Publications

2012

• The impact of the ITER-like wall at JET on disruptions
  
  • de Vries P. C.
  • Arnoux G.
  • Huber A.
  • Flanagan J.
  • Lehnen M.
  • Riccardo V.
  • Reux C.
  • Jachmich S.
  • Lowry C.
  • Calabro G.
  • Frigione D.
  • Tsalas M.
  • Hartmann N.
  • Brezinsek S.
  • Clever M.
  • Douai D.
  • Groth M.
  • Hender T. C.
  • Hodille E.
  • Joffrin E.
  • Kruezi U.
  • Matthews G. F.
  • Morris J.
  • Neu R.
  • Philipps V.
  • Sergienko G.
  • Sertoli M.
  Plasma Physics and Controlled Fusion, IOP Publishing, 2012, 12, pp.124032. The new full-metal ITER-like wall (ILW) at JET was found to have a profound impact on the physics of disruptions. The main difference is a significantly lower fraction (by up to a factor of 5) of energy radiated during the disruption process, yielding higher plasma temperatures after the thermal quench and thus longer current quench times. Thus, a larger fraction of the total energy was conducted to the wall resulting in larger heat loads. Active mitigation by means of massive gas injection became a necessity to avoid beryllium melting already at moderate levels of thermal and magnetic energy (i.e. already at plasma currents of 2 MA). A slower current quench, however, reduced the risk of runaway generation. Another beneficial effect of the ILW is that disruptions have a negligible impact on the formation and performance of the subsequent discharge.Culham Sci Ctr, CCFE EURATOM Assoc, Abingdon OX14 3DB, Oxon, England.EURATOM, Forschungszentrum Julich, Inst Energie & Klimatforsch IEK 4, D-52425 Julich, Germany.Ecole Polytech, CNRS, F-91128 Palaiseau, France.ERM KMS, Assoc Euratom Etat Belge Belg Staat, Brussels, Belgium.Commiss European Communities, B-1049 Brussels, Belgium.Assoc Euratom ENEA Fus, I-00044 Rome, Italy.CEA, IRFM, F-13108 St Paul Les Durance, France.Aalto Univ, Assoc EURATOM Tekes, Espoo, Finland.Ecole Cent Lyon, F-69134 Ecully, France.EURATOM, Max Planck Inst Plasmaphys, D-85748 Garching, Germany.JET EFDA Culham Sci Ctr, Abingdon OX15 3DB, Oxon, England.contract of Association between EURATOM and FOM and was carried outwithin the framework of the European Fusion Development Agreement. Theviews and opinions expressed herein do not necessarily reflect those ofthe European Commission. (10.1088/0741-3335/54/12/124032)
  DOI : 10.1088/0741-3335/54/12/124032
• Multi-scale Cluster observations of reconnection jet fronts/braking regions and associated particle energization in near-Earth magnetotail
  
  • Retinò Alessandro
  • Vaivads A.
  • Zieger B.
  • Fujimoto M.
  • Kasahara S.
  • Nakamura R.
  , 2012. Reconnection jet fronts, the boundaries separating jetting from ambient plasma, and jet braking regions, where jets eventually stop/dissipate, play a key role for the near-Earth magnetotail e.g. in terms of particle energization. Recent Cluster orbits, where two spacecraft are separated by ~ 100 km (sub-proton scales) while being separated from the others by ~ 10000 km (MHD scales), allow the unique possibility to study jet fronts/braking regions and associated particle energization at different scales. Here we present Cluster observations from such orbits, focusing in particular on the datasets from the upcoming Cluster Guest Investigator campaign.
• Solar wind charge exchange X-ray emission from Mars.
  
