Publications

2016

• An expression for the h<SUB>l</SUB> factor in low-pressure electronegative plasma discharges
  
  • Chabert Pascal
  Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (2), pp.025010. The positive ion flux exiting a low-pressure plasma discharge is a crucial quantity in global (volume-averaged) models. In discharges containing only electrons and positive ions (electropositive discharges), it is common to write this flux , where is the central positive ion density, is the positive ion fluid speed at the sheath edge (the Bohm speed), and is the positive ion edge-to-centre density ratio. There are well established formulae for in electropositive discharges, but for discharges containing negative ions (electronegative discharges), the analysis is more complicated. The purpose of this paper is to propose a formula for the factor in low-pressure electronegative discharges. We use the numerical solution of fluid equations with Boltzmann negative ions, including Poisson's equation, as a guide to derive an analytical expression that can easily be incorporated in global models. The parameter space in which the derived expression is valid is discussed at the end of the paper. (10.1088/0963-0252/25/2/025010)
  DOI : 10.1088/0963-0252/25/2/025010
• A comparison between micro hollow cathode discharges and atmospheric pressure plasma jets in Ar/O<SUB>2</SUB> gas mixtures
  
  • Lazzaroni Claudia
  • Chabert Pascal
  Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (6), pp.065015. Using global models, micro hollow cathode discharges (MHCDs) are compared to radiofrequency atmospheric pressure plasma jets (APPJs) in terms of reactive oxygen species (ROS) production. Ar/O 2 gas mixtures are investigated, typically with a small percentage of oxygen in argon. The same chemical reaction set, involving 17 species and 128 chemical reactions in the gas phase, is used for both devices, operated in the typical geometries previously published; the APPJ is driven by a radiofrequency voltage across a 1&#8201;mm gap, at atmospheric pressure, while the MHCD is driven by a DC voltage source, at 100 Torr and in a 400 &#956; m hole. The MHCD may be operated either in the self-pulsing or in the normal (stationary) regime, depending on the driving voltage. The comparison shows that in both regimes, the MHCD produces larger amounts of ##IMG## [http://ej.iop.org/images/0963-0252/25/6/065015/psstaa4123ieqn001.gif] \textO_2^\ast , while the APPJ produces predominantly reactive oxygen ground state species, ##IMG## [http://ej.iop.org/images/0963-0252/25/6/065015/psstaa4123ieqn002.gif] \textO and ##IMG## [http://ej.iop.org/images/0963-0252/25/6/065015/psstaa4123ieqn003.gif] \textO_3 . These large differences in ROS composition are mostly due to the higher plasma density produced in the MHCD. The difference in operating pressure is a second order effect. (10.1088/0963-0252/25/6/065015)
  DOI : 10.1088/0963-0252/25/6/065015
• Transport matrix for particles and momentum in collisional drift waves turbulence in linear plasma devices
  
  • Ashourvan A.
  • Diamond P.H.
  • Gürcan Özgür D.
  Physics of Plasmas, American Institute of Physics, 2016, 23, pp.022309. The relationship between the physics of turbulent transport of particles and azimuthal momentum in a linear plasma device is investigated using a simple model with a background density gradient and zonal flows driven by turbulent stresses. Pure shear flow driven Kelvin-Helmholtz instabilities (k&#8741;=0) relax the flow and drive an outward (down gradient) flux of particles. However, instabilities at finite k&#8741; with flow enhanced pumping can locally drive an inward particle pinch. The turbulent vorticity flux consists of a turbulent viscosity term, which acts to reduce the global vorticity gradient and the residual vorticity flux term, accelerating the zonal flows from rest. Moreover, we use the positivity of the production of fluctuation potential enstrophy to obtain a constraint relation, which tightly links the vorticity transport to the particle transport. This relation can be useful in explaining the experimentally observed correlation between the presence of E×B flow shear and the measured inward particle flux in various magnetically confined plasma devices. (10.1063/1.4942420)
  DOI : 10.1063/1.4942420
• MMS observations of ion-scale magnetic island in the magnetosheath turbulent plasma
  
