Publications

2016

• Kinetics of excited species at high specific energy deposition: quenching by electrons in the afterglow of a nanosecond capillary discharge
  
  • Lepikhin N D
  • Klochko A.V.
  • Popov N A
  • Starikovskaia Svetlana
  , 2016, pp.AIAA-2016-1213.
• Multi-point ignition of Hydrogen/Air mixtures with single pulsed nanosecond surface dielectric barrier discharge. Morphology of the discharge at elevated pressures
  
  • Shcherbanev S.A.
  • Popov N A
  • Starikovskaia Svetlana
  , 2016, pp.AIAA-2016-1692.
• Impact of the Eulerian chaos of magnetic field lines in magnetic reconnection
  
  • Firpo Marie-Christine
  • Ettoumi Wahb
  • Lifschitz A. F.
  • Retinò Alessandro
  • Farengo R F
  • Ferrari H E
  • García-Martínez P L
  Physics of Plasmas, American Institute of Physics, 2016, 23 (12). Stochasticity is an ingredient that may allow the breaking of the frozen-in law in the reconnec-tion process. It will first be argued that non-ideal effects may be considered as an implicit way to introduce stochasticity. Yet there also exists an explicit stochasticity that does not require the invocation of non-ideal effects. This comes from the spatial (or Eulerian) chaos of magnetic field lines that can show up only in a truly three-dimensional description of magnetic reconnection since two-dimensional models impose the integrability of the magnetic field lines. Some implications of this magnetic braiding, such as the increased particle finite-time Lyapunov exponents and increased acceleration of charged particles, are discussed in the frame of tokamak sawteeth that form a laboratory prototype of spontaneous magnetic reconnection. A justification for an increased reconnection rate with chaotic vs integrable magnetic field lines is proposed. Moreover, in 3D, the Eulerian chaos of magnetic field lines may coexist with the Eulerian chaos of velocity field lines, that is more commonly named turbulence. (10.1063/1.4972544)
  DOI : 10.1063/1.4972544
• Cone angle control of the interaction of magnetic clouds with the Earth's bow shock
  
  • Turc Lucile
  • Escoubet C. Philippe
  • Fontaine Dominique
  • Kilpua E. K. J.
  • Enestam S.
  Geophysical Research Letters, American Geophysical Union, 2016, 43, pp.4781-4789. Not Available (10.1002/2016GL068818)
  DOI : 10.1002/2016GL068818
• Ion injection at Quasi-parallel Shocks Seen by the Cluster Spacecraft
  
  • Johlander A.
  • Vaivads A.
  • Khotyaintsev Y. V.
  • Retinò Alessandro
  • Dandouras I.
  The Astrophysical Journal Letters, Bristol : IOP Publishing, 2016, 817 (1), pp.L4. Collisionless shocks in space plasma are known to be capable of accelerating ions to very high energies through diffusive shock acceleration (DSA). This process requires an injection of suprathermal ions, but the mechanisms producing such a suprathermal ion seed population are still not fully understood. We study acceleration of solar wind ions resulting from reflection off short large-amplitude magnetic structures (SLAMSs) in the quasi-parallel bow shock of Earth using in situ data from the four Cluster spacecraft. Nearly specularly reflected solar wind ions are observed just upstream of a SLAMS. The reflected ions are undergoing shock drift acceleration (SDA) and obtain energies higher than the solar wind energy upstream of the SLAMS. Our test particle simulations show that solar wind ions with lower energy are more likely to be reflected off the SLAMS, while high-energy ions pass through the SLAMS, which is consistent with the observations. The process of SDA at SLAMSs can provide an effective way of accelerating solar wind ions to suprathermal energies. Therefore, this could be a mechanism of ion injection into DSA in astrophysical plasmas. (10.3847/2041-8205/817/1/L4)
  DOI : 10.3847/2041-8205/817/1/L4
• Orientation of the X-line in asymmetric magnetic reconnection
  
