Publications

2012

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.
Variation of F2 layer critical frequency with solar cycle at Dakar station
- Thiam N. M.
- Ouattara Frédéric Martial
- Gnabahou Doua Allain
- Amory-Mazaudier Christine
- Fleury Rolland
- Lassurdie-Duchesne P.
Journal des sciences, Université Cheikh Anta Diop, 2012, 11 (2), pp.16-20. ...
Thin current sheets in the presence of a guiding magnetic field in Earth's magnetosphere
- Malova H. V.
- Popov V. Y.
- Mingalev O. V.
- Mingalev I. V.
- Melnik M. N.
- Artemyev A. V.
- Petrukovich A. A.
- Delcourt Dominique C.
- Shen C.
- Zelenyi L. M.
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2012, 117, pp.4212. A self-consistent theory of relatively thin anisotropic current sheets (TCS) in collisionless plasma is developed, taking into account the presence of a guiding field By (all notations are used in the GSM coordinate system). TCS configurations with a finite value of guiding field By are often observed in Earth's magnetotail and are typical for Earth's magnetopause. A characteristic signature of such configurations is the existence of a magnetic field component along the direction of TCS current. A general case is considered in this paper with global sheared magnetic field By = const. Analytical and numerical (particle-in-cell) models for such plasma equilibria are analyzed and compared with each other as well as with Cluster observations. It is shown that, in contrast to the case with By = 0, the character of ``particle-current sheet'' interaction is drastically changed in the case of a global magnetic shear. Specifically, serpentine-like parts of ion trajectories in the neutral plane become more tortuous, leading to a thicker current sheet. The reflection coefficient of particles coming from northern and southern sources also becomes asymmetric and depends upon the value of the By component. As a result, the degree of asymmetry of magnetic field, plasma, and current density profiles appears characteristic of current sheets with a constant By. In addition, in the presence of nonzero guiding field, the curvature current of electrons in the center of the current sheet decreases, yielding an effective thickening of the sheet. Implications of these results for current sheets in Earth's magnetosphere are discussed. (10.1029/2011JA017359)
DOI : 10.1029/2011JA017359
Kinetic equilibrium for an asymmetric tangential layer, Physics of Plasmas
- Belmont Gérard
- Aunai Nicolas
- Smets Roch
Physics of Plasmas, American Institute of Physics, 2012, 19, pp.022108. Finding kinetic (Vlasov) equilibria for tangential current layers is a long standing problem, especially in the context of reconnection studies, when the magnetic field reverses. Its solution is of pivotal interest for both theoretical and technical reasons when such layers must be used for initializing kinetic simulations. The famous Harris equilibrium is known to be limited to symmetric layers surrounded by vacuum, with constant ion and electron flow velocities, and with current variation purely dependent on density variation. It is clearly not suited for the magnetopause-like layers, which separate two plasmas of different densities and temperatures, and for which the localization of the current density j=nδv is due to the localization of the electron-to-ion velocity difference δv and not of the density n. We present here a practical method for constructing a Vlasov stationary solution in the asymmetric case, extending the standard theoretical methods based on the particle motion invariants. We show that, in the case investigated of a coplanar reversal of the magnetic field without electrostatic field, the distribution function must necessarily be a multi-valued function of the invariants to get asymmetric profiles for the plasma parameters together with a symmetric current profile. We show also how the concept of accessibility makes these multi-valued functions possible, due to the particle excursion inside the layer being limited by the Larmor radius. In the presented method, the current profile across the layer is chosen as an input, while the ion density and temperature profiles in between the two asymptotic imposed values are a result of the calculation. It is shown that, assuming the distribution is continuous along the layer normal, these profiles have always a more complex profile than the profile of the current density and extends on a larger thickness. The different components of the pressure tensor are also outputs of the calculation and some conclusions concerning the symmetries of this tensor are pointed out. (10.1063/1.3685707)
DOI : 10.1063/1.3685707
Inductance and near fields of a loop antenna in a cold magnetoplasma in the whistler frequency band
- Korobkov S. V.
- Kostrov A. V.
- Gushchin M. E.
- Zaboronkova T. M.
- Krafft C.
