Publications

2013

• Goniopolarimetry with Coupled Electric and Magnetic Measurements
  
  • Cecconi B.
  • Gautier A.-L.
  • Bergman J.E.S
  • Chust Thomas
  • Marchaudon A.
  • Cavoit C.
  • Santolík O.
  , 2013.
• Building small scales in MHD turbulence
  
  • Verdini Andrea
  • Grappin Roland
  • Pinto Rui
  • Velli Marco
  , 2013, 1539, pp.74-77. Magneto-hydrodynamic turbulence (MHD) with a mean large-scale field is known to produce an anisotropic cascade, with energy mostly in perpendicular scales. We use a shell-model version of the Reduced MHD equations to simulate turbulence in homogeneous periodic conditions, in coronal loops, and in the solar wind. We compare the perpendicular and parallel spectra and show that different regimes of weak turbulence develop in loops and in the solar wind. We briefly comment on the way their characteristic large-scale features influence the weak turbulence spectra and their transition to strong turbulence. (10.1063/1.4810993)
  DOI : 10.1063/1.4810993
• Solar wind turbulence: the fight between the direct turbulent cascade and the (anisotropic) expansion
  
  • Grappin Roland
  , 2013.
• On the origin of the quasi-perpendicular ion foreshock: Full-particle simulations
  
  • Savoini Philippe
  • Lembège Bertrand
  • Stienlet J.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2013, 118 (3), pp.1132-1145. Many space missions have already evidenced the existence of the ion foreshock region located upstream of the Earth's bow shock and populated by energetic backstreaming ions reflected by the shock front. In order to analyse this region, a curved shock is simulated with a 2-D particle-in-cell (PIC) code. The analysis is presently restricted to the quasi-perpendicular angular range defined by 45° ≤ θBn ≤ 90°. In agreement with experimental data, present results evidence two distinct ion populations back-streaming from the shock front along the interplanetary magnetic field: (i) the Field-Aligned Beam population (hereafter "FAB") and (ii) the gyrophase bunched population (hereafter "GPB") which differ from each other by their gyrotropic or non-gyrotropic behavior, respectively. Excluded by a simulation time which is too short, ion instabilities pitch-angle scattering cannot be the source of "GPB". Two new criteria are proposed to identify more precisely each population: their interaction time Δtint with the shock front and their downstream penetration depth. These criteria show that (i) the "FAB" population moves back and forth between the upstream edge of the shock front and the overshoot, and is characterized by a Δtint covering several upstream gyro-periods. (ii) In contrast, the "GPB" ions suffer a short interaction time (i.e. 1 < τci). We observe that the "FAB" ions may have different originsalthough all "GPB" ions seem to be produced by the electrostatic field built up at the shock and are emitted in a burst-like mode rather than in continuous way. (10.1002/jgra.50158)
  DOI : 10.1002/jgra.50158
• Anisotropy of radiation emitted from planar wire arrays
  
  • Kantsyrev Viktor L.
  • Chuvatin Alexandre S.
  • Esaulov A. A.
  • Safronova Alla S.
  • Rudakov Leonid I.
  • Velikovich A. L.
  • Williamson Kenneth M.
  • Osborne Glenn C.
  • Shrestha I. K.
  • Weller Michael E.
  • Shlyaptseva V. V.
  Physics of Plasmas, American Institute of Physics, 2013, 20 (07), pp.070702. The planar wire array (PWA) is a promising load for new multi-source inertial confinement fusion (ICF) hohlraums [B. Jones et al. Phys. Rev. Lett. 104 125001 (2010)]. The hohlraum radiation symmetry is an important issue for ICF. It was found that extreme ultraviolet and sub-keV photon emission from PWAs may have considerable anisotropy in the load azimuthal plane. This experimental result is obtained on the UNR 11.7 MA Zebra generator. The time-dependent anisotropy effect is detected. This feature is studied in 2D numerical simulations and can be explained by initial anisotropy of implosion of those non-cylindrical loads radiating essentially as surface sources in sub-keV quanta and also by radiation absorption in cold magnetized plasma tails forming in the direction of magnetic compression. (10.1063/1.4817023)
  DOI : 10.1063/1.4817023
• A spectroscopic study of ethylene destruction and by-product generation using a three-stage atmospheric packed-bed plasma reactor
  
