Publications

2017

• Influence of neutral pressure on instability enhanced friction and ion velocities at the sheath edge of two-ion-species plasmas
  
  • Adrian P. J.
  • Baalrud S. D.
  • Lafleur T.
  Physics of Plasmas, American Institute of Physics, 2017, 24. The Instability Enhanced Friction theory [Baalrud et al., Phys. Rev. Lett. 103, 205002 (2009)] is extended to account for the influence of neutral pressure in predicting the flow speed of each ion species at the sheath edge of plasmas containing two ion species. Particle-in-cell simulations show that the theory accurately predicts both the neutral pressure cutoff of ion-ion two-stream instabilities and the ion flow speeds at the sheath edge as pressure is varied over several orders of magnitude. The simulations are used to directly calculate the instability-enhanced ion-ion friction force. At sufficiently high neutral pressure, the simulations also provide evidence for collisional modifications to the Bohm criterion. (10.1063/1.4986239)
  DOI : 10.1063/1.4986239
• Enhanced control of the ionization rate in radio-frequency plasmas with structured electrodes via tailored voltage waveforms
  
  • Doyle Scott J.
  • Lafleur Trevor
  • Gibson Andrew R.
  • Tian Peng
  • Kushner Mark J.
  • Dedrick James
  Plasma Sources Science and Technology, IOP Publishing, 2017, 26. Radio-frequency capacitively coupled plasmas that incorporate structured electrodes enable increases in the electron density within spatially localized regions through the hollow cathode effect (HCE). This enables enhanced control over the spatial profile of the plasma density, which is useful for several applications including materials processing, lighting and spacecraft propulsion. However, asymmetries in the powered and grounded electrode areas inherent to the hollow cathode geometry lead to the formation of a time averaged dc self-bias voltage at the powered electrode. This bias alters the energy and flux of secondary electrons leaving the surface of the cathode and consequentially can moderate the increased localized ionization afforded by the hollow cathode discharge. In this work, two-dimensional fluid-kinetic simulations are used to demonstrate control of the dc self-bias voltage in a dual-frequency driven (13.56, 27.12 MHz), hollow cathode enhanced, capacitively coupled argon plasma over the 66.6--200 Pa (0.5--1.5 Torr) pressure range. By varying the phase offset of the 27.12 MHz voltage waveform, the dc self-bias voltage varies by 10%--15% over an applied peak-to-peak voltage range of 600--1000 V, with lower voltages showing higher modulation. Resulting ionization rates due to secondary electrons within the hollow cathode cavity vary by a factor of 3 at constant voltage amplitude, demonstrating the ability to control plasma properties relevant for maintaining and enhancing the HCE. (10.1088/1361-6595/aa96e5)
  DOI : 10.1088/1361-6595/aa96e5
• Controlling plasma properties under differing degrees of electronegativity using odd harmonic dual frequency excitation
  
  • Gibson Andrew R.
  • Gans Timo
  Plasma Sources Science and Technology, IOP Publishing, 2017, 26. The charged particle dynamics in low-pressure oxygen plasmas excited by odd harmonic dual frequency waveforms (low frequency of 13.56 MHz and high frequency of 40.68 MHz) are investigated using a one-dimensional numerical simulation in regimes of both low and high electronegativity. In the low electronegativity regime, the time and space averaged electron and negative ion densities are approximately equal and plasma sustainment is dominated by ionisation at the sheath expansion for all combinations of low and high frequency and the phase shift between them. In the high electronegativity regime, the negative ion density is a factor of 15--20 greater than the low electronegativity cases. In these cases, plasma sustainment is dominated by ionisation inside the bulk plasma and at the collapsing sheath edge when the contribution of the high frequency to the overall voltage waveform is low. As the high frequency component contribution to the waveform increases, sheath expansion ionisation begins to dominate. It is found that the control of the average voltage drop across the plasma sheath and the average ion flux to the powered electrode are similar in both regimes of electronegativity, despite the differing electron dynamics using the considered dual frequency approach. This offers potential for similar control of ion dynamics under a range of process conditions, independent of the electronegativity. This is in contrast to ion control offered by electrically asymmetric waveforms where the relationship between the ion flux and ion bombardment energy is dependent upon the electronegativity. (10.1088/1361-6595/aa8dcd)
  DOI : 10.1088/1361-6595/aa8dcd
• Counterpropagating radiative shock experiments on the Orion laser
  