  • Huang S. Y.
  • Sahraoui Fouad
  • Retinò Alessandro
  • Le Contel Olivier
  • Yuan Z. G.
  • Chasapis A.
  • Aunai Nicolas
  • Breuillard Hugo
  • Deng X. H.
  • Zhou M.
  • Fu H.S.
  • Pang Y.
  • Wang D. D.
  • Torbert R. B.
  • Goodrich K. A.
  • Ergun R. E.
  • Khotyaintsev Y. V.
  • Lindqvist P.-A.
  • Russell C. T.
  • Strangeway R. J.
  • Magnes W.
  • Bromund K.
  • Leinweber H.
  • Plaschke F.
  • Anderson B. J.
  • Pollock C. J.
  • Giles B. L.
  • Moore T. E.
  • Burch J. L.
  Geophysical Research Letters, American Geophysical Union, 2016, 43 (15), pp.7850-7858. In this letter, first observations of ion-scale magnetic island from the Magnetospheric Multiscale mission in the magnetosheath turbulent plasma are presented. The magnetic island is characterized by bipolar variation of magnetic fields with magnetic field compression, strong core field, density depletion, and strong currents dominated by the parallel component to the local magnetic field. The estimated size of magnetic island is about 8 d<SUB>i</SUB>, where d<SUB>i</SUB> is the ion inertial length. Distinct particle behaviors and wave activities inside and at the edges of the magnetic island are observed: parallel electron beam accompanied with electrostatic solitary waves and strong electromagnetic lower hybrid drift waves inside the magnetic island and bidirectional electron beams, whistler waves, weak electromagnetic lower hybrid drift waves, and strong broadband electrostatic noise at the edges of the magnetic island. Our observations demonstrate that highly dynamical, strong wave activities and electron-scale physics occur within ion-scale magnetic islands in the magnetosheath turbulent plasma. (10.1002/2016GL070033)
  DOI : 10.1002/2016GL070033
• Langmuir wave decay in turbulent inhomogeneous solar wind plasmas
  
  • Krafft C.
  • Volokitin A.
  AIP Conference Proceedings, American Institute of Physics, 2016, 1720 (1), pp.040009. Langmuir wave decay in solar wind plasmas typical of type III bursts' source regions near 1 AU have been reported by several spacecraft observations. In such plasmas, due to the presence of random density fluctuations, wave decay occurs usually simultaneously and compete with other coupling effects between the fields and the density irregularities, as reflection, scattering and/or refraction processes. Numerical simulations show that resonant three-wave coupling processes including several cascades of Langmuir wave decay can occur in such plasmas, leading to wave energy transfer to smaller wavenumbers k, as shown in the frame of weak turbulence theory. However, in such conditions, and contrary to what occurs in homogeneous plasmas, the decay process is localized in space at a given time. Moreover, wave-wave coupling plays a significant role in the modulation of the Langmuir waveforms, in agreement with recent space observations. (10.1063/1.4943820)
  DOI : 10.1063/1.4943820
• Tailored-waveform excitation of capacitively coupled plasmas and the electrical asymmetry effect
  
  • Lafleur T.
  Plasma Sources Science and Technology, IOP Publishing, 2016, 25. Unequal areas of the powered and grounded electrodes in single-frequency capacitively coupled plasmas (CCPs) are well-known to generate a DC self-bias voltage and an asymmetric plasma response. By instead applying non-sinusoidal waveforms composed of multiple harmonics---referred to in the literature as arbitrary waveforms, multi-harmonic waveforms or tailored waveforms---an asymmetric plasma response and a DC self-bias can also be produced; even for perfectly geometrically symmetric systems. This electrical asymmetry effect (EAE) has opened the doors to a wide range of novel ideas and interesting new physics that could allow limitations between the control of the ion flux and ion energy in traditional CCPs to be broken; thus helping to develop next-generation industrial plasma processing reactors. This review is dedicated to the current status of the EAE, and highlights important theoretical, numerical and experimental work in the field that has contributed to our understanding. (10.1088/0963-0252/25/1/013001)
  DOI : 10.1088/0963-0252/25/1/013001
• Electron jet of asymmetric reconnection
  
