Publications

2011

• Multi-spacecraft investigation of space turbulence: lessons from Cluster and input to the Cross- Scale mission
  
  • Sahraoui Fouad
  • Goldstein M.L.
  • Belmont Gérard
  • Roux A.
  • Rezeau Laurence
  • Canu Patrick
  • Robert Patrick
  • Cornilleau-Wehrlin Nicole
  • Le Contel Olivier
  • Dudok de Wit Thierry
  • Pinçon Jean-Louis
  • Kiyani K.
  Planetary and Space Science, Elsevier, 2011, 59 (7), pp.Pages 585-591. Investigating space plasma turbulence from single-point measurements is known to be characterized by unavoidable ambiguities in disentangling temporal and spatial variations. Solving this problem has been one of the major goals of the Cluster mission. For that purpose multipoint measurements techniques, such as the $k$-filtering, have been developed. Such techniques combine several time series recorded simultaneously at different points in space to estimate the corresponding energy density in the wavenumber space. Here we apply the technique to both simulated and Cluster magnetometer data in the solar wind (SW) and investigate the errors and limitations that arise due to the separation of the spacecraft and the quality of the tetrahedral configuration. Specifically, we provide an estimation of the minimum and maximum scales that can be accurately measured given a specific distance between the satellites and show the importance of the geometry of the tetrahedron and the relationship of that geometry to spatial aliasing. We also present recent results on characterizing small scale SW turbulence and provide scientific arguments supporting the need of new magnetometers having better sensitivity than the existing ones. Throughout the paper we emphasize technical challenges and their solutions that can be considered for a better preparation of the Cross- Scale mission. (10.1016/j.pss.2010.06.001)
  DOI : 10.1016/j.pss.2010.06.001
• Three-dimensional spatial structures of solar wind turbulence from 10 000-km to 100-km scales
  
  • Narita Y.
  • Glassmeier K.-H.
  • Goldstein M. L.
  • Motschmann U.
  • Sahraoui Fouad
  Annales Geophysicae, European Geosciences Union, 2011, 29, pp.1731-1738. Using the four Cluster spacecraft, we have determined the three-dimensional wave-vector spectra of fluctuating magnetic fields in the solar wind. Three different solar wind intervals of Cluster data are investigated for this purpose, representing three different spatial scales: 10 000 km, 1000 km, and 100 km. The spectra are determined using the wave telescope technique (k-filtering technique) without assuming the validity of Taylor's frozen-in-flow hypothesis nor are any assumptions made as to the symmetry properties of the fluctuations. We find that the spectra are anisotropic on all the three scales and the power is extended primarily in the directions perpendicular to the mean magnetic field, as might be expected of two-dimensional turbulence, however, the analyzed fluctuations are not axisymmetric. The lack of axisymmetry invalidates some earlier techniques using single spacecraft observations that were used to estimate the percentage of magnetic energy residing in quasi-two-dimensional power. However, the dominance of two-dimensional turbulence is consistent with the relatively long mean free paths of cosmic rays in observed in the heliosphere. On the other hand, the spectra also exhibit secondary extended structures oblique from the mean magnetic field direction. We discuss possible origins of anisotropy and asymmetry of solar wind turbulence spectra. (10.5194/angeo-29-1731-2011)
  DOI : 10.5194/angeo-29-1731-2011
• Kelvin-Helmholtz waves under southward interplanetary magnetic field
  
  • Hwang K.-J.
  • Kuznetsova M. M.
  • Sahraoui Fouad
  • Goldstein M. L.
  • Lee E.
  • Parks G. K.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2011, 116, pp.8210. The Kelvin-Helmholtz waves have been observed along the Earth's low-latitude magnetopause and have been suggested to play a certain role in the entry of solar wind plasma into Earth's magnetosphere. In situ observations of the KH waves (KHW) and, in particular, a nonlinear stage of the KH instability, i.e., rolled-up KH vortices (KHVs), have been reported to occur preferentially for northward interplanetary magnetic field (IMF). Using Cluster data, we present the first in situ observation of nonlinearly developed KHW during southward IMF. The analysis reveals that there is a mixture of less-developed and more-developed KHW that shows inconsistent variations in scale size and the magnetic perturbations in the context of the expected evolution of KH structures. A coherence analysis implies that the observed KHW under southward IMF appear to be irregular and intermittent. These irregular and turbulent characteristics are more noticeable than previously reported KHW events that have been detected preferentially during northward IMF. This suggests that under southward IMF KHVs become easily irregular and temporally intermittent, which might explain the preferential in situ detection of KHVs when the IMF is northward. MHD simulation of the present event shows that during southward IMF dynamically active subsolar environments can cause KHV that evolve with considerable intermittency. The MHD simulations appear to reproduce well the qualitative features of the Cluster observations. (10.1029/2011JA016596)
  DOI : 10.1029/2011JA016596
• On the dispersion features of whistler waves in almost pure ion plasmas
  
