Publications

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Below, by year, are the publications listed in the HAL open archive.

2015

Reply to Comment on "A review on ion-ion plasmas created in weakly magnetized electronegative plasmas
- Aanesland Ane
- Bredin Jérôme
- Chabert Pascal
Plasma Sources Science and Technology, IOP Publishing, 2015, 24 (3), pp.038002. This is a reply to the comments made by Bogdanov et al on our paper entitled ?A review on ion?ion plasmas created in weakly magnetized electronegative plasmas?. We have clarified some issues regarding the electronegative plasma stratification and the definition of an ion?ion plasma, and we discuss the issue of extracting negative ions from a steady-state ion?ion plasma formed at the periphery or downstream a magnetized electronegative plasma. (10.1088/0963-0252/24/3/038002)
DOI : 10.1088/0963-0252/24/3/038002
Structure of a laser-driven radiative shock
- Chaulagain Uddhab
- Stehle Chantal
- Larour Jean
- Kozlová Michaela
- Suzuki-Vidal Francisco
- Barroso Patrice
- Cotelo M.
- Velarde P.
- Rodriguez R.
- Gil J. M.
- Ciardi Andrea
- Acef Ouali
- Nejdl Jaroslav
- de Sá Lionel
- Singh Raj Laxmi
- Ibgui Laurent
- Champion Norbert
High Energy Density Physics, Elsevier, 2015, 17, pp.106-113. Radiative shocks are ubiquitous in stellar environments and are characterized by high temperature plasma emitting a considerable fraction of their energy as radiation. The physical structure of these shocks is complex and experimental benchmarks are needed to provide a deeper understanding of the physics at play. In addition, experiments provide unique data for testing radiation hydrodynamics codes which, in turn, are used to model astrophysical phenomena.
Production of nongyrotropic and gyrotropic backstreaming ion distributions in the quasi-perpendicular ion foreshock region
- Savoini Philippe
- Lembège Bertrand
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015, 120 (9), pp.7154–7171. A curved shock is analyzed in the whole quasi-perpendicular propagation region (90° ≥ θBn≥45°) in a supercritical regime with the help of a 2-D particle-in-cell code including self-consistent effects such as the shock front curvature and the time-of-flight effects. Two distinct ion populations are observed within the foreshock: a (gyrotropic) field-aligned beam population, hereafter named “FAB,” and a (nongyrotropic) gyrophase bunched population, hereafter named “GPB.” The origin of these high-energy particles and their corresponding acceleration mechanisms are analyzed in details in the present paper. Both FAB and GPB populations are shown to be produced by the shock front itself and more important, do have exactly the same origin. At the shock front, the two populations gain a nongyrotropic distribution, but FAB population loses its initial phase coherency after suffering several bounces along the curved front. This result has one main consequence: the time evolution of the two populations does not involve some distinct reflection processes as often claimed in the literature, but results only from the particle time history at the shock front. This important result was not expected and greatly simplifies the question of their origin. More precisely, a new parameter, the injection angle θinj has been defined between the shock normal direction and the ion gyrating velocity vector. We found that the FAB population is formed by ions injected almost along the shock front, while GPB population is formed by ions injected almost along the shock normal. (10.1002/2015JA021018)
DOI : 10.1002/2015JA021018
Geodesic acoustic modes in a fluid model of tokamak plasma: the effects of finite beta and collisionality
- Singh Rameswar
- Storelli A.
- Gürcan Özgür D.
- Hennequin Pascale
- Vermare Laure
- Morel Pierre
- Singh R.
Plasma Physics and Controlled Fusion, IOP Publishing, 2015, 57, pp.125002. Starting from the Braginskii equations, relevant for the tokamak edge region, a complete set of nonlinear equations for the geodesic acoustic modes (GAM) has been derived which includes collisionality, plasma beta and external sources of particle, momentum and heat. Local linear analysis shows that the GAM frequency increases with collisionality at low radial wave number and decreases at high . GAM frequency also decreases with plasma beta. Radial profiles of GAM frequency for two Tore Supra shots, which were part of a collisionality scan, are compared with these calculations. A discrepancy between experiment and theory is observed, which seems to be explained by a finite for the GAM when flux surface averaged density \langle n\rangle and temperature are assumed to vanish. It is shown that this agreement is incidental and self-consistent inclusion of and responses enhances the disagreement more with at high . So the discrepancy between the linear GAM calculation and experiment, (which also persist for more 'complete' linear models such as gyrokinetics) can probably not be resolved by simply adding a finite . (10.1088/0741-3335/57/12/125002)
DOI : 10.1088/0741-3335/57/12/125002
WEST Physics Basis
- Bourdelle C.