  • Khotyaintsev Y. V.
  • Graham D. B.
  • Norgren C.
  • Eriksson E.
  • Li W.
  • Johlander A.
  • Vaivads A.
  • André M.
  • Pritchett P. L.
  • Retinò Alessandro
  • Phan T. D.
  • Ergun R. E.
  • Goodrich K. A.
  • Lindqvist P.-A.
  • Marklund G. T.
  • Le Contel Olivier
  • Plaschke F.
  • Magnes W.
  • Strangeway R. J.
  • Russell C. T.
  • Vaith H.
  • Argall M. R.
  • Kletzing C. A.
  • Nakamura R.
  • Torbert R. B.
  • Paterson W. R.
  • Gershman D. J.
  • Dorelli J. C.
  • Avanov L. A.
  • Lavraud B.
  • Saito Y.
  • Giles B. L.
  • Pollock C. J.
  • Turner D. L.
  • Blake J. D.
  • Fennell J. F.
  • Jaynes A.
  • Mauk B. H.
  • Burch J. L.
  Geophysical Research Letters, American Geophysical Union, 2016, 43 (11), pp.5571-5580. We present Magnetospheric Multiscale observations of an electron-scale current sheet and electron outflow jet for asymmetric reconnection with guide field at the subsolar magnetopause. The electron jet observed within the reconnection region has an electron Mach number of 0.35 and is associated with electron agyrotropy. The jet is unstable to an electrostatic instability which generates intense waves with E<SUB>||</SUB> amplitudes reaching up to 300 mV m<SUP>-1</SUP> and potentials up to 20% of the electron thermal energy. We see evidence of interaction between the waves and the electron beam, leading to quick thermalization of the beam and stabilization of the instability. The wave phase speed is comparable to the ion thermal speed, suggesting that the instability is of Buneman type, and therefore introduces electron-ion drag and leads to braking of the electron flow. Our observations demonstrate that electrostatic turbulence plays an important role in the electron-scale physics of asymmetric reconnection. (10.1002/2016GL069064)
  DOI : 10.1002/2016GL069064
• Subsolar magnetopause observation and kinetic simulation of a tripolar guide magnetic field perturbation consistent with a magnetic island
  
  • Eriksson S.
  • Cassak P. A.
  • Retinò Alessandro
  • Mozer F. S.
  Geophysical Research Letters, American Geophysical Union, 2016, 43 (7), pp.3035-3041. The Polar satellite recorded two reconnection exhausts within 6 min on 1 April 2001 across a subsolar magnetopause that displayed a symmetric plasma density, but different out-of-plane magnetic field signatures for similar solar wind conditions. The first magnetopause crossing displayed a bipolar guide field variation in a weak external guide field consistent with a symmetric Hall field from a single X line. The subsequent crossing represents the first observation of a tripolar guide field perturbation at Earth's magnetopause in a strong guide field. This perturbation consists of a significant guide field enhancement between two narrow guide field depressions. A particle-in-cell simulation for the prevailing conditions across this second event resulted in a magnetic island between two simulated X lines across which a tripolar guide field developed consistent with the observation. The simulated island supports a scenario whereby Polar encountered the asymmetric quadrupole Hall magnetic fields between two X lines for symmetric conditions across the magnetopause. (10.1002/2016GL068691)
  DOI : 10.1002/2016GL068691
• Electric field measurements in a kHz-driven He jetthe influence of the gas flow speed
  
  • Sobota Ana
  • Guaitella Olivier
  • Sretenović Gb
  • Krstić Ib
  • Kovačević Vv
  • Obrusník A.
  • Nguyen Yn
  • Zajíčková L.
  • Obradović Bm
  • Kuraica Mm
  Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (6), pp.065026. This report focuses on the dependence of electric field strength in the effluent of a vertically downwards-operated plasma jet freely expanding into room air as a function of the gas flow speed. A 30&#8201;kHz AC-driven He jet was used in a coaxial geometry, with an amplitude of 2&#8201;kV and gas flow between 700&#8201;sccm and 2000&#8201;SCCM. The electric field was measured by means of Stark polarization spectroscopy of the He line at 492.19&#8201;nm. While the minimum and the maximum measured electric fields remained unchanged, the effect of the gas flow speed is to cause stretching of the measured profile in spacethe higher the flow, the longer and less steep the electric field profile. The portion of the effluent in which the electric field was measured showed an increase of electric field with increasing distance from the capillary, for which the probable cause is the contraction of the plasma bullet as it travels through space away from the capillary. There are strong indications that the stretching of the electric field profile with increase in the flow speed is caused by differences in gas mixing as a function of the gas flow speed. The simulated gas composition shows that the amount of air entrained into the gas flow behaves in a similar way to the observed behaviour of the electric field. In addition we have shown that the visible length of the plasma plume is associated with a 0.027 molar fraction of air in the He flow in this configuration, while the maximum electric field measured was associated with a 0.014 molar fraction of air at gas flow rates up to 1500&#8201;SCCM (4.9 m s&#8722;1). At higher flows vortices occur in the effluent of the jet, as seen in Schlieren visualization of the gas flow with and without the discharge. (10.1088/0963-0252/25/6/065026)
  DOI : 10.1088/0963-0252/25/6/065026
• Slope and amplitude asymmetry effects on low frequency capacitively coupled carbon tetrafluoride plasmas
  
