Publications

2016

• Dynamical Monte Carlo methods for plasma-surface reactions
  
  • Guerra V.
  • Marinov Daniil
  Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (4), pp.045001. Different dynamical Monte Carlo algorithms to investigate molecule formation on surfaces are developed, evaluated and compared with the deterministic approach based on reaction-rate equations. These include a null event algorithm, the n-fold way/BKL algorithm and an 'hybrid' variant of the latter. NO2 formation by NO oxidation on Pyrex and O recombination on silica with the formation of O2 are taken as case studies. The influence of the grid size on the CPU calculation time and the accuracy of the results is analysed. The role of LangmuirHinsehlwood recombination involving two physisorbed atoms and the effect of back diffusion and its inclusion in a deterministic formulation are investigated and discussed. It is shown that dynamical Monte Carlo schemes are flexible, simple to implement, describe easily elementary processes that are not straightforward to include in deterministic simulations, can run very efficiently if appropriately chosen and give highly reliable results. Moreover, the present approach provides a relatively simple procedure to describe fully coupled surface and gas phase chemistries. (10.1088/0963-0252/25/4/045001)
  DOI : 10.1088/0963-0252/25/4/045001
• Theory for the anomalous electron transport in Hall effect thrusters. I. Insights from particle-in-cell simulations
  
  • Lafleur Trevor
  • Baalrud S. D.
  • Chabert Pascal
  Physics of Plasmas, American Institute of Physics, 2016, 23 (5), pp.053502. Using a 1D particle-in-cell simulation with perpendicular electric, E0, and magnetic, B0, fields, and modelling the azimuthal direction (i.e., the E0 × B0 direction), we study the cross-field electron transport in Hall effect thrusters (HETs). For low plasma densities, the electron transport is found to be well described by classical electron-neutral collision theory, but at sufficiently high densities (representative of typical HETs), a strong instability is observed to significantly enhance the electron mobility, even in the absence of electron-neutral collisions. This instability is associated with correlated high-frequency (of the order of MHz) and short-wavelength (of the order of mm) fluctuations in both the electric field and the plasma density, which are shown to be the cause of the anomalous transport. Saturation of the instability is observed to occur due to a combination of ion-wave trapping in the E0 × B0 direction, and convection in the E0 direction. (10.1063/1.4948495)
  DOI : 10.1063/1.4948495
• 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
• 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
• 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
• 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  ±2 · 10 13 m −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 −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
• 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
• 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
• 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
• 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
• Scaling of Compressible Magnetohydrodynamic Turbulence in the Fast Solar Wind
  
  • Banerjee Supratik
  • Hadid Lina
  • Sahraoui Fouad
  • Galtier Sébastien
  The Astrophysical Journal Letters, Bristol : IOP Publishing, 2016, 829 (2), pp.L27. The role of compressible fluctuations in the energy cascade of fast solar wind turbulence is studied using a reduced form of an exact law derived recently for compressible isothermal magnetohydrodynamics and in situ observations from the THEMIS B/ARTEMIS P1 spacecraft. A statistical survey of the data revealed a turbulent energy cascade over a range of two decades of scales that is broader than the previous estimates made from an exact incompressible law. A term-by-term analysis of the compressible model reveals new insight into the role played by the compressible fluctuations in the energy cascade. The compressible fluctuations are shown to amplify by two to four times the turbulent cascade rate with respect to the incompressible model in ~ 10 % of the analyzed samples. This new estimated cascade rate is shown to provide the adequate energy dissipation required to account for the local heating of the non-adiabatic solar wind. (10.3847/2041-8205/829/2/L27)
  DOI : 10.3847/2041-8205/829/2/L27
• 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.
• 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
• 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).
• 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
• 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
• 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
• 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
• 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
• 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
• 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
• 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
• 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