  • Suzuki-Vidal Francisco
  • Clayson Thomas
  • Stehlé Chantal
  • Swadling G. F.
  • Foster J.
  • Skidmore J.
  • Graham P.
  • Burdiak G.
  • Lebedev S. V.
  • Chaulagain Uddhab
  • Singh Raj Laxmi
  • Gumbrell E.
  • Patankar S.
  • Spindloe C.
  • Larour Jean
  • Kozlová Michaela
  • Rodriguez Perez R.
  • Gil J. M.
  • Espinosa G.
  • Velarde P.
  • Danson C.
  Physical Review Letters, American Physical Society, 2017, 119 (05), pp.055001. We present new experiments to study the formation of radiative shocks and the interaction between two counterpropagating radiative shocks. The experiments are performed at the Orion laser facility, which is used to drive shocks in xenon inside large aspect ratio gas cells. The collision between the two shocks and their respective radiative precursors, combined with the formation of inherently three-dimensional shocks, provides a novel platform particularly suited for the benchmarking of numerical codes. The dynamics of the shocks before and after the collision are investigated using point-projection x-ray backlighting while, simultaneously, the electron density in the radiative precursor was measured via optical laser interferometry. Modeling of the experiments using the 2D radiation hydrodynamic codes NYM and PETRA shows very good agreement with the experimental results. (10.1103/PhysRevLett.119.055001)
  DOI : 10.1103/PhysRevLett.119.055001
• Long-lived laser-induced arc discharges for energy channeling applications
  
  • Point Guillaume
  • Arantchouk Léonid
  • Thouin Emmanuelle
  • Carbonnel Jérôme
  • Mysyrowicz André
  • Houard Aurélien
  Scientific Reports, Nature Publishing Group, 2017, 7 (1), pp.13801. Laser filamentation offers a promising way for the remote handling of large electrical power in the form of guided arc discharges. We here report that it is possible to increase by several orders of magnitude the lifetime of straight plasma channels from filamentation-guided sparks in atmospheric air. A 30 ms lifetime can be reached using a low-intensity, 100 mA current pulse. Stability of the plasma shape is maintained over such a timescale through a continuous Joule heating from the current. This paves the way for applications based on the generation of straight, long duration plasma channels, like virtual plasma antennas or contactless transfer of electric energy. (10.1038/s41598-017-14054-z)
  DOI : 10.1038/s41598-017-14054-z
• Erratum: "On the Existence of the Kolmogorov Inertial Range in the Terrestrial Magnetosheath Turbulence" (2017, ApJL, 836, L10)
  
  • Huang S. Y.
  • Hadid Lina
  • Sahraoui Fouad
  • Yuan Z. G.
  • Deng X. H.
  The Astrophysical Journal Letters, Bristol : IOP Publishing, 2017, 837 (2), pp.L31. Not Available (10.3847/2041-8213/aa633c)
  DOI : 10.3847/2041-8213/aa633c
• Differential kinetic physics of solar-wind minor ions
  
  • Perrone Denise
  • Valentini F.
  • Servidio S.
  • Stabile S.
  • Pezzi O.
  • Sorriso-Valvo L.
  • de Marco R.
  • Marcucci M. F.
  • Brienza D.
  • Bruno Roberto
  • Lavraud Benoit
  • Retinò Alessandro
  • Vaivads A.
  • Consolini G.
  • de Keyser J.
  • Salatti M.
  • Veltri P.
  , 2017, 19, pp.13382. The solar wind, although predominantly constituted of protons, is also made up of a finite amount of alpha particles, together with a few percent of heavier ions. The kinetic properties of heavy ions in the solar wind are known to behave in a well organized way under most solar-wind flow conditions: their speeds are faster than that of hydrogen by about the local Alfvén speed, and their kinetic temperatures are more than proportional to their mass. Preferential heating and acceleration of heavy ions in the solar wind and corona represent a long-standing theoretical problem in space physics, and are distinct experimental signatures of kinetic processes occurring in collisionless plasmas. However, due to very scarce measurements of heavy ions at time resolutions comparable with their kinetic scales, energy partition between species in turbulent plasma dissipation is basically unexplored. For the moment, most of the information comes from numerical simulations and a crucial support is given by self-consistent, fully nonlinear Vlasov models. Here, hybrid Vlasov-Maxwell simulations are used to investigate the role of kinetic effects in a two-dimensional turbulent multi-ion plasma, composed of kinetic protons and alpha particles, and fluid electrons. The response of different ion species to the fluctuating electromagnetic fields appears to be different. In particular, a significant differential heating of alpha particles with respect to protons is observed, localized nearby the peaks of ion vorticity and where strong deviations from thermodynamic equilibrium are recovered. Then, the understanding of the complex process of particle heating results strongly related to the study of the non-Maxwellian features on the three-dimensional ion velocity distributions. These numerical results highlight the importance for the future space missions to provide detailed ion measurements to make a significant step forward in the problem of heating in turbulent space plasmas.
• MMS observations of whistler waves in electron diffusion region
  