  • Koutroumpa Dimitra
  • Modolo Ronan
  • Chanteur Gérard
  • Chaufray Jean-Yves
  • Kharchenko V.
  • Lallement Rosine
  Astronomy & Astrophysics - A&A, EDP Sciences, 2012, 545, pp.A153. We study the soft X-ray emission induced by charge exchange (CX) collisions between solar-wind, highly charged ions and neutral atoms of the Martian exosphere. A 3D multi species hybrid simulation model with improved spatial resolution (130 km) is used to describe the interaction between the solar wind and the Martian neutrals. We calculated velocity and density distributions of the solar wind plasma in the Martian environment with realistic planetary ions description, using spherically symmetric exospheric H and O profiles. Following that, a 3D test-particle model was developed to compute the X-ray emission produced by CX collisions between neutrals and solar wind minor ions. The model results are compared to XMM-Newton observations of Mars. We calculate projected X-ray emission maps for the XMM-Newton observing conditions and demonstrate how the X-ray emission reflects the Martian electromagnetic structure in accordance with the observed X-ray images. Our maps confirm that X-ray images are a powerful tool for the study of solar wind - planetary interfaces. However, the simulation results reveal several quantitative discrepancies compared to the observations. Typical solar wind and neutral coronae conditions corresponding to the 2003 observation period of Mars cannot reproduce the high luminosity or the corresponding very extended halo observed with XMM-Newton. Potential explanations of these discrepancies are discussed. (10.1051/0004-6361/201219720)
  DOI : 10.1051/0004-6361/201219720
• A hemispherical retarding field energy analyzer to characterize spatially and angularly extended electron beams
  
  • Cipriani Fabrice
  • Leblanc Frédéric
  • Illiano Jean-Marie
  • Berthelier Jean-Jacques
  European Physical Journal: Applied Physics, EDP Sciences, 2012, 60 (2), pp.21002 (7 p.). We have designed and built a hemispherical retarding field energy analyzer in order to facilitate characterization of large area electron emitters (typically field emitter arrays with active areas up to 1 cm2) with large angular aperture. A complete numerical model of the analyzer has been built, including perturbations due to secondary particles, in order to determine the analyzer performances. The analyzer energy resolution is better than 100 meV for an energy range up to 120 eV. The analyzer has a global field of view of 112° and allows measurements of the energy distribution of the beam as a function of the emission angle, as well as measurements of the beam intensity profile along any section of the beam. We have successfully used the analyzer to characterize the electron beam emitted by 1 cm2 Mo microtips-based field emitter arrays. (10.1051/epjap/2012120011)
  DOI : 10.1051/epjap/2012120011
• Hydrogenated microcrystalline silicon thin films deposited by RF-PECVD under low ion bombardment energy using voltage waveform tailoring
  
  • Johnson E.V.
  • Pouliquen S.
  • Delattre Pierre-Alexandre
  • Booth Jean-Paul
  Journal of Non-Crystalline Solids, Elsevier, 2012, 358 (17), pp.1974-1977. We present experimental results for hydrogenated amorphous and microcrystalline silicon (a-Si:H and μc-Si:H) thin films deposited by PECVD while using a voltage waveform tailoring (VWT) technique to create an electrical asymmetry in the reactor. VWT dramatically modifies the mean ion bombardment energy (IBE) during growth, and we show that for a constant peak-to-peak excitation voltage (VPP), waveforms resembling peaks or valleys result in very different material properties. Using Raman scattering spectroscopy, we show that the crystallinity of the material depends strongly on the IBE, as controlled by VWT. A detailed examination of the Raman scattering spectra reveals that the narrow peak at 520 cm− 1 is disproportionately enhanced by lowering the IBE through the VWT technique. We examine this effect for a range of process parameters, varying the pressure, hydrogensilane dilution ratio, and total flow of H2. In addition, the Sisingle bondHX bonding in silicon thin films deposited using VWT is characterised for the first time, showing that the hydrogen bonding character is changed by the IBE. These results demonstrate the potential for VWT in controlling the IBE during thin film growth, thus ensuring that application-appropriate film densities and crystallinities are achieved, independent of the injected RF power. (10.1016/j.jnoncrysol.2012.01.014)
  DOI : 10.1016/j.jnoncrysol.2012.01.014
• Model of a low-pressure radio-frequency inductive discharge in Ar/O<SUB>2</SUB> used for plasma spray deposition
  
