Publications

2010

• Microhollow cathode sustained discharges : comparative studies in micro- and equivalent macro-cell geometries
  
  • Callegari Thierry
  • Aubert X.
  • Rousseau Antoine
  • Buf J.P.
  • Pitchford L.
  The European Physical Journal D : Atomic, molecular, optical and plasma physics, EDP Sciences, 2010, 60 (3), pp.581-587. We present an experimental study of discharge initiation in a three-electrode configuration consisting of a microhollow cathode discharge (MHCD) and a third planar electrode, biased positively and placed some distance away. This work is based on the microcathode sustained (MCS) configuration where the MHCD acts as a plasma cathode and enables the generation of a stable, non-equilibrium plasma at high pressure in the volume between the MHCD and the third electrode. Our experiments were carried out in two different set-ups, one using a MHCD as a cathode and another in an equivalent macrocell geometry, easier to implement and operating at lower pressure in which the same phenomena are observed. Consistent with previous modeling results, we find that the plasma column in the volume between the MHCD and the third electrode is characterized by a low reduced electric field, with values similar to those expected for a positive column. The ignition voltage of the plasma column depends on the voltage difference between the MHCD and the third electrode, the MHCD current, and the gas pressure and gap spacing. (10.1140/epjd/e2010-00229-x)
  DOI : 10.1140/epjd/e2010-00229-x
• Nanocrystalline silicon film growth morphology control through RF waveform tailoring
  
  • Johnson E.V.
  • Verbeke T.
  • Vanel J-C
  • Booth Jean-Paul
  Journal of Physics D: Applied Physics, IOP Publishing, 2010, 43, pp.412001. We demonstrate the application of RF waveform tailoring to generate an electrical asymmetry in a capacitively coupled plasma-enhanced chemical vapour deposition system, and its use to control the growth mode of hydrogenated amorphous and nanocrystalline silicon thin films deposited at low temperature (150&#8201;°C). A dramatic shift in the dc bias potential at the powered electrode is observed when simply inverting the voltage waveform from 'peaks' to 'troughs', indicating an asymmetric distribution of the sheath voltage. By enhancing or suppressing the ion bombardment energy at the substrate (situated on the grounded electrode), the growth of thin silicon films can be switched between amorphous and nanocrystalline modes, as observed using in situ spectroscopic ellipsometry. The effect is observed at pressures sufficiently low that the collisional reduction in average ion bombardment energy is not sufficient to allow nanocrystalline growth (<100&#8201;mTorr). (10.1088/0022-3727/43/41/412001)
  DOI : 10.1088/0022-3727/43/41/412001
• Out-of-Equilibrium Phase Transitions and Time-Asymptotic One-Particle Dynamics in the Vlasov Limit of The Hamiltonian Mean Field Model
  
  • Firpo Marie-Christine
  Transport Theory and Statistical Physics, Taylor & Francis, 2010, 39 (5-7), pp.370-386. When starting from specific initial conditions, the ferromagnetic-like XY Hamiltonian mean field (HMF) model evolves toward quasistationary states, with lifetimes diverging with the number N of degrees of freedom that violate equilibrium statistical mechanics predictions. Phase transitions have been reported between low-energy magnetized quasistationary states and large energy unexpected, antiferromagnetic-like ones with low, but nonvanishing, magnetization. This issue is addressed here in the Vlasov N&#8594;&#8734; limit. It is argued that the time asymptotic states emerging in the Vlasov limit can be related to simple generic time asymptotic forms for the force field. The proposed picture unveils the nature of the out-of-equilibrium phase transitions reported for the ferromagnetic HMF in the second order regime: This is a bifurcation point connecting an effective integrable Vlasov one-particle time-asymptotic dynamic to a partly ergodic one, which means an abrupt open-up of the Vlasov one-particle phase space. This is proposed as a mechanism for second-order phase transitions compatible with nonvanishing time-asymptotic values of the order parameter in mean-field long-range systems. (10.1080/00411450.2011.563707)
  DOI : 10.1080/00411450.2011.563707
• Origin of backstreaming electrons within the quasi-perpendicular foreshock region: Two-dimensional self-consistent PIC simulation
  