  • Cao D.
  • Fu H.S.
  • Cao J.B.
  • Wang T. Y.
  • Graham D. B.
  • Chen Z. Z.
  • Peng F. Z.
  • Huang S. Y.
  • Khotyaintsev Y. V.
  • André M.
  • Russell C. T.
  • Giles B. L.
  • Lindqvist P.-A.
  • Torbert R. B.
  • Ergun R. E.
  • Le Contel Olivier
  • Burch J. L.
  Geophysical Research Letters, American Geophysical Union, 2017, 44 (9), pp.3954-3962. Whistler waves that can produce anomalous resistivity by affecting electrons' motion have been suggested as one of the mechanisms responsible for magnetic reconnection in the electron diffusion region (EDR). Such type of waves, however, has rarely been observed inside the EDR so far. In this study, we report such an observation by Magnetospheric Multiscale (MMS) mission. We find large-amplitude whistler waves propagating away from the X line with a very small wave-normal angle. These waves are probably generated by the perpendicular temperature anisotropy of the 300 eV electrons inside the EDR, according to our analysis of dispersion relation and cyclotron resonance condition; they significantly affect the electron-scale dynamics of magnetic reconnection and thus support previous simulations. (10.1002/2017GL072703)
  DOI : 10.1002/2017GL072703
• Evidence and relevance of spatially chaotic magnetic field lines in MCF devices
  
  • Firpo Marie-Christine
  • Lifschitz Agustin
  • Ettoumi Wahb
  • Farengo Ricardo
  • Ferrari Hugo
  • Garcia-Martinez Pablo Luis
  Plasma Physics and Controlled Fusion, IOP Publishing, 2017, 59 (3). Numerical evidence for the existence of spatially chaotic magnetic field lines about the collapse phase of tokamak sawteeth with incomplete reconnection is presented. This uses the results of extensive test particle simulations in different sets of electromagnetic perturbations tested against experimental JET measurements. In tokamak sawteeth, that form a laboratory prototype of magnetic reconnection, the relative magnetic perturbation δB/B may reach a few percents. This does not apply to tokamak operating regimes dominated by turbulence where δB/B is usually not larger than 10 −4. However, this small magnetic perturbation being sustained by a large spectrum of modes is shown to be sufficient to ensure the existence of stochastic magnetic field lines. This has important consequences for magnetic confinement fusion (MCF) where electrons are dominantly governed by the magnetic force. Indeed some overlap between magnetic resonances can locally induce chaotic magnetic field lines enabling the spatial redistribution of the electron population and of its thermal content. As they are the swiftest plasma particles, electrons feed back the most rapid perturbations of the magnetic field (10.1088/1361-6587/aa570d)
  DOI : 10.1088/1361-6587/aa570d
• Study of Plasma Waves Observed onboard Rosetta in the 67P/ChuryumovGerasimenko Comet Environment Using High Time Resolution Density Data Inferred from RPC-MIP and RPC-LAP Cross-calibration
  
  • Breuillard H.
  • Henri P.
  • Vallières Xavier
  • Eriksson A. I.
  • Odelstad E.
  • Johansson F. L.
  • Richter I.
  • Goetz C.
  • Wattieaux G.
  • Tsurutani B.
  • Hajra R.
  • Le Contel O.
  , 2017, 2017. During two years, the groundbreaking ESA/Rosetta mission was able to escort comet 67P where previous cometary missions were only limited to flybys. This enabled for the first time to make in-situ measurements of the evolution of a comet's plasma environment. The density and temperature measured by Rosetta are derived from RPC-Mutual Impedance Probe (MIP) and RPC-Langmuir Probe (LAP). On one hand, low time resolution electron density are calculated using the plasma frequency extracted from the MIP mutual impedance spectra. On the other hand, high time resolution density fluctuations are estimated from the spacecraft potential measured by LAP. In this study, using a simple spacecraft charging model, we perform a cross-calibration of MIP plasma density and LAP spacecraft potential variations to obtain high time resolution measurements of the electron density. These results are also used to constrain the electron temperature. Then we make use of these new dataset, together with RPC-MAG magnetic field measurements, to investigate for the first time the compressibility and the correlations between plasma and magnetic field variations, for both singing comet waves and steepened waves observed, respectively during low and high cometary outgassing activity, in the plasma environment of comet 67P.
• Absolute ozone densities in a radio-frequency driven atmospheric pressure plasma using two-beam UV-LED absorption spectroscopy and numerical simulations
  
