Publications

2017

• Dynamic probing of plasma-catalytic surface processes: Oxidation of toluene on CeO₂
  
  • Jia Zixian
  • Wang Xianjie
  • Thevenet Frederic
  • Rousseau Antoine
  Plasma Processes and Polymers, Wiley-VCH Verlag, 2017. This article reports the use of innovative diagnostics to monitor toluene adsorption and oxidation on CeO2 surface under non-thermal plasma (NTP) exposure. Two plasma-catalytic configurations are explored, namely: post-plasma catalysis (PPC) and in-plasma catalysis (IPC). Since heterogeneous processes are pointed out as key steps of the plasma-catalyst coupling, the catalyst surface has been monitored by two complementary in situ diagnostics: (i) diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) and (ii) transmission fourier transform infrared spectroscopy using Sorbent track (ST) device. Dielectric barrier discharges (DBD) are used in both PPC and IPC configurations to induce adsorbed toluene oxidation. Toluene in dry air is first adsorbed on the selected catalytic surface: ceria (CeO2). Subsequently, the plasma is switched on. During the experiment, the ceria surface is monitored by infrared to study toluene adsorption and oxidation mechanisms. The adsorption capacity of toluene on ceria is, respectively, measured in the configurations of PPC and IPC by DRIFTS and ST. The oxidation of toluene by plasma follows a first-order reaction regardless of plasma configuration and injected power and IPC is more effective for the toluene removal than PPC. Intermediates of toluene (benzyl alcohol, benzaldehyde and benzoic acid) are also identified on the surface and their respective temporal evolutions as a function of the plasma exposure are studied.ppap201600114-gra-0001 (10.1002/ppap.201600114)
  DOI : 10.1002/ppap.201600114
• Power coupling mode transitions induced by tailored voltage waveforms in capacitive oxygen discharges
  
  • Derzsi A.
  • Bruneau Bastien
  • Gibson Andrew
  • Johnson Erik
  • O'Connell D.
  • Gans T.
  • Booth Jean-Paul
  • Donko Zoltan
  Plasma Sources Science and Technology, IOP Publishing, 2017, 26 (3), pp.034002. Low-pressure capacitively coupled radio frequency discharges operated in O 2 and driven by tailored voltage waveforms are investigated experimentally and by means of kinetic simulations. Pulse-type (peaks/valleys) and sawtooth-type voltage waveforms that consist of up to four consecutive harmonics of the fundamental frequency are used to study the amplitude asymmetry effect as well as the slope asymmetry effect at different fundamental frequencies (5, 10, and 15 MHz) and at different pressures (50?700 mTorr). Values of the DC self-bias determined experimentally and spatio-temporal excitation rates derived from phase resolved optical emission spectroscopy measurements are compared with particle-in-cell/Monte Carlo collisions simulations. The spatio-temporal distributions of the excitation rate obtained from experiments are well reproduced by the simulations. Transitions of the discharge electron heating mode from the drift-ambipolar mode to the ? -mode are induced by changing the number of consecutive harmonics included in the driving voltage waveform or by changing the gas pressure. Changing the number of harmonics in the waveform has a strong effect on the electronegativity of the discharge, on the generation of the DC self-bias and on the control of ion properties at the electrodes, both for pulse-type, as well as sawtooth-type driving voltage waveforms The effect of the surface quenching rate of oxygen singlet delta metastable molecules on the spatio-temporal excitation patterns is also investigated. (10.1088/1361-6595/aa56d6)
  DOI : 10.1088/1361-6595/aa56d6
• Nonlinear radiation generation processes in the auroral acceleration region
  
  • Pottelette Raymond
  • Berthomier Matthieu
  Annales Geophysicae, European Geosciences Union, 2017, 35 (6), pp.1241-1248. It is known from laboratory plasma experiments that double layers (DLs) radiate in the electromagnetic spectrum; but this is only known qualitatively. In these experiments, it was shown that the electron beam created on the high-potential side of a DL generates nonlinear structures which couple to electromagnetic waves and act as a sender antenna. In the Earth auroral region, observations performed by auroral spacecraft have shown that DLs occur naturally in the source region of intense radio emissions called auroral kilometric radiation (AKR). Very high time-, spatial-, and temporal-resolution measurements are needed in order to characterize waves and particle distributions in the vicinity of DLs, which are moving transient structures. We report observations from the FAST satellite of a localized large-amplitude parallel electric field ( 300 mV m<SUP>-1</SUP>) recorded at the edges of the auroral density cavity. In agreement with laboratory experiments, on the high-potential side of the DL, elementary radiation events are detected. They occur substantially above the local electron gyrofrequency and are associated with the presence of electron holes. The velocity of these nonlinear structures can be derived from the measurement of the Doppler-shifted AKR frequency spectrum above the electron gyrofrequency. The generated electron holes appear as the nonlinear evolution of electrostatic waves generated by the electron-electron two-stream instability because they propagate at about half the beam velocity. It is pointed out that, in the vicinity of a DL, the shape of the electron distribution gives rise to a significant power recorded in the left-hand polarized ordinary (LO) mode. (10.5194/angeo-35-1241-2017)
  DOI : 10.5194/angeo-35-1241-2017
• Zipper-like periodic magnetosonic waves: Van Allen Probes, THEMIS, and magnetospheric multiscale observations
  