  • Bruneau Bastien
  • Korolov Ihor
  • Lafleur Trevor
  • Gans T.
  • O'Connell D.
  • Greb Arthur
  • Derzsi A.
  • Donkó Z.
  • Brandt S.
  • Schüngel E.
  • Schulze J.
  • Johnson Erik
  • Booth Jean-Paul
  Journal of Applied Physics, American Institute of Physics, 2016, 119 (16), pp.163301. We report investigations of capacitively coupled carbon tetrafluoride (CF4) plasmas excited with tailored voltage waveforms containing up to five harmonics of a base frequency of 5.5 MHz. The impact of both the slope asymmetry, and the amplitude asymmetry, of these waveforms on the discharge is examined by combining experiments with particle-in-cell simulations. For all conditions studied herein, the discharge is shown to operate in the drift-ambipolar mode, where a comparatively large electric field in the plasma bulk (outside the sheaths) is the main mechanism for electron power absorption leading to ionization. We show that both types of waveform asymmetries strongly influence the ion energy at the electrodes, with the particularity of having the highest ion flux on the electrode where the lowest ion energy is observed. Even at the comparatively high pressure (600 mTorr) and low fundamental frequency of 5.5 MHz used here, tailoring the voltage waveforms is shown to efficiently create an asymmetry of both the ion energy and the ion flux in geometrically symmetric reactors. (10.1063/1.4947453)
  DOI : 10.1063/1.4947453
• Brief review on plasma propulsion with neutralizer-free systems
  
  • Rafalskyi D.V.
  • Aanesland Ane
  Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (4), pp.043001. Electric space propulsion is an intensively developing field addressing new demands and challenges for long-term spacecraft operation. Many novel plasma propulsion concepts aim to find new acceleration principles, use alternative propellants, upscale or downscale thrusters for large thrust or for very small spacecrafts etc. In this work we review the neutralizer-free concepts, where both positive and negative particles are extracted and accelerated from plasmas. We can divide these concepts into three main categories, defined by their acceleration principle: (i) neutral beam generation, (ii) plasma acceleration/expansion and (iii) bipolar beam acceleration. We describe the basic physical principles and evaluate the main advantages and drawbacks in view of general space applications. We also present here further detail on a recent concept where RF voltages are used to accelerate quasi-simultaneously positive ions and electrons from the same source. (10.1088/0963-0252/25/4/043001)
  DOI : 10.1088/0963-0252/25/4/043001
• Plasma dynamics of a laser filamentation-guided spark
  
  • Point Guillaume
  • Arantchouk Léonid
  • Carbonnel Jérôme
  • Mysyrowicz André
  • Houard Aurélien
  Physics of Plasmas, American Institute of Physics, 2016, 23 (9), pp.093505. We investigate experimentally the plasma dynamics of a centimeter-scale, laser filamentation-guided spark discharge. Using electrical and optical diagnostics to study monopolar discharges with varying current pulses we show that plasma decay is dominated by free electron recombination if the current decay time is shorter than the recombination characteristic time. In the opposite case, the plasma electron density closely follows the current evolution. We demonstrate that this criterion holds true in the case of damped AC sparks, and that alternative current is the best option to achieve a long plasma lifetime for a given peak current. (10.1063/1.4962517)
  DOI : 10.1063/1.4962517
• foF2 long-term trend linked to Earth's magnetic field secular variation at a station under the northern crest of the equatorial ionization anomaly
  
  • Pham Thi Thu Hong
  • Amory-Mazaudier Christine
  • Le Huy Minh
  • Elias Ana G. Anagelias@Yahoo.Com
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2016, 121 (1), pp.719–726. Long-term trend of the critical frequency of the F 2 ionospheric region, f o F 2 , at Phu Thuy station (21.03°N, 105.96°E), Vietnam, located under the northern crest of the equatorial ionization anomaly, EIA, is studied. Annual mean data are analyzed at 04 LT and 12 LT for the period 1962–2002 using monthly median values and monthly mean values during magnetically quiet days (am < 20). In both cases we obtain similar trends at 4 LT and 12 LT, which we interpret as an absence of geomagnetic activity effect over trends. The positive trends obtained are not consistent with the negative values expected from greenhouse gases effect at this layer of the upper atmosphere. The increasing trend observed at 12 LT is qualitatively in agreement with the expected effect of the secular displacement of the dip equator over the EIA latitudinal profile. At 04 LT, when the EIA is absent, the positive trend is in qualitative agreement with the secular variation of the Earth's magnetic field inclination, I, and the consequent increase of the sin(I)cos(I) factor at the corresponding location. (10.1002/2015JA021890)
  DOI : 10.1002/2015JA021890
• Influence of surface emission processes on a fast-pulsed dielectric barrier discharge in air at atmospheric pressure
  