  • Lazzaroni Claudia
  • Baba K.
  • Nikravech M.
  • Chabert Pascal
  Journal of Physics D: Applied Physics, IOP Publishing, 2012, 45, pp.485207. A global (volume-averaged) model of a low-pressure radio-frequency (RF) inductive discharge used for nanostructured zinc oxide thin film deposition, the so-called spray-plasma device, is proposed. The plasma reactor is fed with an admixture of argon and oxygen and the pressure is typically several tens of mTorr. In the first step of the modelling, the injector and the substrate holder are not taken into account, and therefore zinc-containing species are not considered. The global model is based on the numerical integration of the particle balance equations and the electronic power balance equation. The model is first run until the steady state is reached to determine the equilibrium discharge parameters that are the species densities and the electron temperature. A parametric study is carried out varying the gas pressure, the RF power and the O2 fraction in the reactor. A parameter of great importance for the deposition process is the flux of the reactive species on the substrate holder and the model allows a fast exploration of this parameter. For continuous plasmas, the ratio of the reactive species flux to the total positive ion flux can be controlled varying the three basic parameters cited before (pressure, power and dilution). In the last part of the paper, we also investigate pulsed plasmas and the effect of the duty cycle variations on the neutral/ion flux ratio. (10.1088/0022-3727/45/48/485207)
  DOI : 10.1088/0022-3727/45/48/485207
• Control of the ion flux and ion energy in CCP discharges using non-sinusoidal voltage waveforms
  
  • Lafleur Trevor
  • Booth Jean-Paul
  Journal of Physics D: Applied Physics, IOP Publishing, 2012, 45, pp.395203. Using particle-in-cell simulations we perform a characterization of the ion flux and ion energy in a capacitively coupled rf plasma reactor excited with non-sinusoidal voltage waveforms. The waveforms used are positive Gaussian type pulses (with a repetition frequency of 13.56 MHz), and as the pulse width is decreased, three main effects are identified that are not present in typical symmetric sinusoidal discharges: (1) the ion flux (and plasma density) rapidly increases, (2) as the pressure increases a significant asymmetry in the ion fluxes to the powered and grounded electrodes develops and (3) the average ion energy on the grounded electrode cannot be made arbitrarily small, but in fact remains essentially constant (together with the bias voltage) for the pressures investigated (20500 mTorr). Effects (1) and (3) potentially offer a new form of control in these types of rf discharges, where the ion flux can be increased while keeping the average ion energy on the grounded electrode constant. This is in contrast with the opposite control mechanism recently identified in Donkó et al (2009 J. Phys. D: Appl. Phys. 42 025205), where by changing the phase angle between applied voltage harmonics the ion flux can be kept constant while the ion energy (and bias voltage) varies. (10.1088/0022-3727/45/39/395203)
  DOI : 10.1088/0022-3727/45/39/395203
• On application of quantum cascade lasers for plasma diagnostics : A Review
  
  • Röpcke J.
  • Davies P.B.
  • Lang N.
  • Rousseau Antoine
  • Welzel S.
  Journal of Physics D: Applied Physics, IOP Publishing, 2012, 45, pp.423001. Over the past few years mid-infrared absorption spectroscopy based on quantum cascade lasers operating over the region from 3 to 12 µm and called quantum cascade laser absorption spectroscopy or QCLAS has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, nitrogen oxides and organo-silicon compounds has led to further applications of QCLAS because most of these compounds and their decomposition products are infrared active. QCLAS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species at time resolutions below a microsecond, which is of particular importance for the investigation of reaction kinetics and dynamics. Information about gas temperature and population densities can also be derived from QCLAS measurements. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of QCLAS techniques to industrial requirements including the development of new diagnostic equipment. The recent availability of external cavity (EC) QCLs offers a further new option for multi-component detection. The aim of this paper is fourfold: (i) to briefly review spectroscopic issues arising from applying pulsed QCLs, (ii) to report on recent achievements in our understanding of molecular phenomena in plasmas and at surfaces, (iii) to describe the current status of industrial process monitoring in the mid-infrared and (iv) to discuss the potential of advanced instrumentation based on EC-QCLs for plasma diagnostics. (10.1088/0022-3727/45/42/423001)
  DOI : 10.1088/0022-3727/45/42/423001
• Implosions of larger size wire arrays at enhanced current of 1.51.7 MA on Zebra with LCM
  