  • Aunai Nicolas
  • Hesse Michael
  • Lavraud B.
  • Dargent Jérémy
  • Smets Roch
  Journal of Plasma Physics, Cambridge University Press (CUP), 2016, 82 (4), pp.535820401. Magnetic reconnection can occur in current sheets separating magnetic fields sheared by any angle and of arbitrarily different amplitudes. In such asymmetric and non-coplanar systems, it is not yet understood what the orientation of the X-line will be. Studying how this orientation is determined locally by the reconnection process is important to understand systems such as the Earth magnetopause, where reconnection occurs in regions with large differences in upstream plasma and field properties. This study aims at determining what the local X-line orientation is for different upstream magnetic shear angles in an asymmetric set-up relevant to the Earth's magnetopause. We use two-dimensional hybrid simulations and vary the simulation plane orientation with regard to the fixed magnetic field profile and search for the plane maximizing the reconnection rate. We find that the plane defined by the bisector of upstream fields maximizes the reconnection rate and this appears not to depend on the magnetic shear angle, domain size or upstream plasma and asymmetries. (10.1017/S0022377816000647)
  DOI : 10.1017/S0022377816000647
• Electron scale structures and magnetic reconnection signatures in the turbulent magnetosheath
  
  • Yordanova E.
  • Vörös Z.
  • Varsani A.
  • Graham D. B.
  • Norgren C.
  • Khotyaintsev Y. V.
  • Vaivads A.
  • Eriksson E.
  • Nakamura R.
  • Lindqvist P.-A.
  • Marklund G.
  • Ergun R. E.
  • Magnes W.
  • Baumjohann W.
  • Fischer D.
  • Plaschke F.
  • Narita Y.
  • Russell C. T.
  • Strangeway R. J.
  • Le Contel Olivier
  • Pollock C.
  • Torbert R. B.
  • Giles B. J.
  • Burch J. L.
  • Avanov L. A.
  • Dorelli J. C.
  • Gershman D. J.
  • Paterson W. R.
  • Lavraud B.
  • Saito Y.
  Geophysical Research Letters, American Geophysical Union, 2016, 43 (12), pp.5969-5978. Collisionless space plasma turbulence can generate reconnecting thin current sheets as suggested by recent results of numerical magnetohydrodynamic simulations. The Magnetospheric Multiscale (MMS) mission provides the first serious opportunity to verify whether small ion-electron-scale reconnection, generated by turbulence, resembles the reconnection events frequently observed in the magnetotail or at the magnetopause. Here we investigate field and particle observations obtained by the MMS fleet in the turbulent terrestrial magnetosheath behind quasi-parallel bow shock geometry. We observe multiple small-scale current sheets during the event and present a detailed look of one of the detected structures. The emergence of thin current sheets can lead to electron scale structures. Within these structures, we see signatures of ion demagnetization, electron jets, electron heating, and agyrotropy suggesting that MMS spacecraft observe reconnection at these scales. (10.1002/2016GL069191)
  DOI : 10.1002/2016GL069191
• Double and Single Planar Wire Arrays on University-Scale Low-Impedance LTD Generator
  
  • Safronova Alla S.
  • Kantsyrev Viktor L.
  • Weller Michael E.
  • Shlyaptseva V. V.
  • Shrestha I. K.
  • Lorance M. Y.
  • Schmidt-Petersen M. T.
  • Stafford A.
  • Cooper M. C.
  • Steiner A. M.
  • Yager-Elorriaga D. A.
  • Patel S. G.
  • Jordan N. M.
  • Gilgenbach R. M.
  • Chuvatin Alexandre S.
  IEEE Transactions on Plasma Science, Institute of Electrical and Electronics Engineers, 2016, 44 (4), pp.432-440. Planar wire array (PWA) experiments were performed on Michigan Accelerator for Inductive Z-pinch Experiments, the University of Michigan's low-impedance linear transformer driver (LTD)-driven generator (0.1 Ω, 0.5-1 MA, and 100-200 ns), for the first time. It was demonstrated that Al wire arrays [both double PWA (DPWA) and single PWA (SPWA)] can be successfully imploded at LTD generator even at the relatively low current of 0.3-0.5 MA. In particular, implosion characteristics and radiative properties of PWAs of different load configurations [for DPWA from Al and stainless steel wires with different wire diameters, interwire gaps, and interplanar gaps (IPGs) and for Al SPWA of different array widths and number of wires] were studied. The major difference from the DPWA experiments on high-impedance Zebra accelerator was in the current rise time that was influenced by the load inductance and was increased up to about 150 ns during the first campaign (and was even longer in the second campaign). The implosion dynamics of DPWAs strongly depends on the critical load parameter, the aspect ratio (the ratio of the array width to IPG) as for Al DPWAs on high-impedance Zebra, but some differences were observed, for low-aspect ratio loads in particular. Analysis of X-ray images and spectroscopy indicates that K-shell Al plasmas from Al PWAs reached the electron temperatures up to more than 450 eV and densities up to 2 x 10²⁰ cm⁻³. Despite the low mass of the loads, opacity effects were observed in the most prominent K-shell Al lines almost in every shot. (10.1109/TPS.2016.2538291)
  DOI : 10.1109/TPS.2016.2538291
• Larger sized planar wire arrays of complex configuration on 1.51.8 MA Z-pinch generator
  