  • Pechereau François
  • Bonaventura Z.
  • Bourdon Anne
  Plasma Sources Science and Technology, IOP Publishing, 2016, 25, pp.044004. This paper presents simulations of an atmospheric pressure air discharge in a point-to-plane geometry with a dielectric layer parallel to the cathode plane. Experimentally, a discharge reignition in the air gap below the dielectrics has been observed. With a 2D fluid model, it is shown that due to the fast rise of the high voltage applied and the sharp point used, a first positive spherical discharge forms around the point. Then this discharge propagates axially and impacts the dielectrics. As the first discharge starts spreading on the upper dielectric surface, in the second air gap with a low preionization density of 10^4~\textc\textm^-3 , the 2D fluid model predicts a rapid reignition of a positive discharge. As in experiments, the discharge reignition is much slower, a discussion on physical processes to be considered in the model to increase the reignition delay is presented. The limit case with no initial seed charges in the second air gap has been studied. First, we have calculated the time to release an electron from the cathode surface by thermionic and field emission processes for a work function φ ∈ ≤ft[3,4\right] eV and an amplification factor β ∈ ≤ft[100,220\right] . Then a 3D Monte Carlo model has been used to follow the dynamics of formation of an avalanche starting from a single electron emitted at the cathode. Due to the high electric field in the second air gap, we have shown that in a few nanoseconds, a Gaussian cloud of seed charges is formed at a small distance from the cathode plane. This Gaussian cloud has been used as the initial condition of the 2D fluid model in the second air gap. In this case, the propagation of a double headed discharge in the second air gap has been observed and the reignition delay is in rather good agreement with experiments. (10.1088/0963-0252/25/4/044004)
  DOI : 10.1088/0963-0252/25/4/044004
• Introduction to Modern Magnetohydrodynamics
  
  • Galtier Sébastien
  , 2016. Preface; Table of physical quantities; Part I. Foundations: 1. Introduction; 2. Magnetohydrodynamics; 3. Conservation laws; Part II. Fundamental Processes: 4. Magnetohydrodynamic waves; 5. Dynamo; 6. Discontinuities and shocks; 7. Magnetic reconnection; Part III. Instabilities and Magnetic Confinement: 8. Static equilibrium; 9. Linear perturbation theory; 10. Study of MHD instabilities; Part IV. Turbulence: 11. Hydrodynamic turbulence; 12. MHD turbulence; 13. Advanced MHD turbulence; Appendix 1. Solutions to the exercises; Appendix 2. Formulary; References; Index.
• Electron Acceleration by Langmuir Waves Produced by a Decay Cascade
  
  • Krafft C.
  • Volokitin A. S.
  The Astrophysical Journal, American Astronomical Society, 2016, 821 (2), pp.99. It was recently reported that a significant part of the Langmuir waveforms observed by the STEREO satellite during type III solar radio bursts are likely consistent with the occurrence of electrostatic decay instabilities, when a Langmuir wave L resonantly interacts with another Langmuir wave L <SUP>\prime</SUP> and an ion sound wave S <SUP>\prime</SUP> through the decay channel L \to L <SUP>\prime</SUP> S <SUP>\prime</SUP> . Usually such wave-wave interactions occur in regions of the solar wind where the presence of electron beams can drive Langmuir turbulence to levels allowing waves L to decay. Moreover, such solar wind plasmas can present long-wavelength, randomly fluctuating density inhomogeneities or monotonic density gradients which can significantly modify the development of such resonant instabilities. If some conditions are met, the waves can encounter a second decay cascade (SDC) according to L <SUP>\prime</SUP> \to L <SUP>\prime\prime</SUP> S <SUP>\prime\prime</SUP> . Analytical estimates and observations based on numerical simulations show that the Langmuir waves L <SUP>\prime\prime</SUP> produced by this SDC can accelerate beam particles up to velocities and kinetic energies exceeding two times the beam drift velocity v<SUB>b</SUB> and half the initial beam energy, respectively. Moreover, this process can be particularly efficient if the scattering effects of waves on the background plasma inhomogeneities have already accelerated a sufficient amount of beam electrons up to the velocity range where the phase velocities of the L <SUP>\prime\prime</SUP> waves are lying. The paper shows that the conditions necessary for such process to occur can be easily met in solar wind plasmas if the beam velocities do not exceed around 35 times the plasma thermal velocity. (10.3847/0004-637X/821/2/99)
  DOI : 10.3847/0004-637X/821/2/99
• Transport of Solar Wind H<SUP>+</SUP> and He<SUP>++</SUP> Ions across Earth's Bow Shock
  
