Publications

2015

• Anisotropy of Third-order Structure Functions in MHD Turbulence
  
  • Verdini Andrea
  • Grappin Roland
  • Hellinger P.
  • Landi Simone
  • Müller Wolf-Christian
  The Astrophysical Journal, American Astronomical Society, 2015, 804, pp.119. The measure of the third-order structure function, \boldsymbolY , is employed in the solar wind to compute the cascade rate of turbulence. In the absence of a mean field B<SUB>0</SUB>=0, \boldsymbolY is expected to be isotropic (radial) and independent of the direction of increments, so its measure yields directly the cascade rate. For turbulence with mean field, as in the solar wind, \boldsymbolY is expected to become more two-dimensional (2D), that is, to have larger perpendicular components, losing the above simple symmetry. To get the cascade rate, one should compute the flux of \boldsymbolY , which is not feasible with single-spacecraft data thus, measurements rely on assumptions about the unknown symmetry. We use direct numerical simulations (DNSs) of magnetohydrodynamic (MHD) turbulence to characterize the anisotropy of \boldsymbolY . We find that for strong guide field B<SUB>0</SUB>=5 the degree of two-dimensionalization depends on the relative importance of shear-Alfvén and pseudo-Alfvén polarizations (the two components of an Alfvén mode in incompressible MHD). The anisotropy also shows up in the inertial range. The more \boldsymbolY is 2D, the more the inertial range extent differs along parallel and perpendicular directions. We finally test the two methods employed in observations and find that the so-obtained cascade rate may depend on the angle between B<SUB>0</SUB> and the direction of increments. Both methods yield a vanishing cascade rate along the parallel direction, contrary to observations, suggesting a weaker anisotropy of solar wind turbulence compared to our DNSs. This could be due to a weaker mean field and/or to solar wind expansion. (10.1088/0004-637X/804/2/119)
  DOI : 10.1088/0004-637X/804/2/119
• 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
• 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
• 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
• 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 <SUB>0</SUB> e <SUB>||</SUB>. 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 <SUB>0</SUB>) 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
• 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
• 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
• 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
• Asymmetric kinetic equilibria: Generalization of the BAS model for rotating magnetic profile and non-zero electric field
  
  • Dorville Nicolas
  • Belmont Gérard
  • Aunai Nicolas
  • Dargent Jérémy
  • Rezeau Laurence
  Physics of Plasmas, American Institute of Physics, 2015, 22 (9), pp.092904. Finding kinetic equilibria for non-collisional/collisionless tangential current layers is a key issue as well for their theoretical modeling as for our understanding of the processes that disturb them, such as tearing or Kelvin Helmholtz instabilities. The famous Harris equilibrium [E. Harris, Il Nuovo Cimento Ser. 10 23, 115121 (1962)] assumes drifting Maxwellian distributions for ions and electrons, with constant temperatures and flow velocities; these assumptions lead to symmetric layers surrounded by vacuum. This strongly particular kind of layer is not suited for the general case: asymmetric boundaries between two media with different plasmas and different magnetic fields. The standard method for constructing more general kinetic equilibria consists in using Jeans theorem, which says that any function depending only on the Hamiltonian constants of motion is a solution to the steady Vlasov equation [P. J. Channell, Phys. Fluids (19581988) 19, 1541 (1976); M. Roth et al., Space Sci. Rev. 76, 251317 (1996); and F. Mottez, Phys. Plasmas 10, 15411545 (2003)]. The inverse implication is however not true: when using the motion invariants as variables instead of the velocity components, the general stationary particle distributions keep on depending explicitly of the position, in addition to the implicit dependence introduced by these invariants. The standard approach therefore strongly restricts the class of solutions to the problem and probably does not select the most physically reasonable. The BAS (Belmont-Aunai-Smets) model [G. Belmont et al., Phys. Plasmas 19, 022108 (2012)] used for the first time the concept of particle accessibility to find new solutions: considering the case of a coplanar-antiparallel magnetic field configuration without electric field, asymmetric solutions could be found while the standard method can only lead to symmetric ones. These solutions were validated in a hybrid simulation [N. Aunai et al., Phys. Plasmas (1994-present) 20, 110702 (2013)], and more recently in a fully kinetic simulation as well [J. Dargent and N. Aunai, Phys. Plasmas (submitted)]. Nevertheless, in most asymmetric layers like the terrestrial magnetopause, one would indeed expect a magnetic field rotation from one direction to another without going through zero [J. Berchem and C. T. Russell, J. Geophys. Res. 87, 81398148 (1982)], and a non-zero normal electric field. In this paper, we propose the corresponding generalization: in the model presented, the profiles can be freely imposed for the magnetic field rotation (although restricted to a 180 rotation hitherto) and for the normal electric field. As it was done previously, the equilibrium is tested with a hybrid simulation. (10.1063/1.4930210)
  DOI : 10.1063/1.4930210
• A Review of General Physical and Chemical Processes Related to Plasma Sources and Losses for Solar System Magnetospheres
  