  • Safronova Alla S.
  • Esaulov A. A.
  • Kantsyrev Viktor L.
  • Stafford A.
  • Weller Michael E.
  • Shlyaptseva V. V.
  • Zunino H. A.
  • Shrestha Ishor
  • Osborne Glenn C.
  • Keim S. F.
  • Chuvatin Alexandre S.
  • Coverdale C. A.
  , 2012, pp.6C7. Experiments on Zebra with a Load Current Multiplier (LCM, which provides an enhanced current of 1.5-1.7 MA) allow the implosion of larger wire array loads than possible with the standard current of 1 MA. Advantages of the larger wire arrays include enhanced energy coupling to plasmas and better diagnostic access to observable plasma regions. Diagnostics, fielded on 10 beam lines, included PCD, XRD, and EUV detectors, X-ray/EUV spectrometers and X-ray pinhole cameras, and laser shadowgraphy. For this work, we collected and analyzed the experimental results from standard and modified triple planar wire arrays (TPWA), as well as cylindrical wire arrays (CWA). The anode-cathode gap in these experiments with LCM was 1 cm, which is half the gap used in the standard mode. For TPWAs, two outer wire planes were made out of mid-atomic-number wire material (Ni or Cu) with the inter-row gap increased from 1.5 or 3 mm (usually used at 1 MA current) up to 4.5 mm. The different designs of a central wire plane from Al were implemented to investigate its role as a magnetic field extruder to prevent the formation of closed magnetic configurations around each wire plane. Previous work has shown that larger double planar wire arrays (at a standard current) can block the inward motion of ablated plasma jets. Therefore, multi-planar wire arrays provide a new test bed to study jet formations in larger wire arrays for laboratory astrophysics. In addition, we measured higher linear radiation yield and the high-temperature precursor Al plasmas, not previously observable in experiments on Zebra at 1 Ma current. However, the observation of high-temperature precursor plasmas from Cu CWAs at enhanced current was consistent with previous findings at a standard current of 1 MA. (10.1109/PLASMA.2012.6384037)
  DOI : 10.1109/PLASMA.2012.6384037
• Upper ionosphere of Mars is not axially symmetrical
  
  • Dubinin E.
  • Fraenz M.
  • Woch J.
  • Modolo Ronan
  • Chanteur Gérard
  • Duru F.
  • Gurnett D. A.
  • Barabash S.
  • Lundin R.
  Earth Planets and Space, Springer / Terra Scientific Publishing Company, 2012, 64 (2), pp.113-120. The measurements carried out by the ASPERA-3 and MARSIS experiments on board the Mars Express (MEX) spacecraft show that the upper Martian ionosphere (h ≥ 400 km) is strongly azimuthally asymmetrical. There are several factors, e.g., the crustal magnetization on Mars and the orientation of the interplanetary magnetic field (IMF) which can give rise to formation of ionospheric swells and valleys. It is shown that expansion of the ionospheric plasma along the magnetic field lines of crustal origin can produce bulges in the plasma density. The absense of a magnetometer on MEX makes the retrieval of an asymmetry caused by the IMF more difficult. However hybrid simulations give a hint that the ionosphere in the hemisphere (E-) to which the motional electric field is pointed occurs more inflated than the ionosphere in the opposite (E+) hemisphere. (10.5047/eps.2011.05.022)
  DOI : 10.5047/eps.2011.05.022
• On the Origin of the 1/f Spectrum in the Solar Wind Magnetic Field
  
  • Verdini Andrea
  • Grappin Roland
  • Pinto Rui
  • Velli Marco
  The Astrophysical Journal Letters, Bristol : IOP Publishing, 2012, 750 (2), pp.L33. We present a mechanism for the formation of the low-frequency 1/f magnetic spectrum based on numerical solutions of a shell-reduced MHD model of the turbulent dynamics inside the sub-Alfvénic solar wind. We assign reasonably realistic profiles to the wind speed and the density along the radial direction, and a radial magnetic field. Alfvén waves of short periodicity (600 s) are injected at the base of the chromosphere, penetrate into the corona, and are partially reflected, thus triggering a turbulent cascade. The cascade is strong for the reflected wave while it is weak for the outward propagating waves. Reflection at the transition region recycles the strong turbulent spectrum into the outward weak spectrum, which is advected beyond the Alfvénic critical point without substantial evolution. There, the magnetic field has a perpendicular power-law spectrum with slope close to the Kolmogorov 5/3. The parallel spectrum is inherited from the frequency spectrum of large (perpendicular) eddies. The shape is a double power law with slopes of sime 1 and 2 at low and high frequencies, respectively, with the position of the break depending on the injected spectrum. We suggest that the double power-law spectrum measured by Helios at 0.3 AU, where the average magnetic field is not aligned with the radial (contrary to our assumptions), results from the combination of such different spectral slopes. At low frequency the parallel spectrum dominates with its characteristic 1/f shape, while at higher frequencies its steep spectral slope (2) is masked by the more energetic perpendicular spectrum (slope 5/3). (10.1088/2041-8205/750/2/L33)
  DOI : 10.1088/2041-8205/750/2/L33
• Analytical-numerical global model of atmospheric-pressure radio-frequency capacitive discharges
  