  • Safronova Alla S.
  • Kantsyrev Viktor L.
  • Weller Michael E.
  • Shlyaptseva V. V.
  • Shrestha I. K.
  • Stafford A.
  • Schmidt-Petersen M. T.
  • Lorance M. Y.
  • Schultz K. A.
  • Chuvatin Alexandre S.
  Physics of Plasmas, American Institute of Physics, 2016, 23, pp.101210. Two new approaches of (i) simultaneous study of implosion and radiative characteristics of different materials in wire array Z-pinch plasmas in one shot and (ii) investigation of larger sized wire arrays (to enhance energy coupling to plasmas and provide better diagnostic access) were developed in experiments with 1.51.8 MA Zebra with a Load Current Multiplier. In particular, the larger sized multi-plane Planar Wire Arrays with two outer planes placed at 9 and 15 mm from each other and then as far as at 19 mm (compared with 6 mm studied before at standard 1 MA current) and with a modified central plane with 8 to 12 empty slots were investigated. Though K-shell Al and L-shell Ni, Cu plasmas have similar electron temperatures and densities, the ablation dynamics and radiation of Al and Ni, Cu planes are somewhat different, which was investigated in detail using the full set of diagnostics and modeling. Advantages of using such wire arrays at higher currents to study plasma flow and radiation from different materials and jets are highlighted. (10.1063/1.4965239)
  DOI : 10.1063/1.4965239
• Etude in-situ de la magnétopause Terrestre, de Cluster à MMS
  
  • Rezeau Laurence
  • Manuzzo Roberto
  • Belmont Gérard
  • Califano F.
  , 2016.
• Distinct responses of the low-latitude ionosphere to CME and HSSWS: The role of the IMF B^z oscillation frequency
  
  • Rodríguez-Zuluaga J.
  • Radicella S. M.
  • Nava B.
  • Amory-Mazaudier Christine
  • Mora-Páez H.
  • Alazo-Cuartas K.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2016. In this work an attempt to identify the role of the interplanetary magnetic field (IMF) in the response of the ionosphere to different solar phenomena is presented. For this purpose, the day-today variability of the equatorial ionospheric anomaly (EIA) and the main ionospheric disturbances are analyzed during one coronal mass ejection (CME) and two high-speed solar wind streams (HSSWSs). The EIA parameters considered are the zonal electric field and both the strength and position of its northern crest. The disturbances being the prompt penetration of magnetospheric electric field (PPMEF) and disturbance dynamo electric field (DDEF) are studied using the magnetic response of their equivalent current systems. In accordance, ground-based Global Navigation Satellite Systems receivers and magnetometers at geomagnetic low latitudes in the American sector are used. During both phenomena, patterns of PPMEF related to fluctuations of the IMF are observed. Diurnal and semidiurnal magnetic oscillations are found to be likely related to DDEF. Comparisons among the EIA parameters and the DDEF magnetic response exhibit poor relation during the CME in contrast to good relation during the HSSWSs. It is concluded that the response of the low-latitude ionosphere to solar phenomena is largely determined through the oscillation frequency of the IMF B z by affecting the generation of the PPMEF and DDEF differently. This is seen as an effect of how the energy from the solar wind is transferred into the magnetosphere-ionosphere system. (10.1002/2016JA022539)
  DOI : 10.1002/2016JA022539
• Solar wind test of the de Broglie-Proca massive photon with Cluster multi-spacecraft data
  
