Publications

2016

• A comparative experimental kinetic study of spontaneous and plasma-assisted cool flames in a rapid compression machine
  
  • Vanhove G.
  • Boumehdi M.-A.
  • Shcherbanev S.A.
  • Fenard Y.
  • Desgroux Pascale
  • Starikovskaia Svetlana
  Proceedings of the Combustion Institute, Elsevier, 2016. Plasma-assisted cool flames of n- heptane were generated in the combustion chamber of a rapid compression machine coupled with a nanosecond dielectric barrier discharge, at a pressure of 1.5 bar and temperature T = 650 K. Increasing of the voltage pulse amplitude at the electrode resulted in a transition from no reactivity to induced cool flame and then to fast ignition. Sampling of the reacting mixture was performed at selected times during the experiments to draw mole fraction profiles of the fuel and major low temperature stable intermediates, showing a gradual increase in the mole fraction of these species after the discharge. Comparison with a spontaneous cool flame case at a slightly higher pressure shows that no new species are formed in the plasma-assisted case, and that after the initiation of reactivity by the discharge at the nanosecond timescale, the distribution and relative importance of the main reaction pathways is conserved at the millisecond timescale. Differences in the shape of the mole fraction and light emission profiles however suggest that the plasma-assisted cool flame is propagative. (10.1016/j.proci.2016.09.007)
  DOI : 10.1016/j.proci.2016.09.007
• Magnetospheric Multiscale observations of large-amplitude, parallel, electrostatic waves associated with magnetic reconnection at the magnetopause
  
  • Ergun R. E.
  • Holmes J. C.
  • Goodrich K. A.
  • Wilder F. D.
  • Stawarz J. E.
  • Eriksson S.
  • Newman D. L.
  • Schwartz S. J.
  • Goldman M. V.
  • Sturner A. P.
  • Malaspina D. M.
  • Usanova M. E.
  • Torbert R. B.
  • Argall M.
  • Lindqvist P.-A.
  • Khotyaintsev Y. V.
  • Burch J. L.
  • Strangeway R. J.
  • Russell C. T.
  • Pollock C. J.
  • Giles B. L.
  • Dorelli J. J. C.
  • Avanov L.
  • Hesse Michael
  • Chen L. J.
  • Lavraud B.
  • Le Contel Olivier
  • Retinò Alessandro
  • Phan T. D.
  • Eastwood Jonathan P.
  • Oieroset M.
  • Drake J. F.
  • Shay M. A.
  • Cassak P. A.
  • Nakamura R.
  • Zhou M.
  • Ashour-Abdalla M.
  • André M.
  Geophysical Research Letters, American Geophysical Union, 2016, 43 (11), pp.5626-5634. We report observations from the Magnetospheric Multiscale satellites of large-amplitude, parallel, electrostatic waves associated with magnetic reconnection at the Earth's magnetopause. The observed waves have parallel electric fields (E<SUB>||</SUB>) with amplitudes on the order of 100 mV/m and display nonlinear characteristics that suggest a possible net E<SUB>||</SUB>. These waves are observed within the ion diffusion region and adjacent to (within several electron skin depths) the electron diffusion region. They are in or near the magnetosphere side current layer. Simulation results support that the strong electrostatic linear and nonlinear wave activities appear to be driven by a two stream instability, which is a consequence of mixing cold (<10 eV) plasma in the magnetosphere with warm (~100 eV) plasma from the magnetosheath on a freshly reconnected magnetic field line. The frequent observation of these waves suggests that cold plasma is often present near the magnetopause. (10.1002/2016GL068992)
  DOI : 10.1002/2016GL068992
• Exact scaling laws for helical three-dimensional two-fluid turbulent plasmas
  