  • Li J.
  • Bortnik J.
  • Li W.
  • Ma Q.
  • Thorne R. M.
  • Kletzing C. A.
  • Kurth W. S.
  • Hospodarsky G. B.
  • Wygant J.
  • Breneman A.
  • Thaller S.
  • Funsten H. O.
  • Mitchell D. G.
  • Manweiler J. W.
  • Torbert R. B.
  • Le Contel Olivier
  • Ergun R. E.
  • Lindqvist P.-A.
  • Torkar Klaus
  • Nakamura R.
  • Andriopoulou M.
  • Russell C. T.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2017, 122 (2), pp.1600-1610. An interesting form of "zipper-like" magnetosonic waves consisting of two bands of interleaved periodic rising-tone spectra was newly observed by the Van Allen Probes, the Time History of Events and Macroscale Interactions during Substorms (THEMIS), and the Magnetospheric Multiscale (MMS) missions. The two discrete bands are distinct in frequency and intensity; however, they maintain the same periodicity which varies in space and time, suggesting that they possibly originate from one single source intrinsically. In one event, the zipper-like magnetosonic waves exhibit the same periodicity as a constant-frequency magnetosonic wave and an electrostatic emission, but the modulation comes from neither density fluctuations nor ULF waves. A statistical survey based on 3.5 years of multisatellite observations shows that zipper-like magnetosonic waves mainly occur on the dawnside to noonside, in a frequency range between 10 f<SUB>cp</SUB> and f<SUB>LHR</SUB>. The zipper-like magnetosonic waves may provide a new clue to nonlinear excitation or modulation process, while its cause still remains to be fully understood. (10.1002/2016JA023536)
  DOI : 10.1002/2016JA023536
• Electron Heating at Kinetic Scales in Magnetosheath Turbulence
  
  • Chasapis A.
  • Matthaeus W. H.
  • Parashar T. N.
  • Le Contel Olivier
  • Retinò Alessandro
  • Breuillard Hugo
  • Khotyaintsev Y. V.
  • Vaivads A.
  • Lavraud B.
  • Eriksson E.
  • Moore T. E.
  • Burch J. L.
  • Torbert R. B.
  • Lindqvist P.-A.
  • Ergun R. E.
  • Marklund G.
  • Goodrich K. A.
  • Wilder F. D.
  • Chutter M.
  • Needell J.
  • Rau D.
  • Dors I.
  • Russell C. T.
  • Le G.
  • Magnes W.
  • Strangeway R. J.
  • Bromund K. R.
  • Leinweber H. K.
  • Plaschke F.
  • Fischer D.
  • Anderson B. J.
  • Pollock C. J.
  • Giles B. L.
  • Paterson W. R.
  • Dorelli J. C.
  • Gershman D. J.
  • Avanov L.
  • Saito Y.
  The Astrophysical Journal, American Astronomical Society, 2017, 836, pp.247. We present a statistical study of coherent structures at kinetic scales, using data from the Magnetospheric Multiscale mission in the Earth's magnetosheath. We implemented the multi-spacecraft partial variance of increments (PVI) technique to detect these structures, which are associated with intermittency at kinetic scales. We examine the properties of the electron heating occurring within such structures. We find that, statistically, structures with a high PVI index are regions of significant electron heating. We also focus on one such structure, a current sheet, which shows some signatures consistent with magnetic reconnection. Strong parallel electron heating coincides with whistler emissions at the edges of the current sheet. (10.3847/1538-4357/836/2/247)
  DOI : 10.3847/1538-4357/836/2/247
• Climatology of ionospheric scintillation over the Vietnam low-latitude region for the period 2006-2014
  