  • Retinò Alessandro
  • Spallicci Alessandro D. A. M.
  • Vaivads Andris
  Astroparticle Physics, Elsevier, 2016, 82, pp.49–55. Our understanding of the universe at large and small scales relies largely on electromagnetic observations. As photons are the messengers, fundamental physics has a concern in testing their properties, including the absence of mass. We use Cluster four spacecraft data in the solar wind at 1 AU to estimate the mass upper limit for the photon. We look for deviations from Ampère’s law, through the curlometer technique for the computation of the magnetic field, and through the measurements of ion and electron velocities for the computation of the current. We show that the upper bound for mγ lies between 1.4×10−491.4×10−49 and 3.4×10−513.4×10−51 kg, and thereby discuss the currently accepted lower limits in the solar wind. (10.1016/j.astropartphys.2016.05.006)
  DOI : 10.1016/j.astropartphys.2016.05.006
• Equatorial noise emissions with a quasi-periodic modulation observed by DEMETER at harmonics of the O⁺ ion gyrofrequency
  
  • Parrot Michel
  • Nĕmec František
  • Santolík Ondřej
  • Cornilleau-Wehrlin Nicole
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2016, 121 (10), pp.10,289–10,302. The analysis of ionospheric equatorial noise (EN) with a quasi-periodic (QP) modulation observed by the DEMETER spacecraft is presented. These EN emissions also called whistler mode or fast magnetosonic waves play an important role in acceleration of radiation belt electrons. A statistical analysis with 103 events shows that they occur just after intense magnetic storms. Usually, they are generated by unstable proton ring distribution close to the magnetic equator at harmonics of the proton gyrofrequency in the inner magnetosphere (2 < L < 8). But at lower L values down in the ionosphere three events have been analyzed and it appears that the EN waves are at harmonics of - or very close to - a O⁺ ion gyrofrequency which can be found close or slightly above the satellite. The wave propagation analysis indicates that these emissions are coming from an area above the satellite. Concerning one event, the EN emissions are observed on several consecutive orbits and there is a temporal coincidence with observations performed by the CLUSTER satellites at higher altitudes in the magnetosphere. EN emissions at lower frequencies have been also observed by the CLUSTER satellites in the same longitudinal sector as DEMETER but at ~5 R_E. The analysis of the STAFF data onboard C1 reveals that the magnetic field spectrogram has peaks close to harmonics of the local proton gyrofrequency as usually reported. It is shown that the DEMETER and CLUSTER EN waves have a similar QP modulation but with slightly different period and frequency. (10.1002/2016JA022989)
  DOI : 10.1002/2016JA022989
• Beyond the Maltese Cross: Geometry of Turbulence Between 0.2 and 1 au
  
  • Verdini Andrea
  • Grappin Roland
  The Astrophysical Journal, American Astronomical Society, 2016, 831. The spectral anisotropy of turbulent structures has been measured in the solar wind since 1990, relying on the assumption of axisymmetry about the mean magnetic field, B <SUB>0</SUB>. However, several works indicate that this hypothesis might be partially wrong, thus raising two questions: (i) is it correct to interpret measurements at 1 au (the so-called Maltese cross) in term of a sum of slab and two-dimensional (2D) turbulence; and (ii) what information is really contained in the Maltese cross? We solve direct numerical simulations of the magnetohydrodynamic equations including the transverse stretching exerted by the solar wind flow and study the genuine 3D anisotropy of turbulence as well as that one resulting from the assumption of axisymmetry about B <SUB>0</SUB>. We show that the evolution of the turbulent spectrum from 0.2 to 1 au depends strongly on its initial anisotropy. An axisymmetric spectrum with respect to B <SUB>0</SUB> keeps its axisymmetry, i.e., resists stretching perpendicular to radial, while an isotropic spectrum becomes essentially axisymmetric with respect to the radial direction. We conclude that close to the Sun, slow-wind turbulence has a spectrum that is axisymmetric around B <SUB>0</SUB> and the measured 2D component at 1 au describes the real shape of turbulent structures. In contrast, fast-wind turbulence has a more isotropic spectrum at the source and becomes radially symmetric at 1 au. Such structure is hidden by the symmetrization applied to the data that instead returns a slab geometry. (10.3847/0004-637X/831/2/179)
  DOI : 10.3847/0004-637X/831/2/179
• Emission spectroscopy of partial discharges in air-filled voids in Unfilled epoxy
  