Physics of Plasmas, American Institute of Physics, 2012, 19, pp.093301. The influence of a magnetoplasma on the inductance of a circular loop antenna oriented perpendicular to the ambient static magnetic field and operated in the whistler frequency band is studied. Based on a strict electrodynamic approach, the analytical treatment of the antenna reactance is performed for a uniform rf current distribution along the antenna wire. Calculations are made for plasma parameters and operating frequencies typical for active ionospheric experiments and laboratory rf (helicon) sources of dense magnetized plasmas. It is shown that the plasma influence on the inductance of the loop antenna remains relatively weak, even for antennas with dimensions close to half of the longitudinal whistler wavelength, when the rf field distribution in the antenna near zone is strongly different from that in vacuum. The theoretical predictions are confirmed by measurements performed on the large KROT plasma device. The results obtained are of crucial importance for the preparation of active ionospheric experiments and for the matching of loop antennas used in laboratory rf sources of dense magnetized plasmas. (10.1063/1.4745611)
DOI : 10.1063/1.4745611
The International Research Group in geophysics, Europa Africa : a laboratory without borders in the Earth Science and Environment
- Amory-Mazaudier Christine
Journal of Life Sciences, 2012, 6, pp.336-342. ...
Observations of turbulence within reconnection jet in the presence of guide field
- Huang S. Y.
- Zhou M.
- Sahraoui Fouad
- Vaivads A.
- Deng X. H.
- André M.
- He J. S.
- Fu H.S.
- Li H. M.
- Yuan Z. G.
- Wang D. D.
Geophysical Research Letters, American Geophysical Union, 2012, 39, pp.L11104. We present the first comprehensive observations of turbulence properties within high speed reconnection jet in the plasma sheet with moderate guide field. The power spectral density index is about −1.73 in the inertial range, and follows the value of −2.86 in the ion dissipation range. The turbulence is strongly anisotropic in the wave-vector space with the major power having its wave-vector highly oblique to the ambient magnetic field, suggesting that the turbulence is quasi-2D. The measured dispersion relations obtained using the k-filtering technique are compared with theory and are found to be consistent with the Alfvén-Whistler mode. In addition, both Probability Distribution Functions and flatness results show that the turbulence in the reconnection jet is intermittent (multifractal) at scales less than the proton gyroradius/inertial lengths. The estimated electric field provided by anomalous resistivity caused by turbulence is about 3 mV/m, which is close to the typical reconnection electric field in the magnetotail. (10.1029/2012GL052210)
DOI : 10.1029/2012GL052210
Determination of TEC by using pseudo range at Koudougou station in Burkina Faso
- Ouattara Frédéric Martial
- Zoundi C.
- Amory-Mazaudier Christine
- Fleury Rolland
- Lassudrie Duchesne P.
Journal des sciences, Université Cheikh Anta Diop, 2012, 11 (1), pp.12-19. ...
Coupling between whistler waves and slow-mode solitary waves
- Tenerani Anna
- Califano F.
- Pegoraro F.
- Le Contel Olivier
Physics of Plasmas, American Institute of Physics, 2012, 19, pp.052103. The interplay between electron- and ion-scale phenomena is of general interest for both laboratory and space plasma physics. In this paper, we investigate the linear coupling between whistler waves and slow magnetosonic solitons through two-fluid numerical simulations. Whistler waves can be trapped in the presence of inhomogeneous external fields such as a density hump or hole where they can propagate for times much longer than their characteristic time scale, as shown by laboratory experiments and space measurements. Space measurements have detected whistler waves also in correspondence to magnetic holes, i.e., to density humps with magnetic field minima extending on ion-scales. This raises the interesting question of how ion-scale structures can couple to whistler waves. Slow magnetosonic solitons share some of the main features of a magnetic hole. Using the ducting properties of an inhomogeneous plasma as a guide, we present a numerical study of whistler waves that are trapped and transported inside propagating slow magnetosonic solitons. (10.1063/1.4717764)
DOI : 10.1063/1.4717764
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
Model of a low-pressure radio-frequency inductive discharge in Ar/O2 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
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
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
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
ON THE NATURE OF THE SOLAR WIND FROM CORONAL PSEUDOSTREAMERS
- Wang Y-M
- Grappin Roland
- Robbrecht E.