  • Lan Tran Thi
  • Minh Le Huy
  • Amory-Mazaudier Christine
  • Fleury Rolland
  Advances in Space Research, Elsevier, 2017. This paper presents the characteristics of the occurrence of ionospheric scintillations at low-latitude, over Vietnam, by using continuous data of three GSV4004 receivers located at PHUT/Hanoï (105.9 o E, 21.0 o N; magnetic latitude 14.4 o N), HUES/Hue (107.6 o E, 16.5 o N; magnetic latitude 9.5 o N) and HOCM/ Ho Chi Minh city (106.6 o E, 10.8 o N; magnetic latitude 3.3 o N) for the period 2006-2014. The results show that the scintillation activity is maximum during equinox months for all the years and depends on solar activity as expected. The correlations between the monthly percentage scintillation occurrence and the F10.7 flux are of 0.40, 0.52 and 0.67 for PHUT, HUES and HOCM respectively. The distribution of scintillation occurrences is dominant in the pre-midnight sector and around the northern crest of the equatorial ionization anomaly (EIA), from the 15 o N to 20 o N geographic latitude with a maximum at 16 o N. The results obtained from the directional analysis show higher distributions of scintillations in the southern sky of PHUT and in the northern sky of HUES and HOCM, and in the elevation angles smaller than 40 o. The correlation between ROTI and S 4 is low and 2 rather good at PHUT (under EIA) than HOCM (near equator). We found better correlation in the post-midnight hours and less correlation in the pre-midnight hours for all stations. When all satellites are considered during the period of 2009-2011, the range of variation of the ration between ROTI and S 4 is from 1 to 7 for PHUT, from 0.3 to 6 for HUES and from 0.7 to 6 for HOCM. (10.1016/j.asr.2017.05.005)
  DOI : 10.1016/j.asr.2017.05.005
• Counter-propagating radiative shock experiments on the Orion laser and the formation of radiative precursors
  
  • Clayson Thomas
  • Suzuki-Vidal Francisco
  • Lebedev S. V.
  • Swadling G. F.
  • Stehlé Chantal
  • Burdiak G. C.
  • Foster J. M.
  • Skidmore J.
  • Graham P.
  • Gumbrell E.
  • Patankar S.
  • Spindloe C.
  • Chaulagain Uddhab
  • Kozlová Michaela
  • Larour Jean
  • Singh Raj Laxmi
  • Rodriguez Perez R.
  • Gil J. M.
  • Espinosa G.
  • Velarde P.
  • Danson C.
  High Energy Density Physics, Elsevier, 2017, 23, pp.60-72. We present results from new experiments to study the dynamics of radiative shocks, reverse shocks and radiative precursors. Laser ablation of a solid piston by the Orion high-power laser at AWE Aldermaston UK was used to drive radiative shocks into a gas cell initially pressurised between 0.1 and 1.0 bar with different noble gases. Shocks propagated at 80 ± 10 km/s and experienced strong radiative cooling resulting in post-shock compressions of × 25 ± 2. A combination of X-ray backlighting, optical self-emission streak imaging and interferometry (multi-frame and streak imaging) were used to simultaneously study both the shock front and the radiative precursor. These experiments present a new configuration to produce counter-propagating radiative shocks, allowing for the study of reverse shocks and providing a unique platform for numerical validation. In addition, the radiative shocks were able to expand freely into a large gas volume without being confined by the walls of the gas cell. This allows for 3-D effects of the shocks to be studied which, in principle, could lead to a more direct comparison to astrophysical phenomena. By maintaining a constant mass density between different gas fills the shocks evolved with similar hydrodynamics but the radiative precursor was found to extend significantly further in higher atomic number gases (∼ 4 times further in xenon than neon). Finally, 1-D and 2-D radiative-hydrodynamic simulations are presented showing good agreement with the experimental data. (10.1016/j.hedp.2017.03.002)
  DOI : 10.1016/j.hedp.2017.03.002
• Statistical analysis of solar events associated with SSC over year of solar maximum during cycle 23: 2. Characterisation on the Sun-Earth path - Geoeffectiveness
  
  • Cornilleau-Wehrlin Nicole
  • Bocchialini Karine
  • Menvielle M.
  • Fontaine Dominique
  • Grison B.
  • Marchaudon A.
  • Pick Monique
  • Pitout F.
  • Schmieder Brigitte
  • Régnier Stéphane
  • Zouganelis Yannis
  • Chambodut Aude
  , 2017, 31. Taking the 32 sudden storm commencements (SSC) listed by the observatory de l'Ebre / ISGI over the year 2002 (maximal solar activity) as a starting point, we performed a statistical analysis of the related solar sources, solar wind signatures, and terrestrial responses. For each event, we characterized and identified, as far as possible, (i) the sources on the Sun (Coronal Mass Ejections -CME-), with the help of a series of criteria (velocities, drag coefficient, radio waves, magnetic field polarity), as well as (ii) the structure and properties in the interplanetary medium, at L1, of the event associated to the SSC: magnetic clouds -MC-, non-MC interplanetary coronal mass ejections -ICME-, co-rotating/stream interaction regions -SIR/CIR-, shocks only and unclear events that we call "miscellaneous" events. The geoeffectiveness of the events, classified by category at L1, is analysed by their signatures in the Earth ionized (magnetosphere and ionosphere) and neutral (thermosphere) environments, using a broad set of in situ, remote and ground based instrumentation. The role of the presence of a unique or of a multiple source at the Sun, of its nature, halo or non halo CME, is also discussed. The set of observations is statistically analyzed so as to evaluate and compare the geoeffectiveness of the events. The results obtained for this set of geomagnetic storms started by SSCs is compared to the overall statistics of year 2002, relying on already published catalogues of events, allowing assessing the relevance of our approach ; for instance all the 12 well identified Magnetic Clouds of 2002 give rise to SSCs.
• Turbulence characteristics of the I-mode confinement regime in ASDEX Upgrade
  