  • Wijaikhum A.
  • Schröder D.
  • Schröter S.
  • Gibson A. R.
  • Niemi K.
  • Friderich J.
  • Greb A.
  • Schulz-von der Gathen V.
  • O'Connell D.
  • Gans T.
  Plasma Sources Science and Technology, IOP Publishing, 2017, 26. The efficient generation of reactive oxygen species (ROS) in cold atmospheric pressure plasma jets (APPJs) is an increasingly important topic, e.g. for the treatment of temperature sensitive biological samples in the field of plasma medicine. A 13.56 MHz radio-frequency (rf) driven APPJ device operated with helium feed gas and small admixtures of oxygen (up to 1%), generating a homogeneous glow-mode plasma at low gas temperatures, was investigated. Absolute densities of ozone, one of the most prominent ROS, were measured across the 11 mm wide discharge channel by means of broadband absorption spectroscopy using the Hartley band centred at lambda = 255 nm. A two-beam setup with a reference beam in Mach--Zehnder configuration is employed for improved signal-to-noise ratio allowing high-sensitivity measurements in the investigated single-pass weak-absorbance regime. The results are correlated to gas temperature measurements, deduced from the rotational temperature of the N<SUB>2</SUB> (C <SUP>3</SUP> {{{\Pi }}}<SUB>u</SUB><SUP> </SUP> \to B <SUP>3</SUP> {{{\Pi }}}<SUB>g</SUB><SUP> </SUP>, upsilo = 0 \to 2) optical emission from introduced air impurities. The observed opposing trends of both quantities as a function of rf power input and oxygen admixture are analysed and explained in terms of a zero-dimensional plasma-chemical kinetics simulation. It is found that the gas temperature as well as the densities of O and O<SUB>2</SUB>(b{}<SUP>1</SUP>{{{Sigma }}}<SUB>g</SUB><SUP> </SUP>) influence the absolute O<SUB>3</SUB> densities when the rf power is varied. (10.1088/1361-6595/aa8ebb)
  DOI : 10.1088/1361-6595/aa8ebb
• Space Weather, from the Sun to the Earth, the key role of GNSS. Part II: Training on daily Global Positioning System (GPS) data
  
  • Amory-Mazaudier Christine
  • Fleury Rolland
  • Gadimova Sharafat
  • Touzani Abderrahmane
  Coordinates, 2017, 13 (3), pp.31-36. The goal of this paper is to give a clear view of the Sun Earth relationships that are complex. The phenomena acting at large scales and essentially related to dynamic and electromagnetic physical processes have been addressed. Besides physics, the work done to develop the training in Space Weather by focusing on Global Navigation Satellite Systems has also been presented. Readers may recall that we published the first part of this article which focused on physics of the relationships Sun, Earth and Meteorology of Space. In this issue, aspects of GNSS training and capacity building are discussed.
• Experimental study of the interaction of two laser-driven radiative shocks at the PALS laser
  
  • Singh R. L.
  • Stehlé C.
  • Suzuki-Vidal F.
  • Kozlova M.
  • Larour Jean
  • Chaulagain U.
  • Clayson T.
  • Rodriguez R.
  • Gil M.
  • Nejdl J.
  • Krus M.
  • Dostal J.
  • Dudzak R.
  • Barroso P.
  • Acef O.
  • Cotelo M.
  • Velarde P.
  High Energy Density Physics, Elsevier, 2017, 23, pp.20 - 30. Radiative shocks (RS) are complex phenomena which are ubiquitous in astrophysical environments. The study of such hypersonic shocks in the laboratory, under controlled conditions, is of primary interest to understand the physics at play and also to check the ability of numerical simulations to reproduce the experimental results. In this context, we conducted, at the Prague Asterix Laser System facility (PALS), the first experiments dedicated to the study of two counter-propagating radiative shocks propagating at non-equal speeds up to 25–50 km/s in noble gases at pressures ranging between 0.1 and 0.6 bar. These experiments highlighted the interaction between the two radiative precursors. This interaction is qualitatively but not quantitatively described by 1D simulations. Preliminary results obtained with XUV spectroscopy leading to the estimation of shock temperature and ion charge of the plasma are also presented. (10.1016/j.hedp.2017.03.001)
  DOI : 10.1016/j.hedp.2017.03.001
• Multipoint Observations of Energetic Particle Injections and Substorm Activity During a Conjunction Between Magnetospheric Multiscale (MMS) and Van Allen Probes
  