  • Seki K.
  • Nagy A.
  • Jackman C. M.
  • Crary F.
  • Fontaine Dominique
  • Zarka P.
  • Wurz Peter
  • Milillo A.
  • Slavin J. A.
  • Delcourt Dominique C.
  • Wiltberger M.
  • Ilie R.
  • Jia X.
  • Ledvina S. A.
  • Liemohn M. W.
  • Schunk R. W.
  Space Science Reviews, Springer Verlag, 2015, 192 (1-4), pp.27-89. The aim of this paper is to provide a review of general processes related to plasma sources, their transport, energization, and losses in the planetary magnetospheres. We provide background information as well as the most up-to-date knowledge of the comparative studies of planetary magnetospheres, with a focus on the plasma supply to each region of the magnetospheres. This review also includes the basic equations and modeling methods commonly used to simulate the plasma sources of the planetary magnetospheres. In this paper, we will describe basic and common processes related to plasma supply to each region of the planetary magnetospheres in our solar system. First, we will describe source processes in Sect. 1. Then the transport and energization processes to supply those source plasmas to various regions of the magnetosphere are described in Sect. 2. Loss processes are also important to understand the plasma population in the magnetosphere and Sect. 3 is dedicated to the explanation of the loss processes. In Sect. 4, we also briefly summarize the basic equations and modeling methods with a focus on plasma supply processes for planetary magnetospheres. (10.1007/s11214-015-0170-y)
  DOI : 10.1007/s11214-015-0170-y
• Distribution of energetic oxygen and hydrogen in the near-Earth plasma sheet
  
  • Kronberg E. A.
  • Grigorenko E. E.
  • Haaland S. E.
  • Daly P. W.
  • Delcourt Dominique C.
  • Luo H.
  • Kistler L. M.
  • Dandouras I.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015, 120 (5), pp.3415-3431. The spatial distributions of different ion species are useful indicators for plasma sheet dynamics. In this statistical study based on 7 years of Cluster observations, we establish the spatial distributions of oxygen ions and protons at energies from 274 to 955 keV, depending on geomagnetic and solar wind (SW) conditions. Compared with protons, the distribution of energetic oxygen has stronger dawn-dusk asymmetry in response to changes in the geomagnetic activity. When the interplanetary magnetic field (IMF) is directed southward, the oxygen ions show significant acceleration in the tail plasma sheet. Changes in the SW dynamic pressure (P<SUB>dyn</SUB>) affect the oxygen and proton intensities in the same way. The energetic protons show significant intensity increases at the near-Earth duskside during disturbed geomagnetic conditions, enhanced SW P<SUB>dyn</SUB>, and southward IMF, implying there location of effective inductive acceleration mechanisms and a strong duskward drift due to the increase of the magnetic field gradient in the near-Earth tail. Higher losses of energetic ions are observed in the dayside plasma sheet under disturbed geomagnetic conditions and enhanced SW P<SUB>dyn</SUB>. These observations are in agreement with theoretical models. (10.1002/2014JA020882)
  DOI : 10.1002/2014JA020882
• Direct identification of predator-prey dynamics in gyrokinetic simulations
  
  • Kobayashi Sumire
  • Gürcan Özgür D.
  • Diamond P.H.
  Physics of Plasmas, American Institute of Physics, 2015, 22 (9), pp.090702. The interaction between spontaneously formed zonal flows and small-scale turbulence in nonlinear gyrokinetic simulations is explored in a shearless closed field line geometry. It is found that when clear limit cycle oscillations prevail, the observed turbulent dynamics can be quantitatively captured by a simple Lotka-Volterra type predator-prey model. Fitting the time traces of full gyrokinetic simulations by such a reduced model allows extraction of the model coefficients. Scanning physical plasma parameters, such as collisionality and density gradient, it was observed that the effective growth rates of turbulence (i.e., the prey) remain roughly constant, in spite of the higher and varying level of primary mode linear growth rates. The effective growth rate that was extracted corresponds roughly to the zonal-flow-modified primary mode growth rate. It was also observed that the effective damping of zonal flows (i.e., the predator) in the parameter range, where clear predator-prey dynamics is observed, (i.e., near marginal stability) agrees with the collisional damping expected in these simulations. This implies that the Kelvin-Helmholtz-like instability may be negligible in this range. The results imply that when the tertiary instability plays a role, the dynamics becomes more complex than a simple Lotka-Volterra predator prey. (10.1063/1.4930127)
  DOI : 10.1063/1.4930127
• Strong Ionization Asymmetry in a Geometrically Symmetric Radio Frequency Capacitively Coupled Plasma Induced by Sawtooth Voltage Waveforms
  
