Publications

2019

• A rotational Raman study under non-thermal conditions in pulsed CO2−N2 and CO2−O2 glow discharges
  
  • Grofulovic Marija
  • Klarenaar Bart
  • Guaitella Olivier
  • Guerra V.
  • Engeln Richard
  Plasma Sources Science and Technology, IOP Publishing, 2019, 28 (4), pp.045014. This work employs in situ rotational Raman spectroscopy to study the effect of N2 and O2 addition to CO2 in pulsed glow discharges in the mbar range. The spatiotemporally resolved measurements are performed in CO2 and 25%, 50% and 75% of N2 or O2 admixture, in a 510 ms on-off cycle, 50 mA plasma current and 6.7 mbar total pressure. The rotational temperature profile is not affected by adding N2, ranging from 400 to 850 K from start to end of the discharge pulse, while the addition of O2 decreases the temperature at corresponding time points. Molecular number densities of CO2, CO, O2 and N2 are determined, showing the spatial homogeneity along the axis of the reactor and uniformity during the cycle. The measurements in the N2 containing mixtures show that CO2 conversion factor α increases from 0.15 to 0.33 when the content of N2 is increased from 0% to 75%, demonstrating the potential of N2 addition to enhance the vibrational pumping of CO2 and its beneficial effect on CO2 dissociation. Furthermore, the influence of admixtures on CO2 vibrations is examined by analysing the vibrationally averaged nuclear spin degeneracy. The difference between the fitted odd averaged degeneracy and the calculated odd degeneracy assuming thermal conditions increases with the addition of N2, demonstrating the growth of vibrational temperatures in CO2. On the other hand, the addition of O2 leads to a decrease of α, which might be attributed to quenched vibrations of CO2, and/or to the influence of the back reaction in the presence of O2. (10.1088/1361-6595/ab1240)
  DOI : 10.1088/1361-6595/ab1240
• Stationarity of I-mode operation and I-mode divertor heat fluxes on the ASDEX Upgrade tokamak
  
  • Happel T.
  • Griener M.
  • Silvagni D.
  • Freethy S. J.
  • Hennequin Pascale
  • Janky F.
  • Manz P.
  • Prisiazhniuk D.
  • Ryter F.
  • Bernert M.
  • Brida D.
  • Eich T.
  • Faitsch M.
  • Gil L.
  • Guimarais L.
  • Merle A.
  • Nille D.
  • Pinzón J R
  • Sieglin B.
  • Stroth U.
  • Viezzer E.
  Nuclear Materials and Energy, Elsevier, 2019, 18, pp.159 - 165. Recent I-mode investigations from the ASDEX Upgrade tokamak are reported. It is shown that neutral-beam-injection heated I-modes can be stationary, which is important in terms of extrapolability towards future fusion devices. Furthermore, detailed studies on the weakly coherent mode are reported. In particular, experimental observations point towards its existence in L-mode, before I-mode starts. Moreover, its impact on density and temperature fluctuations is evaluated. Studies of stationary divertor heat fluxes show that in I-mode, the upstream power fall-off length is between those observed in L-mode and H-mode, and it is connected to the scrape-off layer temperature fall-off length. Moreover, analysis of transient divertor heat loads shows that intermittent turbulent events, observed in the confinement region and linked to the weakly coherent mode, are responsible for a significant part of divertor heat loads. (10.1016/j.nme.2018.12.022)
  DOI : 10.1016/j.nme.2018.12.022
• Whistler Waves Driven by Field-Aligned Streaming Electrons in the Near-Earth Magnetotail Reconnection
  
  • Ren Y.
  • Dai L.
  • Li W.
  • Tao X.
  • Wang C.
  • Tang B.
  • Lavraud B.
  • Wu Y.
  • Burch J. L.
  • Giles B. L.
  • Le Contel Olivier
  • Torbert R. B.
  • Russell C. T.
  • Strangeway R. J.
  • Ergun R. E.
  • Lindqvist P.-A.
  Geophysical Research Letters, American Geophysical Union, 2019, 46 (10), pp.5045-5054. We analyze Magnetospheric Multiscale Mission observations of whistler waves and associated electron field-aligned crescent distribution in the vicinity of the magnetotail near-Earth X-line. The whistler waves propagate outward from the X-line in the neutral sheet. The associated field-aligned streaming electrons exhibit a crescent-like shape, with an inverse slope (df/d|v<SUB>||</SUB>|>0) at 1-5 keV. The parallel phase velocity of the waves is in the range (1-5 keV) of the inverse slope of the field-aligned crescents in the velocity space. We demonstrate that the observed whistler waves are driven by the electron field-aligned crescents through Landau resonance. The cyclotron resonance is at the high-energy tail with negligible free energy of pitch angle anisotropy in these events. (10.1029/2019GL083283)
  DOI : 10.1029/2019GL083283
• Evidence of Electron Acceleration at a Reconnecting Magnetopause
  
