Publications

2012

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
Velocity diffusion in plasma waves excited by electron beams
- Volokitin A.
- Krafft C.
Plasma Physics and Controlled Fusion, IOP Publishing, 2012, 54, pp.085002. New results provided by numerical simulations of the weak instability of a warm electron beam in a collisionless plasma are presented. The theoretical model considers the self-consistent resonant interactions of beam particles with wave packets of broad spectra; it is derived using some of the initial approximations of the standard derivation of the quasilinear diffusion equation in the weak turbulence approach, without, however, the assumption of randomly phased waves. A huge number of particle trajectories calculated over long times by a symplectic code are analyzed using various statistical algorithms. The dynamics of the beam relaxation and the saturation of the wave spectrum are studied and compared with the analytical solutions provided by the quasilinear theory of weak turbulence. The most interesting results concern the presence of strong and persistent irregularities in the wave energy spectrum at saturation, which are linked to large velocity variations observed in the particles' dynamics and to non-Gaussian local diffusion. Quantitative estimates of the diffusion coefficients are given and compared with predictions of the weak turbulence theory. (10.1088/0741-3335/54/8/085002)
DOI : 10.1088/0741-3335/54/8/085002
THEMIS observation of chorus elements without a gap at half the gyrofrequency
- Kurita S.
- Katoh Y.
- Omura Y.
- Angelopoulos V.
- Cully C. M.
- Le Contel Olivier
- Misawa H.
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2012, 117, pp.11223. Using waveform data obtained by one of the THEMIS satellites, we report properties of rising tone chorus elements without a gap at half the gyrofrequency in a region close to the magnetic equator. The wave normal angle of the chorus elements is typically field-aligned in the entire frequency range of both upper-band and lower-band chorus emissions. We find that the observed frequency sweep rates are consistent with the estimation based on the nonlinear wave growth theory of Omura et al. (2008). In addition, we compare the frequency profiles of the chorus wave amplitudes with those of the optimum and threshold wave amplitudes derived from the nonlinear wave growth theory for triggering rising tone chorus emissions. The results of the comparison show a reasonable agreement, indicating that rising tone chorus elements are continually generated through a triggering process which generates elements with the optimum amplitudes for nonlinear growth. (10.1029/2012JA018076)
DOI : 10.1029/2012JA018076
Centrifugally stimulated exospheric ion escape at Mercury
- Delcourt Dominique C.
- Seki K.
- Terada N.
- Moore T. E.
Geophysical Research Letters, American Geophysical Union, 2012, 39, pp.22105. We investigate the transport of ions in the low-altitude magnetosphere of Mercury. We show that, because of small spatial scales, the centrifugal effect due to curvature of the E × B drift paths can lead to significant particle energization in the parallel direction. We demonstrate that because of this effect, ions with initial speed smaller than the escape speed such as those produced via thermal desorption can overcome gravity and escape into the magnetosphere. The escape route of this low-energy exosphere originating material is largely controlled by the magnetospheric convection rate. This escape route spreads over a narrower range of altitudes when the convection rate increases. Bulk transport of low-energy planetary material thus occurs within a limited region of space once moderate magnetospheric convection is established. These results suggest that, via release of material otherwise gravitationally trapped, the E × B related centrifugal acceleration is an important mechanism for the net supply of plasma to the magnetosphere of Mercury. (10.1029/2012GL054085)
DOI : 10.1029/2012GL054085
Dynamics of neutral gas depletion investigated by time- and space-resolved measurement of xenon atom ground state density
- Liard Laurent
- Aanesland Ane
- Chabert Pascal
Journal of Physics D: Applied Physics, IOP Publishing, 2012, 45, pp.235201. The dynamics of neutral gas depletion in high-density plasmas is investigated by time- and space-resolved measurements of the xenon ground state density. Two-photon absorbed laser induced fluorescence experiments were carried out in a helicon reactor operating at 10 mTorr in xenon gas. When the plasma is magnetized, a plasma column is formed from the bottom of the chamber up to the pumping region. In this situation it is found that two phenomena, with different time scales, are responsible for the neutral gas depletion. The magnetized plasma column is ignited in a short (millisecond) time scale leading to a neutral gas depletion at the discharge centre and to an increase of neutral gas density at the reactor walls. This is explained both by neutral gas heating and by the rise of the plasma pressure at the discharge centre. Then, on a much longer (second) time scale, the overall neutral gas density in the reactor decreases due to higher pumping efficiency when the magnetized plasma column is ignited. The pumping enhancement is not observed when the plasma is not magnetized, probably because in this case the dense plasma column vanishes and the plasma is more localized near the antenna. (10.1088/0022-3727/45/23/235201)