  • Turner D. L.
  • Fennell J. F.
  • Blake J. B.
  • Claudepierre S. G.
  • Clemmons J. H.
  • Jaynes A. N.
  • Leonard T.
  • Baker D. N.
  • Cohen I. J.
  • Gkioulidou M.
  • Ukhorskiy A. Y.
  • Mauk B. H.
  • Gabrielse C.
  • Angelopoulos V.
  • Strangeway R. J.
  • Kletzing C. A.
  • Le Contel Olivier
  • Spence H. E.
  • Torbert R. B.
  • Burch J. L.
  • Reeves G. D.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2017, 122 (11), pp.481-504. This study examines multipoint observations during a conjunction between Magnetospheric Multiscale (MMS) and Van Allen Probes on 7 April 2016 in which a series of energetic particle injections occurred. With complementary data from Time History of Events and Macroscale Interactions during Substorms, Geotail, and Los Alamos National Laboratory spacecraft in geosynchronous orbit (16 spacecraft in total), we develop new insights on the nature of energetic particle injections associated with substorm activity. Despite this case involving only weak substorm activity (maximum AE <300 nT) during quiet geomagnetic conditions in steady, below-average solar wind, a complex series of at least six different electron injections was observed throughout the system. Intriguingly, only one corresponding ion injection was clearly observed. All ion and electron injections were observed at <600 keV only. MMS reveals detailed substructure within the largest electron injection. A relationship between injected electrons with energy <60 keV and enhanced whistler mode chorus wave activity is also established from Van Allen Probes and MMS. Drift mapping using a simplified magnetic field model provides estimates of the dispersionless injection boundary locations as a function of universal time, magnetic local time, and L shell. The analysis reveals that at least five electron injections, which were localized in magnetic local time, preceded a larger injection of both electrons and ions across nearly the entire nightside of the magnetosphere near geosynchronous orbit. The larger ion and electron injection did not penetrate to L < 6.6, but several of the smaller electron injections penetrated to L < 6.6. Due to the discrepancy between the number, penetration depth, and complexity of electron versus ion injections, this event presents challenges to the current conceptual models of energetic particle injections. (10.1002/2017JA024554)
  DOI : 10.1002/2017JA024554
• Vibrational kinetics of non-equilibrium CO<SUB>2</SUB> plasma discharge in low-excitation regime
  
  • Grofulovic Marija
  • Silva Tiago
  • Guerra V.
  • Pintassilgo C.D.
  • Klarenaar Bart
  • Engeln Richard
  • Morillo-Candas Ana-Sofia
  • Guaitella Olivier
  , 2017, 62.
• QDB: a new database of plasma chemistries and reactions
  
  • Tennyson Jonathan
  • Rahimi Sara
  • Hill Christian
  • Tse Lisa
  • Vibhakar Anuradha
  • Akello-Egwel Dolica
  • Brown Daniel B
  • Dzarasova Anna
  • Hamilton James R
  • Jaksch Dagmar
  • Mohr Sebastian
  • Wren-Little Keir
  • Bruckmeier Johannes
  • Agarwal Ankur
  • Bartschat Klaus
  • Annemie Bogaerts Annemie
  • Booth Jean-Paul
  • Goeckner Matthew J
  • Hassouni Khaled
  • Itikawa Yukikazu
  • Braams Bastiaan J
  • Krishnakumar E.
  • Laricchiuta Annarita
  • Mason Nigel J
  • Pandey Sumeet
  • Petrovic Zoran Lj
  • Pu Yi-Kang
  • Ranjan Alok
  • Rauf S.
  • Schulze J.
  • Turner M.M.
  • Ventzek Peter
  • Whitehead J.C.
  • Yoon Jung-Sik
  Plasma Sources Science and Technology, IOP Publishing, 2017, 26 (5), pp.055014. One of the most challenging and recurring problems when modeling plasmas is the lack of data on the key atomic and molecular reactions that drive plasma processes. Even when there are data for some reactions, complete and validated datasets of chemistries are rarely available. This hinders research on plasma processes and curbs development of industrial applications. The QDB project aims to address this problem by providing a platform for provision, exchange, and validation of chemistry datasets. A new data model developed for QDB is presented. QDB collates published data on both electron scattering and heavy-particle reactions. These data are formed into reaction sets, which are then validated against experimental data where possible. This process produces both complete chemistry sets and identifies key reactions that are currently unreported in the literature. Gaps in the datasets can be filled using established theoretical methods. Initial validated chemistry sets for SF 6 /CF 4 /O 2 and SF 6 /CF 4 /N 2 /H 2 are presented as examples. (10.1088/1361-6595/aa6669)
  DOI : 10.1088/1361-6595/aa6669
• Promoting lentil germination and stem growth by plasma activated tap water, demineralized water and liquid fertilizer
  