- Sheeley N R
The Astrophysical Journal, American Astronomical Society, 2012, 749 (2), pp.182. Coronal pseudostreamers, which separate like-polarity coronal holes, do not have current sheet extensions, unlike the familiar helmet streamers that separate opposite-polarity holes. Both types of streamers taper into narrow plasma sheets that are maintained by continual interchange reconnection with the adjacent open magnetic field lines. White-light observations show that pseudostreamers do not emit plasma blobs; this important difference from helmet streamers is due to the convergence of like-polarity field lines above the X-point, which prevents the underlying loops from expanding outward and pinching off. The main component of the pseudostreamer wind has the form of steady outflow along the open field lines rooted just inside the boundaries of the adjacent coronal holes. These flux tubes are characterized by very rapid expansion below the X-point, followed by reconvergence at greater heights. Analysis of an idealized pseudostreamer configuration shows that, as the separation between the underlying holes increases, the X-point rises and the expansion factor f ss at the source surface increases. In situ observations of pseudostreamer crossings indicate wind speeds v ranging from ~350 to ~550 km s1, with O7 /O6 ratios that are enhanced compared with those in high-speed streams but substantially lower than in the slow solar wind. Hydrodynamic energy-balance models show that the empirical v-f ss relation overestimates the wind speeds from nonmonotonically expanding flux tubes, particularly when the X-point is located at low heights and f ss is small. We conclude that pseudostreamers produce a "hybrid" type of outflow that is intermediate between classical slow and fast solar wind. (10.1088/0004-637X/749/2/182)
DOI : 10.1088/0004-637X/749/2/182
New Insight into Short-wavelength Solar Wind Fluctuations from Vlasov Theory
- Sahraoui Fouad
- Belmont Gérard
- Goldstein M. L.
The Astrophysical Journal, American Astronomical Society, 2012, 748, pp.100. The nature of solar wind (SW) turbulence below the proton gyroscale is a topic that is being investigated extensively nowadays, both theoretically and observationally. Although recent observations gave evidence of the dominance of kinetic Alfvén waves (KAWs) at sub-ion scales with omega < omegaci, other studies suggest that the KAW mode cannot carry the turbulence cascade down to electron scales and that the whistler mode (i.e., omega > omegaci) is more relevant. Here, we study key properties of the short-wavelength plasma modes under limited, but realistic, SW conditions, typically beta i >~ beta e ~ 1 and for high oblique angles of propagation 80° <= Theta kB < 90° as observed from the Cluster spacecraft data. The linear properties of the plasma modes under these conditions are poorly known, which contrasts with the well-documented cold plasma limit and/or moderate oblique angles of propagation (Theta kB < 80°). Based on linear solutions of the Vlasov kinetic theory, we discuss the relevance of each plasma mode (fast, Bernstein, KAW, whistler) in carrying the energy cascade down to electron scales. We show, in particular, that the shear Alfvén mode (known in the magnetohydrodynamic limit) extends at scales krho i >~ 1 to frequencies either larger or smaller than omegaci, depending on the anisotropy k par/k . This extension into small scales is more readily called whistler (omega > omegaci) or KAW (omega < omegaci), although the mode is essentially the same. This contrasts with the well-accepted idea that the whistler branch always develops as a continuation at high frequencies of the fast magnetosonic mode. We show, furthermore, that the whistler branch is more damped than the KAW one, which makes the latter the more relevant candidate to carry the energy cascade down to electron scales. We discuss how these new findings may facilitate resolution of the controversy concerning the nature of the small-scale turbulence, and we discuss the implications for present and future spacecraft wave measurements in the SW. (10.1088/0004-637X/748/2/100)
DOI : 10.1088/0004-637X/748/2/100
Larger Size Planar Wire Arrays with a Modified Central Plane and Their Applications on Zebra with LCM
- Safronova Alla S.
- Esaulov A.
- Kantsyrev Viktor L.
- Chuvatin Alexandre S.
- Coverdale C.
- Jones B.
- Shlyaptseva V. V.
- Weller Michael E.
- Stafford A.
- Shrestha Ishor
- Osborne Glenn C.
- Keim S. F.
, 2012, 57 (12), pp.BO5.00009.
Thermodynamics of flow generation Momentum Transport, PV Homogenization and Entropy Production
- Gürcan Özgür D.
, 2012. http://canopus.cnu.ac.kr/lsap2012
Entropy Generation across Earth's Collisionless Bow Shock
- Parks G. K.