  • Fu H.S.
  • Peng F. Z.
  • Liu C. M.
  • Burch J. L.
  • Gershman D. G.
  • Le Contel Olivier
  Geophysical Research Letters, American Geophysical Union, 2019, 46 (11), pp.5645-5652. It is still unknown nowadays whether magnetic reconnection−-a process occurring both in the magnetotail and at the magnetopause−-can intrinsically accelerate energetic electrons. Observations in the Earth's magnetotail usually indicate strong electron acceleration during magnetic reconnection, while observations at the Earth's magnetopause rarely show such features. With the recently launched Magnetospheric Multiscale (MMS) mission, here we report the first evidence of energetic-electron acceleration at a reconnecting magnetopause. We find that the acceleration of electrons, with energy up to 70 times their thermal energy, occurs in the magnetosheath side of the ion diffusion region and is associated with strong whistler waves. Such acceleration−-not contaminated by the magnetospheric population−-is attributed to nonadiabatic wave-particle interactions, as supported by analyses of the resonance condition. It manifests that energetic-electron acceleration can happen at the reconnecting magnetopause, like that in the tail. (10.1029/2019GL083032)
  DOI : 10.1029/2019GL083032
• Electron-Driven Dissipation in a Tailward Flow Burst
  
  • Chen Z. Z.
  • Fu H.S.
  • Liu C. M.
  • Wang T. Y.
  • Ergun R. E.
  • Cozzani Giulia
  • Huang S. Y.
  • Khotyaintsev Y. V.
  • Le Contel Olivier
  • Giles B. L.
  • Burch J. L.
  Geophysical Research Letters, American Geophysical Union, 2019, 46 (11), pp.5698-5706. Traditionally, the magnetotail flow burst outside the diffusion region is known to carry ions and electrons together (V<SUB>i</SUB> = V<SUB>e</SUB>), with the frozen-in condition well satisfied (E V<SUB>e</SUB> × B = 0). Such picture, however, may not be true, based on our analyses of the high-resolution MMS (Magnetospheric Multiscale mission) data. We find that inside the flow burst the electrons and ions can be decoupled (V<SUB>e</SUB> != V<SUB>i</SUB>), with the electron speed 5 times larger than the ion speed. Such super-Alfvenic electron jet, having scale of 10 d<SUB>i</SUB> (ion inertial length) in X<SUB>GSM</SUB> direction, is associated with electron demagnetization (E V<SUB>e</SUB> × B != 0), electron agyrotropy (crescent distribution), and O-line magnetic topology but not associated with the flow reversal and X-line topology; it can cause strong energy dissipation and electron heating. We quantitatively analyze the dissipation and find that it is primarily attributed to lower hybrid drift waves. These results emphasize the non-MHD (magnetohydrodynamics) behaviors of magnetotail flow bursts and the role of lower hybrid drift waves in dissipating energies. (10.1029/2019GL082503)
  DOI : 10.1029/2019GL082503
• Erratum: Multi frequency matching for voltage waveform tailoring <A href="/abs/">(2018 Plasma Sources Sci. Technol. 27 095012</A>)
  
  • Schmidt Frederik
  • Schulze Julian
  • Johnson Erik
  • Booth Jean-Paul
  • Keil Douglas
  • French David M.
  • Trieschmann Jan
  • Mussenbrock Thomas
  Plasma Sources Science and Technology, IOP Publishing, 2019, 28, pp.019601. (10.1088/1361-6595/aaeb4b)
  DOI : 10.1088/1361-6595/aaeb4b
• Groupe de Travail Soleil Heliosphere-Magnetospheres (SHM)
  