  • Manz P.
  • Happel T.
  • Ryter F.
  • Bernert M.
  • Birkenmeier G.
  • Conway G. D.
  • Dunne M.
  • Guimarais L.
  • Hennequin Pascale
  • Hetzenecker A.
  • Honoré Cyrille
  • Lauber P.
  • Maraschek M.
  • Nikolaeva V.
  • Prisiazhniuk D.
  • Stroth U.
  • Viezzer E.
  Nuclear Fusion, IOP Publishing, 2017, 57 (8), pp.086022. Besides strong geodesic acoustic mode (GAM) activity, turbulence in the I-mode confinement regime of ASDEX Upgrade exhibits two prominent features, the weakly coherent mode (WCM) and strongly intermittent solitary density perturbations. The nonlinear interaction between these structures is studied in detail by means of a conditional averaged wavelet-bicoherence analysis. The wavelet analysis reveals that these density perturbations are at the WCM frequency. The GAM is coupled to all frequency scales of the velocity fluctuations via a modulational instability. The WCM shows coupling to higher frequencies prior to the bursts, indicating a process resembling wave-steepening. A possible mechanism for the generation of such solitary density perturbations by a Korteweg?de Vries-like nonlinearity is discussed. (10.1088/1741-4326/aa7476)
  DOI : 10.1088/1741-4326/aa7476
• Electrostatic analyzer with a 3-D instantaneous field of view for fast measurements of plasma distribution functions in space
  
  • Morel Xavier
  • Berthomier Matthieu
  • Berthelier Jean-Jacques
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2017, 122 (3), pp.3397-3410. We describe the concept and properties of a new electrostatic optic which aims to provide a 2π sr instantaneous field-of-view to characterize space plasmas. It consists of a set of concentric toroidal electrodes that form a number of independent energy selective channels. Charged particles are deflected towards a common imaging planar detector. The full 3D distribution function of charged particles is obtained through a single energy sweep. Angle and energy resolution of the optics depends on the number of toroidal electrodes, on their radii of curvature, on their spacing, and on the angular aperture of the channels. We present the performances, as derived from numerical simulations, of an initial implementation of this concept that would fit the need of many space plasma physics applications. The proposed instrument has 192 entrance windows corresponding to 8 polar channels each with 24 azimuthal sectors. The initial version of this 3D plasma analyzer may cover energies from a few eV up to 30 keV, typically with a channel dependent energy resolution varying from 10% to 7%. The angular acceptance varies with the direction of the incident particle from 3° to 12°. With a total geometric factor of two sensor heads reaching ~ 0.23 cm² · sr · eV/eV, this “donut” shape analyzer has enough sensitivity to allow very fast measurements of plasma distribution functions in most terrestrial and planetary environments on three-axis stabilized as well as on spinning satellites. (10.1002/2016JA023596)
  DOI : 10.1002/2016JA023596
• Hall thruster microturbulence under conditions of modified electron wall emission
  
  • Tsikata Sédina
  • Héron A.
  • Honoré Cyrille
  Physics of Plasmas, American Institute of Physics, 2017, 24, pp.053519. In recent numerical, theoretical, and experimental papers, the short-scale electron cyclotron drift instability (ECDI) has been studied as a possible contributor to the anomalous electron current observed in Hall thrusters. In this work, features of the instability, in the presence of a zero-electron emission material at the thruster exit plane, are analyzed using coherent Thomson scattering. Limiting the electron emission at the exit plane alters the localization of the accelerating electric field and the expected drift velocity profile, which in turn modifies the amplitude and localization of the ECDI. The resulting changes to the standard thruster operation are expected to favor an increased contribution by the ECDI to electron current. Such an operation is associated with a degradation of thruster performance and stability. (10.1063/1.4984255)
  DOI : 10.1063/1.4984255
• Alternative derivation of exact law for compressible and isothermal magnetohydrodynamics turbulence
  