  • Savoini Philippe
  • Lembège Bertrand
  • Stienlet J.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2010, 115 (A9), pp.A09104. The foreshock region is populated by energetic backstreaming particles (electrons and ions) issued from the shock after having interacted with it. Several aspects concerning the origin of these high-energy particles and their corresponding acceleration mechanisms are still unresolved. The present study is focused on a quasi-perpendicular curved shock and associated electron foreshock region (i.e., for 90° ≥ θBn ≥ 45°, where θBn is the angle between the shock normal and the upstream magnetostatic field). Two-dimensional full-particle simulation is used in order to include self-consistently the electron and ion dynamics, the full dynamics of the shock, the curvature effects and the time-of-flight effects. All expected salient features of the bow shock are recovered both for particles and for electromagnetic fields. Present simulations evidence that the fast-Fermi acceleration (magnetic mirror) mechanism, which is commonly accepted, is certainly not the unique process responsible for the formation of energetic backstreaming electrons. Other mechanisms also contribute. More precisely, three different classes of backstreaming electrons are identified according to their individual penetration depth within the shock front: (i) “magnetic mirrored” electrons which only suffer a specular reflection at the front, (ii) “trapped” electrons which succeed to penetrate the overshoot region and suffer a local trapping within the parallel electrostatic potential at the overshoot, and (iii) “leaked” electrons which penetrate even much deeper into the downstream region. “Trapped” and “leaked” electrons succeed to find appropriate conditions to escape from the shock and to be reinjected back upstream. All these different types of electrons contribute together to the formation of energetic field-aligned beam. The acceleration mechanisms associated to each electron class and/or escape conditions are analyzed and discussed. (10.1029/2010JA015263)
  DOI : 10.1029/2010JA015263
• Physical characterization of molybdenum microtips field emitter arrays
  
  • Cipriani Fabrice
  • Leblanc François
  • Illiano Jean-Marie
  • Berthelier Jean-Jacques
  • Müller F.
  Journal of Physics D: Applied Physics, IOP Publishing, 2010, 43 (6), pp.065501 (12pp). We used a dedicated hemispherical energy analyzer to measure energetic and angular distributions of electrons emitted from Molybdenum microtips integrated in a 1 cm² Field Emitter Array designed by the CEA/LETI laboratory. Such cathodes typically deliver about 25mA at an extraction voltage of 100V, and are studied in order to replace heated wires as electron sources for space applications. We find that the energy distribution of the beam strongly depends on the extraction voltage, and is therefore expected to vary across the emission life time of the device, at a rate depending both on the alteration of the resistive structure with time, and on the fate of adsorbed contaminants at the tip surface. A semi-empirical model of the emitters is proposed and used to determine parameters of energetic and angular distributions. The energy dispersion of the beam is found to increase from 2eV±20% up to 9eV±20% eV, for extraction voltages varying from 40 to 100 V. The mean angular dispersion of the beam is found to be 42° ±20% when a null electric field is set at the grid extraction surface. (10.1088/0022-3727/43/6/065501)
  DOI : 10.1088/0022-3727/43/6/065501
• Influence of charges deposition on breakdown synchronisation and propagation velocity of streamer in dielectric barrier discharge
  
  • Guaitella Olivier
  • Marinov Ilya
  • Rousseau Antoine
  , 2010.
• Whistler radiation in plasmas with cylindrical magnetic field irregularities
  
  • Krafft C.
  • Zaboronkova T. M.
  Journal of Plasma Physics, Cambridge University Press (CUP), 2010, 76 (02), pp.193-207. The radiation of whistler waves by linear dipole sources immersed in magnetoplasmas with cylindrical magnetic field inhomogeneities are studied. Two types of irregularities are investigated: magnetic field enhancements and depletions. A theoretical analysis is developed for comparatively weak local perturbations of the ambient magnetic field. Results are provided by numerical calculations performed for physical conditions typical of laboratory experiments involving artificially created magnetic field irregularities. It is shown that plasma regions with locally enhanced (depleted) magnetic field intensities can increase (decrease) the amplitudes of whistler waves radiated by dipole sources, regardless of their orientation with respect to the ambient magnetic field. Results are relevant to space and laboratory experiments on very low-frequency wave radiation. (10.1017/S0022377809990249)
  DOI : 10.1017/S0022377809990249
• Magnetic energy distribution in the four-dimensional frequency and wave vector domain in the solar wind
  