  • Shcherbanev S.A.
  • Nadinov I.U.
  • Auvray Philippe
  • Starikovskaia Svetlana
  • Pancheshnyi Sergey V.
  • Herrmann L.G.
  IEEE Transactions on Plasma Science, Institute of Electrical and Electronics Engineers, 2016, 44 (7), pp.1219-1227. Emission optical spectroscopy is used as a technique to study partial discharges (PDs) in four unfilled epoxy samples encompassing an artificial air-filled cavity. It is shown that emission spectroscopy can be used to estimate the density and the chemical composition of a gas from the spectrally resolved emission and time-resolved pulse shape of the PDs at any time during the aging process. Two scenarios are observed: either PD continues until the sample breaks down (observed in 1 out of 4 samples) or PD stops at a certain point without sample breakdown (3 out of 4 samples). For both the scenarios, a stable initial phase with a gradual decrease of emission intensity from the discharge is typical for a few hundreds of hours of continuous discharge operation. At this stage, the spectrum of the second positive system of molecular nitrogen dominates in the entire spectral range of 350500 nm studied in this work. Furthermore, time-resolved measurements indicate two types of discharges of very different frequencies and magnitudes as well as a decrease of the pressure in the voids as a function of aging time. Then, a sharp 500% increase of the N2 emission is observed 2 days before the breakdown; during the last day a spectrum of CO and some other C-N-O-H containing molecules is observed instead of the spectrum of molecular nitrogen. This allows predicting a breakdown at least a few hours before it happens by analyzing the broad emission spectra behavior. Additionally, the possible role of surface conductivity increase during aging on PD inhibition is discussed. (10.1109/TPS.2016.2576560)
  DOI : 10.1109/TPS.2016.2576560
• Influence of the angular scattering of electrons on the runaway threshold in air
  
  • Chanrion Olivier
  • Bonaventura Z.
  • Bourdon Anne
  • Neubert Torsten
  Plasma Physics and Controlled Fusion, IOP Publishing, 2016, 58, pp.044001. The runaway electron mechanism is of great importance for the understanding of the generation of x- and gamma rays in atmospheric discharges. In 1991, terrestrial gamma-ray flashes (TGFs) were discovered by the Compton Gamma-Ray Observatory. Those emissions are bremsstrahlung from high energy electrons that run away in electric fields associated with thunderstorms. In this paper, we discuss the runaway threshold definition with a particular interest in the influence of the angular scattering for electron energy close to the threshold. In order to understand the mechanism of runaway, we compare the outcome of different FokkerPlanck and Monte Carlo models with increasing complexity in the description of the scattering. The results show that the inclusion of the stochastic nature of collisions smooths the probability to run away around the threshold. Furthermore, we observe that a significant number of electrons diffuse out of the runaway regime when we take into account the diffusion in angle due to the scattering. Those results suggest using a runaway threshold energy based on the FokkerPlanck model assuming the angular equilibrium that is 1.6 to 1.8 times higher than the one proposed by [1, 2], depending on the magnitude of the ambient electric field. The threshold also is found to be 5 to 26 times higher than the one assuming forward scattering. We give a fitted formula for the threshold field valid over a large range of electric fields. Furthermore, we have shown that the assumption of forward scattering is not valid below 1 MeV where the runaway threshold usually is defined. These results are important for the thermal runaway and the runaway electron avalanche discharge mechanisms suggested to participate in the TGF generation. (10.1088/0741-3335/58/4/044001)
  DOI : 10.1088/0741-3335/58/4/044001
• A computational analysis of the vibrational levels of molecular oxygen in low-pressure stationary and transient radio-frequency oxygen plasma
  