  • Zhang Shiqiang
  • Rousseau Antoine
  • Dufour Thierry
  RSC Advances, Royal Society of Chemistry, 2017, 7 (50), pp.31244–31251. Tap water, demineralized water and liquid fertilizer have been activated using an atmospheric pressure plasma jet (APPJ) to investigate their benefits for the germination rate and stem elongation rate of lentils from Puy-en-Velay (France). By plasma-activating tap water, we have obtained germination rates as high as 80% (instead of 30% with tap water). Also, higher stem elongation rates and final stem lengths were obtained using activated tap water compared with commercial fertilizer. We show that these rates of germination and stem growth strongly depend on the combination of two radicals generated in the liquids by the plasma: hydrogen peroxide and nitrate. This synergy appears to be a condition for releasing seed dormancy through the endogenous production of NO radicals. (10.1039/C7RA04663D)
  DOI : 10.1039/C7RA04663D
• Stable and unstable roots of ion temperature gradient driven mode using curvature modified plasma dispersion functions
  
  • Gultekin Ozgur
  • Gürcan Özgür D.
  Plasma Physics and Controlled Fusion, IOP Publishing, 2017, 60 (2), pp.025021. Basic, local kinetic theory of ion temperature gradient driven (ITG) mode, with adiabatic electrons is reconsidered. Standard unstable, purely oscillating as well as damped solutions of the local dispersion relation are obtained using a bracketing technique that uses the argument principle. This method requires computing the plasma dielectric function and its derivatives, which are implemented here using modified plasma dispersion functions with curvature and their derivatives, and allows bracketing/following the zeros of the plasma dielectric function which corresponds to different roots of the ITG dispersion relation. We provide an open source implementation of the derivatives of modified plasma dispersion functions with curvature, which are used in this formulation. Studying the local ITG dispersion, we find that near the threshold of instability the unstable branch is rather asymmetric with oscillating solutions towards lower wave numbers (i.e. drift waves), and damped solutions toward higher wave numbers. This suggests a process akin to inverse cascade by coupling to the oscillating branch towards lower wave numbers may play a role in the nonlinear evolution of the ITG, near the instability threshold. Also, using the algorithm, the linear wave diffusion is estimated for the marginally stable ITG mode. (10.1088/1361-6587/aa9e27)
  DOI : 10.1088/1361-6587/aa9e27
• Capacitively coupled hydrogen plasmas sustained by tailored voltage waveforms: vibrational kinetics and negative ions control
  
  • Diomede P.
  • Bruneau Bastien
  • Longo S.
  • Johnson E.V.
  • Booth Jean-Paul
  Plasma Sources Science and Technology, IOP Publishing, 2017, 26 (7), pp.075007. A comprehensive hybrid model of a hydrogen capacitively coupled plasma, including a detailed description of the molecular vibrational kinetics, has been applied to the study of the effect of tailored voltage waveforms (TVWs) on the production kinetics and transport of negative ions in these discharges. Two kinds of TVWs are considered, valleys-to-peaks and saw-tooth, with amplitude and slope asymmetry respectively. By tailoring the voltage waveform only, it is possible to exert substantial control over the peak density and position of negative ions inside the discharge volume. This control is particularly effective for saw-tooth waveforms. Insight into the mechanisms allowing this control is provided by an analysis of the model results. This reveals the roles of the vibrational distribution function and of the electron energy distribution and their correlations, as well as changes in the negative ion transport in the electric field when using different TVWs. Considering the chemical reactivity of H ? ions, the possibility of a purely electrical control of the negative ion cloud in a reactor operating with a feedstock gas diluted by hydrogen may find interesting applications. This is the first study of vibrational kinetics in the context of TVWs in molecular gases. (10.1088/1361-6595/aa752c)
  DOI : 10.1088/1361-6595/aa752c
• Turbulence of Weak Gravitational Waves in the Early Universe
  