  • Andrés Nahuel
  • Sahraoui Fouad
  Physical Review E, American Physical Society (APS), 2017, 96 (5), pp.053205. The exact law for fully developed homogeneous compressible magnetohydrodynamics (CMHD) turbulence is derived. For an isothermal plasma, without the assumption of isotropy, the exact law is expressed as a function of the plasma velocity field, the compressible Alfvén velocity, and the scalar density, instead of the Elsasser variables used in previous works. The theoretical results show four different types of terms that are involved in the nonlinear cascade of the total energy in the inertial range. Each category is examined in detail, in particular, those that can be written either as source or flux terms. Finally, the role of the background magnetic field B<SUB>0</SUB> is highlighted and a comparison with the incompressible MHD (IMHD) model is discussed. This point is particularly important when testing this exact law on numerical simulations and in situ observations in space plasmas. (10.1103/PhysRevE.96.053205)
  DOI : 10.1103/PhysRevE.96.053205
• On the Existence of the Kolmogorov Inertial Range in the Terrestrial Magnetosheath Turbulence
  
  • Huang S. Y.
  • Hadid Lina
  • Sahraoui Fouad
  • Yuan Z. G.
  • Deng X. H.
  The Astrophysical Journal Letters, Bristol : IOP Publishing, 2017, 836 (1), pp.L10. In the solar wind, power spectral density (PSD) of the magnetic field fluctuations generally follow the so-called Kolmogorov spectrum f <SUP>-5/3</SUP> in the inertial range, where the dynamics is thought to be dominated by nonlinear interactions between counter-propagating incompressible Alfvén wave parquets. These features are thought to be ubiquitous in space plasmas. The present study gives a new and more complex picture of magnetohydrodynamic (MHD) turbulence as observed in the terrestrial magnetosheath. The study uses three years of in situ data from the Cluster mission to explore the nature of the magnetic fluctuations at MHD scales in different locations within the magnetosheath, including flanks and subsolar regions. It is found that the magnetic field fluctuations at MHD scales generally have a PSD close to f <SUP>-1</SUP> (shallower than the Kolmogorov one f <SUP>-5/3</SUP>) down to the ion characteristic scale, which recalls the energy-containing scales of solar wind turbulence. The Kolmogorov spectrum is observed only away from the bow shock toward the flank and the magnetopause regions in 17% of the analyzed time intervals. Measuring the magnetic compressibility, it is shown that only a fraction (35%) of the observed Kolmogorov spectra was populated by shear Alfvénic fluctuations, whereas the majority of the events (65%) was found to be dominated by compressible magnetosonic-like fluctuations, which contrasts with well-known turbulence properties in the solar wind. This study gives a first comprehensive view of the origin of the f <SUP>-1</SUP> and the transition to the Kolmogorov inertial range; both questions remain controversial in solar wind turbulence. (10.3847/2041-8213/836/1/L10)
  DOI : 10.3847/2041-8213/836/1/L10
• Charge Proportional and Weakly Mass-Dependent Acceleration of Different Ion Species in the Earth's Magnetotail
  
  • Catapano F.
  • Zimbardo G.
  • Perri S.
  • Greco A.
  • Delcourt Dominique C.
  • Retinò Alessandro
  • Cohen I. J.
  Geophysical Research Letters, American Geophysical Union, 2017, 44 (20), pp.10,10810,115. Energetic particles with energies from tens of keV to a few hundred keV are frequently observed in the Earth's magnetotail. Here we study, by means of a test particle numerical simulation, the acceleration of different ion species (H<SUP> </SUP>, He<SUP> </SUP>, He<SUP> </SUP>, and O<SUP>n </SUP> with n = 1-6) in the presence of transient electromagnetic perturbations. All the considered ions develop power law tails at high energies, except for O<SUP> </SUP> ions. This is strongly correlated to the time that the particle spends in the current sheet. Ion acceleration is found to be proportional to the charge state, while it grows in a weaker way with the ion mass. We find that O<SUP>5 /6 </SUP> can reach energies higher than 500 keV. These results may explain the strong oxygen acceleration observed in the magnetotail. (10.1002/2017GL075092)
  DOI : 10.1002/2017GL075092
• Coordinated observations of two types of diffuse auroras near magnetic local noon by Magnetospheric Multiscale mission and ground all-sky camera
  