  • Narita Y.
  • Sahraoui Fouad
  • Goldstein M. L.
  • Glassmeier K.-H.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2010, 115, pp.4101. We present a measurement of the energy distribution in the four-dimensional (4-D) frequency and wave vector domain of magnetic field fluctuations in the solar wind. The measurement makes use of the wave telescope technique that has been developed particularly for multispacecraft data analysis. We review briefly the theoretical background and then present a numerical test using synthetic data; the technique is then applied to magnetic field data obtained while the Cluster spacecraft was in the solar wind. The energy distribution is determined in the flow rest frame in the frequency range below 0.2 rad/s and the wave number range below 0.0015 rad/km, corrected for the Doppler shift. We find the following properties in the energy distribution in the rest frame: (1) a double anisotropy in the wave vector domain associated with the mean magnetic field and the flow directions, (2) a symmetric distribution with respect to the sign of wave vector, and (3) no evidence for a linear dispersion relation in the frequency and wave number domain. Since the flow direction in the analyzed time interval is close to the normal direction to the bow shock, the anisotropy may well be associated with the bow shock. These results suggest that the solar wind is in a state of well-developed strong turbulence and justifies the theoretical picture of quasi-two-dimensional turbulence that obtains in the presence of a (relatively) strong DC magnetic field. However, the fluctuations are not axisymmetric around the mean field and the energy distribution is extended in the perpendicular direction to the flow or shock normal. Anisotropy associated with the boundary is reminiscent of previously reported magnetosheath turbulence. This study opens a way to investigate solar wind turbulence in the full 4-D frequency and wave vector space. (10.1029/2009JA014742)
  DOI : 10.1029/2009JA014742
• A transient narrow poleward extrusion from the diffuse aurora and the concurrent magnetotail activity
  
  • Liu W. W.
  • Le Contel Olivier
  • Zong Q.-G.
  • Lui A. T. Y.
  • Spanswick E.
  • Donovan E. F.
  • Liang Jun
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2010, 115, pp.10210. We report observation of a transient narrow auroral feature extruding from the poleward boundary of the diffuse aurora on March 19, 2009. It moved westward and poleward initially to form part of a vortex pattern, followed by its equatorward-dawnward retreat later. During this auroral activity, THEMIS satellites, projected near the same magnetic local time of the auroral feature, detected appreciable plasma flows, increase in the ratio of the ion energy over the electron energy, and some enhancements of electrostatic waves. The plasma flows were initially duskward-earthward and changed to duskward-tailward later. The overall development of the observed plasma flow pattern was detected during the equatorward-dawnward retreat of the auroral feature when the Alfvén transit time between the magnetotail and the ionosphere is taken into account. This suggests that THEMIS satellites remotely sensed a counter-clockwise flow vortex (viewed from above the equatorial plane) in the magnetotail with decreasing strength. We suggest that the process generating the auroral feature is related to the flow vortex in association with the depletion of the electron energy relative to the ion energy and wave-particle interaction. An estimate of the possible associated current density is made. We provide reasoning for this auroral feature to be an auroral streamer and not a ``failed'' transpolar arc. (10.1029/2010JA015449)
  DOI : 10.1029/2010JA015449
• Wave properties in the magnetic reconnection diffusion region with high Beta: Application of the k-filtering method to Cluster multispacecraft data
  
  • Huang S. Y.
  • Zhou M.
  • Sahraoui Fouad
  • Deng X. H.
  • Pang Y.
  • Yuan Z. G.
  • Wei Q.
  • Wang J. F.
  • Zhou X. M.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2010, 115 (A12), pp.12211. Magnetic reconnection is a crucial physical process in laboratory and astrophysical plasmas. Plasma waves are believed to provide the dissipation mechanism in magnetic reconnection. In this paper we analyze the properties of low-frequency waves in a magnetotail reconnection diffusion region with a small guide field and high &#946;. Using the k-filtering method on the magnetic field data measured by Cluster spacecraft, we found that low-frequency waves in the diffusion region were highly oblique propagating mode. We compared the measured dispersion relation with theoretical ones calculated using the linear (hot) two-fluid and Vlasov-Maxwell theory. It is found that the observed waves in the diffusion region (with high plasma &#946;) follow the dispersion relation of the Alfvén-Whistler wave mode. Comparisons with previous simulations and observational results are also discussed. (10.1029/2010JA015335)
  DOI : 10.1029/2010JA015335
• Charged particle acceleration in the hermean magnetosphere: the role of dipolarizations, plasma turbulence and induction electric fields
  