  • Galtier Sébastien
  • Nazarenko Sergey V.
  Physical Review Letters, American Physical Society, 2017, 119 (22), pp.221101. We study the statistical properties of an ensemble of weak gravitational waves interacting nonlinearly in a flat space-time. We show that the resonant three-wave interactions are absent and develop a theory for four-wave interactions in the reduced case of a 2.5+1 diagonal metric tensor. In this limit, where only plus-polarized gravitational waves are present, we derive the interaction Hamiltonian and consider the asymptotic regime of weak gravitational wave turbulence. Both direct and inverse cascades are found for the energy and the wave action, respectively, and the corresponding wave spectra are derived. The inverse cascade is characterized by a finite-time propagation of the metric excitations—a process similar to an explosive nonequilibrium Bose–Einstein condensation, which provides an efficient mechanism to ironing out small-scale inhomogeneities. The direct cascade leads to an accumulation of the radiation energy in the system. These processes might be important for understanding the early Universe where a background of weak nonlinear gravitational waves is expected. (10.1103/PhysRevLett.119.221101)
  DOI : 10.1103/PhysRevLett.119.221101
• Interplay between Alfvén and magnetosonic waves in compressible magnetohydrodynamics turbulence
  
  • Andrés Nahuel
  • Leoni P. Clark Di
  • Mininni P. D.
  • Dmitruk P.
  • Sahraoui Fouad
  • Matthaeus W. H.
  Physics of Plasmas, American Institute of Physics, 2017, 24, pp.102314. Using spatio-temporal spectra, we show direct evidence of excitation of magnetosonic and Alfvén waves in three-dimensional compressible magnetohydrodynamic turbulence at small Mach numbers. For the plasma pressure dominated regime, or the high beta regime (with beta the ratio between fluid and magnetic pressure), and for the magnetic pressure dominated regime, or the low beta regime, we study magnetic field fluctuations parallel and perpendicular to a guide magnetic field B<SUB>0</SUB>. In the low beta case, we find excitation of compressible and incompressible fluctuations, with a transfer of energy towards Alfvénic modes and to a lesser extent towards magnetosonic modes. In particular, we find signatures of the presence of fast magnetosonic waves in a scenario compatible with that of weak turbulence. In the high beta case, fast and slow magnetosonic waves are present, with no clear trace of Alfvén waves, and a significant part of the energy is carried by two-dimensional turbulent eddies. (10.1063/1.4997990)
  DOI : 10.1063/1.4997990
• Intrinsic non-inductive current driven by ETG turbulence in tokamaks
  
  • Kaw P. K.
  • Singh R.
  • Gürcan Özgür D.
  Physics of Plasmas, American Institute of Physics, 2017, 24, pp.102303. Motivated by observations and physics understanding of the phenomenon of intrinsic rotation, it is suggested that similar considerations for electron dynamics may result in intrinsic current in tokamaks. We have investigated the possibility of intrinsic non-inductive current in the turbulent plasma of tokamaks. Ohm's law is generalized to include the effect of turbulent fluctuations in the mean field approach. This clearly leads to the identification of sources and the mechanisms of non-inductive current drive by electron temperature gradient turbulence. It is found that a mean parallel electro-motive force and hence a mean parallel current can be generated by (1) the divergence of residual current flux density and (2) a non-flux like turbulent source from the density and parallel electric field correlations. Both residual flux and the non-flux source require parallel wave-number k&#8741; symmetry breaking for their survival which can be supplied by various means like mean E&#8201;×&#8201;B shear, turbulence intensity gradient, etc. Estimates of turbulence driven current are compared with the background bootstrap current in the pedestal region. It is found that turbulence driven current is nearly 10% of the bootstrap current and hence can have a significant influence on the equilibrium current density profiles and current shear driven modes. (10.1063/1.4990746)
  DOI : 10.1063/1.4990746
• Three-dimensional Simulations and Spacecraft Observations of Sub-ion Scale Turbulence in the Solar Wind: Influence of Landau Damping
  