  • Lazzaroni Claudia
  • Chabert Pascal
  • Lieberman M.A.
  • Lichtenberg A.J.
  • Leblanc A.
  Plasma Sources Science and Technology, IOP Publishing, 2012, 21, pp.035013. A one-dimensional hybrid analyticalnumerical global model of atmospheric-pressure, radio-frequency (rf) driven capacitive discharges is developed. The feed gas is assumed to be helium with small admixtures of oxygen or nitrogen. The electrical characteristics are modeled analytically as a current-driven homogeneous discharge. The electron power balance is solved analytically to determine a time-varying Maxwellian electron temperature, which oscillates on the rf timescale. Averaging over the rf period yields effective rate coefficients for gas phase activated processes. The particle balance relations for all species are then integrated numerically to determine the equilibrium discharge parameters. The coupling of analytical solutions of the time-varying discharge and electron temperature dynamics, and numerical solutions of the discharge chemistry, allows for a fast solution of the discharge equilibrium. Variations of discharge parameters with discharge composition and rf power are determined. Comparisons are made to more accurate but numerically costly fluid models, with space and time variations, but with the range of parameters limited by computational time. (10.1088/0963-0252/21/3/035013)
  DOI : 10.1088/0963-0252/21/3/035013
• A load current multiplier of the MIG terawatt generator
  
  • Chaikovsky S. A.
  • Chuvatin Alexandre S.
  • Oreshkin V. I.
  Instruments and Experimental Techniques, MAIK Nauka/Interperiodica, 2012, 55 (2), pp.209-217. The design of the load current multiplier with a 1.75-current enlargement factor, when the pulse amplitude of the current through a 3-MA load of the MIG terawatt pulse generator (multifunction pulse generator), is described, and its operation is demonstrated. The design of the multiplier is sufficiently simple, and it is easily demounted, allowing one to use the MIG generator in other operation modes with different-impedance loads. It is shown that it is expedient to use the multiplier for operation with static low-inductance loads, e.g., in studies of the skin electric explosion or nonlinear diffusion of megagauss magnetic fields. In this case, the multiplier application is intended to ensure a one-and-one-half increase in the current through the load as compared to the standard operation mode of the &#1052;&#1048;&#1043; generator.
• Microcrystalline silicon solar cells deposited using a plasma process excited by tailored voltage waveforms
  
  • Johnson E.V.
  • Delattre Pierre-Alexandre
  • Booth Jean-Paul
  Applied Physics Letters, American Institute of Physics, 2012, 100, pp.133504. Thin film solar cells in a p-i-n structure with an absorbing layer of intrinsic hydrogenated microcrystalline silicon (&#956;c-Si:H) deposited through plasma enhanced chemical vapour deposition excited by tailored voltage waveforms have been prepared. The use of an asymmetric voltage waveform decouples the ion-bombardment energy at the growth surface from the injected power and allows the growth of good quality &#956;c-Si:H at reasonable deposition rates (3&#8201;Å/s) using low pressure, powder-free conditions. Unoptimized photovoltaic devices with an efficiency of 6.1% are demonstrated using an i-layer deposited at 1.3&#8201;Å/s and a process pressure of 500 mTorr. (10.1063/1.3699222)
  DOI : 10.1063/1.3699222
• Spatio-temporal evolution of the L -> I -> H transition
  