  • Hubner Antoine
  • Guaitella Olivier
  • Rousseau Antoine
  • Roepcke J.
  Journal of Applied Physics, American Institute of Physics, 2013, 114, pp.033301. Using a three-stage dielectric packed-bed plasma reactor at atmospheric pressure, the destruction of ethylene, a typical volatile organic compound, and the generation of major by-products have been studied by means of Fourier Transform Infrared Spectroscopy. A test gas mixture air at a gas flow of 1 slm containing 0.12% humidity with 0.1% ethylene has been used. In addition to the fragmentation of the precursor gas, the evolution of the concentration of ten stable reaction products, CO, CO2, O3, NO2, N2O, HCN, H2O, HNO3, CH2O, and CH2O2 has been monitored. The concentrations of the by-products range between 5&#8201;ppm, in the case of NO2, and 1200&#8201;ppm, for H2O. By the application of three sequentially working discharge cells at a frequency of f&#8201;=&#8201;4 kHz and voltage values between 9 and 12&#8201;kV, a nearly complete decomposition of C2H4 could be achieved. Furthermore, the influence of the specific energy deposition (SED) on the destruction process has been studied and the maximum value of SED was about 900 J l&#8722;1. The value of the characteristic energy &#946;, characterizing the energy efficiency of the ethylene destruction in the reactor, was found to be 330 J l&#8722;1. It was proven that the application of three reactor stages suppresses essentially the production of harmful by-products as formaldehyde, formic acid, and NO2 compared to the use of only one or two stages. Based on the multi-component detection, the carbon balance of the plasma chemical conversion of ethylene has been analyzed. The dependence of the fragmentation efficiencies of ethylene (RF(C2H4)&#8201;=&#8201;5.5&#8201;×&#8201;1019 molecules J&#8722;1) and conversion efficiencies to the produced molecular species (RC&#8201;=&#8201;(0.13)&#8201;×&#8201;1016 molecules J&#8722;1) on the discharge conditions could be estimated in the multistage plasma reactor. (10.1063/1.4813409)
  DOI : 10.1063/1.4813409
• Physics of Stimulated L->H Transitions
  
  • Miki K.
  • Diamond P.H.
  • Hahn S. -H.
  • Xiao W. W.
  • Gürcan Özgür D.
  • Tynan G.R.
  Physical Review Letters, American Physical Society, 2013, 110, pp.195002. We report on model studies of stimulated L&#8594;H transitions. These studies use a novel reduced mesoscale model. Studies reveal that L&#8594;H transitions can be triggered by particle injection into a subcritical state (i.e., P<PThresh). Particle injection alters the edge mean flow shear via changes of density and temperature gradients. The change of edge mean flow shear is critical to turbulence collapse and the subsequent stimulated transition. For low ambient heating, strong injection is predicted to trigger a transient turbulence collapse. We predict that repetitive injection can maintain the turbulence collapse and so sustain a driven H-mode-like state. The total number of particles required to induce a transition by either injection or gas puffing is estimated. Results indicate that the total number of injected particles required is much smaller than that required for inducing a transition by gas puffing. Thus, we show that internal injection is more efficient than gas puffing of comparable strength. We also observe that zonal flows do not play a critical role in stimulated transitions. (10.1103/PhysRevLett.110.195002)
  DOI : 10.1103/PhysRevLett.110.195002
• CLUSTER observation of polar electron precipitation above the polar caps during periods of Northward IMF
  