  • Zelenyi L. M.
  • Korgov A.
  • Malova H. V.
  • Popov V. Y.
  • Artemyev A. V.
  • Delcourt Dominique
  , 2010, 19, pp.9.
• Les Plasmas Spatiaux au LPP
  
  • Rezeau Laurence
  Flash X - La revue scientifique de l'Ecole polytechnique, Ecole polytechnique, 2010, 12, pp.31-32. ISSN : 1775-0385
• Shell models and the possibility of application to fusion plasmas
  
  • Gürcan Özgür D.
  • Hennequin Pascale
  • Vermare Laure
  • Garbet X.
  • Diamond P.H.
  Plasma Physics and Controlled Fusion, IOP Publishing, 2010, 52, pp.045002. An extensive study of spectral shell models with possibilities for application to fusion plasmas is discussed. A set of shell models addressing various aspects of the characteristics of fusion plasmas have been derived. Difficulties associated with plasma medium, namely its intrinsic excitability, and importance of mescals have been discussed. The numerical implementation of shell models is discussed. It was observed that depending on the parameter regime, they may lead to steady state or display characteristics of predatorprey dynamics. (10.1088/0741-3335/52/4/045002)
  DOI : 10.1088/0741-3335/52/4/045002
• Wave-Vector Dependence of Magnetic-Turbulence Spectra in the Solar Wind
  
  • Narita Y.
  • Glassmeier K.-H.
  • Sahraoui Fouad
  • Goldstein M. L.
  Physical Review Letters, American Physical Society, 2010, 104 (17), pp.171101. Using four-point measurements of the Cluster spacecraft, the energy distribution was determined for magnetic field fluctuations in the solar wind directly in the three-dimensional wave-vector domain in the range |k|&#8804;1.5×10&#8722;3&#8201;&#8201;rad/km. The energy distribution exhibits anisotropic features characterized by a prominently extended structure perpendicular to the mean field preferring the ecliptic north direction and also by a moderately extended structure parallel to the mean field. From the three-dimensional energy distribution wave vector anisotropy is estimated with respect to directions parallel and perpendicular to the mean magnetic field, and the result suggests the dominance of quasi-two-dimensional turbulence toward smaller spatial scales. (10.1103/PhysRevLett.104.171101)
  DOI : 10.1103/PhysRevLett.104.171101
• Nonlinear dynamics of resonant interactions between wave packets and particle distributions with loss-cone-like structures
  
  • Krafft C.
  • Volokitin A.
  • Zaslavsky A.
  Physical Review E, American Physical Society (APS), 2010, 82, pp.066402. The paper studies the nonlinear mechanisms at work in magnetized plasmas when wave packets interact resonantly with particle distributions presenting loss-cone-like structures. Lower hybrid waves are considered in view of the great importance, in space and laboratory plasmas, of waves with frequencies below the electron cyclotron frequency. Owing to a three-dimensional Hamiltonian model and a numerical symplectic code, the authors study the nonlinear stage of the loss-cone instability for various particle distributions and wave spectra involving symmetric and asymmetric features. In particular, the wave-particle interaction process of dynamical resonance merging, which results from an instability of the trapped particles' motion and leads to complex stochastic phenomena, is discussed. Whereas interactions at normal cyclotron resonances are mostly considered, the role of the Landau and the anomalous cyclotron resonances is also studied to explain thoroughly the nonlinear wave-particle dynamics as well as the competition between loss-cone, fan, and beam instabilities. The relaxed particle distributions and the saturated wave spectra are analyzed. The time necessary for filling the loss-cone structures is determined as a function of the characteristics of the particle distributions. Whereas most of the previous works analyzed the asymptotic stage of the system's evolution in the frame of the well-known quasilinear theory, the paper considers the case when the energy carried by the wave packet is sufficiently large so that the description of the physical processes at work cannot be limited to the frame of weak turbulence theories. (10.1103/PhysRevE.82.066402)
  DOI : 10.1103/PhysRevE.82.066402