  • Han D.-S.
  • Li J.-X.
  • Nishimura Y.
  • Lyons L.R.
  • Bortnik J.
  • Zhou M.
  • Liu J.-J.
  • Hu Z.-J.
  • Hu H.-Q.
  • Yang H.-G.
  • Fuselier S. A.
  • Le Contel Olivier
  • Ergun R. E.
  • Malaspina D.
  • Lindqvist P.-A.
  • Pollock C. J.
  Geophysical Research Letters, American Geophysical Union, 2017, 44 (16), pp.8130-8139. Structured diffuse auroras are often observed near magnetic local noon (MLN), but their generation mechanisms are poorly understood. We have found that two types of structured diffuse auroras with obviously different dynamical properties often coexist near MLN. One type usually drifts from low to high latitude with higher speed and shows pulsation. The other type is always adjacent to the discrete aurora oval and drifts together with nearby discrete aurora with much lower speed. Using coordinated observations from MMS and ground all-sky imagers, we found that the two types of diffuse auroras are well correlated with number density increase of O (from the ionosphere) and of He2 (from magnetosheath) ions, respectively. These observations indicate that mangetosheath particles penetrated into the magnetosphere also can play an important role for producing the dayside diffuse aurora. In addition, for the first time, electron cyclotron harmonic waves are observed associated with dayside diffuse aurora. (10.1002/2017GL074447)
  DOI : 10.1002/2017GL074447
• Measurement of the isotope shift of the 2p<SUP>4</SUP> -> 2p<SUP>3</SUP>3p <SUP>3</SUP>P<SUB>2</SUB> two-photon transition of O I and a revision of the triplet energy levels of atomic oxygen
  
  • Marinov Daniil
  • Booth Jean-Paul
  • Drag Cyril
  • Blondel Christophe
  Journal of Physics B: Atomic, Molecular and Optical Physics, IOP Publishing, 2017, 50 (6), pp.065003. Two-photon induced fluorescence of atomic oxygen, which is routinely used for plasma and flame diagnostics, is implemented with two counterpropagating laser beams of sufficient monochromaticity to permit Doppler-free spectroscopy. A single-mode injection-seeded pulsed Ti:sapphire laser has been frequency-doubled twice to produce narrow-band pulsed radiation at wavelengths around 225.6 nm, suitable to excite the ground-state-to- ##IMG## [http://ej.iop.org/images/0953-4075/50/6/065003/jpbaa5a88ieqn3.gif] 2\rmp^33\rmp§^3P two-photon resonance line of oxygen. Accurate measurement of the injection-seeding wavelength provides new data on the excitation energy of the ##IMG## [http://ej.iop.org/images/0953-4075/50/6/065003/jpbaa5a88ieqn4.gif] 3\rmp§^3P states. The experiment was done both sequentially and simultaneously on 16 O and 18 O, which makes it possible to test recent calculations of the isotope shift. Having an absolute measurement of the excitation wave-number from the ground level, which has been the case only twice in previous spectroscopic studies of O I, we can re-examine the energy levels of the subset of triplet states and present an updated set of optimised energy values. (10.1088/1361-6455/aa5a88)
  DOI : 10.1088/1361-6455/aa5a88
• Energy budget and mechanisms of cold ion heating in asymmetric magnetic reconnection
  
  • Toledo-Redondo Sergio
  • André M.
  • Khotyaintsev Y. V.
  • Lavraud Benoit
  • Vaivads A.
  • Graham Daniel B.
  • Li Wenya
  • Perrone Denise
  • Fuselier Stephen
  • Gershman D. J.
  • Aunai Nicolas
  • Dargent Jérémy
  • Giles B. L.
  • Le Contel Olivier
  • Lindqvist Per-Arne
  • Ergun Robert E.
  • Russell Christopher T.
  • Burch James L.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2017, 122 (9), pp.9396-9413. Cold ions (few tens of eV) of ionospheric origin are commonly observed on the magnetospheric side of the Earth's dayside magnetopause. As a result, they can participate in magnetic reconnection, changing locally the reconnection rate and being accelerated and heated. We present four events where cold ion heating was observed by the Magnetospheric Multiscale mission, associated with the magnetospheric Hall E field region of magnetic reconnection. For two of the events the cold ion density was small compared to the magnetosheath density, and the cold ions were heated roughly to the same temperature as magnetosheath ions inside the exhaust. On the other hand, for the other two events the cold ion density was comparable to the magnetosheath density and the cold ion heating observed was significantly smaller. Magnetic reconnection converts magnetic energy into particle energy, and ion heating is known to dominate the energy partition. We find that at least 10-25% of the energy spent by reconnection into ion heating went into magnetospheric cold ion heating. The total energy budget for cold ions may be even higher when properly accounting for the heavier species, namely helium and oxygen. Large E field fluctuations are observed in this cold ion heating region, i.e., gradients and waves, that are likely the source of particle energization. (10.1002/2017JA024553)
  DOI : 10.1002/2017JA024553
• Density profile and turbulence evolution during L-H transition studied with the ultra-fast swept reflectometer on ASDEX Upgrade
  