  • Lundin B. V.
  • Krafft C.
  Physics of Plasmas, American Institute of Physics, 2011, 18, pp.102114. It is shown that in a multi-ion plasma of moderate density enriched by a large amount of negatively charged ions and/or heavy particulates, the lower cutoff frequencies of the electron whistler and the Z-mode (extraordinary) waves tend to each other, approaching the ion plasma frequency omegapi. The evolution of the dispersion curves omega(k, theta) of both wave modes is studied as a function of the relative electron density, beginning from the case of a moderate density plasma with comparable values of electron gyro- and plasma frequencies and omegapi essentially exceeding the ions' gyrofrequencies. When the fraction of free electrons is very small the transparency frequency domain of the electron whistler mode becomes very narrow, being located in the vicinity of omegapi. If the negatively charged ions have the smallest specific charges among other ion species then, under similar conditions, the so-called crossover effect and the accompanying polarization sense reverse can occur at frequencies essentially greater than the ions' gyrofrequencies. The revealed effects are characteristic of plasmas with excess of electronegative gas molecules or dust particulates highly adhesive to electrons, i.e., almost pure ion plasmas and/or pair-ion plasmas with extra small fractions of free electrons. Moreover, it is found that the vanishingly small electron fraction providing the same value of the electron whistler ion cutoff frequency normalized to the ion plasma frequency is essentially less for pair fullerene C<SUB>60</SUB><SUP> /-</SUP> than for pair hydrogen H /- plasmas. The technique to determine omegapi using the evolution of the whistler wave magnetic field polarization ellipse is also described. (10.1063/1.3650075)
  DOI : 10.1063/1.3650075
• Collective Light Scattering for the Study of Fluctuations in Magnetized Plasmas: The Hall Thruster Case Study
  
  • Tsikata Sedina
  • Honoré Cyrille
  • Grésillon D.
  • Lemoine N.
  Contributions to Plasma Physics, Wiley-VCH Verlag, 2011, 51 (2-3), pp.119-125. Collective light scattering (CLS) has been recently extended to the observation of Hall effect thruster plasmas. A better understanding of the physics of Hall thrusters is key to improving thruster operation and lifetimes. In particular, the origins of thruster phenomena such as anomalous electron transport need to be determined. A high-performance CLS diagnostic has therefore been developed for identifying and measuring electron density fluctuations in the thruster, at millimetric length scales and MHz frequencies. Such modes are believed to play a role in anomalous transport, and experiments so far performed have provided information on their dispersion relations, amplitude and directivity. This work describes the technical aspects of the optical bench and the range of accessible experiments (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) (10.1002/ctpp.201000057)
  DOI : 10.1002/ctpp.201000057
• Numerical analysis of electronegative plasma in the extraction region of negative hydrogen ion sources
  
  • Kuppel S.
  • Matsushita D.
  • Hatayama A.
  • Bacal M.
  Journal of Applied Physics, American Institute of Physics, 2011, 109 (1), pp.013305. This numerical study focuses on the physical mechanisms involved in the extraction of volume-produced H&#8722; ions from a steady state laboratory negative hydrogen ion source with one opening in the plasma electrode (PE) on which a dc-bias voltage is applied. A weak magnetic field is applied in the source plasma transversely to the extracted beam. The goal is to highlight the combined effects of the weak magnetic field and the PE bias voltage (upon the extraction process of H&#8722; ions and electrons). To do so, we focus on the behavior of electrons and volume-produced negative ions within a two-dimensional model using the particle-in-cell method. No collision processes are taken into account, except for electron diffusion across the magnetic field using a simple random-walk model at each time step of the simulation. The results show first that applying the magnetic field (without PE bias) enhances H&#8722; ion extraction, while it drastically decreases the extracted electron current. Secondly, the extracted H&#8722; ion current has a maximum when the PE bias is equal to the plasma potential, while the extracted electron current is significantly reduced by applying the PE bias. The underlying mechanism leading to the above results is the gradual opening by the PE bias of the equipotential lines towards the parts of the extraction region facing the PE. The shape of these lines is due originally to the electron trapping by the magnetic field. (10.1063/1.3530454)
  DOI : 10.1063/1.3530454
• Electron-cylotron maser radiation from electron holes: upward current region
  
