruster Case Study
  
  • Tsikata Sedina
  • Honoré Cyrille
  • Grésillon D.
  • Lemoine N.
  Contributions to Plasma Physics, Wiley-VCH Verlag, 2011, 51 (2-3), pp.119-125. Collective light scattering (CLS) has been recently extended to the observation of Hall effect thruster plasmas. A better understanding of the physics of Hall thrusters is key to improving thruster operation and lifetimes. In particular, the origins of thruster phenomena such as anomalous electron transport need to be determined. A high-performance CLS diagnostic has therefore been developed for identifying and measuring electron density fluctuations in the thruster, at millimetric length scales and MHz frequencies. Such modes are believed to play a role in anomalous transport, and experiments so far performed have provided information on their dispersion relations, amplitude and directivity. This work describes the technical aspects of the optical bench and the range of accessible experiments (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) (10.1002/ctpp.201000057)
  DOI : 10.1002/ctpp.201000057
• Discharge resistance and power dissipation in the self-pulsing regime of micro-hollow cathode discharges
  
  • Lazzaroni Claudia
  • Chabert Pascal
  Plasma Sources Science and Technology, IOP Publishing, 2011, 20, pp.055004. Micro-hollow cathode discharges (MHCDs), driven by a dc voltage source, may operate in a self-pulsing regime during which the voltage and the current across the discharge are pulsed with a frequency of several tens of kilohertz. A model for the self-pulsing regime of MHCDs is proposed based on an equivalent electrical circuit of the whole device. The discharge itself is modeled by a non-linear resistance in series with an inductance, and it is placed in parallel with the capacitance of the electrodedielectricelectrode sandwich hosting the discharge micro-hole. The capacitance of the coaxial cable used to feed the device is also included. It is shown that a detailed comparison between theory and experiments allows the discharge non-linear resistance to be accurately determined. When the discharge current is maximum, this resistance is about 2500&#8201;&#937; at 50&#8201;Torr and decreases to about 750&#8201;&#937; at 150&#8201;Torr. The absorbed power is calculated throughout the self-pulsing cycle and may be above 100&#8201;W at the current peak, while the time-averaged power remains a fraction of a Watt. (10.1088/0963-0252/20/5/055004)
  DOI : 10.1088/0963-0252/20/5/055004
• On plasma rotation with toroidal magnetic field ripple and no external momentum input
  
  • Fenzi C.
  • Garbet X.
  • Trier Elisée
  • Hennequin Pascale
  • Bourdelle C.
  • Aniel Thierry
  • Colledani G.
  • Devynck P.
  • Gil C.
  • Gürcan Özgür D.
  • Manenc L.
  • Schneider M.
  • Segui J.-L.
  • Tore Supra Team
  Nuclear Fusion, IOP Publishing, 2011, 51, pp.103038. Ripple-induced thermal loss effect on plasma rotation is investigated in a set of Ohmic L-mode plasmas performed in Tore Supra, and comparisons with neoclassical predictions including ripple are performed. Adjusting the size of the plasma, the ripple amplitude has been varied from 0.5% to 5.5% at the plasma boundary, keeping the edge safety factor constant. The toroidal flow dynamics is understood as being likely dominated by turbulence transport driven processes at low ripple amplitude, while the ripple-induced toroidal friction becomes dominant at high ripple. In the latter case, the velocity tends remarkably towards the neoclassical prediction (counter-current rotation). The radial electric field is not affected by the ripple variation and remains well described by its neoclassical prediction. Finally, the poloidal velocity is of the order of the neoclassical prediction at high ripple amplitude, but significantly departs from it at low ripple. (10.1088/0029-5515/51/10/103038)
  DOI : 10.1088/0029-5515/51/10/103038
• Numerical analysis of electronegative plasma in the extraction region of negative hydrogen ion sources
  
  • Kuppel S.
  • Matsushita D.
  • Hatayama A.
  • Bacal M.
  Journal of Applied Physics, American Institute of Physics, 2011, 109 (1), pp.013305. This numerical study focuses on the physical mechanisms involved in the extraction of volume-produced H&#8722; ions from a steady state laboratory negative hydrogen ion source with one opening in the plasma electrode (PE) on which a dc-bias voltage is applied. A weak magnetic field is applied in the source plasma transversely to the extracted beam. The goal is to highlight the combined effects of the weak magnetic field and the PE bias voltage (upon the extraction process of H&#8722; ions and electrons). To do so, we focus on the behavior of electrons and volume-produced negative ions within a two-dimensional model using the particle-in-cell method. No collision processes are taken into account, except for electron diffusion across the magnetic field using a simple random-walk model at each time step of the simulation. The results show first that applying the magnetic field (without PE bias) enhances H&#8722; ion extraction, while it drastically decreases the extracted electron current. Secondly, the extracted H&#8722; ion current has a maximum when the PE bias is equal to the plasma potential, while the extracted electron current is significantly reduced by applying the PE bias. The underlying mechanism leading to the above results is the gradual opening by the PE bias of the equipotential lines towards the parts of the extraction region facing the PE. The shape of these lines is due originally to the electron trapping by the magnetic field. (10.1063/1.3530454)
  DOI : 10.1063/1.3530454
• Multi-spacecraft investigation of space turbulence: lessons from Cluster and input to the Cross- Scale mission
  
  • Sahraoui Fouad
  • Goldstein M.L.
  • Belmont Gérard
  • Roux A.
  • Rezeau Laurence
  • Canu Patrick
  • Robert Patrick
  • Cornilleau-Wehrlin Nicole
  • Le Contel Olivier
  • Dudok de Wit Thierry
  • Pinçon Jean-Louis
  • Kiyani K.
  Planetary and Space Science, Elsevier, 2011, 59 (7), pp.Pages 585-591. Investigating space plasma turbulence from single-point measurements is known to be characterized by unavoidable ambiguities in disentangling temporal and spatial variations. Solving this problem has been one of the major goals of the Cluster mission. For that purpose multipoint measurements techniques, such as the $k$-filtering, have been developed. Such techniques combine several time series recorded simultaneously at different points in space to estimate the corresponding energy density in the wavenumber space. Here we apply the technique to both simulated and Cluster magnetometer data in the solar wind (SW) and investigate the errors and limitations that arise due to the separation of the spacecraft and the quality of the tetrahedral configuration. Specifically, we provide an estimation of the minimum and maximum scales that can be accurately measured given a specific distance between the satellites and show the importance of the geometry of the tetrahedron and the relationship of that geometry to spatial aliasing. We also present recent results on characterizing small scale SW turbulence and provide scientific arguments supporting the need of new magnetometers having better sensitivity than the existing ones. Throughout the paper we emphasize technical challenges and their solutions that can be considered for a better preparation of the Cross- Scale mission. (10.1016/j.pss.2010.06.001)
  DOI : 10.1016/j.pss.2010.06.001