- Lee E.
- Mccarthy M.
- Goldstein M. L.
- Fu S. Y.
- Cao J.B.
- Canu Patrick
- Lin N.
- Wilber M.
- Dandouras I.
- Rème H.
- Fazakerley A.
Physical Review Letters, American Physical Society, 2012, 108, pp.061102. Earth's bow shock is a collisionless shock wave but entropy has never been directly measured across it. The plasma experiments on Cluster and Double Star measure 3D plasma distributions upstream and downstream of the bow shock allowing calculation of Boltzmann's entropy function H and his famous H theorem, dH/dt<=0. The collisionless Boltzmann (Vlasov) equation predicts that the total entropy does not change if the distribution function across the shock becomes nonthermal, but it allows changes in the entropy density. Here, we present the first direct measurements of entropy density changes across Earth's bow shock and show that the results generally support the model of the Vlasov analysis. These observations are a starting point for a more sophisticated analysis that includes 3D computer modeling of collisionless shocks with input from observed particles, waves, and turbulences. (10.1103/PhysRevLett.108.061102)
DOI : 10.1103/PhysRevLett.108.061102
Transition from Weak to Strong Cascade in MHD Turbulence
- Verdini Andrea
- Grappin Roland
Physical Review Letters, American Physical Society, 2012, 109, pp.025004. The transition from weak to strong turbulence when passing from large to small scales in magneto-hydrodynamic (MHD) turbulence with guide field is a cornerstone of anisotropic turbulence theory. We present the first check of this transition, using the Shell-RMHD, which combines a shell model of perpendicular nonlinear coupling and linear propagation along the guide field. This model allows us to reach Reynolds numbers around 106. We obtain surprisingly good agreement with the theoretical predictions, with a reduced perpendicular energy spectrum scaling as k^-2 at large scales and as k_⊥^-5/3 at small scales, where critical balance between nonlinear and propagation time is reached. However, even in the strong regime, a high level of excitation is found in the weak coupling region of Fourier space, which is due to the rich frequency spectrum of large eddies. A corollary is that the reduced parallel spectral slope is not a definite test of the spectral anisotropy, contrary to standard belief. (10.1103/PhysRevLett.109.025004)
DOI : 10.1103/PhysRevLett.109.025004
Afterglow kinetics in oxygen pulsed discharges
- Guerra V.
- Marinov Daniil
- Guaitella Olivier
- Rousseau Antoine
, 2012.
Radiative properties of mixed nested cylindrical wire arrays on Zebra at UNR
- Weller Michael E.
- Safronova Alla S.
- Kantsyrev Viktor L.
- Esaulov A. A.
- Coverdale C. A.
- Chuvatin Alexandre S.
- Ouart N. D.
- Williamson Kenneth M.
- Shrestha Ishor
- Osborne Glenn C.
- Shlyaptseva V. V.
- Keim S. F.
- Stafford A.
High Energy Density Physics, Elsevier, 2012, 8 (2), pp.184-189. The dynamics of mixed nested cylindrical wire arrays were studied at the UNR Zebra generator with our existing theoretical and experimental tools to better understand the contributions of each array to the emitted radiation. In particular, experimental results of mixed brass (70% Cu, 30% Zn) and Al (5056, 5% Mg) nested cylindrical wire arrays are analyzed and compared. The loads used brass in the inner array and Al in the outer array, or alternately, Al in the inner array and brass in the outer array, with a mass ratio of 1:1 (outer to inner). Consequently, radiative properties of K-shell Al and Mg ions and L-shell Cu and Zn ions are compared as functions of the placements of the brass and Al wires on the inner and outer arrays. Results show that the placement of brass and Al, whether on the inner or outer array, dramatically affects the intensity of the X-ray emission. Specifically, the ratio of Cu L-shell to Al K-shell emissions changed from 4 when Al is in the outer array to 40 when brass is in the outer array, and the total radiated yield was highest when the brass was on the outer array (18 kJ, versus 15 kJ when brass is on the inner array). Each load was fielded twice to vary the timing of the time-gated imaging and spectral diagnostics. This provides a more complete understanding of the evolution of the plasma parameters over the X-ray pulse and highlights the importance of the time-gated diagnostics. (10.1016/j.hedp.2012.03.004)
DOI : 10.1016/j.hedp.2012.03.004
Global model of a gridded-ion thruster powered by a radiofrequency inductive coil
- Chabert Pascal
- Arancibia Monreal J.