  • Treumann R. A.
  • Baumjohann W.
  • Pottelette R.
  Annales Geophysicae, European Geosciences Union, 2011, 29, pp.1885-1904. Electron holes are suggested to be an important source for generation of electron-cyclotron maser radiation. We demonstrate that electron holes generated in a ring-horseshoe distribution in the auroral-kilometric radiation source region have the capacity to emit band-limited radiation. The radiation is calculated in the proper frame of a circular model hole and shown to be strictly perpendicular in this frame. Its bandwidth under auroral conditions is of the order of ~1 kHz, which is a reasonable value. It is also shown that much of the drift of fine structure in the radiation can be interpreted as Doppler shift. Estimates based on data are in good agreement with theory. Growth and absorption rates have been obtained for the emitted radiation. However, the growth rate of a single hole obtained under conservative conditions is small, too small for reproducing the observed fine structure flux. Trapping of radiation inside the hole for the hole's lifetime helps amplifying the radiation additionally but introduces other problems. This entire set of questions is discussed at length and compared to radiation from the global horseshoe distribution. The interior of the hole produces a weak absorption at slightly higher frequency than emission. The absorptivity is roughly two orders of magnitude below the growth rate of the radiation thus being weak even when the emission and absorption bands overlap. Transforming to the stationary observer's frame it is found that the radiation becomes oblique against the magnetic field. For approaching holes the radiated frequencies may even exceed the local electron cyclotron frequency. (10.5194/angeo-29-1885-2011)
  DOI : 10.5194/angeo-29-1885-2011
• On plasma rotation with toroidal magnetic field ripple and no external momentum input
  
  • Fenzi C.
  • Garbet X.
  • Trier Elisée
  • Hennequin Pascale
  • Bourdelle C.
  • Aniel Thierry
  • Colledani G.
  • Devynck P.
  • Gil C.
  • Gürcan Özgür D.
  • Manenc L.
  • Schneider M.
  • Segui J.-L.
  • Tore Supra Team
  Nuclear Fusion, IOP Publishing, 2011, 51, pp.103038. Ripple-induced thermal loss effect on plasma rotation is investigated in a set of Ohmic L-mode plasmas performed in Tore Supra, and comparisons with neoclassical predictions including ripple are performed. Adjusting the size of the plasma, the ripple amplitude has been varied from 0.5% to 5.5% at the plasma boundary, keeping the edge safety factor constant. The toroidal flow dynamics is understood as being likely dominated by turbulence transport driven processes at low ripple amplitude, while the ripple-induced toroidal friction becomes dominant at high ripple. In the latter case, the velocity tends remarkably towards the neoclassical prediction (counter-current rotation). The radial electric field is not affected by the ripple variation and remains well described by its neoclassical prediction. Finally, the poloidal velocity is of the order of the neoclassical prediction at high ripple amplitude, but significantly departs from it at low ripple. (10.1088/0029-5515/51/10/103038)
  DOI : 10.1088/0029-5515/51/10/103038
• Discharge resistance and power dissipation in the self-pulsing regime of micro-hollow cathode discharges
  
  • Lazzaroni Claudia
  • Chabert Pascal
  Plasma Sources Science and Technology, IOP Publishing, 2011, 20, pp.055004. Micro-hollow cathode discharges (MHCDs), driven by a dc voltage source, may operate in a self-pulsing regime during which the voltage and the current across the discharge are pulsed with a frequency of several tens of kilohertz. A model for the self-pulsing regime of MHCDs is proposed based on an equivalent electrical circuit of the whole device. The discharge itself is modeled by a non-linear resistance in series with an inductance, and it is placed in parallel with the capacitance of the electrodedielectricelectrode sandwich hosting the discharge micro-hole. The capacitance of the coaxial cable used to feed the device is also included. It is shown that a detailed comparison between theory and experiments allows the discharge non-linear resistance to be accurately determined. When the discharge current is maximum, this resistance is about 2500&#8201;&#937; at 50&#8201;Torr and decreases to about 750&#8201;&#937; at 150&#8201;Torr. The absorbed power is calculated throughout the self-pulsing cycle and may be above 100&#8201;W at the current peak, while the time-averaged power remains a fraction of a Watt. (10.1088/0963-0252/20/5/055004)
  DOI : 10.1088/0963-0252/20/5/055004
• Modeling the superstorm in November 2003
  