  • Kemaneci Efe
  • Booth Jean-Paul
  • Chabert Pascal
  • van Dijk Jan
  • Mussenbrock Thomas
  • Brinkmann Ralf Peter
  Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (2), pp.025025. Vibrational levels of molecular oxygen, O 2 ( v &#8201;&#8201;<&#8201;&#8201;42), are investigated in continuous and pulse-modulated low-pressure radio-frequency oxygen plasma with a global modelling approach. The model is benchmarked against a variety of pressure-, power- and time-resolved measurements of several inductive and asymmetric capacitive discharges available in the literature, and a good agreement is obtained. The sensitivity of the model with respect to the vibrational kinetics, the wall reactions and the spatial inhomogeneity of the charged particles are presented. The simulations without the vibrational levels are also shown for the sake of comparison. (10.1088/0963-0252/25/2/025025)
  DOI : 10.1088/0963-0252/25/2/025025
• The Earth: Plasma Sources, Losses, and Transport Processes
  
  • Welling D. T.
  • André M.
  • Dandouras Iannis
  • Delcourt Dominique
  • Fazakerley A.
  • Fontaine Dominique
  • Foster John
  • Ilie R.
  • Kistler L. M.
  • Lee J. H.
  • Liemohn M. W.
  • Slavin J. A.
  • Wang Chih-Ping
  • Wiltberger M.
  • Yau Andrew
  • Blanc Michel
  • Chappell Charles R.
  • Krupp N.
  , 2016, pp.145. Not Available
• Charge transfer to a dielectric target by guided ionization waves using electric field measurements
  
  • Slikboer Elmar
  • Garcia-Caurel Enric
  • Guaitella Olivier
  • Sobota Ana
  Plasma Sources Science and Technology, IOP Publishing, 2016. A kHz-operated atmospheric pressure plasma jet is investigated by measuring charge transferred to a dielectric electro-optic surface (BSO crystal) allowing for the measurement of electric field by exploiting the Pockels effect. The electric field values, distribution of the surface discharge and amount of deposited charge are obtained for various parameters, including gas flow, applied voltage, target distance and the length of the capillary from ground to the end. A newly formed surface discharge emerges at the target when enough charge is deposited at the impact point and electric fields are high enough, i.e. 200 pC and 9 ± 2 kV cm&#8722;1. The maximum amount of charge transferred by a single ionization wave ('plasma bullet') is 350 ± 40 pC. Due to the emerging new surface discharge behind the impact point, the total charge deposited on the surface of the dielectric target can increase up to 950 pC. The shape of the secondary discharge on the target is found to be mainly driven by gas flow, while the applied voltage allows us to utilize longer distances within the boundaries set by this gas mixing. Finally the ionization wave is found to lose charge along its propagation on the inner walls of the capillary. The loss is estimated to be approximately 7.5 pC mm&#8722;1 of travel distance inside the capillary. (10.1088/1361-6595/aa53fe)
  DOI : 10.1088/1361-6595/aa53fe
• Experimental and simulation study of a capacitively coupled oxygen discharge driven by tailored voltage waveforms
  
  • Derzsi A.
  • Lafleur Trevor
  • Booth Jean-Paul
  • Korolov Ihor
  • Donko Zoltan
  Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (1), pp.015004. We report experimental and particle-based kinetic simulation studies of low-pressure capacitively coupled oxygen plasmas driven by tailored voltage waveforms that consist of up to four harmonics of base frequency 13.56 MHz. Experimentally determined values of DC self-bias and electrical power deposition, as well as flux density and flux-energy distribution of the positive ions at the grounded electrode are compared with simulation data for a wide range of operating conditions. Very good agreement is found for self-bias and flux-energy distribution of the positive ions at the electrodes, while a fair agreement is reached for discharge power and ion flux data. The simulated spatial and temporal behaviour of the electric field, electron density, electron power absorption, ionization rate and mean electron energy shows a transition between sheath expansion heating and drift-ambipolar discharge modes, induced by changing either the number of harmonics comprising the excitation waveform or the gas pressure. The simulations indicate that under our experimental conditions the plasma operates at high electronegativity, and also reveal the crucial role of singlet metastable molecules in establishing discharge behavior via the fast destruction of negative ions within the bulk plasma. (10.1088/0963-0252/25/1/015004)
  DOI : 10.1088/0963-0252/25/1/015004
• Hybrid global model of water cluster ions in atmospheric pressure Ar/H<SUB>2</SUB>O RF capacitive discharges
  