DOI : 10.1088/0022-3727/45/23/235201
Electron energy distribution function and plasma parameters across magnetic filters
- Aanesland Ane
- Bredin Jérôme
- Chabert Pascal
- Godyak V.
Applied Physics Letters, American Institute of Physics, 2012, 100, pp.044102. The electron energy distribution function (EEDF) is measured across a magnetic filter in inductively coupled plasmas. The measured EEDFs are found to be Maxwellian in the elastic energy range with the corresponding electron temperature monotonously decreasing along the positive gradient of the magnetic field. At the maximum of the magnetic field, the electron temperature reaches its minimum and remains nearly constant in the area of the negative gradient of the field, where the plasma density distribution exhibits a local minimum. (10.1063/1.3680088)
DOI : 10.1063/1.3680088
An electromagnetic theory of turbulence driven poloidal rotation
- Mcdevitt C.J.
- Gürcan Özgür D.
Physics of Plasmas, American Institute of Physics, 2012, 19, pp.102311. An electromagnetic theory of turbulence driven poloidal rotation is developed with particular emphasis on understanding poloidal rotation in finite-β plasmas. A relation linking the flux of polarization charge to the divergence of the total turbulent stress is derived for electromagnetic gyrokinetic modes. This relation is subsequently utilized to derive a constraint on the net electromagnetic turbulent stress exerted on the poloidal flow. Various limiting cases of this constraint are considered, where it is found that electromagnetic contributions to the turbulent stress may either enhance or reduce the net turbulent stress depending upon the branch of turbulence excited. (10.1063/1.4764078)
DOI : 10.1063/1.4764078
Quasilinear transport modelling at low magnetic shear
- Citrin Jonathan
- Bourdelle C.
- Cottier P.
- Escande D.F.
- Gürcan Özgür D.
- Hatch D.R.
- Hogeweij G.M.D.
- Jenko F.
- Pueschel M.J.
Physics of Plasmas, American Institute of Physics, 2012, 19, pp.062305. Accurate and computationally inexpensive transport models are vital for routine and robust predictions of tokamak turbulent transport. To this end, the QuaLiKiz [Bourdelle et al., Phys. Plasmas 14, 112501 (2007)] quasilinear gyrokinetic transport model has been recently developed. QuaLiKiz flux predictions have been validated by non-linear simulations over a wide range in parameter space. However, a discrepancy is found at low magnetic shear, where the quasilinear fluxes are significantly larger than the non-linear predictions. This discrepancy is found to stem from two distinct sources: the turbulence correlation length in the mixing length rule and an increase in the ratio between the quasilinear and non-linear transport weights, correlated with increased non-linear frequency broadening. Significantly closer agreement between the quasilinear and non-linear predictions is achieved through the development of an improved mixing length rule, whose assumptions are validated by non-linear simulations. (10.1063/1.4719697)
DOI : 10.1063/1.4719697
Symmetry breaking effects of density gradient on parallel momentum transport: A new rau s * effect
- Singh Rameswar
- Singh R.
- Kaw P.
- Gürcan Özgür D.
- Diamond P.H.
- Nordman H.
Physics of Plasmas, American Institute of Physics, 2012, 19, pp.012301. Symmetry breaking effects of density gradient on parallel momentum transport is studied via quasilinear theory. It is shown that finite ρ*s(≡ρs/Ln), where ρs is ion sound radius and Ln is density scale length, leads to symmetry breaking of the ion temperature gradient (ITG) eigenfunction. This broken symmetry persists even in the absence of mean poloidal (from radial electric field shear) and toroidal flows. This effect, as explained in the text, originates from the divergence of polarization particle current in the ion continuity equation. The form of the eigenfunction allows the microturbulence to generate parallel residual stress via 〈k‖〉 symmetry breaking. Comparison with the →E×→B shear driven parallel residual stress, parallel polarization stress and turbulence intensity gradient driven parallel residual stress are discussed. It is shown that this ρ*s driven parallel residual stress may become comparable to →E×→B shear driven parallel residual stress in small Ln region. In the regular drift wave ordering, where ρ*s≪1, this effect is found to be of the same order as the parallel polarization stress. This ρ*s driven parallel residual stress can also overtake the turbulence intensity gradient driven parallel residual stress in strong density gradient region whereas the later one is dominant in the strong profile curvature region. The parallel momentum diffusivity is found to remain undisturbed by this ρ*s effect as long as the turbulence intensity inhomogenity is not important. (10.1063/1.3672518)
DOI : 10.1063/1.3672518
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
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. ...
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. ...
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
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
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

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