  • Miki K.
  • Diamond P.H.
  • Gürcan Özgür D.
  • Tynan G.R.
  • Estrada T.
  • Schlitz L.
  • Xu G.S.
  Physics of Plasmas, American Institute of Physics, 2012, 19, pp.092306. We investigate the dynamics of the low(L)&#8201;&#8594;&#8201;high(H) transition using a time-dependent, one dimensional (in radius) model which self-consistently describes the time evolution of zonal flows (ZFs), mean flows (MFs), poloidal spin-up, and density and pressure profiles. The model represents the physics of ZF and MF competition, turbulence suppression via E×B shearing, and poloidal flows driven by turbulence. Numerical solutions of this model show that the L&#8594;H transition can occur via an intermediate phase (I-phase) which involves oscillations of profiles due to ZF and MF competition. The I-phase appears as a nonlinear transition wave originating at the edge boundary and propagates inward. Locally, I-phase exhibits the characteristics of a limit-cycle oscillation. All these observations are consistent with recent experimental results. We examine the trigger of the L&#8594;H transition, by defining a ratio of the rate of energy transfer from the turbulence to the zonal flow to the rate of energy input into the turbulence. When the ratio exceeds order unity, ZF shear gains energy, and a net decay of the turbulence is possible, thus triggering the L&#8594;H transition. Numerical calculations indicate that the L&#8594;H transition is triggered by this peak of the normalized ZF shearing. Zonal flows act as reservoir, in which to store increasing fluctuation energy without increasing transport, thus allowing the mean flow shear to increase and lock in the transition. A counterpart of the L &#8594; I&#8594;H transition, i.e., an L&#8594;H transition without I-phase, is obtained in a fast power ramp, for which I-phase is compressed into a single burst of ZF, which triggers the transition. Effects of neutral charge exchange on the L&#8594;H transition are studied by varying ZF damping and neoclassical viscosity. Results show that the predicted L&#8594;H transition power increases when either ZF damping or viscosity increase, suggesting a link between recycling, ZF damping, and the L&#8594;H threshold. Studies of fueling effects on the transition and pedestal structure with an emphasis on the particle pinch are reported. (10.1063/1.4753931)
  DOI : 10.1063/1.4753931
• Device convolution effects on the collective scattering signal of the E x B mode from Hall thruster experiments: 2D dispersion relation
  
  • Grésillon D.
  • Cavalier J.
  • Lemoine N.
  • Bonhomme Gildas
  • Tsikata Sedina
  • Honoré Cyrille
  Physics of Plasmas, American Institute of Physics, 2012, 19, pp.082117. The effect of the collective light scattering diagnostic transfer function is considered in the context of the dispersion relation of the unstable E×B mode previously reported. This transfer function is found to have a contribution to the measured frequencies and mode amplitudes which is more or less significant depending on the measurement wavenumbers and angles. After deconvolution, the experimental data are found to be possibly compatible with the idea that the mode frequency in the jet frame (after subtraction of the Doppler effect due to the plasma motion along the thruster axis) is independent of the orientation of the wave vector in the plane orthogonal to the local magnetic field. (10.1063/1.4748286)
  DOI : 10.1063/1.4748286
• Adsorption and reactivity of nitrogen atoms on dielectric surfaces under
  
  • Marinov Daniil
  • Guaitella Olivier
  • Rousseau Antoine
  , 2012.
• VOC elimination using plasma enhanced catalysis in a post situ configuration
  
  • Barakat Christelle
  • Gravejat Paul
  • Guaitella Olivier
  • Rousseau Antoine
  , 2012.
• Reflection of solar wind protons on the Martian bow shock: Investigations by means of 3-dimensional simulations
  
  • Richer Emilie
  • Chanteur Gérard
  • Modolo Ronan
  • Dubinin Eduard
  Geophysical Research Letters, American Geophysical Union, 2012, 39, pp.L17101. The reflection of solar wind protons on the Martian bow shock (BS) is investigated by means of three-dimensional simulation models. A two steps approach is adopted to allow a detailed analysis of the reflected population. Firstly, the 3-dimensional hybrid model of Modolo et al. (2005) is used to compute a stationary state of the interaction of the solar wind (SW) with Mars. Secondly, the motion of test particles is followed in the electromagnetic field computed by the hybrid simulation meanwhile detection criteria defined to identify reflected protons are applied. This study demonstrates some effects of the large curvature of a planetary BS on the structure of the foreshock. Reflected protons encounter the BS in a region encompassing parts of the quasi-perpendicular and quasi-parallel shocks, and exit the shock mainly from the quasi-parallel region. The energy spectrum of all reflected protons extends from 0 to almost 15keV. A virtual omnidirectional detector (VOD) is used to compute the local omnidirectional flux of reflected protons at various locations upstream of the BS. Spatial variations of this omnidirectional flux indicate the location and spatial extent of the proton foreshock and demonstrate its shift, increasing with the distance downstream, in the direction opposite to the motional electric field of the SW. Local energy spectra computed from the VOD observations demonstrate the existence of an energy gradient along the direction of the convection electric field. (10.1029/2012GL052858)
  DOI : 10.1029/2012GL052858
• La science au service du développement
  