- Artaud J.F.
- Basiuk Vincent
- Bécoulet M.
- Brémond S.
- Bucalossi J.
- Bufferand H.
- Ciraolo G.
- Colas L.
- Corre Y.
- Courtois X.
- Decker J.
- Delpech L.
- Devynck P.
- Dif-Pradalier Guilhem
- Doerner R.P.
- Douai D.
- Dumont Rémi
- Ekedahl A.
- Fedorczak N.
- Fenzi C.
- Firdaouss M.
- Garcia J.
- Ghendrih Philippe
- Gil C.
- Giruzzi G.
- Goniche M.
- Grisolia C.
- Grosman A.
- Guilhem D.
- Guirlet R.
- Gunn J.
- Hennequin Pascale
- Hillairet J.
- Hoang T.
- Imbeaux Frédéric
- Ivanova-Stanik Irena
- Joffrin E.
- Kallenbach A.
- Linke J.
- Loarer T.
- Lotte P.
- Maget P.
- Marandet Yannick
- Mayoral M.L.
- Meyer O.
- Missirlian M.
- Mollard P.
- Monier-Garbet P.
- Moreau P.
- Nardon Eric
- Pégourié B.
- Peysson Y.
- Sabot R.
- Saint-Laurent F.
- Schneider M.
- Travère J. M.
- Tsitrone E.
- Vartanian S.
- Vermare Laure
- Yoshida M.
- Zagorski R.
- Jet Contributors
Nuclear Fusion, IOP Publishing, 2015, 55 (6), pp.063017. With WEST (Tungsten Environment in Steady State Tokamak) (Bucalossi et al 2014 Fusion Eng. Des. 89 [http://dx.doi.org/10.1016/j.fusengdes.2014.01.062] 907?12 ), the Tore Supra facility and team expertise (Dumont et al 2014 Plasma Phys. Control. Fusion 56 [http://dx.doi.org/10.1088/0741-3335/56/7/075020] 075020 ) is used to pave the way towards ITER divertor procurement and operation. It consists in implementing a divertor configuration and installing ITER-like actively cooled tungsten monoblocks in the Tore Supra tokamak, taking full benefit of its unique long-pulse capability. WEST is a user facility platform, open to all ITER partners. This paper describes the physics basis of WEST: the estimated heat flux on the divertor target, the planned heating schemes, the expected behaviour of the L?H threshold and of the pedestal and the potential W sources. A series of operating scenarios has been modelled, showing that ITER-relevant heat fluxes on the divertor can be achieved in WEST long pulse H-mode plasmas. (10.1088/0029-5515/55/6/063017)
DOI : 10.1088/0029-5515/55/6/063017
Experimental turbulence studies for gyro-kinetic code validation using advanced microwave diagnostics
- Stroth U.
- Bañón Navarro A.
- Conway G. D.
- Görler T.
- Happel T.
- Hennequin Pascale
- Lechte C.
- Manz P.
- Simon P.
- Biancalani A.
- Blanco E.
- Bottereau C.
- Clairet F.
- Coda S.
- Eibert T.
- Estrada T.
- Fasoli A.
- Guimarais L.
- Gürcan Özgür D.
- Huang Z.
- Jenko F.
- Kasparek W.
- Koenen C.
- Krämer-Flecken A.
- Manso M.-E.
- Medvedeva A.
- Molina D.
- Nikolaeva V.
- Plaum B.
- Porte L.
- Prisiazhniuk D.
- Ribeiro T.
- Scott B.D.
- Siart U.
- Storelli A.
- Vermare Laure
- Wolf S.