  • Bruneau Bastien
  • Gans T.
  • O'Connell D.
  • Greb Arthur
  • Johnson E.V.
  • Booth Jean-Paul
  Physical Review Letters, American Physical Society, 2015, 114, pp.125002. The ionization dynamics in geometrically symmetric parallel plate capacitively coupled plasmas driven by radio frequency tailored voltage waveforms is investigated using phase resolved optical emission spectroscopy (PROES) and particle-in-cell (PIC) simulations. Temporally asymmetric waveforms induce spatial asymmetries and offer control of the spatiotemporal dynamics of electron heating and associated ionization structures. Sawtooth waveforms with different rise and fall rates are employed using truncated Fourier series approximations of an ideal sawtooth. Experimental PROES results obtained in argon plasmas are compared with PIC simulations, showing excellent agreement. With waveforms comprising a fast voltage drop followed by a slower rise, the faster sheath expansion in front of the powered electrode causes strongly enhanced ionization in this region. The complementary waveform causes an analogous effect in front of the grounded electrode. (10.1103/PhysRevLett.114.125002)
  DOI : 10.1103/PhysRevLett.114.125002
• Predator-prey model for the self-organization of stochastic oscillators in dual populations.
  
  • Moradi S.
  • Anderson J.
  • Gürcan Özgür D.
  Physical Review E, American Physical Society (APS), 2015, 92, pp.06293. A predator-prey model of dual populations with stochastic oscillators is presented. A linear cross-coupling between the two populations is introduced following the coupling between the motions of a Wilberforce pendulum in two dimensions: one in the longitudinal and the other in torsional plain. Within each population a Kuramoto-type competition between the phases is assumed. Thus, the synchronization state of the whole system is controlled by these two types of competitions. The results of the numerical simulations show that by adding the linear cross-coupling interactions predator-prey oscillations between the two populations appear, which results in self-regulation of the system by a transfer of synchrony between the two populations. The model represents several important features of the dynamical interplay between the drift wave and zonal flow turbulence in magnetically confined plasmas, and a novel interpretation of the coupled dynamics of drift wave-zonal flow turbulence using synchronization of stochastic oscillator is discussed. (10.1103/PhysRevE.92.062930)
  DOI : 10.1103/PhysRevE.92.062930
• Turbulence elasticity: a key concept to a unified paradigm of L -> I -> H transition
  
  • Guo Z. B.
  • Diamond P.H.
  • Kosuga Y.
  • Gürcan Özgür D.
  Nuclear Fusion, IOP Publishing, 2015, 55 (4), pp.043022. We present a theory of turbulence elasticity, which follows from delayed response of drift waves (DWs) to zonal flow (ZF) shears. It is shown that when |&#12296;V&#12297;'ZF|/&#916;&#969;k &#8805; 1, with |&#12296;V&#12297;'ZF| the ZF shearing rate and &#916;&#969;k the local turbulence decorrelation rate, the ZF evolution equation is converted from a diffusion equation to a telegraph equation. This insight provides a natural framework for understanding temporally periodic ZF structures, e.g., propagation of the ZF/turbulence intensity fronts. Furthermore, by incorporating the elastic property of the DWZF turbulence, we propose a unified paradigm of low-confinement-mode to intermediate-confinement-mode to high-confinement-mode (L &#8594; I &#8594; H) transitions. In particular, we predict the onset and termination conditions of the limit cycle oscillations, i.e. the I-mode. The transition from an unstable L-mode to I-mode is predicted to occur when &#916;&#969;k < |&#12296;V&#12297;'ZF|<&#12296;V&#12297;'cr, where &#12296;V&#12297;'cr is a critical flow shearing rate and is derived explicitly. If |&#12296;V&#12297;'E×B| > &#12296;V&#12297;'cr(&#12296;V&#12297;E×B is mean E × B shear flow driven by edge radial electrostatic field), the I-mode will terminate and spiral into the H-mode. (10.1088/0029-5515/55/4/043022)
  DOI : 10.1088/0029-5515/55/4/043022
• Magnetic noise contribution of the ferromagnetic core of induction magnetometers
  