  • Tavant Antoine
  • Lieberman M.A.
  Journal of Physics D: Applied Physics, IOP Publishing, 2016, 49 (46), pp.465201. Water is a trace gas of strong interest for plasma-based medical applications. We use a hybrid global model to simulate a chemically complex Ar/ atmospheric pressure, radio frequency capacitive discharge, including 47 species with positive ion clusters up to . For a discharge gap of 1.5&#8201;mm driven at 27.12 MHz, we determine the discharge properties over a range of rf currents (150500 A m&#8722;2) and initial concentrations (0.252%). An isothermal plug-flow model is used with a gas residence time of 0.2&#8201;s for most calculations, with the gas temperature calculated self-consistently from the input power. The cluster density distributions are determined, and we find that the higher mass cluster densities decrease rapidly with increasing gas temperature. A simplified cluster dynamics analytic model is developed and solved to determine the cluster density distributions, which is in good agreement with the hybrid simulation results. (10.1088/0022-3727/49/46/465201)
  DOI : 10.1088/0022-3727/49/46/465201
• Numerical and experimental study of the dynamics of a $\mu$s helium plasma gun discharge with various amounts of N$_2$ admixture
  
  • Bourdon Anne
  • Darny Thibault
  • Pechereau François
  • Pouvesle Jean-Michel
  • Viegas Pedro
  • Iséni Sylvain
  • Robert Eric
  Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (3), pp.035002. This paper presents a combined 2D numerical and experimental study of the influence of N$_2$ admixture on the dynamics of a He–N$_2$ discharge in the 10 cm long dielectric tube of a plasma gun setup. First, the comparison between experiments and simulations is carried out on the ionization front propagation velocity in the tube. The importance of taking into account a detailed kinetic scheme for the He–N$_2$ mixture in the simulations to obtain a good agreement with the experiments is put forward. For the μs driven plasma gun, the two-and three-body Penning reactions occurring in the plasma column behind the ionization front, are shown to play a key role on the discharge dynamics. In the experiments and simulations, the significant influence of the amplitude of the applied voltage on the ionization front propagation velocity is observed. As the amount of N$_2$ varies, simulation results show that the ionization front velocity, depends on a complex coupling between the kinetics of the discharge, the photoionization and the 2D structure of the discharge in the tube. Finally, the time evolution of axial and radial components of the electric field measured by an electro-optic probe set outside the tube are compared with simulation results. A good agreement is obtained on both components of the electric field. In the tube, simulations show that the magnitude of the axial electric field on the discharge axis depends weakly on the amount of N 2 conversely to the magnitude of the off-axis peak electric field. Both, simulations and first measurements in the tube or within the plasma plume show peak electric fields of the order of 45 kV·cm$^{−1}$ . (10.1088/0963-0252/25/3/035002)
  DOI : 10.1088/0963-0252/25/3/035002
• Electron power absorption dynamics in capacitive radio frequency discharges driven by tailored voltage waveforms in CF<SUB>4</SUB>
  