  • Auchère F.
  • Astafyeva E.
  • Baudin F.
  • Briand C.
  • Brun S.
  • Célestin Sebastien
  • Génot V.
  • Kretzschmar Matthieu
  • Leblanc François
  • Rouillard A.
  • Sahraoui F.
  CNES: Rapport du Groupe de Travail Soleil Heliosphere-Magnetospheres (SHM), 2019, pp.1-28. Les grandes questions scientifiques abordées dans le cadre de la thématique Soleil Héliosphère et Magnétosphères (SHM) couvrent l'ensemble des problématiques liées aux relations entre notre étoile et le système solaire. Cela commence par l'étude de la structure interne du Soleil à travers l'observation multi-spectrales, l'hélio-sismologie et la modélisation ; par l'étude de l'origine de la couronne solaire, son chauffage et l'accélération du vent solaire et des particules énergétiques solaires lors d'événements énergétiques solaires ; et enfin par l'étude de la propagation du vent solaire et des mécanismes de chauffage de celui-ci par dissipation turbulente.
• Universality of Lower Hybrid Waves at Earth's Magnetopause
  
  • Graham D. B.
  • Khotyaintsev Yu. V.
  • Norgren C.
  • Vaivads A.
  • André M.
  • Drake J. F.
  • Egedal J.
  • Zhou M.
  • Le Contel O.
  • Webster J. M.
  • Lavraud B.
  • Kacem I.
  • Génot V.
  • Jacquey C.
  • Rager A. C.
  • Gershman D. J.
  • Burch J. L.
  • Ergun R. E.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2019, 124, pp.8727-8760. Waves around the lower hybrid frequency are frequently observed at Earth's magnetopause and readily reach very large amplitudes. Determining the properties of lower hybrid waves is crucial because they are thought to contribute to electron and ion heating, cross-field particle diffusion, anomalous resistivity, and energy transfer between electrons and ions. All these processes could play an important role in magnetic reconnection at the magnetopause and the evolution of the boundary layer. In this paper, the properties of lower hybrid waves at Earth's magnetopause are investigated using the Magnetospheric Multiscale mission. For the first time, the properties of the waves are investigated using fields and direct particle measurements. The highest-resolution electron moments resolve the velocity and density fluctuations of lower hybrid waves, confirming that electrons remain approximately frozen in at lower hybrid wave frequencies. Using fields and particle moments, the dispersion relation is constructed and the wave-normal angle is estimated to be close to 90° to the background magnetic field. The waves are shown to have a finite parallel wave vector, suggesting that they can interact with parallel propagating electrons. The observed wave properties are shown to agree with theoretical predictions, the previously used single-spacecraft method, and four-spacecraft timing analyses. These results show that single-spacecraft methods can accurately determine lower hybrid wave properties. (10.1029/2019JA027155)
  DOI : 10.1029/2019JA027155
• Electron Distribution Functions Around a Reconnection X-Line Resolved by the FOTE Method
  
  • Wang Z.
  • Fu H. S.
  • Liu C. M.
  • Liu Y. Y.
  • Cozzani G.
  • Giles B. L.
  • Hwang K. -J.
  • Burch J. L.
  Geophysical Research Letters, American Geophysical Union, 2019, 46, pp.1195. Using data from the MMS mission and the First-Order Taylor Expansion(FOTE) method, here we reveal electron distribution functions around areconnection X-line at the Earth's magnetopause. We find cigardistribution of electrons in both the magnetosphere-side andmagnetosheath-side inflow regions, isotropic distribution of electronsat the separatrix, and loss of high-energy electrons in the antiparalleldirection in the magnetosheath-side inflow region. We interpret theformation of cigar distribution in the inflow regions using the Fermimechanism?as suggested in previous simulations, the loss of high-energyelectrons in the magnetosheath side using the parallel electricfields?which evacuate electrons to escape the diffusion region along theantiparallel direction, and the isotropic distribution at the separatrixusing the pitch angle scattering by whistler waves?which existfrequently at the separatrix. We also find that the electrondistribution functions can change rapidly (within 60 ms) from isotropicto cigar as the spacecraft moves slightly away from the separatrix. (10.1029/2018GL081708)
  DOI : 10.1029/2018GL081708
• Sign singularity of the local energy transfer in space plasma turbulence
  