  • Coillot C
  • El Moussalim M
  • Brun E
  • Rhouni A
  • Lebourgeois R
  • Sou Gérard
  • Mansour Malik
  Journal of Sensors and Sensor Systems, Copernicus Publ, 2015, 4, pp.229 - 237. The performance of induction magnetometers, in terms of resolution, depends both on the induction sensor and the electronic circuit. To investigate accurately the sensor noise sources, an induction sensor, made of a ferrite ferromagnetic core, is combined with a dedicated low voltage and current noise preamplifier, designed in CMOS 0.35 µm technology. A modelling of the contribution of the ferromagnetic core to the noise through the complex permeability formalism is performed. Its comparison with experimental measurements highlight another possible source for the dominating noise near the resonance. (10.5194/jsss-4-229-2015)
  DOI : 10.5194/jsss-4-229-2015
• Wide-banded NTC radiation: local to remote observations by the four Cluster satellites
  
  • Décréau Pierrette
  • Aoutou S.
  • Denazelle A.
  • Galkina I.
  • Rauch Jean-Louis
  • Vallières Xavier
  • Canu Patrick
  • Rochel Grimald S.
  • El-Lemdani Mazouz Farida
  • Darrouzet F.
  Annales Geophysicae, European Geosciences Union, 2015, 33 (10), pp.1285-1300. The Cluster multi-point mission offers a unique collection of non-thermal continuum (NTC) radio waves observed in the 2-80 kHz frequency range over almost 15 years, from various view points over the radiating plasmasphere. Here we present rather infrequent case events, such as when primary electrostatic sources of such waves are embedded within the plasmapause boundary far from the magnetic equatorial plane. The spectral signature of the emitted electromagnetic waves is structured as a series of wide harmonic bands within the range covered by the step in plasma frequency encountered at the boundary. Developing the concept that the frequency distance df between harmonic bands measures the magnetic field magnitude B at the source (df = F<SUB>ce</SUB>, electron gyrofrequency), we analyse three selected events. The first one (studied in Grimald et al., 2008) presents electric field signatures observed by a Cluster constellation of small size (~ 200 to 1000 km spacecraft separation) placed in the vicinity of sources. The electric field frequency spectra display frequency peaks placed at frequencies fs = n df (n being an integer), with df of the order of F<SUB>ce</SUB> values encountered at the plasmapause by the spacecraft. The second event, taken from the Cluster tilt campaign, leads to a 3-D view of NTC waves ray path orientations and to a localization of a global source region at several Earth radii (R<SUB>E</SUB>) from Cluster (Décréau et al., 2013). The measured spectra present successive peaks placed at fs ~ (n 1/2) df. Next, considering if both situations might be two facets of the same phenomenon, we analyze a third event. The Cluster fleet, configured into a constellation of large size (~ 8000 to 25 000 km spacecraft separation), allows us to observe wide-banded NTC waves at different distances from their sources. Two new findings can be derived from our analysis. First, we point out that a large portion of the plasmasphere boundary layer, covering a large range of magnetic latitudes, is radiating radio waves. The radio waves are issued from multiple sources of small size, each related to a given fs series and radiating inside a beam of narrow cone angle, referred to as a beamlet. The beamlets illuminate different satellites simultaneously, at different characteristic fs values, according to the latitude at which the satellite is placed. Second, when an observing satellite moves away from its assumed source region (the plasmapause surface), it is illuminated by several beamlets, issued from nearby sources with characteristic fs values close to each other. The addition of radio waves blurs the spectra of the overall received electric field. It can move the signal peaks such that their position fs satisfiesfs = (n alpha) df, with 0 < alpha < 1. These findings open new perspectives for the interpretation of NTC events displaying harmonic signatures. (10.5194/angeo-33-1285-2015)
  DOI : 10.5194/angeo-33-1285-2015
• Is collisionless heating in capacitively coupled plasmas really collisionless?
  
  • Lafleur Trevor
  • Chabert Pascal
  Plasma Sources Science and Technology, IOP Publishing, 2015, 24 (4), pp.044002. By performing a combination of test-particle and particle-in-cell simulations, we investigate electron heating in single frequency capacitively coupled plasmas (CCPs). In agreement with previous theoretical considerations highlighted in Kaganovich et al (1996 Appl. Phys. Lett. 69 3818), we show that the level of true collisionless/stochastic heating in typical CCPs is significantly smaller than that due to collisional interactions; even at very low pressures and wide gap lengths. Fundamentally electron heating is a collisional phenomenon whereby particle collisions provide the vital phase randomization and stochastization mechanism needed to generate both a local (or ohmic) heating component, and a non-local (or hybrid) heating component. (10.1088/0963-0252/24/4/044002)
  DOI : 10.1088/0963-0252/24/4/044002
• Plasma acceleration using a radio frequency self-bias effect
  