  • Parks G. K.
  • Lee E.
  • Fu S. Y.
  • Kim H. E.
  • Ma Y. Q.
  • Yang Z. W.
  • Liu Y.
  • Lin N.
  • Hong J.
  • Canu Patrick
  • Dandouras I.
  • Rème H.
  • Goldstein M. L.
  The Astrophysical Journal Letters, Bristol : IOP Publishing, 2016, 825 (2), pp.L27. We have investigated the dependence of mass, energy, and charge of solar wind (SW) transport across Earth?s bow shock. An examination of 111 crossings during quiet SW in both quasi-perpendicular and quasi-parallel shock regions shows that 64 crossings had various degrees of heating and thermalization of SW. We found 22 crossings where the SW speed was <400 km s ?1 . The shock potential of a typical supercritical quasi-perpendicular shock estimated from deceleration of the SW and cutoff energy of electron flat top distribution is ?50 Volts. We find that the temperatures of H and He beams that penetrate the shock can sometimes be nearly the same in the upstream and downstream regions, indicating little or no heating had occurred crossing the bow shock. None of the models predict that the SW can cross the bow shock without heating. Our observations are important constraints for new models of collisionless shocks. (10.3847/2041-8205/825/2/L27)
  DOI : 10.3847/2041-8205/825/2/L27
• Improved Design of a Multistage Axial Vircator With Reflectors for Enhanced Performances
  
  • Champeaux Stéphanie
  • Gouard Philippe
  • Cousin Richard
  • Larour Jean
  IEEE Transactions on Plasma Science, Institute of Electrical and Electronics Engineers, 2016, 44 (1), pp.31-38. The basic design of an axial virtual cathode oscillator (vircator) with axial extraction operating in TM01 mode is modified by introducing thin conducting disks, also called reflectors, into the cylindrical waveguide. The operation principal of this novel type of device relies on the formation of a series of virtual cathodes, located at the center of adjacent quasi-cavities. The behavior of this new type of multistage vircator is numerically investigated using CST Particle Studio 3-D particle-in-cell code. Progressively decreasing the radii of the reflectors installed upstream in the tube allows the mitigation of spurious modes. Tapering the radii of the reflectors turns out to be crucial in focusing the electron beam on axis in the downstream region and maximizing the TM01 power conversion efficiency. This novel architecture enables a five-reflector vircator operating with an injected electron beam of 508-kV mean voltage and 19-kA mean current to deliver up to 2-GW mean power sustained only by the TM01 mode in the S-band with a power conversion efficiency close to 21%. (10.1109/TPS.2015.2502432)
  DOI : 10.1109/TPS.2015.2502432
• Diffusion of Energetic Electrons in Turbulent Plasmas of the Solar Wind
  
  • Volokitin A. S.
  • Krafft C.
  The Astrophysical Journal, American Astronomical Society, 2016, 833 (2), pp.166. A method of calculation of the diffusion coefficients D (v) of particles in velocity space, based on the statistical analysis of the motion of a great number of test electrons, is proposed. In the case of Langmuir turbulence developing in plasmas with fluctuating density inhomogeneities such as the solar wind, simulations provide coefficients D (v) which mainly depend on the Langmuir wave spectra and agree well with the analytical predictions D <SUB>th</SUB>(v) of the quasilinear theory of weak turbulence. Nevertheless, some noticeable differences exist with this theory: in the range of phase velocity of the short waves where the main part of the wave energy is concentrated, D (v) is noticeably smaller than D <SUB>th</SUB>(v), due to the scattering, the reflection, and the focusing effects encountered by the Langmuir waves when they interact with the plasma density inhomogeneities. Moreover, the probability of large velocity jumps in the particles' trajectories essentially exceeds the probability of a Gaussian distribution. These large jumps, which are connected with the waves' transformation processes, modify the nature of the particle diffusion, which is no more classical. These higher order effects cause the discrepancies observed with the quasilinear theory, which does not take them into account in its perturbative approach. The solar wind plasmas, which present fluctuating density inhomogeneities of noticeable average levels, are a very good laboratory to study such diffusion processes, which can eventually influence significantly on the development of essential physical phenomena, as electromagnetic radio emissions by type III solar radio bursts, for example. (10.3847/1538-4357/833/2/166)
  DOI : 10.3847/1538-4357/833/2/166
• Controlling the shape of the ion energy distribution at constant ion flux and constant mean ion energy with tailored voltage waveforms
  