- Bredin Jérôme
- Popelier Lara
- Aanesland Ane
Physics of Plasmas, American Institute of Physics, 2012, 19, pp.195201. A global (volume-averaged) model of a gridded-ion thruster is proposed. The neutral propellant (xenon gas) is injected into the thruster chamber at a fixed rate and a plasma is generated by circulating a radiofrequency current in an inductive coil. The ions generated in this plasma are accelerated out of the thruster by a pair of DC biased grids. The neutralization downstream is not treated. Xenon atoms also flow out of the thruster across the grids. The model, based on particle and energy balance equations, solves for four global variables in the thruster chamber: the plasma density, the electron temperature, the neutral gas (atom) density, and the neutral gas temperature. The important quantities to evaluate the thruster efficiency and performances are calculated from these variables and from the voltage across the grids. It is found that the mass utilization efficiency rapidly decreases with the gas flow rate. However, the radiofrequency power transfer efficiency increases significantly with the injected gas flow rate. Therefore, there is a compromise to be found between these two quantities. (10.1063/1.4737114)
DOI : 10.1063/1.4737114
Seasonal TEC Variability in West Africa Equatorial Anomaly Region
- Zoundi Christian
- Ouattara Frédéric
- Fleury Rolland
- Amory-Mazaudier Christine
- Lassudrie-Duchesne Patrick
European Journal of Scientific Research, EuroJournals, 2012, 77 (3), pp.309-319. This paper presented the seasonal variability of TEC/ GPS data recorded at Ouagadougou a West Africa GPS station located near the magnetic equator. Seasonal data TEC time variations are compared to those of TEC derived from IGS GPS network maps. The present study showed that TEC map model predicts well data TEC during equinoctial months and fairly well during solstice months. The best prediction is obtained during spring and the worst during winter. The analysis of seasonal TEC profiles highlighted that model accuracy shows seasonal variations with respect to the complexity of TEC time variations. This work pointed out that model accuracy depends not only on the integration of station data in IGS GPS network data but also on the presence of daytime multi-peaks and/or the presence and the amplitude of night time peak. Solstice and summer data TEC analysis showed that those present F2 layer annual anomaly.
Analysis of radiation from silver HED plasma sources with the potential for lasing
- Weller Michael E.
- Safronova Alla S.
- Kantsyrev Viktor L.
- Esaulov A. A.
- Stafford A.
- Shrestha Ishor
- Osborne Glenn C.
- Shlyaptseva V. V.
- Keim S. F.
- Zunino H. A.
- Chuvatin Alexandre S.
- Apruzese J. P.
- Golovkin I. E.
- Macfarlane J. J.
, 2012, pp.2P78. Silver (Ag) high energy density plasmas were produced using uniform and combined single planar wire array (SPWA) z-pinches. Ag SPWAs were recently introduced as an efficient x-ray radiator and have shown to create L-shell plasmas that have the highest electron temperature (>;1.8 keV) observed on Zebra so far and upwards of 30 kJ of energy output, which is of interest for future applications of inertial confinement fusion1. A set of diagnostics included fast, filtered x-ray diodes; a Ni bolometer; laser shadowgraphy and optical streak cameras; time-gated and time-integrated x-ray pinhole cameras; and time-integrated spatially resolved (TISR) and time-gated spatially-integrated (TGSI) x-ray spectrometers. In particular, a new time-gated hard x-ray spectrometer was fielded to attain first results to understand how Ag plasmas evolve in time. In addition, an important question about such Ag plasmas is whether lasing occurs in the Na-like and Ne-like soft x-ray range, and if so, at what gains? Our suite of theoretical diagnostics was expanded with HELIOS-CR code, that was utilized to study implosion characterisitcs and radiative characteristics of Ag wire arrays as well as to calculate possible lasing gains. Lastly, the results of new experiments on Zebra with the load current multiplier (LCM) at enhanced current of 1.5 - 1.7 MA were analyzed and compared to those of standard configurations. (10.1109/PLASMA.2012.6383634)
DOI : 10.1109/PLASMA.2012.6383634

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