  • Fontaine Dominique
  • Maggiolo R.
  AGU Fall Meeting Abstracts, AGU, 2013, 21, pp.07. The CLUSTER spacecraft revealed the presence of successive current sheets of opposite polarity above the polar caps during periods of northward or weak IMF. We first present the general electrodynamical context. At CLUSTER altitude (5-7 RE), the upward part of this current system consists of ion beams accelerated by quasi-static electric fields, associated with precipitating electrons. They are surrounded by low energy upflowing electron beams carrying a downward current. We then focus on the precipitating electrons above the polar cap which form acceleration structures at about 100 - 300 eV. This acceleration is interpreted as the effect of an electrostatic potential along magnetic field lines located above CLUSTER altitude, i.e. typically above 5-7 RE. We present statistics on the characteristics of these precipitating electron structures and we discuss the source regions and the mechanisms possibly at their origin.
• Nonlinear saturation of wave packets excited by low-energy electron horseshoe distributions
  
  • Krafft C.
  • Volokitin A.
  Physical Review E, American Physical Society (APS), 2013, 87, pp.053107. Horseshoe distributions are shell-like particle distributions that can arise in space and laboratory plasmas when particle beams propagate into increasing magnetic fields. The present paper studies the stability and the dynamics of wave packets interacting resonantly with electrons presenting low-energy horseshoe or shell-type velocity distributions in a magnetized plasma. The linear instability growth rates are determined as a function of the ratio of the plasma to the cyclotron frequencies, of the velocity and the opening angle of the horseshoe, and of the relative thickness of the shell. The nonlinear stage of the instability is investigated numerically using a symplectic code based on a three-dimensional Hamiltonian model. Simulation results show that the dynamics of the system is mainly governed by wave-particle interactions at Landau and normal cyclotron resonances and that the high-order normal cyclotron resonances play an essential role. Specific features of the dynamics of particles interacting simultaneously with two or more waves at resonances of different natures and orders are discussed, showing that such complex processes determine the main characteristics of the wave spectrum's evolution. Simulations with wave packets presenting quasicontinuous spectra provide a full picture of the relaxation of the horseshoe distribution, revealing two main phases of the evolution: an initial stage of wave energy growth, characterized by a fast filling of the shell, and a second phase of slow damping of the wave energy, accompanied by final adjustments of the electron distribution. The influence of the density inhomogeneity along the horseshoe on the wave-particle dynamics is also discussed. (10.1103/PhysRevE.87.053107)
  DOI : 10.1103/PhysRevE.87.053107
• Action diffusion and lifetimes of quasistationary states in the Hamiltonian Mean Field model
  
  • Ettoumi Wahb
  • Firpo Marie-Christine
  Physical Review E, American Physical Society (APS), 2013, 87, pp.030102(R). Out-of-equilibrium quasistationary states (QSSs) are one of the signatures of a broken ergodicity in long-range interacting systems. For the widely studied Hamiltonian Mean-Field model, the lifetime of some QSSs has been shown to diverge with the number N of degrees of freedom with a puzzling N¹.7 scaling law, contradicting the otherwise widespread N scaling law. It is shown here that this peculiar scaling arises from the locality properties of the dynamics captured through the computation of the diffusion coefficient in terms of the action variable. The use of a mean first passage time approach proves to be successful in explaining the non-trivial scaling at stake here, and sheds some light on another case, where lifetimes diverging as e^N above some critical energy have been reported. (10.1103/PhysRevE.87.030102)
  DOI : 10.1103/PhysRevE.87.030102
• Anomalous collisionality in low-pressure plasmas
  
  • Lafleur Trevor
  • Chabert Pascal
  • Turner M.M.
  • Booth Jean-Paul
  Physics of Plasmas, American Institute of Physics, 2013, 20 (12), pp.124503. Based on a theoretical argument from fundamental kinetic theory, by way of simple worked examples, and through the use of particle-in-cell simulations of capacitively coupled plasmas, we demonstrate that conventional methods for calculating the momentum transfer collision frequency in low-pressure plasmas can be seriously erroneous. This potentially plays an important and previously unconsidered role in many low-pressure discharges, and at least in part provides a possible explanation for anomalous behaviour often encountered in these plasmas. (10.1063/1.4859155)
  DOI : 10.1063/1.4859155
• Theory for the self-bias formation in capacitively coupled plasmas excited by arbitrary waveforms
  