  • Bruneau Bastien
  • Lafleur Trevor
  • Booth Jean-Paul
  • Johnson Erik
  Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (2), pp.025006. In this paper, we investigate the excitation of a capacitively coupled plasma using a non-sinusoidal voltage waveform whose amplitude- and slope-asymmetry varies continuously with a period which is a multiple of the fundamental RF period. We call this period the ?beating? period. Through particle-in-cell (PIC) simulations, we show that such waveforms cause oscillation of the self-bias at this beating frequency, corresponding to the charging and discharging of the external capacitor. The amplitude of this self-bias oscillation depends on the beating period, the value of the external capacitor, and the ion flux to the electrodes. This self-bias oscillation causes temporal modulation of the ion flux distribution function (IFDF), albeit at a constant ion flux and constant mean ion energy, and allows the energy width of the IFDF (averaged over the beating period) to be varied in a controlled fashion. (10.1088/0963-0252/25/2/025006)
  DOI : 10.1088/0963-0252/25/2/025006
• Effect of gas properties on the dynamics of the electrical slope asymmetry effect in capacitive plasmas: comparison of Ar, H<SUB>2</SUB> and CF<SUB>4</SUB>
  
  • Bruneau Bastien
  • Lafleur Trevor
  • Gans T.
  • O'Connell D.
  • Greb Arthur
  • Korolov Ihor
  • Derzsi A.
  • Donkó Z.
  • Brandt S.
  • Schüngel E.
  • Schulze J.
  • Diomede P.
  • Economou D. J.
  • Longo S.
  • Johnson E.V.
  • Booth Jean-Paul
  Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (1), pp.01LT02. Tailored voltage excitation waveforms provide an efficient control of the ion energy (through the electrical asymmetry effect) in capacitive plasmas by varying the ?amplitude? asymmetry of the waveform. In this work, the effect of a ?slope? asymmetry of the waveform is investigated by using sawtooth-like waveforms, through which the sheath dynamic can be manipulated. A remarkably different discharge dynamic is found for Ar, H 2 , and CF 4 gases, which is explained by the different dominant electron heating mechanisms and plasma chemistries. In comparison to Argon we find that the electrical asymmetry can even be reversed by using an electronegative gas such as CF 4 . Phase resolved optical emission spectroscopy measurements, probing the spatiotemporal distribution of the excitation rate show excellent agreement with the results of particle-in-cell simulations, confirming the high degree of correlation between the excitation rates with the dominant heating mechanisms in the various gases. It is shown that, depending on the gas used, sawtooth-like voltage waveforms may cause a strong asymmetry. (10.1088/0963-0252/25/1/01LT02)
  DOI : 10.1088/0963-0252/25/1/01LT02
• Turbulence Heating ObserveR - satellite mission proposal
  
  • Vaivads A.
  • Retinò Alessandro
  • Soucek J.
  • Khotyaintsev Yu. V. V
  • Valentini F.
  • Escoubet C. Philippe
  • Alexandrova Olga
  • André M.
  • Bale S. D.
  • Balikhin M. A.
  • Burgess D.
  • Camporeale E.
  • Caprioli D.
  • Chen C. H. K.
  • Clacey E.
  • Cully C. M.
  • de Keyser J.
  • Eastwood Jonathan P.
  • Fazakerley A. N.
  • Eriksson S.
  • Goldstein M. L.
  • Graham D. B.
  • Haaland S.
  • Hoshino M.
  • Ji H.
  • Karimabadi H.
  • Kucharek H.
  • Lavraud B.
  • Marcucci F.
  • Matthaeus W. H.
  • Moore T. E.
  • Nakamura R.
  • Narita Y.
  • Nemecek Z.
  • Norgren C.
  • Opgenoorth H.
  • Palmroth M.
  • Perrone D.
  • Pinçon Jean-Louis
  • Rathsman P.
  • Rothkaehl H.
  • Sahraoui Fouad
  • Servidio S.
  • Sorriso-Valvo L.
  • Vainio R.
  • Vörös Z.
  • Wimmer-Schweingruber R. F.
  Journal of Plasma Physics, Cambridge University Press (CUP), 2016, 82 (5), pp.905820501 (16 pages). The Universe is permeated by hot, turbulent, magnetized plasmas. Turbulent plasma is a major constituent of active galactic nuclei, supernova remnants, the intergalactic and interstellar medium, the solar corona, the solar wind and the Earth's magnetosphere, just to mention a few examples. Energy dissipation of turbulent fluctuations plays a key role in plasma heating and energization, yet we still do not understand the underlying physical mechanisms involved. THOR is a mission designed to answer the questions of how turbulent plasma is heated and particles accelerated, how the dissipated energy is partitioned and how dissipation operates in different regimes of turbulence. THOR is a single-spacecraft mission with an orbit tuned to maximize data return from regions in near-Earth space - magnetosheath, shock, foreshock and pristine solar wind - featuring different kinds of turbulence. Here we summarize the THOR proposal submitted on 15 January 2015 to the `Call for a Medium-size mission opportunity in ESAs Science Programme for a launch in 2025 (M4)'. THOR has been selected by European Space Agency (ESA) for the study phase. (10.1017/S0022377816000775)
  DOI : 10.1017/S0022377816000775
• Observations of turbulence in a Kelvin-Helmholtz event on 8 September 2015 by the Magnetospheric Multiscale mission
  