  • Brandt S.
  • Berger B.
  • Schüngel E.
  • Korolov Ihor
  • Derzsi A.
  • Bruneau Bastien
  • Johnson E.V.
  • Lafleur Trevor
  • O'Connell D.
  • Koepke M.
  • Gans T.
  • Booth Jean-Paul
  • Donkó Z.
  • Schulze J.
  Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (4), pp.045015. The power absorption dynamics of electrons and the electrical asymmetry effect in capacitive radio-frequency plasmas operated in CF4 and driven by tailored voltage waveforms are investigated experimentally in combination with kinetic simulations. The driving voltage waveforms are generated as a superposition of multiple consecutive harmonics of the fundamental frequency of 13.56 MHz. Peaks/valleys and sawtooth waveforms are used to study the effects of amplitude and slope asymmetries of the driving voltage waveform on the electron dynamics and the generation of a DC self-bias in an electronegative plasma at different pressures. Compared to electropositive discharges, we observe strongly different effects and unique power absorption dynamics. At high pressures and high electronegativities, the discharge is found to operate in the drift-ambipolar (DA) heating mode. A dominant excitation/ionization maximum is observed during sheath collapse at the edge of the sheath which collapses fastest. High negative-ion densities are observed inside this sheath region, while electrons are confined for part of the RF period in a potential well formed by the ambipolar electric field at this sheath edge and the collapsed (floating potential) sheath at the electrode. For specific driving voltage waveforms, the plasma becomes divided spatially into two different halves of strongly different electronegativity. This asymmetry can be reversed electrically by inverting the driving waveform. For sawtooth waveforms, the discharge asymmetry and the sign of the DC self-bias are found to reverse as the pressure is increased, due to a transition of the electron heating mode from the &#945;-mode to the DA-mode. These effects are interpreted with the aid of the simulation results. (10.1088/0963-0252/25/4/045015)
  DOI : 10.1088/0963-0252/25/4/045015
• Two types of whistler waves in the hall reconnection region
  
  • Huang S. Y.
  • Fu H.S.
  • Yuan Z. G.
  • Vaivads A.
  • Khotyaintsev Y. V.
  • Retinò Alessandro
  • Zhou M.
  • Graham D. B.
  • Fujimoto K.
  • Sahraoui Fouad
  • Deng X. H.
  • Ni B.
  • Pang Y.
  • Fu S. Y.
  • Wang D. D.
  • Zhou X. M.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2016, 121 (7), pp.6639-6646. Whistler waves are believed to play an important role during magnetic reconnection. Here we report the near-simultaneous occurrence of two types of the whistler-mode waves in the magnetotail Hall reconnection region. The first type is observed in the magnetic pileup region of downstream and propagates away to downstream along the field lines and is possibly generated by the electron temperature anisotropy at the magnetic equator. The second type, propagating toward the X line, is found around the separatrix region and probably is generated by the electron beam-driven whistler instability or Cerenkov emission from electron phase-space holes. These observations of two different types of whistler waves are consistent with recent kinetic simulations and suggest that the observed whistler waves are a consequence of magnetic reconnection. (10.1002/2016JA022650)
  DOI : 10.1002/2016JA022650
• Full particle-in-cell simulations of kinetic equilibria and the role of the initial current sheet on steady asymmetric magnetic reconnection
  
  • Dargent Jérémy
  • Aunai Nicolas
  • Belmont Gérard
  • Dorville Nicolas
  • Lavraud B.
  • Hesse M.
  Journal of Plasma Physics, Cambridge University Press (CUP), 2016, 82 (03), pp.905820305. Tangential current sheets are ubiquitous in space plasmas and yet hard to describe with a kinetic equilibrium. In this paper, we use a semi-analytical model, the BAS model, which provides a steady ion distribution function for tangential asymmetric current sheet and we prove that an ion kinetic equilibrium produced by this model remains steady in a fully kinetic Particle-In-Cell simulation even if the electron distribution function does not satisfy the time independent Vlasov equation. We then apply this equilibrium to look at the dependence of magnetic reconnection simulations upon their initial condition. We show that, as the current sheet evolves from symmetric to asymmetric upstream plasmas, the reconnection rate is impacted, the X line and the electron flow stagnation point separate from one another and start to drift. For the simulated systems, we investigate the overall evolution of the reconnection process via the classical signatures discussed in the literature and searched in the Magnetospheric MultiScale data. We show that they seem robust and do not depend on the specific details of the internal structure of the initial current sheet. (10.1017/S002237781600057X)
  DOI : 10.1017/S002237781600057X