  • Lafleur Trevor
  • Chabert Pascal
  • Turner M.M.
  • Booth Jean-Paul
  Plasma Sources Science and Technology, IOP Publishing, 2013, 22 (6), pp.065013. We develop a semi-analytical theory for the self-bias formation in capacitively coupled plasmas excited by arbitrary radio-frequency (rf) waveforms. The requirement of rf current continuity and voltage balance across the discharge results in the need for a self-bias voltage to develop with non-sinusoidal excitations, even in geometrically symmetric systems. The theory is compared extensively with a wide range of experimental and particle-in-cell (PIC) simulation data within the literature, and is found to be in excellent agreement. Furthermore, it is shown that the present theory is formally equivalent to the original model proposed by Heil et al (2008 J. Phys. D: Appl. Phys. 41 165202), but goes further by explicitly allowing the time-varying sheath voltages and symmetry parameter to be evaluated without input from PIC simulations. (10.1088/0963-0252/22/6/065013)
  DOI : 10.1088/0963-0252/22/6/065013
• Capacitively coupled radio-frequency plasmas excited by tailored voltage waveforms
  
  • Lafleur Trevor
  • Delattre Pierre-Alexandre
  • Johnson E.V.
  • Booth Jean-Paul
  Plasma Physics and Controlled Fusion, IOP Publishing, 2013, 55 (12), pp.124002. By applying certain types of ?tailored? voltage waveforms (TVWs) to capacitively coupled plasmas, a dc self-bias and an asymmetric plasma response can be produced, even in geometrically symmetric reactors. Furthermore, these arbitrary applied waveforms can produce a number of interesting phenomena that are not present in typical single-frequency sinusoidal discharges. This electrical asymmetry effect presents emerging possibilities for the improved control of the ion energy and ion flux in these systems; parameters of vital importance to both etching and deposition applications for materials processing. With a combined research approach utilizing both experimental measurements, and particle-in-cell simulations, we review and extend recent investigations that study a particular class of TVW. The waveforms used have a pulse-type shape and are composed of a varying number of harmonic frequencies. This allows a strong self-bias to be produced, and causes most of the applied voltage to be dropped across a single sheath. Additionally, decreasing the pulse width (by increasing the number of harmonics), allows the plasma density and ion flux to be increased. Simulation and experimental results both demonstrate that this type of waveform can be used to separately control the ion flux and ion energy, while still producing a uniform plasma over large area (50 cm diameter) rf electrodes. (10.1088/0741-3335/55/12/124002)
  DOI : 10.1088/0741-3335/55/12/124002
• Statistical properties of planetary heavy-ion precipitations toward the Martian ionosphere obtained from Mars Express
  
  • Hara T.
  • Seki K.
  • Futaana Y.
  • Yamauchi M.
  • Barabash S.
  • Fedorov A. O.
  • Yagi M.
  • Delcourt Dominique C.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2013, 118 (8), pp.5348-5357. The interplanetary magnetic field (IMF) embedded in the solar wind interacts with the Martian crustal magnetic field and atmosphere. The IMF orientation is one of the important parameters to control the acceleration and precipitation of planetary heavy ions (PHIs). We statistically investigate the effects of the IMF orientation on PHI precipitations toward the ionosphere based on observations by Mars Express (MEX). We identified 59 PHI precipitation events between July 2007 and September 2009. To estimate the IMF orientation without magnetometer that MEX does not carry, we used the velocity distribution of exospheric-origin pickup protons. We estimated the IMF orientation without its polarity for 10 events. The results show that the precipitations of PHIs tend to be observed around pole regions in the MSE (Mars-centered, solar electrical) coordinates determined from the solar wind electric field (Esw), in which the pole axis directs to the parallel or antiparallel to Esw due to the ambiguity in the IMF polarity determination. The observed precipitating PHIs are accelerated only up to a few keV. This feature may reflect the short distance from the picked-up region. For one of these 10 events, we estimated the IMF polarity by comparing the velocity distribution of exospheric-origin pickup protons observed by MEX with those obtained from statistical trajectory tracing simulations under two cases of possible IMF polarity conditions. The estimated polarity indicates that the PHI precipitation in this event is observed in the downward electric field hemisphere in MSE, where Esw points to Mars in the pole region. (10.1002/jgra.50494)
  DOI : 10.1002/jgra.50494
• Simulation benchmarks for low-pressure plasmas: Capacitive discharges
  