  • Fok M.-C. H.
  • Moore T. E.
  • Slinker Steve P.
  • Fedder Joel A.
  • Delcourt Dominique C.
  • Nosé Masahito
  • Chen Sheng-Hsien
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2011, 116. The superstorm on 20-21 November 2003 was the largest geomagnetic storm in solar cycle 23 as measured by Dst, which attained a minimum value of -422 nT. We have simulated this storm to understand how particles originating from the solar wind and ionosphere get access to the magnetosphere and how the subsequent transport and energization processes contribute to the buildup of the ring current. The global electromagnetic configuration and the solar wind H<SUP> </SUP> distribution are specified by the Lyon-Fedder-Mobarry (LFM) magnetohydrodynamics model. The outflow of H<SUP> </SUP> and O<SUP> </SUP> ions from the ionosphere are also considered. Their trajectories in the magnetosphere are followed by a test-particle code. The particle distributions at the inner plasma sheet established by the LFM model and test-particle calculations are then used as boundary conditions for a ring current model. Our simulations reproduce the rapid decrease of Dst during the storm main phase and the fast initial phase of recovery. Shielding in the inner magnetosphere is established at early main phase. This shielding field lasts several hours and then breaks down at late main phase. At the peak of the storm, strong penetration of ions earthward to L shell of 1.5 is revealed in the simulation. It is surprising that O<SUP> </SUP> is significant but not the dominant species in the ring current in our calculation for this major storm. It is very likely that substorm effects are not well represented in the models and O<SUP> </SUP> energization is underestimated. Ring current simulation with O<SUP> </SUP> energy density at the boundary set comparable to Geotail observations produces excellent agreement with the observed symH. As expected in superstorms, ring current O<SUP> </SUP> is the dominant species over H<SUP> </SUP> during the main to midrecovery phase of the storm. (10.1029/2010JA015720)
  DOI : 10.1029/2010JA015720
• Collisionality scaling in Tore Supra: detailed energy confinement analysis, turbulence measurements and gyrokinetic modelling
  
  • Bourdelle C.
  • Gerbaud T.
  • Vermare Laure
  • Casati A.
  • Aniel Thierry
  • Artaud J.F.
  • Basiuk Vincent
  • Bucalossi J.
  • Clairet F.
  • Corre Y.
  • Devynck P.
  • Falchetto G.
  • Fenzi C.
  • Garbet X.
  • Guirlet R.
  • Gürcan Özgür D.
  • Heuraux S.
  • Hennequin Pascale
  • Hoang G.T.
  • Imbeaux Frédéric
  • Manenc L.
  • Monier-Garbet P.
  • Moreau P.
  • Sabot R.
  • Segui J.-L.
  • Sirinelli A.
  • Villegas D.
  • Tore Supra Team
  Nuclear Fusion, IOP Publishing, 2011, 51, pp.063037. This paper presents the first observation of geodesic acoustic modes (GAMs) on Tore Supra plasmas. Using the Doppler backscattering system, the oscillations of the plasma flow velocity, localized between r/a = 0.85 and r/a = 0.95, and with a frequency, typically around 10 kHz, have been observed at the plasma edge in numerous discharges. When the additional heating power is varied, the frequency is found to scale with Cs/R. The MUltiple SIgnal Classification (MUSIC) algorithm is employed to access the temporal evolution of the perpendicular velocity of density fluctuations. The method is presented in some detail, and is validated and compared against standard methods, such as the conventional fast Fourier transform method, using a synthetic signal. It stands out as a powerful data analysis method to follow the Doppler frequency with a high temporal resolution, which is important in order to extract the dynamics of GAMs. (10.1088/0029-5515/51/6/063037)
  DOI : 10.1088/0029-5515/51/6/063037
• Wavenumber spectrum of micro-turbulence in tokamak plasmas / Spectre en nombre d'onde de la micro-turbulence dans les plasmas de tokamak
  