  • Sorriso-Valvo L.
  • de Vita Gaetano
  • Fraternale Federico
  • Gurchumelia Alexandre
  • Perri S.
  • Nigro Giuseppina
  • Catapano F.
  • Retinò Alessandro
  • Chen Christopher H. K.
  • Yordanova E.
  • Pezzi O.
  • Chargazia Khatuna
  • Kharshiladze Oleg
  • Kvaratskhelia Diana
  • Vásconez Christian L.
  • Marino Raffaele
  • Le Contel Olivier
  • Giles B. L.
  • Moore T. E.
  • Torbert Roy B.
  • Burch James L.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2019, 7, pp.108. In weakly collisional space plasmas, the turbulent cascade provides most of the energy that is dissipated at small scales by various kinetic processes. Understanding the characteristics of such dissipative mechanisms requires the accurate knowledge of the fluctuations that make energy available for conversion at small scales, as different dissipation processes are triggered by fluctuations of a different nature. The scaling properties of different energy channels are estimated here using a proxy of the local energy transfer, based on the third-order moment scaling law for magnetohydrodynamic turbulence. In particular, the sign-singularity analysis was used to explore the scaling properties of the alternating positive-negative energy fluxes, thus providing information on the structure and topology of such fluxes for each of the different type of fluctuations. The results show the highly complex geometrical nature of the flux, and that the local contributions associated with energy and cross-helicity nonlinear transfer have similar scaling properties. Consequently, the fractal properties of current and vorticity structures are similar to those of the Alfvénic fluctuations. (10.3389/fphy.2019.00108)
  DOI : 10.3389/fphy.2019.00108
• Evolution of Turbulence in the Kelvin-Helmholtz Instability in the Terrestrial Magnetopause
  
  • Di Mare Francesca
  • Sorriso-Valvo L.
  • Retinò Alessandro
  • Malara Francesco
  • Hasegawa H.
  Atmosphere, MDPI, 2019, 10 (9), pp.561. The dynamics occurring at the terrestrial magnetopause are investigated by using Geotail and THEMIS spacecraft data of magnetopause crossings during ongoing KelvinHelmholtz instability. Properties of plasma turbulence and intermittency are presented, with the aim of understanding the evolution of the turbulence as a result of the development of KelvinHelmholtz instability. The data have been tested against standard diagnostics for intermittent turbulence, such as the autocorrelation function, the spectral analysis and the scale-dependent statistics of the magnetic field increments. A quasi-periodic modulation of different scaling exponents may exist along the direction of propagation of the KelvinHelmholtz waves along the Geocentric Solar Magnetosphere coordinate system (GSM), and it is visible as a quasi-periodic modulation of the scaling exponents we have studied. The wave period associated with such oscillation was estimated to be approximately 6.4 Earth Radii ( RE ). Furthermore, the amplitude of such modulation seems to decrease as the measurements are taken further away from the Earth along the magnetopause, in particular after X(GSM)&#8818;&#8722;15RE . The observed modulation seems to persist for most of the parameters considered in this analysis. This suggests that a kind of signature related to the development of the KelvinHelmholtz instabilities could be present in the statistical properties of the magnetic turbulence. (10.3390/atmos10090561)
  DOI : 10.3390/atmos10090561
• Nonlinear interaction of whistler waves in a magnetized plasma with density ducts
  
  • Zaboronkova T. M.
  • Krafft Catherine
  • Yashina N. F.
  Physics of Plasmas, American Institute of Physics, 2019, 26, pp.102104. The nonlinear resonant interactions between whistler waves guided bydensity ducts surrounded by a uniform magnetized plasma are studied. Itis shown that, under specific conditions that are determined, a time-harmonic external electromagnetic field can drive the parametricinstability of guided whistlers. Both cases of cylindrical and planarducts are considered, of either decreased or increased plasma density.The frequency interval where the magnetized plasma may be resonant isanalyzed. The growth rate and the threshold of the parametricinstability are determined. Numerical calculations are presented forparameters typical of "space plasmas and" modeling laboratoryexperiments where guided whistler propagation was observed. (10.1063/1.5110958)
  DOI : 10.1063/1.5110958
• Nonlinear Diffusion Models for Gravitational Wave Turbulence
  