  • Turner M.M.
  • Derzsi A.
  • Donkó Z.
  • Eremin D.
  • Kelly S.J.
  • Lafleur Trevor
  • Mussenbrock T.
  Physics of Plasmas, American Institute of Physics, 2013, 20, pp.013507. Benchmarking is generally accepted as an important element in demonstrating the correctness of computer simulations. In the modern sense, a benchmark is a computer simulation result that has evidence of correctness, is accompanied by estimates of relevant errors, and which can thus be used as a basis for judging the accuracy and efficiency of other codes. In this paper, we present four benchmark cases related to capacitively coupled discharges. These benchmarks prescribe all relevant physical and numerical parameters. We have simulated the benchmark conditions using five independently developed particle-in-cell codes. We show that the results of these simulations are statistically indistinguishable, within bounds of uncertainty that we define. We, therefore, claim that the results of these simulations represent strong benchmarks, which can be used as a basis for evaluating the accuracy of other codes. These other codes could include other approaches than particle-in-cell simulations, where benchmarking could examine not just implementation accuracy and efficiency, but also the fidelity of different physical models, such as moment or hybrid models. We discuss an example of this kind in the Appendix. Of course, the methodology that we have developed can also be readily extended to a suite of benchmarks with coverage of a wider range of physical and chemical phenomena. (10.1063/1.4775084)
  DOI : 10.1063/1.4775084
• Structures of dayside whistler-mode waves deduced from conjugate diffuse aurora
  
  • Nishimura Y.
  • Bortnik J.
  • Li W.
  • Thome R.M.
  • Ni B.
  • Lyons L.R.
  • Angelopoulos V.
  • Ebihara Y.
  • Bonnell J. W.
  • Le Contel Olivier
  • Auster U.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2013, 118 (2), pp.664-673. [1] We present simultaneous measurements of dayside diffuse aurora and whistler-mode waves made by the South Pole all-sky imager and two of the THEMIS spacecraft. We found a high correlation between the diffuse aurora intensity at 557.7&#8201;nm near the footprint latitudes of THEMIS and whistler-mode wave intensity measured on board. The power in other wave modes was negligibly small in most cases, indicating that the dayside diffuse aurora is driven by precipitating energetic electrons resonating with whistler-mode waves. The high correlation over a wide L* range (6&#8201;<&#8201;L*&#8201;<&#8201;11) further allowed us to magnetically link the wave and magnetospheric plasma distributions with the auroral patterns. Two distinct regions of whistler-mode waves and ambient plasma density were found outside the plasmasphere near the equator: (1) intense waves in a smooth, low density and (2) moderate waves with enhanced and fluctuating density. The whistler-mode wave intensity in the fluctuating plasma density region is positively correlated with the ambient density variations. The corresponding auroral images show an azimuthally elongated diffuse auroral band on the field lines connected to the low density region, as opposed to a structured diffuse aurora on the fluctuating density field lines. Each structured diffuse auroral patch was stable for a few tens of minutes and slowly drifted azimuthally. The high correlation of waves and auroras indicates that the structured diffuse auroral pattern reflects the spatial distribution of whistler-mode waves and ambient plasma density in space. The enhanced density measured by the spacecraft is quasi-spatial and contributes to enhanced growth of whistler-mode waves. (10.1029/2012JA018242)
  DOI : 10.1029/2012JA018242
• Interaction of plasma transport and turbulence on particle fuelling
  