  • Amory-Mazaudier Christine
  , 2012. Dans cet ouvrage est présentée une expérience humaine d'un réseau de scientifiques, GIRGEA, qui a réuni, et qui réunit toujours plusieurs centaines d'étudiants, ingénieurs, techniciens, chercheurs, enseignants, etc. autour d'un objectif central : développer la recherche en sciences de l'espace en Afrique en installant des instruments, en formant des chercheurs de niveau international qui pourront constituer des équipes de recherche dans leur pays et assurer ainsi la pérennité du réseau. Il semble impensable, et pourtant, qu'au XXIe siècle un scientifique puisse prétendre développer des études planétaires des phénomènes physiques de l'environnement terrestre sans mesures sur l'ensemble du globe. Le Nord est indispensable au Sud et le Sud est indispensable au Nord. L'éruption du volcan Eyjafjöll en 2010 a révélé la fragilité des pays du Nord techniquement avancés. Le tsunami du 26 décembre 2004 a mis en évidence la grande vulnérabilité des habitants de certaines régions du Sud ayant des réseaux d'alerte au tsunami défaillants ou inexistants. Mais ces événements ont aussi surtout démontré nos liens, nous vivons tous sur la même planète : la Terre est un petit village et nous dépendons les uns des autres. Le GIRGEA propose donc une méthode pour allier science de l'espace au niveau international et règles éthiques afin de favoriser une recherche à l'échelle planétaire indispensable à l'avancée des connaissances et profitable à tous.
• Numerical modelling of the properties of an ion beam extracted from a low-pressure wire discharge
  
  • Gueroult R
  • Elias Paul-Quentin
  • Packan Denis
  • Rax Jean-Marcel
  Journal of Physics D: Applied Physics, IOP Publishing, 2012, 45 (24), pp.245203. The combination of a dedicated acceleration stage with direct extraction of ions from a low-pressure wire discharge enables one to generate a narrow band ion beam of tunable energy. In order to optimize this ion source concept, we conduct a numerical study emphasizing the main physical processes and the parameters controlling them. An analysis of the three-dimensional non-collisional ion trajectories inside the ion source reveals that the total beam current can be tuned by modifying the depth and radius of the diaphragm connecting the wire discharge chamber to the acceleration chamber, while the inter-electrode distance controls the beam focusing. A particle-in-cell (PIC) modelling of the beam acceleration and propagation indicates that, despite the low operating pressure (~10−2 mbar), elastic and charge-exchange collisions are responsible for a decrease in the beam current by a factor of four. Charge-exchange collisions are also shown to be accountable for the existence of a low-energy ion population being non-uniform throughout a beam cross section. (10.1088/0022-3727/45/24/245203)
  DOI : 10.1088/0022-3727/45/24/245203
• Coupling Between Whistler Waves and Ion-Scale Solitary Waves: Cluster Measurements in the Magnetotail During a Substorm
  
  • Tenerani Anna
  • Le Contel Olivier
  • Califano F.
  • Pegoraro F.
  • Robert Patrick
  • Cornilleau-Wehrlin Nicole
  • Sauvaud J.-A.
  Physical Review Letters, American Physical Society, 2012, 109, pp.155005. We present a new model of self-consistent coupling between low frequency, ion-scale coherent structures with high frequency whistler waves in order to interpret Cluster data. The idea relies on the possibility of trapping whistler waves by inhomogeneous external fields where they can be spatially confined and propagate for times much longer than their characteristic electronic time scale. Here we take the example of a slow magnetosonic soliton acting as a wave guide in analogy with the ducting properties of an inhomogeneous plasma. The soliton is characterized by a magnetic dip and density hump that traps and advects high frequency waves over many ion times. The model represents a new possible way of explaining space measurements often detecting the presence of whistler waves in correspondence to magnetic depressions and density humps. This approach, here given by means of slow solitons, but more general than that, is alternative to the standard approach of considering whistler wave packets as associated with nonpropagating magnetic holes resulting from a mirror-type instability. (10.1103/PhysRevLett.109.155005)
  DOI : 10.1103/PhysRevLett.109.155005
• Negative hydrogen ion production in fusion dedicated ion sources
  