Nuclear Fusion, IOP Publishing, 2015, 55 (8), pp.083027. For a comprehensive comparison with theoretical models and advanced numerical turbulence simulations, a large spectrum of fluctuation parameters was measured on the devices ASDEX Upgrade, TCV, and Tore-Supra. Radial profiles of scale-resolved turbulence levels in H-mode discharges are measured and compared with GENE simulations in the transition range from ion-temperature-gradient to trapped-electron-mode turbulence. Correlation reflectometry is used to study the microscopic structure of turbulence and GAMs in discharges where poloidal flow damping was varied by means of variations of the shape of the poloidal plasma cross-section and collisionality. Full-wave codes and synthetic diagnostics are applied for the interpretation of the data. (10.1088/0029-5515/55/8/083027)
DOI : 10.1088/0029-5515/55/8/083027
Comprehensive comparisons of geodesic acoustic mode characteristics and dynamics between Tore Supra experiments and gyrokinetic simulations
- Storelli A.
- Vermare Laure
- Hennequin Pascale
- Gürcan Özgür D.
- Dif-Pradalier Guilhem
- Sarazin Y.
- Garbet X.
- Görler T.
- Singh Rameswar
- Morel Pierre
- Grandgirard Virginie
- Ghendrih Philippe
- Tore Supra Team
Physics of Plasmas, American Institute of Physics, 2015, 22 (6). In a dedicated collisionality scan in Tore Supra, the geodesic acoustic mode (GAM) is detected and identified with the Doppler backscattering technique. Observations are compared to the results of a simulation with the gyrokinetic code GYSELA. We found that the GAM frequency in experiments is lower than predicted by simulation and theory. Moreover, the disagreement is higher in the low collisionality scenario. Bursts of non harmonic GAM oscillations have been characterized with filtering techniques, such as the Hilbert-Huang transform. When comparing this dynamical behaviour between experiments and simulation, the probability density function of GAM amplitude and the burst autocorrelation time are found to be remarkably similar. In the simulation, where the radial profile of GAM frequency is continuous, we observed a phenomenon of radial phase mixing of the GAM oscillations, which could influence the burst autocorrelation time. (10.1063/1.4922845)
DOI : 10.1063/1.4922845
Finding the Elusive E × B Staircase in Magnetized Plasmas
- Dif-Pradalier Guilhem
- Hornung G
- Ghendrih Philippe
- Sarazin Yanick
- Clairet F
- Vermare L
- Diamond P H
- Abiteboul J
- Cartier-Michaud T
- Ehrlacher C
- Estève Daniel
- Garbet Xavier
- Grandgirard Virginie
- Gürcan Özgür D.
- Hennequin P
- Kosuga Y
- Latu Guillaume
- Maget P
- Morel Pierre
- Norscini C
- Sabot R
- Storelli A.
Physical Review Letters, American Physical Society, 2015, 114, pp.085004. Turbulence in hot magnetized plasmas is shown to generate permeable localized transport barriers that globally organize into the so-called "ExB staircase" [G. Dif-Pradalier et al., Phys. Rev. E, 82, 025401(R) (2010)]. Its domain of existence and dependence with key plasma parameters is discussed theoretically. Based on these predictions, staircases are observed experimentally in the Tore Supra tokamak by means of high-resolution fast-sweeping X-mode reflectometry. This observation strongly emphasizes the critical role of mesoscale self-organization in plasma turbulence and may have far-reaching consequences for turbulent transport models and their validation. (10.1103/PhysRevLett.114.085004)
DOI : 10.1103/PhysRevLett.114.085004
Different types of whistler mode chorus in the equatorial source region
- Taubenschuss U.
- Santolík O.
- Graham Daniel B.
- Fu H.S.
- Khotyaintsev Y. V.