  • Tamain Patrick
  • Bonhomme Gérard
  • Brochard Frédéric
  • Clairet Frédéric
  • Gil C.
  • Gunn J.
  • Hennequin P.
  • Hornung G.
  • Segui J. L.
  • Vermare L.
  • Ghendrih Philippe
  • Team Tore Supra
  Journal of Nuclear Materials, Elsevier, 2013, 438 (S), pp.S148-S154. We report the results of an experimental investigation of the impact of Supersonic Molecular Beam Injection in the Tore Supra tokamak. Several diagnostics were synchronised with the injection to extract a global picture of the physics at play from the time scale of turbulence (similar to 10 mu s) to the full-recovery time (similar to 1 s). As previously reported, a strong impact of the injection on density and temperature profiles is observed. Both fields exhibit a complex dynamic response involving different phases and time scales. In particular, we show that the effective particle fuelling efficiency is determined by a period of degraded confinement that follows the injection, during which the edge density collapses, in some cases, lower than the initial one. This phase is characterised by a dramatic change in the turbulent transport, with a drop of the frequency spectrum and the observation of large coherent structures as opposed to small intermittent fluctuations before the injection. (10.1016/j.jnucmat.2013.01.023)
  DOI : 10.1016/j.jnucmat.2013.01.023
• Study of reconnection physics using laser-generated B fields
  
  • Smets Roch
  • Belmont Gérard
  • Aunai N.
  • Fuchs J. C.
  • Boniface C.
  • Aulanier G.
  , 2013.
• Commutation rapide déclenchée par filamentation laser femtoseconde
  
  • Larour Jean
  • Arantchouk Léonid
  • Houard Aurélien
  Flash X - La revue scientifique de l'Ecole polytechnique, Ecole polytechnique, 2013, 15, pp.55-57. La revue scientifique de l'Ecole polytechnique (parution annuelle) ISSN : 1775-0385 Le phénomène de filamentation laser dans lair a été décrit dans un précédent numéro (Flash X n° 12) par A. Houard et A. Mysyrowicz, chercheurs du LOA (Laboratoire dOptique appliquée). Rappelons ici quil apparaît spontanément le long dun faisceau laser de durée femtoseconde (fs) quand sa puissance dépasse un seuil critique de quelques GW. un équilibre dynamique entre la diffraction naturelle du faisceau, leffet Kerr, et la défocalisation par lair ionisé, maintient lintensité dans le coeur du faisceau sur une distance qui peut dépasser un mètre, entraînant la formation dun canal de plasma dans le sillage de limpulsion. Le point de démarrage, la longueur des canaux de plasma et leur distribution transverse sont contrôlables en jouant sur les paramètres de limpulsion laser initiale (énergie, durée, diamètre du faisceau).
• Role of the terrestrial bow shock on magnetic clouds structure: 2. 3D analytical MHD model
  
  • Turc Lucile
  • Fontaine Dominique
  • Kilpua E. K. J.
  • Savoini Philippe
  , 2013. Magnetic clouds (MC) figure among the most important drivers of magnetic storms. In the solar wind, they present a very distinctive structure. However, before reaching the magnetosphere, MCs encounter the bow shock which modifies their structure, and therefore may influence their geoeffectivity. In order to understand how the magnetic structure of MCs is altered by the shock, a simple 3D MHD model is used to calculate the magnetic field strength and direction inside the magnetosheath. We present several outputs of the model, corresponding to different MC axis orientations and to different impact parameters. The variation of the magnetic field direction from the solar wind to the magnetosheath appears to be strongly driven by the shock obliquity. Asymmetries due to different shock configurations may arise inside the magnetosheath. Moreover, the Bz component can even reverse in some parts of the magnetosheath. The model outputs are compared with spacecraft observations. Finally, we discuss the impact of our conclusions on MCs geoeffectivity.
• Energetic electron acceleration by unsteady magnetic reconnection
  