  • Galtier Sébastien
  • Nazarenko Sergey V.
  • Buchlin Éric
  • Thalabard Simon
  Physica D: Nonlinear Phenomena, Elsevier, 2019, 390, pp.84-88. A fourth-order and a second-order nonlinear diffusion model in spectral space are proposed to describe gravitational wave turbulence in the approximation of strongly local interactions. We show analytically that the model equations satisfy the conservation of energy and wave action, and reproduce the power law solutions previously derived from the kinetic equations with a direct cascade of energy and an explosive inverse cascade of wave action. In the latter case, we show numerically by computing the second-order diffusion model that the non-stationary regime exhibits an anomalous scaling which is understood as a self-similar solution of the second kind with a front propagation following the law kf∼(t∗−t)3.296 , with t<t∗ . These results are relevant to better understand the dynamics of the primordial universe where potent sources of gravitational waves may produce space–time turbulence. (10.1016/j.physd.2019.01.007)
  DOI : 10.1016/j.physd.2019.01.007
• Measuring the magnetic structure velocity for the 11 July 2017 magnetotail reconnection event
  
  • Denton R. E.
  • Hasegawa H.
  • Torbert R. B.
  • Manuzzo Roberto
  • Sonnerup B. U. Ö.
  • Genestreti K. J.
  • Dors I.
  • Belmont Gérard
  • Rezeau Laurence
  • Califano F.
  , 2019. Velocities in magnetic reconnection events, such as those of particles, are best understood in the frame of reference of the magnetic structure that is ultimately responsible for the reconnection process. We discuss four different methods for evaluating the velocity of the magnetic structure, and use those methods to find the magnetic structure velocity for the 11 July 2017 magnetotail reconnection event studied by Torbert et al. (Science, 2018). The four methods are timing analysis, the SpatioTemporal Difference (STD) method of Shi et al. (JGR, 2006), Electron Magnetohydrodynamic (EMHD) reconstruction (Sonnerup et al., JGR, 2016), and polynomial reconstruction of the magnetic field in the vicinity of the spacecraft using the magnetic field and particle current density as input to the model. The relative merits of the different techniques will be discussed, and the different results compared.
• Waves in Kinetic-Scale Magnetic Dips: MMS Observations in the Magnetosheath
  
  • Yao S. T.
  • Shi Q. Q.
  • Yao Z. H.
  • Li J. X.
  • Yue C.
  • Tao X.
  • Degeling A. W.
  • Zong Q. G.
  • Wang X. G.
  • Tian A. M.
  • Russell C. T.
  • Zhou X. Z.
  • Guo R. L.
  • Rae I. J.
  • Fu H.S.
  • Zhang H.
  • Li L.
  • Le Contel Olivier
  • Torbert R. B.
  • Ergun R. E.
  • Lindqvist P.-A.
  • Pollock C. J.
  • Giles B. L.
  Geophysical Research Letters, American Geophysical Union, 2019, 46 (2), pp.523-533. Kinetic-scale magnetic dips (KSMDs), with a significant depression in magnetic field strength, and scale length close to and less than one proton gyroradius, were reported in the turbulent plasmas both in recent observation and numerical simulation studies. These KSMDs likely play important roles in energy conversion and dissipation. In this study, we present observations of the KSMDs that are labeled whistler mode waves, electrostatic solitary waves, and electron cyclotron waves in the magnetosheath. The observations suggest that electron temperature anisotropy or beams within KSMD structures provide free energy to generate these waves. In addition, the occurrence rates of the waves are higher in the center of the magnetic dips than at their edges, implying that the KSMDs might be the origin of various kinds of waves. We suggest that the KSMDs could provide favorable conditions for the generation of waves and transfer energy to the waves in turbulent magnetosheath plasmas. (10.1029/2018GL080696)
  DOI : 10.1029/2018GL080696
• Experimental study of pulsed microwave discharges at pressures ranging over five orders of magnitude
  