  • Kobayashi Sumire
  • Sahraoui Fouad
  • Passot T.
  • Laveder D.
  • Sulem P.
  • Huang S. Y.
  • Henri Pierre
  • Smets R.
  The Astrophysical Journal, American Astronomical Society, 2017, 839 (2), pp.122. Three-dimensional nonlinear finite Larmor radius (FLR)–Landau fluid simulations, which include some small-scale $({k}_{\perp }{\rho }_{i}\gtrsim 1)$ kinetic effects, are performed to explore the nature of the sub-ion scale turbulence in the solar wind and to investigate the role of Landau damping and FLR corrections. The resulting steady-state magnetic power spectrum in the dispersive range display exponents that vary within a range of values compatible with statistical results reported from in situ spacecraft measurements of solar wind turbulence as well as from gyrokinetic simulations. The spectral slopes are shown to depend on the strength of the nonlinear effects and on the scale at which turbulent fluctuations are driven in the simulations. The influence of Landau damping is addressed by comparison with simulations where the double-adiabatic closure is imposed. The role of FLR corrections is also analyzed. Comparison with in situ observations in the solar wind are performed to enlighten the influence of the fluctuations power at different scales on the spectral slopes in the sub-ion range. Using diagnosis of both magnetic compressibility and frequency-wavenumber spectra, it is shown that in spite of the evidence of the presence of fast-magnetosonic modes, the magnetic energy is mostly distributed around the kinetic Alfvén waves and the slow modes, in agreement with solar wind measurements. The observed large broadening about the linear dispersion relations may reflect the presence of coherent structures. (10.3847/1538-4357/aa67f2)
  DOI : 10.3847/1538-4357/aa67f2
• Induction effects of geomagnetic disturbances in the geo-electric field variations at low latitudes
  
  • Doumbia Vafi
  • Boka Kouadio
  • Kouassi Nguessan
  • Grodji Oswald Didier Franck
  • Amory-Mazaudier Christine
  • Menvielle Michel
  Annales Geophysicae, European Geosciences Union, 2017, 35 (1), pp.39 - 51. In this study we examined the influences of geomagnetic activity on the Earth surface electric field variations at low latitudes. During the International Equatorial Electrojet Year (IEEY) various experiments were performed along 5° W in West Africa from 1992 to 1995. Among other instruments, 10 stations equipped with magnetometers and telluric electric field lines operated along a meridian chain across the geomagnetic dip equator from November 1992 to December 1994. In the present work, the induced effects of space-weather-related geomagnetic disturbances in the equatorial electrojet (EEJ) influence area in West Africa were examined. For that purpose, variations in the north–south (E_x) and east–west (E_y) components of telluric electric field were analyzed, along with that of the three components (H, D and Z) of the geomagnetic field during the geomagnetic storm of 17 February 1993 and the solar flare observed on 4 April 1993. The most important induction effects during these events are associated with brisk impulses like storm sudden commencement (ssc) and solar flare effect (sfe) in the geomagnetic field variations. For the moderate geomagnetic storm that occurred on 17 February 1993, with a minimum Dst index of −110 nT, the geo-electric field responses to the impulse around 11:00 LT at LAM are E_x= 520 mV km⁻¹ and E_y = 400 mV km⁻¹. The geo-electric field responses to the sfe that occurred around 14:30 LT on 4 April 1993 are clearly observed at different stations as well. At LAM the crest-to-crest amplitude of the geo-electric field components associated with the sfe are E_x = 550 mV km⁻¹ and E_y = 340 mV km⁻¹. Note that the sfe impact on the geo-electric field variations decreases with the increasing distance of the stations from the subsolar point, which is located at about 5.13° N on 4 April. This trend does not reflect the sfe increasing amplitude near the dip equator due the high Cowling conductivity in the EEJ belt. (10.5194/angeo-35-39-2017)
  DOI : 10.5194/angeo-35-39-2017
• Hemispheric asymmetries in the ionosphere response observed during the high-speed solar wind streams of the 24-28 August 2010
  
  • Zaourar N.
  • Amory-Mazaudier Christine
  • Fleury Rolland
  Advances in Space Research, Elsevier, 2017. This paper presents the geomagnetic and ionospheric responses to a high speed solar wind stream (HSS) impacting the magnetosphere on 24 August 2010. We focus our study on the interhemispheric conjugated behavior. The solar wind speed remained very high during 5 days from 24 to 28 August 2010. By using magnetometer and ground-based GPS data from various approximately conjugated magnetic observatories and GPS stations, we studied the hemispheric asymmetries in the magnetic signature, Vertical Total Electron Content (VTEC) and scintillation activity during this HSS event. Geomagnetic activity reveals larger disturbances in amplitude in the Northern Hemisphere (NH) than in the southern Hemisphere (SH), and stronger asymmetries at higher latitudes, than at lower latitudes, between the conjugate observatories. VTEC variations reveal large increases in amplitude in the NH; while these effects are less pronounced in the SH. We investigate also the GPS scintillation activities occurring in the conjugated polar regions under HSSs conditions. At auroral latitudes, our results show a good correlation between the rate of VTEC index (ROTI) and auroral Al index, with more intense phase fluctuations in the NH. (10.1016/j.asr.2017.01.048)
  DOI : 10.1016/j.asr.2017.01.048