  • Andrés Nahuel
  • Galtier Sébastien
  • Sahraoui Fouad
  Physical Review E, American Physical Society (APS), 2016, 94 (6), pp.063206. We derive exact scaling laws for a three-dimensional incompressible helical two-fluid plasma, without the assumption of isotropy. For each ideal invariant of the two-fluid model, i.e., the total energy, the electron helicity, and the proton helicity, we derive simple scaling laws in terms of two-point increment correlation functions expressed in terms of the velocity field of each species and the magnetic field. These variables are appropriate for comparison with direct numerical simulation data and with in situ measurements in the near-Earth space over a broad range of spatial scales. Finally, using the exact scaling laws and dimensional analysis we predict the magnetic energy and electron helicity spectra for different ranges of scales. (10.1103/PhysRevE.94.063206)
  DOI : 10.1103/PhysRevE.94.063206
• In situ observations of flux rope at the separatrix region of magnetic reconnection
  
  • Pang Y.
  • Wang D. D.
  • Huang S. Y.
  • Retinò Alessandro
  • Phan T. D.
  • Daughton W.
  • Vaivads A.
  • Karimabadi H.
  • Zhou M.
  • Sahraoui Fouad
  • Li G. L.
  • Yuan Z. G.
  • Deng X. H.
  • Fu H.S.
  • Fu S. Y.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2016, 121 (1), pp.205-213. We present the first in situ observations of a small-scale flux rope locally formed at the separatrix region of magnetic reconnection without large guide field. Bidirectional electron beams (cold and hot beams) and density cavity accompanied by intense wave activity substantiate the crossing of the separatrix region. Density compression and one parallel electron beam are detected inside the flux rope. We suggest that this flux rope is locally generated at the separatrix region due to the tearing instability within the separatrix current layer. This observation sheds new light on the 3-D picture of magnetic reconnection in space plasma. (10.1002/2015JA021468)
  DOI : 10.1002/2015JA021468
• Mars-solar wind interaction: LatHyS, an improved parallel 3-D multispecies hybrid model
  
  • Modolo Ronan
  • Hess Sebastien
  • Mancini Marco
  • Leblanc François
  • Chaufray Jean-Yves
  • Brain David
  • Leclercq Ludivine
  • Esteban Hernandez Rosa
  • Chanteur Gérard
  • Weill Philippe
  • González-Galindo Francisco
  • Forget François
  • Yagi Manabu
  • Mazelle Christian
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2016, 121 (7), pp.6378–6399. In order to better represent Mars-Solar wind interaction, we present an unprecedented model achieving spatial resolution down to 50 km, a so far unexplored resolution for global kinetic models of the Martian ionized environment. Such resolution approaches the ionospheric plasma scale height. In practice, the model is derived from a first version described in Modolo et al. [2005]. An important effort of parallelization has been conducted and is presented here. A better description of the ionosphere was also implemented including ionospheric chemistry, electrical conductivities and a drag force modelling the ion-neutral collisions in the ionosphere. This new version of the code, named LatHyS (Latmos Hybrid Simulation), is here used to characterize the impact of various spatial resolutions on simulation results. In addition, and following a global model challenge effort [Brain et al., 2010], we present the results of simulation run for three cases which allows addressing the effect of the supra-thermal corona and of the solar EUV activity on the magnetospheric plasma boundaries and on the global escape. Simulation results showed that global patterns are relatively similar for the different spatial resolution runs but finest grid runs provide a better representation of the ionosphere and display more details of the planetary plasma dynamic. Simulation results suggest that a significant fraction of escaping O+ ions is originated from below 1200 km altitude. (10.1002/2015JA022324)
  DOI : 10.1002/2015JA022324
• Electron density measurements in highly electronegative plasmas
  