- Le Contel Olivier
Geophysical Research Letters, American Geophysical Union, 2015, 42 (20), pp.8271-8279. The Time History of Events and Macroscale Interactions during Substorms-D spacecraft crossed an active equatorial source region of whistler mode chorus rising tones on 23 October 2008. Rising tones are analyzed in terms of spectral and polarization characteristics, with special emphasis on wave normal angles. The latter exhibit large variations from quasi-parallel to oblique, even within single bursts, but seem to follow a definite pattern, which enables an unambiguous classification into five different groups. Furthermore, we discuss the frequently observed splitting of chorus bursts into a lower and an upper band around one half of the local electron cyclotron frequency. At chorus frequencies close to the gap, we find significantly lowered wave planarities and a tendency of wave normal angles to approach the Gendrin angle. (10.1002/2015GL066004)
DOI : 10.1002/2015GL066004
Thin Current Sheets and Associated Electron Heating in Turbulent Space Plasma
- Chasapis A.
- Retinò Alessandro
- Sahraoui Fouad
- Vaivads A.
- Khotyaintsev Y. V.
- Sundkvist D.
- Greco A.
- Sorriso-Valvo L.
- Canu Patrick
The Astrophysical Journal Letters, Bristol : IOP Publishing, 2015, 804 (1). Intermittent structures, such as thin current sheets, are abundant in turbulent plasmas. Numerical simulations indicate that such current sheets are important sites of energy dissipation and particle heating occurring at kinetic scales. However, direct evidence of dissipation and associated heating within current sheets is scarce. Here, we show a new statistical study of local electron heating within proton-scale current sheets by using high-resolution spacecraft data. Current sheets are detected using the Partial Variance of Increments (PVI) method which identifies regions of strong intermittency. We find that strong electron heating occurs in high PVI (>3) current sheets while no significant heating occurs in low PVI cases (<3), indicating that the former are dominant for energy dissipation. Current sheets corresponding to very high PVI (>5) show the strongest heating and most of the time are consistent with ongoing magnetic reconnection. This suggests that reconnection is important for electron heating and dissipation at kinetic scales in turbulent plasmas. (10.1088/2041-8205/804/1/L1)
DOI : 10.1088/2041-8205/804/1/L1
Observations of discrete harmonics emerging from equatorial noise
- Balikhin M. A.
- Shprits Y. Y.
- Walker S. N.
- Chen Lunjin
- Cornilleau-Wehrlin Nicole
- Dandouras Iannis
- Santolík O.
- Carr Christopher
- Yearby K. H.
- Weiss Benjamin
Nature Communications, Nature Publishing Group, 2015, 6. A number of modes of oscillations of particles and fields can exist in space plasmas. Since the early 1970s, space missions have observed noise-like plasma waves near the geomagnetic equator known as `equatorial noise'. Several theories were suggested, but clear observational evidence supported by realistic modelling has not been provided. Here we report on observations by the Cluster mission that clearly show the highly structured and periodic pattern of these waves. Very narrow-banded emissions at frequencies corresponding to exact multiples of the proton gyrofrequency (frequency of gyration around the field line) from the 17th up to the 30th harmonic are observed, indicating that these waves are generated by the proton distributions. Simultaneously with these coherent periodic structures in waves, the Cluster spacecraft observes `ring' distributions of protons in velocity space that provide the free energy for the waves. Calculated wave growth based on ion distributions shows a very similar pattern to the observations. (10.1038/ncomms8703)
DOI : 10.1038/ncomms8703
Some statistical equilibrium mechanics and stability properties of a class of two-dimensional Hamiltonian mean-field models
- Maciel J. M.
- Firpo Marie-Christine
- Amato M. A.
Physica A: Statistical Mechanics and its Applications, Elsevier, 2015, 424, pp.34-43. Abstract A two-dimensional class of mean-field models that may serve as a minimal model to study the properties of long-range systems in two space dimensions is considered. The statistical equilibrium mechanics is derived in the microcanonical ensemble using Monte Carlo simulations for different combinations of the coupling constants in the potential leading to fully repulsive, fully attractive and mixed attractive?repulsive potential along the Cartesian axis and diagonals. Then, having in mind potential realizations of long-range systems using cold atoms, the linear theory of this two-dimensional mean-field Hamiltonian models is derived in the low temperature limit. (10.1016/j.physa.2014.12.030)
DOI : 10.1016/j.physa.2014.12.030
Towards an emerging understanding of non-locality phenomena and non-local transport
- Ida K.
- Shi Z.
- Sun H. J.
- Inagaki S.
- Kamiya K.
- Rice J.E.
- Tamura N.