  • Rafalskyi D.V.
  • Aanesland Ane
  Physics of Plasmas, American Institute of Physics, 2015, 22 (6), pp.063502. In this work plasma acceleration using a RF self-bias effect is experimentally studied. The experiments are conducted using a novel plasma accelerator system, called Neptune, consisting of an inductively coupled plasma source and a RF-biased set of grids. The plasma accelerator can operate in a steady state mode, producing a plasma flow with separately controlled plasma flux and velocity without any magnetic configuration. The operating pressure at the source output is as low as 0.2&#8201;mTorr and can further be decreased. The ion and electron flows are investigated by measuring the ion and electron energy distribution functions both space resolved and with different orientations with respect to the flow direction. It is found that the flow of electrons from the source is highly anisotropic and directed along the ion flow and this global flow of accelerated plasma is well localized in the plasma transport chamber. The maximum flux is about 7.5·1015 ions s&#8722;1 m&#8722;2 (at standard conditions) on the axis and decreasing to almost zero at a radial distances of more than 15&#8201;cm from the flow axis. Varying the RF acceleration voltage in the range 20350&#8201;V, the plasma flow velocity can be changed between 10 and 35&#8201;km/s. The system is prospective for different technology such as space propulsion and surface modification and also interesting for fundamental studies for space-related plasma simulations and investigation of the dynamo effect using accelerated rotating plasmas. I. INTRODUCT (10.1063/1.4922065)
  DOI : 10.1063/1.4922065
• Numerical experiment to estimate the validity of negative ion diagnostic using photo-detachment combined with Langmuir probing
  
  • Oudini N.
  • Sirse Nishant
  • Benallal R.
  • Taccogna F.
  • Aanesland Ane
  • Bendib A.
  • Ellingboe A. R.
  Physics of Plasmas, American Institute of Physics, 2015, 22 (7), pp.073509. This paper presents a critical assessment of the theory of photo-detachment diagnostic method used to probe the negative ion density and electronegativity &#945;&#8201;=&#8201;n-/ne. In this method, a laser pulse is used to photo-detach all negative ions located within the electropositive channel (laser spot region). The negative ion density is estimated based on the assumption that the increase of the current collected by an electrostatic probe biased positively to the plasma is a result of only the creation of photo-detached electrons. In parallel, the background electron density and temperature are considered as constants during this diagnostics. While the numerical experiments performed here show that the background electron density and temperature increase due to the formation of an electrostatic potential barrier around the electropositive channel. The time scale of potential barrier rise is about 2&#8201;ns, which is comparable to the time required to completely photo-detach the negative ions in the electropositive channel (&#8764;3&#8201;ns). We find that neglecting the effect of the potential barrier on the background plasma leads to an erroneous determination of the negative ion density. Moreover, the background electron velocity distribution function within the electropositive channel is not Maxwellian. This is due to the acceleration of these electrons through the electrostatic potential barrier. In this work, the validity of the photo-detachment diagnostic assumptions is questioned and our results illustrate the weakness of these assumptions. (10.1063/1.4926826)
  DOI : 10.1063/1.4926826
• 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
• Solar illumination control of ionospheric outflow above polar cap arcs
  
  • Maes Lukas
  • Maggiolo R.
  • de Keyser J.
  • Dandouras I.
  • Fear R. C.
  • Fontaine Dominique
  • Haaland S.
  Geophysical Research Letters, American Geophysical Union, 2015, 42 (5), pp.1304-1311. We measure the flux density, composition, and energy of outflowing ions above the polar cap, accelerated by quasi-static electric fields parallel to the magnetic field and associated with polar cap arcs, using Cluster. Mapping the spacecraft position to its ionospheric foot point, we analyze the dependence of these parameters on the solar zenith angle (SZA). We find a clear transition at SZA between 94° and 107°, with the O<SUP> </SUP> flux higher above the sunlit ionosphere. This dependence on the illumination of the local ionosphere indicates that significant O<SUP> </SUP> upflow occurs locally above the polar ionosphere. The same is found for H<SUP> </SUP>, but to a lesser extent. This effect can result in a seasonal variation of the total ion upflow from the polar ionosphere. Furthermore, we show that low-magnitude field-aligned potential drops are preferentially observed above the sunlit ionosphere, suggesting a feedback effect of ionospheric conductivity. (10.1002/2014GL062972)
  DOI : 10.1002/2014GL062972