  • Bacal M.
  Chemical Physics, Elsevier, 2012, 398, pp.3-6. A brief description is given of the basic processes in negative ion sources dedicated to fusion. It is considered that in these sources negative ions are produced by ions and atoms interacting with a caesiated surface, but this mechanism is not unique: the volume production, based on dissociative electron attachment to rovibrationally excited molecules, is also active. We suggest that in RF sources the acceleration of positive ions to a few tens of eV by the plasma potential difference between the driver and the extraction regions can have an important effect on negative ion production by enhancing the negative ion yield from caesiated surfaces, and by charge exchange reactions with caesium atoms. The presence of energetic positive ions can have other implications (modifying the virtual cathode in front of the plasma grid, participating in caesium ionization). (10.1016/j.chemphys.2011.04.002)
  DOI : 10.1016/j.chemphys.2011.04.002
• Producing Kiloelectronvolt L-Shell Plasmas on Zebra at UNR
  
  • Safronova Alla S.
  • Kantsyrev Viktor L.
  • Esaulov A. A.
  • Shrestha Ishor
  • Shlyapstseva V. V.
  • Weller Michael E.
  • Ouart N. D.
  • Osborne Glenn C.
  • Stafford A.
  • Keim S. F.
  • Velikovich A. L.
  • Giuliani J. L.
  • Chuvatin Alexandre S.
  IEEE Transactions on Plasma Science, Institute of Electrical and Electronics Engineers, 2012, 40 (12), pp.3347-3353. Experiments with various wire loads from mid-atomic-number wires, which were performed on the university-scale 1-MA Zebra generator at the University of Nevada, Reno, during the last few years, are analyzed to assess the highest electron temperature reached. In particular, the results from experiments with planar wire arrays (PWAs) were considered. Load materials from mid-atomic-number such as stainless steel, Alumel, Cu, brass, Mo, and up to Ag were used to generate L-shell plasmas and to study plasma parameters. Though the full diagnostic set was utilized, the main focus was on X-ray spectroscopic data and on the non-local thermodynamic equilibrium kinetic modeling. As a result, the scaling of the maximum Te with the load material atomic number is presented for the first time in the range from Fe to Ag for L-shell plasmas from PWAs. The highest values of the electron temperature in L-shell plasmas, which are estimated from the modeling, were from both Ag PWAs and X-pinches. This work is important for the development of efficient X-ray radiators on university-scale Z-pinch generators. (10.1109/TPS.2012.2222451)
  DOI : 10.1109/TPS.2012.2222451
• New probing techniques of radiative shocks
  
  • Stehlé Chantal
  • Kozlova Michaela
  • Larour Jean
  • Nejdl Jaroslav
  • Champion Norbert
  • Barroso Patrice
  • Suzuki-Vidal Francisco
  • Acef Ouali
  • Delattre Pierre-Alexandre
  • Dostal Jan
  • Krus Miroslav
  • Chièze Jean-Pierre
  Optics Communications, Elsevier, 2012, 285, pp.64-69. Radiative shock waves propagating in xenon at a low pressure have been produced using 60 joules of iodine laser (&#955; = 1.315 &#956;m) at PALS center. The shocks have been probed by XUV imaging using a Zn X-raylaser (&#955; = 21 nm) generated with a 20-ns delay after the shock creating pulse. Auxiliary high-speed silicon diodes allowed performing space- and time-resolved measurement of plasma self-emission in the visible and XUV. The results show the generation of a shock wave propagating at 60 km/s preceded by a radiative precursor. This demonstrates the feasibility of radiative shock generation using high power infrared lasers and the use of XRL backlighting as a suitable diagnostic for shock imaging. (10.1016/j.optcom.2011.09.008)
  DOI : 10.1016/j.optcom.2011.09.008