- Diamond P.H.
- Dif-Pradalier Guilhem
- Zou X. L.
- Itoh K.
- Sugita S.
- Gürcan Özgür D.
- Estrada T.
- Hidalgo C.
- Hahm T.S.
- Field A.
- Ding X. T.
- Sakamoto Y.
- Oldenburger S.
- Yoshinuma M.
- Kobayashi T.
- Jiang M.
- Hahn S. -H.
- Jeon Y. M.
- Hong S. H.
- Kosuga Y.
- Dong J.
- Itoh S. I.
Nuclear Fusion, IOP Publishing, 2015, 55 (1), pp.013022. In this paper, recent progress on experimental analysis and theoretical models for non-local transport (non-Fickian fluxes in real space) is reviewed. The non-locality in the heat and momentum transport observed in the plasma, the departures from linear flux-gradient proportionality, and externally triggered non-local transport phenomena are described in both L-mode and improved-mode plasmas. Ongoing evaluation of 'fast front' and 'intrinsically non-local' models, and their success in comparisons with experimental data, are discussed (10.1088/0029-5515/55/1/013022)
DOI : 10.1088/0029-5515/55/1/013022
Entanglement of helicity and energy in kinetic Alfvén wave/whistler turbulence
- Galtier Sébastien
- Meyrand Romain
Journal of Plasma Physics, Cambridge University Press (CUP), 2015, 81 (1), pp.325810106. The role of magnetic helicity is investigated in kinetic Alfvén wave and oblique whistler turbulence in presence of a relatively intense external magnetic field b 0 e ||. In this situation, turbulence is strongly anisotropic and the fluid equations describing both regimes are the reduced electron magnetohydrodynamics (REMHD) whose derivation, originally made from the gyrokinetic theory, is also obtained here from compressible Hall magnetohydrodynamics (MHD). We use the asymptotic equations derived by Galtier and Bhattacharjee (2003 Phys. Plasmas 10, 3065-3076) to study the REMHD dynamics in the weak turbulence regime. The analysis is focused on the magnetic helicity equation for which we obtain the exact solutions: they correspond to the entanglement relation, n ñ = -6, where n and ñ are the power law indices of the perpendicular (to b 0) wave number magnetic energy and helicity spectra, respectively. Therefore, the spectra derived in the past from the energy equation only, namely n = -2.5 and ñ = -3.5, are not the unique solutions to this problem but rather characterize the direct energy cascade. The solution ñ = -3 is a limit imposed by the locality condition; it is also the constant helicity flux solution obtained heuristically. The results obtained offer a new paradigm to understand solar wind turbulence at sub-ion scales where it is often observed that -3 < n < -2.5. (10.1017/S0022377814000774)
DOI : 10.1017/S0022377814000774
Imprints of Expansion on the Local Anisotropy of Solar Wind Turbulence
- Verdini Andrea
- Grappin Roland
The Astrophysical Journal Letters, Bristol : IOP Publishing, 2015, 808, pp.L34. We study the anisotropy of II-order structure functions (SFs) defined in a frame attached to the local mean field in three-dimensional (3D) direct numerical simulations of magnetohydrodynamic turbulence, with the solar wind expansion both included and not included. We simulate spacecraft flybys through the numerical domain by taking increments along the radial (wind) direction that form an angle of 45° with the ambient magnetic field. We find that only when expansion is taken into account do the synthetic observations match the 3D anisotropy observed in the solar wind, including the change of anisotropy with scale. Our simulations also show that the anisotropy changes dramatically when considering increments oblique to the radial directions. Both results can be understood by noting that expansion reduces the radial component of the magnetic field at all scales, thus confining fluctuations in the plane perpendicular to the radial. Expansion is thus shown to affect not only the (global) spectral anisotropy, but also the local anisotropy of second-order SF by influencing the distribution of the local mean field, which enters this higher-order statistics. (10.1088/2041-8205/808/2/L34)
DOI : 10.1088/2041-8205/808/2/L34
Electron-less negative ion extraction from ion-ion plasmas
- Rafalskyi D.V.