  • Rafalskyi D.V.
  • Lafleur Trevor
  • Aanesland Ane
  Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (4), pp.047001. In this paper we present experimental measurements of the electron density in very electronegative ionion ArSF 6 plasmas where previous investigations using Langmuir probes have observed electronegativities of up to 5000. The electron density is measured using a short matched dipole probe technique that provides a tolerance better than&#8201;&#8201;±2 · 10 13 m &#8722;3 . The results demonstrate that the electron density in the low pressure plasma source (which contains a magnetic filter) can be reduced to around 2.7 · 10 13 m &#8722;3 with a corresponding plasma electronegativity of about 4000; close to that from fluid simulation predictions. The highest electronegativity, and lowest electron density, is achieved with a pure SF 6 plasma, while adding only 6% SF 6 to Ar allows the electronegativity to be increased from 0 to a few hundred with a corresponding decrease in the electron density by more than a thousand. The impedance probe based on a short matched dipole appears to be a practical diagnostic that can be used for independent measurements of the electron density in very electronegative plasmas, and opens up the possibility to further investigate and optimize electronegative plasma sources. (10.1088/0963-0252/25/4/047001)
  DOI : 10.1088/0963-0252/25/4/047001
• The Baseline Th17 Lymphocytes Level Is a Predictive Marker of Good Response to Biologics in Rheumatoid Arthritis
  
  • Salomon Sarah
  • Guignant Caroline
  • Morel Pierre
  • Gubler Brigitte
  • Fardellone Patrice
  • Marolleau Jean-Pierre
  • Goeb Vincent
  Arthritis & rheumatology, Wiley, 2016, 68 (10).
• A Review of General Physical and Chemical Processes Related to Plasma Sources and Losses for Solar System Magnetospheres
  
  • Seki K.
  • Nagy A.
  • Jackman C. M.
  • Crary F.
  • Fontaine Dominique
  • Zarka P.
  • Wurz Peter
  • Milillo A.
  • Slavin J. A.
  • Delcourt Dominique
  • Wiltberger M.
  • Ilie R.
  • Jia X.
  • Ledvina S. A.
  • Liemohn M. W.
  • Schunk R. W.
  • Blanc Michel
  • Chappell Charles R.
  • Krupp N.
  , 2016, pp.27-89. The aim of this paper is to provide a review of general processes related to plasma sources, their transport, energization, and losses in the planetary magnetospheres. We provide background information as well as the most up-to-date knowledge of the comparative studies of planetary magnetospheres, with a focus on the plasma supply to each region of the magnetospheres. This review also includes the basic equations and modeling methods commonly used to simulate the plasma sources of the planetary magnetospheres. In this paper, we will describe basic and common processes related to plasma supply to each region of the planetary magnetospheres in our solar system. First, we will describe source processes in Sect. 1. Then the transport and energization processes to supply those source plasmas to various regions of the magnetosphere are described in Sect. 2. Loss processes are also important to understand the plasma population in the magnetosphere and Sect. 3 is dedicated to the explanation of the loss processes. In Sect. 4, we also briefly summarize the basic equations and modeling methods with a focus on plasma supply processes for planetary magnetospheres.
• Tailored voltage waveform capacitively coupled plasmas in electronegative gases: frequency dependence of asymmetry effects
  
  • Schüngel E.
  • Korolov Ihor
  • Bruneau Bastien
  • Derzsi A.
  • Johnson E.V.
  • O'Connell D.
  • Gans T.
  • Booth Jean-Paul
  • Donkó Z.
  • Schulze J.
  Journal of Physics D: Applied Physics, IOP Publishing, 2016, 49 (26), pp.265203. Capacitively coupled radio frequency plasmas operated in an electronegative gas (CF 4 ) and driven by voltage waveforms composed of four consecutive harmonics are investigated for different fundamental driving frequencies using PIC/MCC simulations and an analytical model. As has been observed previously for electropositive gases, the application of peak-shaped waveforms (that are characterized by a strong amplitude asymmetry) results in the development of a DC self-bias due to the electrical asymmetry effect (EAE), which increases the energy of ions arriving at the powered electrode. In contrast to the electropositive case (Korolov et al 2012 J. Phys. D: Appl. Phys . 45 465202) the absolute value of the DC self-bias is found to increase as the fundamental frequency is reduced in this electronegative discharge, providing an increased range over which the DC self-bias can be controlled. The analytical model reveals that this increased DC self-bias is caused by changes in the spatial profile and the mean value of the net charge density in the grounded electrode sheath. The spatio-temporally resolved simulation data show that as the frequency is reduced the grounded electrode sheath region becomes electronegative. The presence of negative ions in this sheath leads to very different dynamics of the power absorption of electrons, which in turn enhances the local electronegativity and plasma density via ionization and attachment processes. The ion flux to the grounded electrode (where the ion energy is lowest) can be up to twice that to the powered electrode. At the same time, while the mean ion energies at both electrodes are quite different, their ratio remains approximately constant for all base frequencies studied here. (10.1088/0022-3727/49/26/265203)
  DOI : 10.1088/0022-3727/49/26/265203
• Chiral exact relations for helicities in Hall magnetohydrodynamic turbulence
  