  • Vermare Laure
  • Gürcan Özgür D.
  • Hennequin Pascale
  • Honoré Cyrille
  • Garbet X.
  • Giacalone J-C.
  • Sabot R.
  • Clairet F.
  • Tore Supra Team
  Comptes Rendus. Physique, Académie des sciences (Paris), 2011, 12 (2), pp.115 - 122. A better understanding of turbulent transport in a tokamak plasma requires precise comparisons between experimental observation and theoretical prediction of micro-turbulence characteristics. The repartition of fluctuation energy over different spatial scales, which contains detailed information about the character of underlying instabilities and the mechanisms involved in energy transfer between different scales, is one of the few quantities allowing a high detail comparison. The present article reports the investigation performed on the Tore Supra tokamak on the wavenumber spectrum of micro-turbulence using Doppler backscattering. The theoretical approach consists of the derivation of spectral models that include interactions between fluctuations and large scale flow structures. Une meilleure compréhension du transport turbulent dans les plasmas de tokamak exige des comparaisons précises entre les observations expérimentales et les prédictions théoriques des caractéristiques de la micro-turbulence. La répartition de l&#700;énergie des fluctuations sur les différentes échelles spatiales, qui contient des informations sur le type instabilités sous-jacentes et sur les mécanismes de transfert d&#700;énergie entre échelles spatiales, est l&#700;une des rares quantités permettant une comparaison de niveau élevé. Cet article présente le travail mené sur le tokamak Tore Supra sur l&#700;étude du spectre en nombre d&#700;onde de la micro-turbulence mesuré par rétro-diffusion Doppler. L&#700;approche théorique consiste en la dérivation de modéles spectraux qui inclut les interactions entre les fluctuations et les structures d&#700;écoulement de grandes échelles. (10.1016/j.crhy.2010.11.003)
  DOI : 10.1016/j.crhy.2010.11.003
• Proton acceleration in antiparallel collisionless magnetic reconnection: Kinetic mechanisms behind the fluid dynamics
  
  • Aunai Nicolas
  • Belmont Gérard
  • Smets Roch
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2011, 116, pp.9232. This paper investigates the proton kinetic mechanisms leading to the formation of plasma jets in antiparallel magnetic reconnection. In particular, the interaction of the protons with the Hall electric field in the proton non-ideal region is discussed. The study, based on a two-dimensional hybrid simulation, details the important role of the proton pressure force in the acceleration process and its role in maintaining open and steady the proton outflow channel. When no fluid closure is assumed, it is found that this force arises from a strong anisotropy in velocity space which comes from kinetic effect. By analyzing the distribution functions and the individual particle dynamics, it is shown that the mixing of protons bouncing in a divergent electrostatic potential well associated to the Hall effect statistically couples the two in-plane velocity components of the particles. This coupling results, from the macroscopic point of view, in off-diagonal components of the pressure tensor. (10.1029/2011JA016688)
  DOI : 10.1029/2011JA016688
• The proton pressure tensor as a new proxy of the proton decoupling region in collisionless magnetic reconnection
  
  • Aunai Nicolas
  • Retinò Alessandro
  • Belmont Gérard
  • Smets Roch
  • Lavraud B.
  • Vaivads A.
  Annales Geophysicae, European Geosciences Union, 2011, 29, pp.1571-1579. Cluster data is analyzed to test the proton pressure tensor variations as a proxy of the proton decoupling region in collisionless magnetic reconnection. The Hall electric potential well created in the proton decoupling region results in bounce trajectories of the protons which appears as a characteristic variation of one of the in-plane off-diagonal components of the proton pressure tensor in this region. The event studied in this paper is found to be consistent with classical Hall field signatures with a possible 20% guide field. Moreover, correlations between this pressure tensor component, magnetic field and bulk flow are proposed and validated, together with the expected counterstreaming proton distribution functions. (10.5194/angeo-29-1571-2011)
  DOI : 10.5194/angeo-29-1571-2011
• Vibrational relaxation of N<SUB>2</SUB> studied by IR titration with time-resolved Qantum Cascade Laser diagnostics
  