  • Shcherbanev S.A.
  • Ali Cherif Mhedine
  • Starikovskaia Svetlana
  • Ikeda Yuji
  Plasma Sources Science and Technology, 2019, 28, pp.045009 (10pp). Microwave discharge igniter (MDI) is a discharge system developed to initiate combustion in automotive engines. The MDI uses a sequence of N = 700 microwave (2.45 GHz) pulses 100 ns in duration separated by 1 &#956;s. The initial breakdown is provided by the &#64257;rst microwave pulse, 5 &#956;s in duration. The aim of pulsing the microwave signal is to keep an optimal combination of parameters when, even at elevated pressures, (i) the discharge propagates over the largest possible volume; (ii) the plasma is non-equilibrium. Properties of plasma produced by MDI igniter in non-combustible gas mixtures at ambient gas temperature and gas pressure in the range between 0.2 mbar and 8 bar were studied experimentally. Discharge spatial structure was analyzed with the help of time-resolved ICCD imaging. Near-UV optical emission spectra taken in different pulses provided the information about rotational and vibrational temperatures. The electric &#64257;eld was estimated on the basis of ratio of emission of the second positive and the &#64257;rst negative systems of molecular nitrogen. (10.1088/1361-6595/aae765)
  DOI : 10.1088/1361-6595/aae765
• Solar Wind Properties and Geospace Impact of Coronal Mass Ejection‐Driven Sheath Regions: Variation and Driver Dependence
  
  • Kilpua E. K. J.
  • Fontaine D.
  • Moissard C.
  • Ala‐lahti M.
  • Palmerio E.
  • Yordanova E.
  • Good S.
  • Kalliokoski M. M. H.
  • Lumme E.
  • Osmane A.
  • Palmroth M.
  • Turc L.
  Space Weather: The International Journal of Research and Applications, American Geophysical Union (AGU), 2019, 17 (8), pp.1257-1280. We present a statistical study of interplanetary conditions and geospace response to 89 coronal mass ejection‐driven sheaths observed during Solar Cycles 23 and 24. We investigate in particular the dependencies on the driver properties and variations across the sheath. We find that the ejecta speed principally controls the sheath geoeffectiveness and shows the highest correlations with sheath parameters, in particular in the region closest to the shock. Sheaths of fast ejecta have on average high solar wind speeds, magnetic (B) field magnitudes, and fluctuations, and they generate efficiently strong out‐of‐ecliptic fields. Slow‐ejecta sheaths are considerably slower and have weaker fields and field fluctuations, and therefore they cause primarily moderate geospace activity. Sheaths of weak and strong B field ejecta have distinct properties, but differences in their geoeffectiveness are less drastic. Sheaths of fast and strong ejecta push the subsolar magnetopause significantly earthward, often even beyond geostationary orbit. Slow‐ejecta sheaths also compress the magnetopause significantly due to their large densities that are likely a result of their relatively long propagation times and source near the streamer belt. We find the regions near the shock and ejecta leading edge to be the most geoeffective parts of the sheath. These regions are also associated with the largest B field magnitudes, out‐of‐ecliptic fields, and field fluctuations as well as largest speeds and densities. The variations, however, depend on driver properties. Forecasting sheath properties is challenging due to their variable nature, but the dependence on ejecta properties determined in this work could help to estimate sheath geoeffectiveness through remote‐sensing coronal mass ejection observations (10.1029/2019SW002217)
  DOI : 10.1029/2019SW002217
• Filamentary nanosecond surface dielectric barrier discharge. Plasma properties in the filaments
  
  • Shcherbanev S.A.
  • Ding Chenyang
  • Starikovskaia Svetlana
  • Popov N.A.
  Plasma Sources Science and Technology, IOP Publishing, 2019, 28 (6), pp.065013. Streamer-to-filament transition is a general feature of nanosecond discharges at elevated pressure. The transition is observed in different discharges by different groups: in the nanosecond surface dielectric barrier discharges (nSDBDs) in a single shot regime at high pressure (2-15 bar), in the point-to-point or point-to-plane open electrodes discharges at high repetitive frequency (so-called nanosecond repetitive pulsed discharges, NRPDs) at atmospherics pressure. The present paper contains experimental analysis of plasma properties in the filamentary nSDBD: the electrical current, the specific deposited energy, the electron density and the electron temperature were measured for a wide range of pressures and voltages. A model explaining plasma properties in filamentary nanosecond discharges and the role of excited species in streamer-to-filament transition is suggested and discussed. (10.1088/1361-6595/ab2230)
  DOI : 10.1088/1361-6595/ab2230
• Filamentary nanosecond surface dielectric barrier discharge. Experimental comparison of the streamer-to-filament transition for positive and negative polarities.
  