  • Banerjee Supratik
  • Galtier Sébastien
  Physical Review E, American Physical Society (APS), 2016, 93, pp.033120. Besides total energy, three-dimensional incompressible Hall magnetohydrodynamics (MHD) possesses two inviscid invariants, which are the magnetic helicity and the generalized helicity. Exact relations are derived for homogeneous (nonisotropic) stationary Hall MHD turbulence (and also for its inertialess electron MHD limit) with nonzero helicities and in the asymptotic limit of large Reynolds numbers. The universal laws are written only in terms of mixed second-order structure functions, i.e., the scalar product of two different increments. It provides, therefore, a direct measurement of the dissipation rates for the corresponding invariant flux. This study shows that the generalized helicity cascade is strongly linked to the left polarized fluctuations, while the magnetic helicity cascade is linked to the right polarized fluctuations. (10.1103/PhysRevE.93.033120)
  DOI : 10.1103/PhysRevE.93.033120
• Direct Evidence of the Transition from Weak to Strong Magnetohydrodynamic Turbulence
  
  • Meyrand Romain
  • Galtier Sébastien
  • Kiyani K. H.
  Physical Review Letters, American Physical Society, 2016, 116, pp.105002. One of the most important predictions in magnetohydrodynamics is that in the presence of a uniform magnetic field b<SUB>0</SUB>e<SUB>^||</SUB> a transition from weak to strong wave turbulence should occur when going from large to small perpendicular scales. This transition is believed to be a universal property of several anisotropic turbulent systems. We present, for the first time, direct evidence of such a transition using a decaying three-dimensional direct numerical simulation of incompressible balanced magnetohydrodynamic turbulence with a grid resolution of 3072<SUP>2</SUP>×256 . From large to small scales, the change of regime is characterized by (i) a change of slope in the energy spectrum going from approximately -2 to -3 /2 , (ii) an increase of the ratio between the wave and nonlinear times, with a critical ratio of chi<SUB>c</SUB>1 /3 , (iii) a modification of the isocontours of energy revealing a transition from a purely perpendicular cascade to a cascade compatible with the critical-balance-type phenomenology, and (iv) an absence followed by a dramatic increase of the communication between Alfvén modes. The changes happen at approximately the same transition scale and can be seen as manifest signatures of the transition from weak to strong wave turbulence. Furthermore, we observe a significant nonlocal three-wave coupling between strongly and weakly nonlinear modes resulting in an inverse transfer of energy from small to large scales. (10.1103/PhysRevLett.116.105002)
  DOI : 10.1103/PhysRevLett.116.105002
• Experimental investigation of electron transport across a magnetic field barrier in electropositive and electronegative plasmas
  
  • Thomas M B
  • Rafalskyi D.V.
  • Lafleur Trevor
  • Aanesland Ane
  Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (4), pp.045018. In this paper we experimentally investigate the drift of electrons in low temperature plasmas containing a magnetic field barrier; a plasma configuration commonly used in gridded negative ion sources. A planar Langmuir probe array is developed to quantify the drift of electrons over the cross-section of the ion-extraction region of an ionion plasma source. The drift is studied as a function of pressure using both electropositive plasmas (Ar), as well electronegative plasmas (Ar and SF 6 mixtures), and is demonstrated to result from an interaction of the applied magnetic field and the electric fields in the sheath and pre-sheath near the transverse boundaries. The drift enhances electron transport across the magnetic field by more than two orders of magnitude compared with simple collisional transport, and is found to be strongly dependant on pressure. The lowest pressure resulted in the highest influence of the drift across the extraction area and is found to be 30%. (10.1088/0963-0252/25/4/045018)
  DOI : 10.1088/0963-0252/25/4/045018
• Differential kinetic dynamics and heating of ions in the turbulent solar wind
  
