Publications

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Sont listées ci-dessous, par année, les publications figurant dans l'archive ouverte HAL.

2016

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 di, where di 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
Velocity diffusion of energetic electrons in the solar wind
- Volokitin A.
- Krafft C.
AIP Conference Proceedings, American Institute of Physics, 2016, 1720 (1), pp.070007. Particle diffusion in velocity space is studied on the basis of 1D simulations of Langmuir turbulence generated by electron beams in solar wind plasmas. Using a large amount of particle trajectories calculated with a great accuracy and over long times and analyzing them with statistical algorithms, the diffusion coefficients of particles in wave packets are estimated, as well as their relation to the waves' intensities and spectra and their dependence on the average level of background plasma density fluctuations. Results are compared with analytical solutions provided by the quasilinear theory of weak turbulence. (10.1063/1.4943844)
DOI : 10.1063/1.4943844
The Alfvén Mission for the ESA M5 Call: Mission Concept
- Fazakerley A.
- Berthomier Matthieu
- Pottelette Raymond
- Forsyth C.
, 2016, 18, pp.EPSC2016-16890. This poster will present the proposed Alfvén mission concept and is complemented by a presentation of the mission scientific goals planned for the ST1.5 session. The Alfvén mission has the scientific objective of studying particle acceleration and other forms of electromagnetic energy conversion in a collisionless low beta plasma. The mission is proposed to operate in the Earth's Auroral Acceleration Region (AAR), the most accessible laboratory for investigating plasmas at an interface where ideal magneto-hydrodynamics does not apply. Alfvén is designed to answer questions about where and how the particles that create the aurorae are accelerated, how and why they emit auroral kilometric radiation, what creates and maintains large scale electric fields aligned with the magnetic field, and to elucidate the ion outflow processes which are slowly removing the Earth's atmosphere. The mission will provide the required coordinated two-spacecraft observations within the AAR several times a day. From well designed separations along or across the magnetic field lines, using a comprehensive suite of inter-calibrated particles and field instruments, it will measure the parallel electric fields, variations in particle flux, and wave energy that will answer open questions on energy conversion. It will use onboard auroral imagers to determine how this energy conversion occurs in the regional context and, together with its orbit design, this makes the mission ideally suited to resolving spatio-temporal ambiguities that have plagued previous auroral satellite studies. The spacecraft observations will be complemented by coordinated observations with the existing dense network of ground based observatories, for more detailed ionospheric and auroral information when Alfvén overflights occur.
Plasma Sources of Solar System Magnetospheres
- Fontaine Dominique
- Delcourt Dominique
, 2016.
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
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
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
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
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
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 kHz AC-driven He jet was used in a coaxial geometry, with an amplitude of 2 kV and gas flow between 700 sccm and 2000 SCCM. The electric field was measured by means of Stark polarization spectroscopy of the He line at 492.19 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 SCCM (4.9 m s−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
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|| amplitudes reaching up to 300 mV m-1 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
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
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.
The 2π charged particles analyzer: All-sky camera concept and development for space missions
- Vaisberg Oleg
- Berthelier Jean-Jacques
- Moore T.
- Avanov L.
- Leblanc François
- Leblanc Frédéric
- Moiseev Pavel P.
- Moiseenko D.
- Becker Joël
- Collier Michael R.
- Laky G.
- Keller J.
- Koynash G.
- Lichtenneger H.
- Leibov A. W.
- Zhuravlev R.
- Shestakov A.
- Burch J.
- Mccomas D.
- Shuvalov S.
- Chornay D.
- Torkar K.
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2016, 121 (12), pp.11,750–11,765. Increasing the temporal resolution and instant coverage of velocity space of space plasma measurements is one of the key issues for experimentalists. Today the top-hat plasma analyzer appears to be the favorite solution due to its relative simplicity and the possibility to extend its application by adding a mass-analysis section and an electrostatic angular scanner. Similarly, great success has been achieved in MMS mission using such multiple top-hat analyzers to achieve unprecedented temporal resolution. An instantaneous angular coverage of charged particles measurements is an alternative approach to pursuing the goal of high time resolution. This was done with FONEMA 4-D and, to a lesser extent, by DYMIO instruments for Mars-96 and with the FIPS instrument for MESSENGER mission. In this paper we describe, along with precursors, a plasma analyzer with a 2π electrostatic mirror that was developed originally for the Phobos-Soil mission with a follow-up in the frame of the BepiColombo mission, and is under development for future Russian missions. Different versions of instrument are discussed along with their advantages and drawbacks. (10.1002/2016JA022568)
DOI : 10.1002/2016JA022568
Alfvén-dynamo balance and magnetic excess in magnetohydrodynamic turbulence
- Grappin Roland
- Müller Wolf-Christian
- Verdini Andrea