- Aanesland Ane
Applied Physics Letters, American Institute of Physics, 2015, 106 (10), pp.104101. This paper presents experimental results showing that continuous negative ion extraction, without co-extracted electrons, is possible from highly electronegative SF6 ion-ion plasma at low gas pressure (1 mTorr). The ratio between the negative ion and electron densities is more than 3000 in the vicinity of the two-grid extraction and acceleration system. The measurements are conducted by both magnetized and non-magnetized energy analyzers attached to the external grid. With these two analyzers, we show that the extracted negative ion flux is almost electron-free and has the same magnitude as the positive ion flux extracted and accelerated when the grids are biased oppositely. The results presented here can be used for validation of numerical and analytical models of ion extraction from ion-ion plasma. (10.1063/1.4914507)
DOI : 10.1063/1.4914507
Magnetic Reconnection in Different Environments
- Jackman C. M.
- Delamere Peter A.
- Hesse Michael
- Aunai N.
- Kuznetsova M. M.
- Zenitani Seiji
- Birn Joachim
, 2015, 207, pp.259-267. This chapter addresses two important aspects of magnetic reconnection: time-dependent rates affected by island formation and the preferred direction of the reconnection line if merging magnetic field components are not anti-parallel. While it is widely known that magnetic reconnection facilitates mass, momentum, and energy transport in plasmas, it is illustrative to consider an analytical approach to describing the efficacy of reconnection as a transport mechanism. The chapter performs a detailed analysis of the correlation between diffusion region dimensions and reconnection rate. For this purpose, it employs open boundary condition calculations of a continuously driven reconnecting system. In summary, results indicate that the magnetic reconnection line in asymmetric systems is preferentially oriented in such a way that it bisects the direction of the asymptotic magnetic field direction on both inflow sides. This orientation is identical to the one for which the product of available magnetic energy is maximized.
Ignition of methane and n-butane containing mixtures at high pressures by pulsed nanosecond discharge
- Boumehdi M.A.
- Stepanyan S.A.
- Vanhove Guillaume
- Desgroux Pascale
- Starikovskaia Svetlana
Combustion and Flame, Elsevier, 2015, 162, pp.1336-1349. A novel experimental scheme to study the ignition of combustible mixtures at high pressures under the action of a high-voltage nanosecond discharge has been developed. The experiments were performed in the combustion chamber of a Rapid Compression Machine (RCM) with a specially designed system of electrodes. A nanosecond surface dielectric barrier discharge (SDBD) provided two-dimensional low-temperature non-equilibrium plasma in the vicinity of the end plate of the combustion chamber. Radially symmetric plasma channels triggered multi-point ignition of gas mixtures at controlled pressure and temperature. Ignition delay times and energies deposited in the gaseous mixtures by the discharge were measured for different parameters of high voltage pulse, for positive or negative high-voltage pulses. The propagation of the subsequent flame in the combustion chamber was recorded with the help of high repetition rate imaging. Preliminary numerical analysis of the ignition under the action of a pulsed nanosecond discharge has been made; it was shown that production of atomic oxygen by the discharge, will modify the ignition chemistry by perturbation of the radical pool. Experiments and calculations were performed in methaneoxygen and n-butaneoxygen mixtures with equivalence ratios between 0.3 and 1 diluted by 7076% of Ar or nitrogen for temperatures between 600 and 1000 K and pressures between 6 and 16 bar. (10.1016/j.combustflame.2014.11.006)
DOI : 10.1016/j.combustflame.2014.11.006
Intensities and spatiotemporal variability of equatorial noise emissions observed by the Cluster spacecraft
- Nemec F.
- Santolík O.
- Hrbackova Z.