  • Valentini F.
  • Perrone D.
  • Stabile S.
  • Pezzi O.
  • Servidio S.
  • de Marco R.
  • Marcucci M. F.
  • Bruno Roberto
  • Lavraud B.
  • de Keyser J.
  • Consolini G.
  • Brienza D.
  • Sorriso-Valvo L.
  • Retinò Alessandro
  • Vaivads A.
  • Salatti M.
  • Veltri P.
  New Journal of Physics, Institute of Physics: Open Access Journals, 2016, 18, pp.125001. The solar wind plasma is a fully ionized and turbulent gas ejected by the outer layers of the solar corona at very high speed, mainly composed by protons and electrons, with a small percentage of helium nuclei and a significantly lower abundance of heavier ions. Since particle collisions are practically negligible, the solar wind is typically not in a state of thermodynamic equilibrium. Such a complex system must be described through self-consistent and fully nonlinear models, taking into account its multi-species composition and turbulence. We use a kinetic hybrid Vlasov-Maxwell numerical code to reproduce the turbulent energy cascade down to ion kinetic scales, in typical conditions of the uncontaminated solar wind plasma, with the aim of exploring the differential kinetic dynamics of the dominant ion species, namely protons and alpha particles. We show that the response of different species to the fluctuating electromagnetic fields is different. In particular, a significant differential heating of alphas with respect to protons is observed. Interestingly, the preferential heating process occurs in spatial regions nearby the peaks of ion vorticity and where strong deviations from thermodynamic equilibrium are recovered. Moreover, by feeding a simulator of a top-hat ion spectrometer with the output of the kinetic simulations, we show that measurements by such spectrometer planned on board the Turbulence Heating ObserveR (THOR mission), a candidate for the next M4 space mission of the European Space Agency, can provide detailed three-dimensional ion velocity distributions, highlighting important non-Maxwellian features. These results support the idea that future space missions will allow a deeper understanding of the physics of the interplanetary medium. (10.1088/1367-2630/18/12/125001)
  DOI : 10.1088/1367-2630/18/12/125001
• Two interacting X lines in magnetotail: Evolution of collision between the counterstreaming jets
  
  • Alexandrova Alexandra
  • Nakamura R.
  • Panov Evgeny V.
  • Sasunov Yury L.
  • Nakamura T. K. M.
  • Vörös Z.
  • Retinò Alessandro
  • Semenov Vladimir S.
  Geophysical Research Letters, American Geophysical Union, 2016, 43 (15), pp.7795-7803. We study the process of collision between the counterstreaming jets flowing out from two reconnection sites in the Earth's magnetotail. The X lines, bracketing the region of jets collision, were passing by two Cluster probes successively in tailward direction. Two probes observed two different stages of the collision process. At the jets collision site, a probe first observed an ion-scale current sheet-like structure, while the other probe observed more compressed one later. The strong wave activities on both ion and electron scales were seen within the compressed layer. Such evolution of the jets collision resulting in the formation of the compressed boundary between the active X lines shows an example of interaction between the X lines during multiple reconnection. (10.1002/2016GL069823)
  DOI : 10.1002/2016GL069823
• Multispacecraft analysis of dipolarization fronts and associated whistler wave emissions using MMS data
  