Astronomy & Astrophysics - A&A, EDP Sciences, 2016, 589. Context. Three-dimensional magnetohydrodynamic (3D MHD) turbulent flows with initially magnetic and kinetic energies at equipartition spontaneously develop a magnetic excess (or residual energy) in both numerical simulations and the solar wind. Closure equations obtained in 1983 describe the residual spectrum as resulting from a balance between a dynamo source proportional to the total energy spectrum and a linear Alfvén damping term. A good agreement was found in 2005 with incompressible simulations; however, recent solar wind measurements disagree with these results. Aims: The previous dynamo-Alfvén theory is generalized to a family of models, leading to simple relations between residual and total energy spectra. We want to assess these models in detail against MHD simulations and solar wind data. Methods: We tested the family of models against compressible decaying MHD simulations with a low Mach number, low cross-helicity, and zero-mean magnetic field with or without expansion terms (EBM; expanding box model). Results: A single dynamo-Alfvén model is found to describe correctly both solar wind scalings and compressible simulations without or with expansion. This model is equivalent to the 1983-2005 closure equation, but it incorporates the critical balance of nonlinear turnover and linear Alfvén times, while the dynamo source term remains unchanged. We elucidate the discrepancy with previous incompressible simulations. The model predicts a linear relation between the spectral slopes of total and residual energies mR = -1/2 3/2mT. By examining previous solar wind data, our relation is found to be valid for any cross-helicity, and is even better at high cross-helicity with the total energy slope varying from 1.7 to 1.55. (10.1051/0004-6361/201628097)
DOI : 10.1051/0004-6361/201628097
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 th(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 th(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
Transport of Solar Wind H+ and He++ 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
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 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.
Solar quiet current response in the African sector due to a 2009 sudden stratospheric warming event
- Bolaji O. S.
- Oyeyemi E. O.
- Owolabi O. P.
- Yamazaki Y.
- Rabiu A. B.
- Okoh D.
- Fujimoto A.
- Amory-Mazaudier Christine
- Seemala G. K.
- Yoshikawa A.
- Onanuga O. K.
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2016. We present solar quiet (S q) variation of the horizontal (H) magnetic field intensity deduced from Magnetic Data Acquisition System (MAGDAS) records over Africa during an unusual strong and prolonged 2009 sudden stratospheric warming (SSW) event. A reduction in the S q H magnitude that enveloped the geomagnetic latitudes between 21.13°N (Fayum FYM) in Egypt and 39.51°S (Durban DRB) in South Africa was observed, while the stratospheric polar temperature was increasing and got strengthened when the stratospheric temperature reached its maximum. Another novel feature associated with the hemispheric reduction is the reversal in the north-south asymmetry of the S q H, which is indicative of higher S q H magnitude in the Northern Hemisphere compared to the Southern Hemisphere during SSW peak phase. The reversal of the equatorial electrojet (EEJ) or the counter electrojet (CEJ) was observed after the polar stratospheric temperature reached its maximum. The effect of additional currents associated with CEJ was observed in the Southern Hemisphere at middle latitude. Similar changes were observed in the EEJ at the South America, Pacific Ocean, and Central Asia sectors. The effect of the SSW is largest in the South American sector and smallest in the Central Asian sector. (10.1002/2016JA022857)
DOI : 10.1002/2016JA022857
Effect of gas properties on the dynamics of the electrical slope asymmetry effect in capacitive plasmas: comparison of Ar, H2 and CF4
- 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
Pressure broadening of atomic oxygen two-photon absorption laser induced fluorescence
- Marinov Daniil
- Drag Cyril
- Blondel Christophe
- Guaitella Olivier
- Golda Judith
- Klarenaar Bart
- Engeln Richard
- Gathen Volker Schulz-von Der
- Booth Jean-Paul
Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (6), pp.06LT03. Atomic oxygen, considered to be a determining reactant in plasma applications at ambient pressure, is routinely detected by two-photon absorption laser induced fluorescence (TALIF). Here, pressure broadening of the (2 p 4 3 P 2 ?????3 p 3 P J =0,1,2 ) two-photon transition in oxygen atoms was investigated using a high-resolution TALIF technique in normal and Doppler-free configurations. The pressure broadening coefficients determined were ##IMG## [http://ej.iop.org/images/0963-0252/25/6/06LT03/psstaa4481ieqn001.gif] γ_\textO_2 ??=??0.40??±??0.08? cm ?1 /bar for oxygen molecules and ##IMG## [http://ej.iop.org/images/0963-0252/25/6/06LT03/psstaa4481ieqn002.gif] γ_\textHe ??=??0.46??±??0.03?cm ?1 /bar for helium atoms. These correspond to pressure broadening rate constants ##IMG## [http://ej.iop.org/images/0963-0252/25/6/06LT03/psstaa4481ieqn003.gif] k_\textPB^\textO_2 ??=??9 · 10 ?9 cm 3 s ?1 and ##IMG## [http://ej.iop.org/images/0963-0252/25/6/06LT03/psstaa4481ieqn004.gif] k_\textPB^\textHe ??=??4 · 10 ?9 cm 3 s ?1 , respectively. The well-known quenching rate constants of O(3 p 3 P J ) by O 2 and He are at least one order of magnitude smaller, which signifies that non-quenching collisions constitute the main line-broadening mechanism. In addition to providing new insights into collisional processes of oxygen atoms in electronically excited 3 p 3 P J state, reported pressure broadening parameters are important for quantification of oxygen TALIF line profiles when both collisional and Doppler broadening mechanisms are important. Thus, the Doppler component (and hence the temperature of oxygen atoms) can be accurately determined from high resolution TALIF measurements in a broad range of conditions. (10.1088/0963-0252/25/6/06LT03)
DOI : 10.1088/0963-0252/25/6/06LT03

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