  • Fu H.S.
  • Khotyaintsev Y. V.
  • Vaivads A.
  • Retinò Alessandro
  • André M.
  Nature Physics, Nature Publishing Group [2005-....], 2013, 9, pp.426-430. The mechanism that produces energetic electrons during magnetic reconnection is poorly understood. This is a fundamental process responsible for stellar flares, substorms, and disruptions in fusion experiments. Observations in the solar chromosphere and the Earth's magnetosphere indicate significant electron acceleration during reconnection, whereas in the solar wind, energetic electrons are absent. Here we show that energetic electron acceleration is caused by unsteady reconnection. In the Earth's magnetosphere and the solar chromosphere, reconnection is unsteady, so energetic electrons are produced; in the solar wind, reconnection is steady, so energetic electrons are absent. The acceleration mechanism is quasi-adiabatic: betatron and Fermi acceleration in outflow jets are two processes contributing to electron energization during unsteady reconnection. The localized betatron acceleration in the outflow is responsible for at least half of the energy gain for the peak observed fluxes. (10.1038/nphys2664)
  DOI : 10.1038/nphys2664
• Radial correlation of density fluctuations by coupling IPP and LPP W-band Doppler reflectometers on ASDEX Upgrade
  
  • Hennequin Pascale
  • Happel T.
  • Conway G. D.
  • Honoré Cyrille
  • Vermare Laure
  • Pisarev V.
  • Giacalone J-C.
  • Gürcan Özgür D.
  • Asdex Upgrade Team
  , 2013 (oral).
• Effects of the surface conductivity and the IMF strength on the dynamics of planetary ions in Mercury's magnetosphere
  
  • Seki Kanako
  • Terada Naoki
  • Yagi Manabu
  • Delcourt Dominique C.
  • Leblanc François
  • Ogino Tatsuki
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2013, 118 (6), pp.3233-3242. To examine the effects of planetary surface conductivity and the southward IMF strength on ion dynamics, systematic trajectory tracings of Na⁺ ions were performed in the electric and magnetic field configurations obtained from magnetohydrodynamics (MHD) simulations of the solar wind-Mercury interaction. Comparison with a previous study, which used an analytical model that rescaled the Earth's magnetosphere and assumed the existence of the distant neutral line (DNL) in Mercury's magnetotail, shows a drastic change in the Na⁺ precipitation pattern onto due to the formation of the near-Mercury neutral line (NMNL) in MHD simulations. The Na⁺ precipitation band at approximately 30 degrees of latitude (LAT), which was obtained in the previous study, disappeared in the equivalent low-conductivity MHD case due to the NMNL formation, while the NMNL formation causes high-energy Na⁺ precipitation in the equatorial region. The change in strength of the southward IMF (sBz) alters the location of the NMNL and the Na⁺ precipitation pattern. In the low-conductivity sBz = 5 case, both the equatorial precipitation and the Na⁺ band at approximately LAT = 30 are formed. In the high-conductivity sBz = 5 case, magnetospheric convection through the polar regions is suppressed, which results in a region of dense Na⁺ near the planet. These results suggest that the precipitation pattern of planetary ions onto Mercury's surface changes significantly with the activity level of Mercury's magnetosphere. It is also suggested that observations of the magnetospheric convection, the distribution of Na⁺ ions around the planet, or the precipitation pattern of Na⁺ ions onto the planetary surface can provide us information about the surface conductivity. (10.1002/jgra.50181)
  DOI : 10.1002/jgra.50181
• Microsecond ramp compression of a metallic liner driven by a 5 MA current on the SPHINX machine using a dynamic load current multiplier pulse shaping
  