  • Breuillard Hugo
  • Le Contel Olivier
  • Retinò Alessandro
  • Chasapis A.
  • Chust Thomas
  • Mirioni Laurent
  • Graham D. B.
  • Wilder F. D.
  • Cohen I.
  • Vaivads A.
  • Khotyaintsev Y. V.
  • Lindqvist P.-A.
  • Marklund G. T.
  • Burch J. L.
  • Torbert R. B.
  • Ergun R. E.
  • Goodrich K. A.
  • Macri J.
  • Needell J.
  • Chutter M.
  • Rau D.
  • Dors I.
  • Russell C. T.
  • Magnes W.
  • Strangeway R. J.
  • Bromund K. R.
  • Plaschke F.
  • Fischer D.
  • Leinweber H. K.
  • Anderson B. J.
  • Le G.
  • Slavin J. A.
  • Kepko E. L.
  • Baumjohann W.
  • Mauk B.
  • Fuselier S. A.
  • Nakamura R.
  Geophysical Research Letters, American Geophysical Union, 2016, 43 (14), pp.7279-7286. Dipolarization fronts (DFs), embedded in bursty bulk flows, play a crucial role in Earth's plasma sheet dynamics because the energy input from the solar wind is partly dissipated in their vicinity. This dissipation is in the form of strong low-frequency waves that can heat and accelerate energetic electrons up to the high-latitude plasma sheet. However, the dynamics of DF propagation and associated low-frequency waves in the magnetotail are still under debate due to instrumental limitations and spacecraft separation distances. In May 2015 the Magnetospheric Multiscale (MMS) mission was in a string-of-pearls configuration with an average intersatellite distance of 160 km, which allows us to study in detail the microphysics of DFs. Thus, in this letter we employ MMS data to investigate the properties of dipolarization fronts propagating earthward and associated whistler mode wave emissions. We show that the spatial dynamics of DFs are below the ion gyroradius scale in this region (500 km), which can modify the dynamics of ions in the vicinity of the DF (e.g., making their motion nonadiabatic). We also show that whistler wave dynamics have a temporal scale of the order of the ion gyroperiod (a few seconds), indicating that the perpendicular temperature anisotropy can vary on such time scales. (10.1002/2016GL069188)
  DOI : 10.1002/2016GL069188
• 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
• Estimating some parameters of the equatorial ionosphere electrodynamics from ionosonde data in West Africa
  
  • Grodji F.O.
  • Doumbia V.
  • Boka K.
  • Amory-Mazaudier Christine
  • Cohen Y.
  • Fleury Rolland
  Advances in Space Research, Elsevier, 2016. During the International Equatorial Electrojet Year (IEEY), an IPS-42 ionosonde located at Korhogo (9.33°N, 5.42°W, -1.88°dip-lat) and a meridian chain of 10 magnetic stations were setup in West Africa (5°West longitude). In this work, some characteristic parameters of the equatorial electrojet were estimated on the basis of the IPS-42 ionosonde data at Korhogo during the years 1993 and 1994. The study consisted of determining the zonal electric field through an estimate of the plasma vertical drift velocity. The daytime plasma vertical drift velocity was estimated from the time rates of change of the F-layer virtual height variations and a correction term that takes into account the ionization production and recombination effects. This method resulted in an improved vertical drift velocity, which was found to be comparable to the results of previous studies. The estimated vertical drift velocity was used in a semi-empirical approach which involved the IRI-2012 model for the Pedersen and Hall conductivities and the IGRF-10 model for the geomagnetic main field intensity. Thus the zonal and polarization electric fields on one hand, and the eastward Pedersen, Hall and the equatorial electrojet current densities on the other hand, were estimated. Furthermore the integrated peak current density at the EEJ center was estimated from ionosonde observations and compared with that inferred from magnetometer data. The integrated EEJ peak current densities obtained from both experiments were found to be in the same order and their seasonal variations exhibit the same trends as well. (10.1016/j.asr.2016.09.004)
  DOI : 10.1016/j.asr.2016.09.004
• 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.
• 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
• Near-field plume properties of an ion beam formed by alternating extraction and acceleration of oppositely charged ions
  