  • Medvedeva A.
  • Bottereau C.
  • Clairet F.
  • Hennequin Pascale
  • Stroth U.
  • Birkenmeier G.
  • Cavedon M.
  • Conway G D
  • Happel T.
  • Heuraux Stéphane
  • Molina D.
  • Silva A.
  • Willensdorfer M.
  • Team Asdex Upgrade
  Plasma Physics and Controlled Fusion, IOP Publishing, 2017, 59 (12), pp.125014. The ultra-fast swept reflectometer with the sweep time of 1 ? s, inferior to the characteristic turbulent time scale, has provided the measurements of the fast density and density fluctuation evolution across major parts of tokamak plasma radius. The L-H transitions in a series of plasma discharges in ASDEX Upgrade have been studied with a high temporal resolution. The comparison of the density fluctuation behaviour in L- and H-mode is presented. The I-phase oscillation dynamics has been described in terms of the density fluctuation level, the radial electric field and the normalised electron pressure gradient. Indications for a phase shift between the turbulence and the radial electric field are observed in the beginning of the I-phase, where the turbulence grows first and the radial electric field increase follows. In the established I-phase the electric field and the turbulence are in phase. (10.1088/1361-6587/aa9251)
  DOI : 10.1088/1361-6587/aa9251
• Crystallographic characters of {11¯22} twin-twin junctions in titanium
  
  • Xu Shun
  • Gong Mingyu
  • Xie Xinyan
  • Liu Yue
  • Schuman Christophe
  • Lecomte J.S.
  • Wang Jian
  Philosophical Magazine Letters, Taylor & Francis, 2017, 97 (11), pp.429-441. Contraction twins that are commonly activated in α-titanium interact to each other and form three types of twin–twin junctions (, , TTJs) corresponding to the crystallography of six twin variants (i = 1,2, … , 6). We detected 243 TTJs in rolled pure α-titanium sheets. Electron backscatter diffraction analysis reveals that TTJs are profuse, 79.8% among three types while and TTJs take up 17.7 and 2.5%. Twin transmission does not occur. Consequently, boundaries associated with twin–twin interactions block twin propagation and influence twin growth. We explain structural features of TTJs according to the Schmid factor analysis and the reaction mechanism of twinning dislocations. The knowledge regarding TTJs provides insight for improving the predictive capability of meso/macro-scale crystal plasticity models for hexagonal metals. (10.1080/09500839.2017.1402132)
  DOI : 10.1080/09500839.2017.1402132
• Dynamics of a shell model of bounced averaged gyrokinetic Vlasov Equation
  
  • Gürcan Özgür D.
  , 2017.
• Linear gyrokinetic investigation of the geodesic acoustic modes in realistic tokamak configurations
  
  • Novikau I.
  • Biancalani A.
  • Bottino A.
  • Conway G. D.
  • Gürcan Özgür D.
  • Manz P.
  • Morel Pierre
  • Poli E.
  • Siena A. Di
  • Team Asdex Upgrade
  Physics of Plasmas, American Institute of Physics, 2017, 24 (12), pp.122117. In order to provide scaling formulae for the geodesic acoustic mode (GAM) frequency and damping rate, GAMs are studied by means of the gyrokinetic global particle-in-cell code ORB5. Linear electromagnetic simulations in the low-&#946;e limit have been performed in order to separate acoustic and Alfvnic time scales and obtain more accurate measurements. The dependence of the frequency and damping rate on several parameters such as the safety factor, the GAM radial wavenumber, and the plasma elongation is studied. All simulations have been performed with kinetic electrons with a realistic electron/ion mass ratio. Interpolating formulae for the GAM frequency and damping rate, based on the results of the gyrokinetic simulations, have been derived. Using these expressions, the influence of the temperature gradient on the damping rate is also investigated. Finally, the results are applied to the study of a real discharge of the ASDEX Upgrade tokamak. (10.1063/1.5003784)
  DOI : 10.1063/1.5003784
• Geodesic acoustic modes with poloidal mode couplings ad infinitum
  
  • Singh Rameswar
  • Gürcan Özgür D.
  Physics of Plasmas, American Institute of Physics, 2017, 24, pp.022507. Geodesic acoustic modes (GAMs) are studied including all poloidal mode (m) couplings within a drift reduced Braginskii framework. An exact analytical formula for GAM frequency is given within the toroidal Hasegawa Mima model with the full finite larmor radius effect and poloidal mode couplings ad infinitum using a scalar continued fraction formulation, which results from reduction of the semi-infinite chain of interactions that is obtained from the nearest neighbor coupling pattern due to geodesic curvature. This pattern can be described by a semi-infinite chain model of the GAM with the mode-mode coupling matrix elements proportional to the radial wave number kr. In the more general case of multi-field description of the GAM, the infinite chain can be reduced to a renormalized bi-nodal chain with a matrix continued fraction formulation. The convergence study of the linear GAM dispersion with respect to kr and the m-spectra confirms that the coupling beyond m&#8201;=&#8201;1 is sustained only when kr&#8201;&#8800;&#8201;0 and the higher m couplings become important with increasing kr and increasing ion to electron temperature ratio &#964;i. (10.1063/1.4976116)
  DOI : 10.1063/1.4976116
• Explaining the isotope effect on heat transport in L-mode with the collisional electron-ion energy exchange
  