- Cornilleau-Wehrlin Nicole
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015, 120, pp.1620-1632. Equatorial noise (EN) emissions are electromagnetic waves observed in the equatorial region of the inner magnetosphere at frequencies between the proton cyclotron frequency and the lower hybrid frequency. We present the analysis of 2229 EN events identified in the Spatio-Temporal Analysis of Field Fluctuations (STAFF) experiment data of the Cluster spacecraft during the years 2001-2010. EN emissions are distinguished using the polarization analysis, and their intensity is determined based on the evaluation of the Poynting flux rather than on the evaluation of only the electric/magnetic field intensity. The intensity of EN events is analyzed as a function of the frequency, the position of the spacecraft inside/outside the plasmasphere, magnetic local time, and the geomagnetic activity. The emissions have higher frequencies and are more intense in the plasma trough than in the plasmasphere. EN events observed in the plasma trough are most intense close to the local noon, while EN events observed in the plasmasphere are nearly independent on magnetic local time (MLT). The intensity of EN events is enhanced during disturbed periods, both inside the plasmasphere and in the plasma trough. Observations of the same events by several Cluster spacecraft allow us to estimate their spatiotemporal variability. EN emissions observed in the plasmasphere do not change on the analyzed spatial scales (DeltaMLT<0.2h, Deltar<0.2 RE), but they change significantly on time scales of about an hour. The same appears to be the case also for EN events observed in the plasma trough, although the plasma trough dependencies are less clear. (10.1002/2014JA020814)
DOI : 10.1002/2014JA020814
TALIF measurements of oxygen atom density in the afterglow of a capillary nanosecond discharge
- Klochko A.V.
- Lemainque J.
- Booth Jean-Paul
- Starikovskaia Svetlana
Plasma Sources Science and Technology, IOP Publishing, 2015, 24 (2), pp.025010. The atomic oxygen density has been measured in the afterglow of a capillary nanosecond discharge in 2430 mbar synthetic air (N 2  : O 2 = 4 : 1) by the two-photon absorption laser-induced fluorescence (TALIF) technique, combined with absolute calibration by comparison with xenon TALIF. The discharge was initiated by a train of 30 ns FWHM pulses of alternating positivenegativepositive polarity, separated by 250 ns, with a train repetition frequency of 10 Hz. The amplitude of the first pulse was 10 kV in the cable. A flow of synthetic air through the tube provided complete gas renewal between pulse trains. The O-atom density measurements were made over the time interval200 ns2 µ s after the initial pulse. The gas temperature was determined by analysis of the molecular nitrogen second positive system optical emission spectrum. The influence of the gas temperature on the atom density measurements, and the reactions producing O atoms, are discussed. (10.1088/0963-0252/24/2/025010)
DOI : 10.1088/0963-0252/24/2/025010
Control and optimization of the slope asymmetry effect in tailored voltage waveforms for capacitively coupled plasmas
- Bruneau Bastien
- Novikova T.
- Lafleur Trevor
- Booth Jean-Paul
- Johnson E.V.
Plasma Sources Science and Technology, IOP Publishing, 2015, 24 (1), pp.015021. Through the use of particle-in-cell simulations, we study the ion flux asymmetry in an argon discharge that is induced by a ?sawtooth-like? excitation voltage waveform. In a previous article we have shown that, due to their differing rising and falling slopes, these waveforms can create a plasma with a significantly higher ion flux to one electrode in a geometrically symmetric reactor. Furthermore, they have the unique property of providing a lower ion energy at the electrode with a higher ion flux. In the present work, we show that a refined waveform allows the ion flux asymmetry to be increased for a given number of harmonics by reducing the ionization rate in front of the low-flux electrode. The flux asymmetry is found to disappear at low pressure due to the increased electron energy transport, which causes a transition from sheath edge ionization to bulk ionization. Changing the fundamental frequency is shown to have two counterbalancing effects: reducing the ionization on the low ion-flux electrode and shifting the maximum ionization to the center of the discharge. Under the representative conditions that we have studied, a maximum asymmetry is found for a base frequency of 3.4 MHz. Finally, it is shown that, by adjusting the rise- to fall-time ratio of the refined waveforms, the ion-flux asymmetry can be continuously shifted from one electrode to the other. (10.1088/0963-0252/24/1/015021)
DOI : 10.1088/0963-0252/24/1/015021
Highly vibrationally excited O2 molecules in low-pressure inductively-coupled plasmas detected by high sensitivity ultra-broad-band optical absorption spectroscopy
- Foucher Mickaël
- Marinov Daniil
- Carbone Emile
- Chabert Pascal
- Booth Jean-Paul
Plasma Sources Science and Technology, IOP Publishing, 2015, 24 (4), pp.042001. Inductively-coupled plasmas in pure O 2 (at pressures of 5?80?mTorr and radiofrequency power up to 500?W) were studied by optical absorption spectroscopy over the spectral range 200?450?nm, showing the presence of highly vibrationally excited O 2 molecules (up to v? = 18) by Schumann?Runge band absorption. Analysis of the relative band intensities indicates a vibrational temperature up to 10,000?K, but these hot molecules only represent a fraction of the total O 2 density. By analysing the (11-0) band at higher spectral resolution the O 2 rotational temperature was also determined, and was found to increase with both pressure and power, reaching 900?K at 80?mTorr 500?W. These measurements were achieved using a new high-sensitivity ultra-broad-band absorption spectroscopy setup, based on a laser-plasma light source, achromatic optics and an aberration-corrected spectrograph. This setup allows the measurement of weak broadband absorbances due to a baseline variability lower than 2???×???10 ?5 across a spectral range of 250?nm. (10.1088/0963-0252/24/4/042001)
DOI : 10.1088/0963-0252/24/4/042001
Experimental determination of the dispersion relation of magnetosonic waves
- Balikhin M. A.