  • Oudini N.
  • Aanesland Ane
  • Chabert Pascal
  • Lounes-Mahloul S.
  • Bendib A.
  Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (5), pp.055013. This paper is devoted to study the expansion of a beam composed of packets of positively and negatively charged ions generated by alternating extraction and acceleration. This beam is extracted from an ionion plasma, i.e. the electron density is negligible compared to the negative ion density. The alternating acceleration of ions is ensured by two grids placed in the ionion plasma region. The screen grid in contact with the plasma is biased with a square voltage waveform while the acceleration grid is grounded. A two-dimensional particle-in-cell (2D-PIC) code and an analytical model are used to study the properties of the near-field plume downstream of the acceleration grid. It is shown that the possible operating bias frequency is delimited by an upper limit and a lower one that are in the low MHz range. The simulations show that alternating acceleration with bias frequencies close to the upper frequency limit for the system can achieve high ion exhaust velocities, similar to traditional gridded ion thrusters, and with lower beam divergence than in classical systems. Indeed, ionion beam envelope might be reduced to 15° with 70% of ion flux contained within an angle of 3°. Thus, this alternating acceleration method is promising for electric space propulsion. (10.1088/0963-0252/25/5/055013)
  DOI : 10.1088/0963-0252/25/5/055013
• 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
• Turbulence intermittency linked to the weakly coherent mode in ASDEX Upgrade I-mode plasmas
  
  • Happel T.
  • Manz P.
  • Ryter F.
  • Hennequin Pascale
  • Hetzenecker A.
  • Conway G. D.
  • Guimarais L.
  • Honoré Cyrille
  • Stroth U.
  • Viezzer E.
  • The Asdex Upgrade Team
  Nuclear Fusion, IOP Publishing, 2016, 56 (6), pp.064004. This letter shows for the first time a pronounced increase of extremely intermittent edge density turbulence behavior inside the confinement region related to the I-mode confinement regime in the ASDEX Upgrade tokamak. With improving confinement, the perpendicular propagation velocity of density fluctuations in the plasma edge increases together with the intermittency of the observed density bursts. Furthermore, it is shown that the weakly coherent mode, a fluctuation feature generally observed in I-mode plasmas, is connected to the observed bursts. It is suggested that the large amplitude density bursts could be generated by a non-linearity similar to that in the Korteweg?de-Vries equation which includes the radial temperature gradient. (10.1088/0029-5515/56/6/064004)
  DOI : 10.1088/0029-5515/56/6/064004
• Capacitively coupled hydrogen plasmas sustained by tailored voltage waveforms: excitation dynamics and ion flux asymmetry
  
  • Bruneau B.
  • Diomede P.
  • Economou D. J.
  • Longo S.
  • Gans T.
  • O'Connell D.
  • Greb A.
  • Johnson E.
  • Booth Jean-Paul
  Plasma Sources Science and Technology, IOP Publishing, 2016, 25. Parallel plate capacitively coupled plasmas in hydrogen at relatively high pressure (~1 Torr) are excited with tailored voltage waveforms containing up to five frequencies. Predictions of a hybrid model combining a particle-in-cell simulation with Monte Carlo collisions and a fluid model are compared to phase resolved optical emission spectroscopy measurements, yielding information on the dynamics of the excitation rate in these discharges. When the discharge is excited with amplitude asymmetric waveforms, the discharge becomes electrically asymmetric, with different ion energies at each of the two electrodes. Unexpectedly, large differences in the \text{H}<SUB>2</SUB><SUP> </SUP> fluxes to each of the two electrodes are caused by the different \text{H}<SUB>3</SUB><SUP> </SUP> energies. When the discharge is excited with slope asymmetric waveforms, only weak electrical asymmetry of the discharge is observed. In this case, electron power absorption due to fast sheath expansion at one electrode is balanced by electron power absorption at the opposite electrode due to a strong electric field reversal. (10.1088/0963-0252/25/4/045019)
  DOI : 10.1088/0963-0252/25/4/045019
• Plasma Sources of Solar System Magnetospheres
  
  • Fontaine Dominique
  • Delcourt Dominique
  , 2016.