  • d'Almeida Thierry
  • Lassalle Francis
  • Morell Alain
  • Grunenwald Julien
  • Zucchini Frédéric
  • Loyen Arnaud
  • Maysonnave Thomas
  • Chuvatin Alexandre S.
  Physics of Plasmas, American Institute of Physics, 2013, 20 (09), pp.092512. SPHINX is a 6 MA, 1-&#956;s Linear Transformer Driver (LTD) operated by the CEA Gramat (France) and primarily used for imploding Z-pinch loads for radiation effects studies. Among the options that are currently being evaluated to improve the generator performances are an upgrade to a 20&#8201;MA, 1-&#956;s LTD machine and various power amplification schemes, including a compact Dynamic Load Current Multiplier (DLCM). A method for performing magnetic ramp compression experiments, without modifying the generator operation scheme, was developed using the DLCM to shape the initial current pulse in order to obtain the desired load current profile. In this paper, we discuss the overall configuration that was selected for these experiments, including the choice of a coaxial cylindrical geometry for the load and its return current electrode. We present both 3-D Magneto-hydrodynamic and 1D Lagrangian hydrodynamic simulations which helped guide the design of the experimental configuration. Initial results obtained over a set of experiments on an aluminium cylindrical liner, ramp-compressed to a peak pressure of 23&#8201;GPa, are presented and analyzed. Details of the electrical and laser Doppler interferometer setups used to monitor and diagnose the ramp compression experiments are provided. In particular, the configuration used to field both homodyne and heterodyne velocimetry diagnostics in the reduced access available within the liner's interior is described. Current profiles measured at various critical locations across the system, particularly the load current, enabled a comprehensive tracking of the current circulation and demonstrate adequate pulse shaping by the DLCM. The liner inner free surface velocity measurements obtained from the heterodyne velocimeter agree with the hydrocode results obtained using the measured load current as the input. An extensive hydrodynamic analysis is carried out to examine information such as pressure and particle velocity history profiles or magnetic diffusion across the liner. The potential of the technique in terms of applications and achievable ramp pressure levels lies in the prospects for improving the DLCM efficiency through the use of a closing switch (currently under development), reducing the load dimensions and optimizing the diagnostics. (10.1063/1.4823720)
  DOI : 10.1063/1.4823720
• Electron nongyrotropy in the context of collisionless magnetic reconnection
  
  • Aunai Nicolas
  • Hesse Michael
  • Kuznetsova M. M.
  Physics of Plasmas, American Institute of Physics, 2013, 20, pp.2903. Collisionless magnetized plasmas have the tendency to isotropize their velocity distribution function around the local magnetic field direction, i.e., to be gyrotropic, unless some spatial and/or temporal fluctuations develop at the particle gyroscales. Electron gyroscale inhomogeneities are well known to develop during the magnetic reconnection process. Nongyrotropic electron velocity distribution functions have been observed to play a key role in the dissipative process breaking the field line connectivity. In this paper, we present a new method to quantify the deviation of a particle population from gyrotropy. The method accounts for the full 3D shape of the distribution and its analytical formulation allows fast numerical computation. Regions associated with a significant degree of nongyrotropy are shown, as well as the kinetic origin of the nongyrotropy and the fluid signature it is associated with. Using the result of 2.5D Particle-In-Cell simulations of magnetic reconnection in symmetric and asymmetric configurations, it is found that neither the reconnection site nor the topological boundaries are generally associated with a maximized degree of nongyrotropy. Nongyrotropic regions do not correspond to a specific fluid behavior as equivalent nongyrotropy is found to extend over the electron dissipation region as well as in non-dissipative diamagnetic drift layers. The localization of highly nongyrotropic regions in numerical models and their correlation with other observable quantities can, however, improve the characterization of spatial structures explored by spacecraft missions. (10.1063/1.4820953)
  DOI : 10.1063/1.4820953