- Shklyar D. R.
- Yearby K. H.
- Canu Patrick
- Carr C. M.
- Dandouras I.
- Walker S. N.
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015, 120, pp.9632-9650. Magnetosonic waves are commonly observed in the vicinity of the terrestrial magnetic equator. It has been proposed that within this region they may interact with radiation belt electrons, accelerating some to high energies. These wave-particle interactions depend upon the characteristic properties of the wave mode. Hence, determination of the wave properties is a fundamental part of understanding these interaction processes. Using data collected during the Cluster Inner Magnetosphere Campaign, this paper identifies an occurrence of magnetosonic waves, discusses their generation and propagation properties from a theoretical perspective, and utilizes multispacecraft measurements to experimentally determine their dispersion relation. Their experimental dispersion is found to be in accordance with that based on cold plasma theory. (10.1002/2015JA021746)
DOI : 10.1002/2015JA021746
Weak magnetohydrodynamic turbulence and intermittency
- Meyrand Romain
- Kiyani K. H.
- Galtier Sébastien
Journal of Fluid Mechanics, Cambridge University Press (CUP), 2015, 770. Three-dimensional numerical simulation is used to investigate intermittency in incompressible weak magnetohydrodynamic turbulence with a strong uniform magnetic field and zero cross-helicity. At leading order, this asymptotic regime is achieved via three-wave resonant interactions with the scattering of a wave on a 2D mode for which . When the interactions with the 2D modes are artificially reduced, we show numerically that the system exhibits an energy spectrum with , whereas the expected exact solution with is recovered with the full nonlinear system. In the latter case, strong intermittency is found when the vector separation of structure functions is taken transverse to . This result may be explained by the influence of the 2D modes whose regime belongs to strong turbulence. In addition to shedding light on the origin of this intermittency, we derive a log-Poisson law, , which fits the data perfectly and highlights the important role of parallel current sheets. (10.1017/jfm.2015.141)
DOI : 10.1017/jfm.2015.141
Wave-number spectrum of dissipative drift waves and a transition scale
- Ghantous K.
- Gürcan Özgür D.
Physical Review E, American Physical Society (APS), 2015, 92 (3), pp.033107. We study the steady state spectrum of the Hasegawa-Wakatani (HW) equations that describe drift wave turbulence. Beyond a critical scale kc, which appears as a balance between the nonlinear time and the parallel conduction time, the adiabatic electron response breaks down nonlinearly and an internal energy density spectrum of the form F(k⊥)∝k−3⊥, associated with the background gradient, is established. More generally a dual power law spectrum, approximately of the form F(k⊥)∝k−3⊥(k−2c k−2⊥) is obtained, which captures this transition. Using dimensional analysis, an expression of the form kc∝C/κ is derived for the transition scale, where C and κ are normalized parameters of the HW equations signifying the electron adiabaticity and the density gradient, respectively. The results are numerically confirmed using a shell model developed and used for the Hasegawa-Wakatani system. (10.1103/PhysRevE.92.033107)
DOI : 10.1103/PhysRevE.92.033107

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