  • Stawarz J. E.
  • Eriksson S.
  • Wilder F. D.
  • Ergun R. E.
  • Schwartz S. J.
  • Pouquet A.
  • Burch J. L.
  • Giles B. L.
  • Khotyaintsev Y. V.
  • Le Contel Olivier
  • Lindqvist P.-A.
  • Magnes W.
  • Pollock C. J.
  • Russell C. T.
  • Strangeway R. J.
  • Torbert R. B.
  • Avanov L. A.
  • Dorelli J. C.
  • Eastwood Jonathan P.
  • Gershman D. J.
  • Goodrich K. A.
  • Malaspina D. M.
  • Marklund G. T.
  • Mirioni Laurent
  • Sturner A. P.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2016, 121 (11), pp.11,021-11,034. Spatial and high-time-resolution properties of the velocities, magnetic field, and 3-D electric field within plasma turbulence are examined observationally using data from the Magnetospheric Multiscale mission. Observations from a Kelvin-Helmholtz instability (KHI) on the Earth's magnetopause are examined, which both provides a series of repeatable intervals to analyze, giving better statistics, and provides a first look at the properties of turbulence in the KHI. For the first time direct observations of both the high-frequency ion and electron velocity spectra are examined, showing differing ion and electron behavior at kinetic scales. Temporal spectra exhibit power law behavior with changes in slope near the ion gyrofrequency and lower hybrid frequency. The work provides the first observational evidence for turbulent intermittency and anisotropy consistent with quasi two-dimensional turbulence in association with the KHI. The behavior of kinetic-scale intermittency is found to have differences from previous studies of solar wind turbulence, leading to novel insights on the turbulent dynamics in the KHI. (10.1002/2016JA023458)
  DOI : 10.1002/2016JA023458
• Observations of cold ion heating inside the magnetospheric separatrix region
  
  • Toledo-Redondo Sergio
  • Andre M.
  • Vaivads A.
  • Khotyaintsev Y. V.
  • Lavraud Benoit
  • Graham Daniel
  • Divin A. V.
  • Aunai N.
  , 2016, 18, pp.EPSC2016-14007. Several studies have shown that cold ions (energies up to tens of eV) of ionospheric origin can be found in different regions of the magnetosphere, including the dayside magnetopause. They can be very abundant, up to ~100 cc, e.g. plasmaspheric plumes, and become the dominant population of the magnetosphere. Cold ions, when present, participate in magnetic reconnection at the dayside magnetopause, mass loading the magnetospheric side and adding a new length-scale into the system owing to their smaller gyroradius. At the same time, reconnection accelerates and heats the cold ions. Based on multi-spacecraft observations, we report observations of cold ion heating inside the separatrix region when reconnection is ongoing and study the mechanisms that energize the cold ions. The heating is not always observed and our observations indicate that cold ion heating is more effective next to the X-line. We find that large electric field gradients and wave-particle interactions are consistent with the heating observed.
• On the Existence of the Kolmogorov Inertial Range in the Terrestrial Magnetosheath Turbulence
  
  • Huang S. Y.
  • Sahraoui Fouad
  • Hadid L. Z.
  • Yuan Z. G.
  , 2016, 18, pp.EPSC2016-6911. In solar wind, it is well established that at MHD scales the magnetic energy spectra generally follow the so-called Kolmogorov inertial range spectrum f-5/3 in solar wind. In this study, we used three years data from the Cluster mission to statistically investigate the existence of the Kolmogorov inertial range in the whole magnetosheath, including flanks and subsolar regions. Statistical results show that Kolmogorov inertial range is not ubiquitous in the magnetosheath. Indeed, most spectra were found to be shallower than the Kolmogorov one and have a scaling f-1recalling the energy containing scales of solarwind turbulence. The Kolmogorov scaling is observed only faraway from the bow shock and in the flanks region. These results suggest that random-like fluctuations are generated behind the shock, which reach a fully developed turbulence state only after some time corresponding to their propagation (or advection) away from the shock. These fluctuations are shown to be essentially compressible and not Alfvenic as in the case of the solar wind. Implications on the new theoretical modeling of space plasma turbulence will be discussed.