  • Marinov Daniil
  • Lopatik D.
  • Guaitella Olivier
  • Roepcke J.
  • Rousseau Antoine
  , 2011.
• Stochastic treatment of finite-N effects in mean-field systems and its application to the lifetimes of coherent structures
  
  • Ettoumi Wahb
  • Firpo Marie-Christine
  Physical Review E, American Physical Society (APS), 2011, 84, pp.030103(R). A stochastic treatment yielding to the derivation of a general Fokker-Planck equation is presented to model the slow convergence toward equilibrium of mean-field systems due to finite-N effects. The thermalization process involves notably the disintegration of coherent structures that may sustain out-of-equilibrium quasistationary states. The time evolution of the fraction of particles remaining close to a mean-field potential trough is analytically computed. This indicator enables to estimate the lifetime of coherent structures and thermalization time scale in mean-field systems. (10.1103/PhysRevE.84.030103)
  DOI : 10.1103/PhysRevE.84.030103
• Predator prey oscillations in a simple cascade model of drift wave turbulence
  
  • Berionni Vincent
  • Gürcan Özgür D.
  Physics of Plasmas, American Institute of Physics, 2011, 18, pp.112301. A reduced three shell limit of a simple cascade model of drift wave turbulence, which emphasizes nonlocal interactions with a large scale mode, is considered. It is shown to describe both the well known predator prey dynamics between the drift waves and zonal flows and to reduce to the standard three wave interaction equations. Here, this model is considered as a dynamical system whose characteristics are investigated. The analytical solutions for the purely nonlinear limit are given in terms of the Jacobi elliptic functions. An approximate analytical solution involving Jacobi elliptic functions and exponential growth is computed using scale separation for the case of unstable solutions that are observed when the energy injection rate is high. The fixed points of the system are determined, and the behavior around these fixed points is studied. The system is shown to display periodic solutions corresponding to limit cycle oscillations, apparently chaotic phase space orbits, as well as unstable solutions that grow slowly while oscillating rapidly. The period doubling route to transition to chaos is examined. (10.1063/1.3656953)
  DOI : 10.1063/1.3656953
• Whistler eigenmodes of magnetic flux tubes in a magnetoplasma
  
  • Kudrin A. V.
  • Bakharev P. V.
  • Zaboronkova T. M.
  • Krafft C.
  Plasma Physics and Controlled Fusion, IOP Publishing, 2011, 53, pp.065005. Guided propagation of whistler waves along cylindrical non-uniformities of a dc magnetic field is studied within the framework of a full-wave approach. Conditions are revealed under which such guiding structures, commonly known as magnetic flux tubes, can support volume and surface eigenmodes in the whistler range. The dispersion properties and field structures of whistler eigenmodes guided by flux tubes with an enhanced magnetic field are calculated and analysed for plasma parameters typical of laboratory experiments. The results obtained are useful in understanding the basic features of whistler wave guidance by magnetic flux tubes and can be applied to interpreting the data of the relevant experiments. (10.1088/0741-3335/53/6/065005)
  DOI : 10.1088/0741-3335/53/6/065005
• Control of Nanocrystalline Silicon Growth Phase and Deposition Rate through Voltage Waveform Tailoring during PECVD
  
  • Johnson E.V.
  • Pouliquen S.
  • Delattre Pierre-Alexandre
  • Booth Jean-Paul
  MRS Online Proceedings Library, Cambridge University Press, 2011, 1339, pp.mrss11-1339-s04-5. The use of Voltage Waveform Tailoring (VWT) that is the use of non-sinusoidal waveforms with a period equivalent to RF frequencies is shown to be effective in modifying the electric field distribution in a parallel plate, capacitively coupled laboratory plasma deposition reactor, and thus in changing the growth mode of silicon thin films from amorphous to nanocrystalline. The use of the VWT technique allows one to decouple the power injected into the plasma from the ion-bombardment energy at the film surface without changing any other deposition parameters, such as pressure or gas mixture. Material results are presented for an H2/SiH4 gas composition. A peaks type waveform increases the ion-bombardment energy at the RF electrode and reduces it at the substrate, resulting in more nanocrystalline growth. The use of a valleys-type waveform has the opposite effect, and results in more amorphous growth. We show the dependence of the process on silane dilution and pressure, including results on changes to the deposition rate when changing the excitation voltage waveform. (10.1557/opl.2011.993)
  DOI : 10.1557/opl.2011.993
• Physics of Radio-Frequency Plasmas
  