  • Ding Chenyang
  • Khomenko A.Yu.
  • Shcherbanev S.A.
  • Starikovskaia Svetlana
  Plasma Sources Science and Technology, IOP Publishing, 2019, 28 (8), pp.085005. Streamer-to-filament transition is a general feature of high pressure high voltage nanosecond surface dielectric barrier discharges (nSDBDs) for mixtures containing molecular gases. The transition is observed at high pressures and voltages in a single-shot experiment a few nanoseconds after the start of the discharge. A set of experimental results comparing streamer-to-filament transition and properties of plasma in the filaments for the identical high voltage pulses of negative and positive polarity is presented. The transition curves in voltage-pressure coordinates are obtained for N 2 :O 2 mixtures with different content of molecular oxygen, from 0 to 20%, at the pressure range 1-12 bar. Continuous optical spectra are compared for both polarities in 6 bar synthetic air. Electron density is calculated from Stark broadening of H α line at λ = 656.5 nm in the discharge and in early afterglow, 40 nanoseconds after the end of the high voltage pulse. Hydrodynamic perturbations are measured using schlieren imaging in 1-6 bar air for streamer and filamentary mode for both polarities. The review of common and distinctive features of the filamentary single-shot nSDBD for two polarities of the applied pulse is provided. (10.1088/1361-6595/ab2d7a)
  DOI : 10.1088/1361-6595/ab2d7a
• Electric field induced second harmonic (E-FISH) generation for characterization of fast ionization wave discharges at moderate and low pressures
  
  • Chng Tat Loon
  • Orel Inna
  • Starikovskaia Svetlana
  • Adamovich I.V.
  Plasma Sources Science and Technology, IOP Publishing, 2019, 28 (4), pp.045004 (8pp). The electric &#64257;eld in an ionization wave discharge in nitrogen at 20100 mbar, initiated by positive polarity, high-voltage, ns duration pulses, is measured by ps second harmonic generation. The axial electric &#64257;eld component is determined both during the propagation of the ionization wave along the discharge tube, and after the wave reaches the grounded electrode, spanning the entire discharge gap. The temporal resolution of the present measurements is 200 ps, with the spatial resolution in the axial direction of approximately 0.5 mm. The second harmonic signal exhibits a quadratic dependence on the Laplacian electric &#64257;eld but indicates that in this pressure range most of the signal is generated within the wall of the tube. Absolute calibration of the signal is obtained from the current shunt data, after the ionization wave has reached the grounded electrode. Comparison of the data taken at different pressures shows that the peak value of the axial electric &#64257;eld in the wave front, 811 kV cm&#8722;1, has a fairly weak dependence on pressure, with the peak reduced electric &#64257;eld reaching &#8776;2000 Td at 20 mbar. Reducing the pressure from 100 to 20 mbar, while keeping the discharge pulse voltage waveform the same, steepens the ionization wave front considerably, from 3.0 to 1.0 ns full width at half maximum. The results demonstrate that ps second harmonic generation may be employed for electric &#64257;eld measurements in low-pressure discharges, discharges sustained in small diameter capillary tubes, and discharges sustained in gas mixtures with low nonlinear susceptibility, at the conditions when the detection of the signal generated directly in the plasma is challenging. High temporal resolution of the present measurements indicates a possibility of detection of non-local electron kinetics effects induced by a rapidly va (10.1088/1361-6595/ab0b22)
  DOI : 10.1088/1361-6595/ab0b22
• Introduction to plasma physics
  
  • Belmont Gérard
  • Rezeau Laurence
  • Riconda C.
  • Zaslavsky A.
  , 2019.
• Experimental investigation of the tilt angle of turbulent structures in the core of fusion plasmas
  
  • Pinzon Javier
  • Happel T.
  • Hennequin Pascale
  • Angioni Clemente
  • Estrada Teresa
  • Lebschy Alexander
  • Stroth Ulrich
  • Asdex Upgrade Team The
  Nuclear Fusion, IOP Publishing, 2019, 59, pp.074002. The tilt angle of turbulent structures stands for the anisotropy of turbulence which is essential for understanding the dynamics of magnetized plasmas. It is a quantity predicted by theory and simulations, that provides information on the interplay between turbulence, micro-instabilities and plasma flows. A new method for measuring the tilt angle of turbulent structures in the core region of fusion plasmas using Doppler reflectometry is presented. First measurements of this type on the ASDEX Upgrade tokamak have shown a significant difference of tilt angle for different plasma conditions. The dominance of sheared flows in determining the structure tilt is experimentally demonstrated for different turbulence regimes. (10.1088/1741-4326/ab227e)
  DOI : 10.1088/1741-4326/ab227e
• Non-Isothermal Sheath Model for Low Pressure Plasmas
  