  • Schneider Philip
  • Bustos Andres
  • Hennequin Pascale
  • Ryter F.
  • Bernert Matthias
  • Cavedon Marco
  • Dunne M.
  • Fischer R.
  • Görler T.
  • Happel T.
  • Igochine V.
  • Kurzan B.
  • Lebschy A.
  • Mcdermott R.M.
  • Morel Pierre
  • Willensdorfer M.
  • The Asdex Upgrade Team
  • The Eurofusion Mst1 Team
  Nuclear Fusion, IOP Publishing, 2017, 57 (6), pp.066003. In ASDEX Upgrade (AUG), the normalised gyroradius ##IMG## [http://ej.iop.org/images/0029-5515/57/6/066003/nfaa65b3ieqn001.gif] ρ_★ was varied via a hydrogen isotope scan while keeping other dimensionless parameters constant. This was done in L-mode, to minimise the impact of pedestal stability on confinement. Power balance and perturbative transport analyses reveal that the electron heat transport is unaffected by the differences in isotope mass. Nonlinear simulations with the G ene code suggest that these L-mode discharges are ion temperature gradient (ITG) dominated. The different gyroradii due to the isotope mass do not necessarily result in a change of the predicted heat fluxes. This result is used in simulations with the A stra transport code to match the experimental profiles. In these simulations the experimental profiles and confinement times are reproduced with the same transport coefficients for hydrogen and deuterium plasmas. The mass only enters in the energy exchange term between electrons and ions. These numerical observations are supported by additional experiments which show a lower ion energy confinement compared to that of the electrons. Additionally, hydrogen and deuterium plasmas have a similar confinement when the energy exchange time between electrons and ions is matched. This strongly suggests that the observed isotope dependence in L-mode is not dominated by a gyroradius effect, but a consequence of the mass dependence in the collisional energy exchange between electrons and ions. (10.1088/1741-4326/aa65b3)
  DOI : 10.1088/1741-4326/aa65b3
• Occurrence rate of whistler waves in the magnetotail reconnection region
  
  • Huang S. Y.
  • Yuan Z. G.
  • Sahraoui Fouad
  • Fu H.S.
  • Pang Y.
  • Zhou M.
  • Fujimoto K.
  • Deng X. H.
  • Retinò Alessandro
  • Wang D. D.
  • Yu X. D.
  • Li H. M.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2017, 122 (7), pp.7188-7196. It has long been suggested that whistler waves play an active role during magnetic reconnection. However, all previous observations were based on case studies and could not give a detailed picture as to where the whistler waves occur in the reconnection region. In this paper, a statistical study by using the Cluster data is performed to investigate the spatial distribution and the occurrence rate of whistler waves in the magnetotail reconnection region. It is found that the occurrence rate of the whistler waves is large in the separatrix region (|B<SUB>x</SUB>/B<SUB>0</SUB>| > 0.4) and in the pileup region (|B<SUB>x</SUB>/B<SUB>0</SUB>| < 0.2, |theta| > 45°, where theta = arctan(B<SUB>z</SUB>/B<SUB>x</SUB>)), but is very small in the vicinity of the X-line. These statistical results are well consistent with recent kinetic simulations and with previous observational case studies. (10.1002/2016JA023670)
  DOI : 10.1002/2016JA023670
• Electric field and temperature in a target induced by a plasma jet imaged using Mueller polarimetry
  
  • Slikboer Elmar
  • Sobota Ana
  • Guaitella Olivier
  • Garcia-Caurel Enric
  Journal of Physics D: Applied Physics, IOP Publishing, 2017, 51, pp.025204. Mueller polarimetry is used to investigate the behavior of an electro optic target (BSO crystal) under exposure of guided ionization waves produced by an atmospheric pressure plasma jet. For the first time, this optical technique is time resolved to obtain the complete Mueller matrix of the sample right before and after the impact of the discharges. By analyzing the induced birefringence, the spatial profiles and local values are obtained of both the electric field and temperature in the sample. Electric fields are generated due to deposited surface charges and a temperature profile is present, due to the heat transferred by the plasma jet. The study of electric field dynamics and local temperature increase at the target, due to the plasma jet is important for biomedical applications, as well as surface functionalization. This work shows how Mueller polarimetry can be used as a novel diagnostic to simultaneously acquire the spatial distribution and local values of both the electric field and temperature, by coupling the external source of anisotropy to the measured induced birefringence via the symmetry point group of the examined material. (10.1088/1361-6463/aa9b17)
  DOI : 10.1088/1361-6463/aa9b17