  • Chabert Pascal
  • Braithwaite Nicholas
  , 2011. Low-temperature radio frequency plasmas are essential in various sectors of advanced technology, from micro-engineering to spacecraft propulsion systems and efficient sources of light. The subject lies at the complex interfaces between physics, chemistry and engineering. Focusing mostly on physics, this book will interest graduate students and researchers in applied physics and electrical engineering. The book incorporates a cutting-edge perspective on RF plasmas. It also covers basic plasma physics including transport in bounded plasmas and electrical diagnostics. Its pedagogic style engages readers, helping them to develop physical arguments and mathematical analyses. Worked examples apply the theories covered to realistic scenarios, and over 100 in-text questions let readers put their newly acquired knowledge to use and gain confidence in applying physics to real laboratory situations.
• Lower hybrid resonances stimulated by the four CLUSTER relaxation sounders deep inside the plasmasphere: observations and inferred plasma characteristics
  
  • Kougblénou S
  • Lointier G
  • Décréau Pierrette
  • Trotignon Jean-Gabriel
  • Rauch Jean-Louis
  • Vallières Xavier
  • Canu Patrick
  • Masson A
  • Pickett J
  Annales Geophysicae, European Geosciences Union, 2011, 29, pp.2003–2018. The frequency range of the WHISPER relaxation sounder instrument on board CLUSTER, 4–80 kHz, has been chosen so as to encompass the electron gyro-frequency, F ce , and the electron plasma frequency, F p , in most regions to be explored. Measurement of those frequencies, which are triggered as resonances by the sounder, provides a direct estimation of in situ fundamental plasma characteristics: electron density and magnetic field intensity. In the late mission phase, CLUSTER penetrated regions deep inside the plas-masphere where F ce and F p are much higher than the upper frequency of the sounder's range. However, they are of the right order of magnitude as to place the lower hybrid frequency , F lh , in the 4–15 kHz band. This characteristic frequency , placed at a resonance of the medium, is triggered by the sounder's transmitter and shows up as an isolated peak in the received spectrum, not present in spectra of naturally occuring VLF waves. This paper illustrates, from analysis of case events, how measured F lh values give access to a plasma diagnostic novel of its kind. CLUSTER, travelling along its orbit, encounters favourable conditions where F ce is increasing and F p decreasing, such that F ce /F p increases from values below unity to values above unity. Measured F lh values thus give access, in turn, to the effective mass, M eff , indicative of plasma ion composition, and to the core plasma-sphere electron density value, a parameter difficult to measure. The analysed case events indicate that the estimated quantities (M eff in the 1.0–1.4 range, N e in the 5 × 10 2 – 10 4 cm −3 range) are varying with external factors (altitude, L value, geomagnetic activity) in a plausible way. Although covering only a restricted region (mid-latitude, low altitude inner plasmasphere), these measurements are available, since Correspondence to: S. Kougblénou (sena.kougblenou@cnrs-orleans.fr) late 2009, for all CLUSTER perigee passes not affected by eclipses (on average, roughly a third of a total of ∼200 passes per year) and offer multipoint observations previously unavailable in this region. (10.5194/angeo-29-2003-2011)
  DOI : 10.5194/angeo-29-2003-2011
• Gas sensing properties of multiwall carbon nanotubes decorated with rhodium nanoparticles
  
  • Leghrib R
  • Dufour Thierry
  • Demoisson F
  • Claessens N
  • Reniers F
  • Llobet E
  Sensors and Actuators B: Chemical, Elsevier, 2011, B160, pp.974-980. In the present work, multiwalled carbon nanotubes were decorated with rhodium nanoparticles using a colloidal solution in the post-discharge of an RF atmospheric plasma of argon (Ar) or argon/oxygen (Ar:O 2). The properties of these hybrid materials towards the room temperature detection of NO 2 , C 2 H 4 , CO, C 6 H 6 and moisture were investigated and discussed in view of compositional and morphological studies. It was found that the presence of oxygen in the plasma treatment is essential to significantly enhance the gas response of Rh-decorated multiwalled carbon nanotubes and to avoid response saturation even at low gas/vapor concentrations. These desirable effects are attributed to the presence of oxygen during the CNT plasma treatment since oxygenated vacancies act both as active adsorption sites for gases and as anchoring sites for Rh nanoparticles (the presence of Rh nanoclusters is nearly doubled in Ar:O 2 treated samples as compared to Ar treated samples). The oxygen treatment also makes easier the charge transfer between Rh nanoparticles and carbon nanotubes upon gas adsorption. The method for treating and decorating multiwalled carbon nanotubes used here is simple, fast and scalable for producing gas sensitive nanohybrid materials with uniform and well-controlled properties. (10.1016/j.snb.2011.09.014)
  DOI : 10.1016/j.snb.2011.09.014
• Turbulence propagation in heat flux-driven plasmas: implications for temperature profile structure
  