  • Tavant Antoine
  • Lucken Romain
  • Bourdon Anne
  • Chabert Pascal
  Plasma Sources Science and Technology, IOP Publishing, 2019, 28 (7), pp.075007. The evolution of the electron mean energy in the pre-sheath and the sheath of a low pressure plasma bounded by two planes is investigated with 1D particle in cell simulations. We observed that the electron mean energy is not constant in the sheath, but instead decreases significantly from the bulk towards the wall. From the simulations, a polytropic state law is proposed, allowing us to close the fluid equations for the electrons without the isothermal hypothesis. A comparison between the fluid model and the simulations show that the non-isothermal sheath model is more accurate than the isothermal model. The impact of the electron mean energy variation on the potential sheath drop and the electron particle and heat flux is evaluated. (10.1088/1361-6595/ab279b)
  DOI : 10.1088/1361-6595/ab279b
• Turbulent Heating in the Accelerating Region Using a Multishell Model
  
  • Verdini Andrea
  • Grappin Roland
  • Montagud-Camps Victor
  Solar Physics, Springer Verlag, 2019, 294. Recent studies of turbulence-driven solar winds indicate that fast winds are obtained only at the price of unrealistic bottom boundary conditions: too large wave amplitudes and small frequencies. In this work, the incompressible turbulent dissipation is modeled with a large-scale von Karman-Howarth-Kolmogorov-like phenomenological expression (Q_K41<SUP>0</SUP>). An evaluation of the phenomenology is thus necessary to understand if unrealistic boundary conditions result from physical or model limitations. To assess the validity of the Kolmogorov-like expression, Q_K41<SUP>0</SUP>, one needs to compare it to exact heating, which requires describing the cascade in detail. This has been done in the case of homogeneous MHD turbulence, including expansion, but not in the critical accelerating region. To assess the standard incompressible turbulent heating in the accelerating region, we use a reduced MHD model (multishell model) in which the perpendicular turbulent cascade is described by a shell model, allowing to reach a Reynolds number of 10<SUP>6</SUP>. We first consider the homogeneous and expanding cases, and find that primitive MHD and multishell equations give remarkably similar results. We thus feel free to use the multishell model in the accelerating region. The results indicate that the large-scale phenomenology is inaccurate and it overestimates the heating by a factor at least 20, thus invalidating earlier studies of winds driven by incompressible turbulence. We conclude that realistic 1D wind models cannot be based solely on incompressible turbulence, but probably need an addition of compressible turbulence and shocks to increase the wave reflection and thus the heating. (10.1007/s11207-019-1458-y)
  DOI : 10.1007/s11207-019-1458-y
• High-Frequency Wave Generation in Magnetotail Reconnection: Linear Dispersion Analysis
  
  • Burch J. L.
  • Dokgo K.
  • Hwang K.-J.
  • Torbert R. B.
  • Graham D. B.
  • Webster J. M.
  • Ergun R. E.
  • Giles B. L.
  • Allen R. C.
  • Chen L.-J.
  • Wang S.
  • Genestreti K. J.
  • Russell C. T.
  • Strangeway R. J.
  • Le Contel Olivier
  Geophysical Research Letters, American Geophysical Union, 2019, 46 (8), pp.4089-4097. Plasma and wave measurements from the NASA Magnetospheric Multiscale mission are presented for magnetotail reconnection events on 3 July and 11 July 2017. Linear dispersion analyses were performed using distribution functions comprising up to six drifting bi-Maxwellian distributions. In both events electron crescent-shaped distributions are shown to be responsible for upper hybrid waves near the X-line. In an adjacent location within the 3 July event a monodirectional field-aligned electron beam drove parallel-propagating beam-mode waves. In the 11 July event an electron distribution consisting of a drifting core and two crescents was shown to generate upper-hybrid and beam-mode waves at three different frequencies, explaining the observed broadband waves. Multiple harmonics of the upper hybrid waves were observed but cannot be explained by the linear dispersion analysis since they result from nonlinear beam interactions. (10.1029/2019GL082471)
  DOI : 10.1029/2019GL082471