  • Wang Z.H.
  • Diamond P.H.
  • Gürcan Özgür D.
  • Garbet X.
  • Wang X.G.
  Nuclear Fusion, IOP Publishing, 2011, 51, pp.073009. Turbulence propagation and temperature profile evolution are studied in heat flux-driven plasmas. A simple model consisting of coupled non-linear reactiondiffusion equations for both turbulence and heat transport is proposed to elucidate several aspects of apparent non-local profile dynamics. Self-consistent E × B shear feedback on turbulence intensity growth and transport is also included in the model. Temperature profile evolution is studied in the presence of an intensity pulse propagating inwards but also interacting with an outward propagating heat pulse. It is found that as the heat flux Q increases, the intensity pulse speed first grows as √ Q and then decays as 1/Q, while the heat pulse speed finally saturates at the level given by neoclassical transport. Intensity pulse propagation can be effectively saturated at or above a critical heat flux, so that the formation of an internal transport barrier (ITB) can be triggered. This suggests that the ITB location is ultimately determined by both heat flux and edge turbulence conditions, and thus the ITB inhibits both the inward turbulence propagation and the outward turbulent heat transport. As a test of turbulence spreading dynamics, the intensity pulse propagation through gaps in turbulence excitation and its implications for profile response to off-axis heat deposition are also investigated. It is shown that the profile resilience phenomena can be recovered by taking into account intensity pulse propagation. (10.1088/0029-5515/51/7/073009)
  DOI : 10.1088/0029-5515/51/7/073009
• Successive Nanosecond Discharges in Water
  
  • Marinov Ilya
  • Guaitella Olivier
  • Rousseau Antoine
  • Starikovskaia Svetlana
  IEEE Transactions on Plasma Science, Institute of Electrical and Electronics Engineers, 2011, 39 (11), pp.2672-2673. Successive discharges of alternating polarities in distilled water have been studied. Cathode-initiated discharges develop in two possible configurations: slow and weakly luminescent bushlike mode or fast and luminous treelike mode with well-pronounced branching. Under our experimental conditions, the two modes originate with approximately equal probability. Positive discharge demonstrates only one discharge morphology with two hemispherical weakly luminescent structures. Successive negative pulse results in the formation of one of the negative modes, bushlike or treelike. (10.1109/TPS.2011.2147337)
  DOI : 10.1109/TPS.2011.2147337
• Triple Q : A three channel quantum cascade laser absorption spectrometer for fast multiple species concentration measurements
  
  • Hübner M.
  • Welzel S.
  • Marinov Daniil
  • Guaitella Olivier
  • Glitsch S.
  • Rousseau Antoine
  • Röpcke J.
  Review of Scientific Instruments, American Institute of Physics, 2011, 82, pp.093102. A compact and transportable three channel quantum cascade laser system (TRIPLE Q) based on mid-infrared absorption spectroscopy has been developed for time-resolved plasma diagnostics. The TRIPLE Q spectrometer encompasses three independently controlled quantum cascade lasers (QCLs), which can be used for chemical sensing, particularly for gas phase analysis of plasmas. All three QCLs are operated in the intra-pulse mode with typical pulse lengths of the order of 150 ns. Using a multiplexed detection, a time resolution shorter than 1 &#956;s can be achieved. Hence, the spectrometer is well suited to study kinetic processes of multiple infrared active compounds in reactive plasmas. A special data processing and analysis technique has been established to account for time jitter effects of the infrared emission of the QCLs. The performance of the TRIPLE Q system has been validated in pulsed direct current plasmas containing N2O/air and NO2/air. (10.1063/1.3633952)
  DOI : 10.1063/1.3633952