Publications

2012

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
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
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
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
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
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. ...
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. ...
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
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 (μc-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 μc-Si:H at reasonable deposition rates (3 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 Å/s and a process pressure of 500 mTorr. (10.1063/1.3699222)
DOI : 10.1063/1.3699222
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
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
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) → 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→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→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→H transition. Numerical calculations indicate that the L→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 → I→H transition, i.e., an L→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→H transition are studied by varying ZF damping and neoclassical viscosity. Results show that the predicted L→H transition power increases when either ZF damping or viscosity increase, suggesting a link between recycling, ZF damping, and the L→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
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 МИГ generator.
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.
Multiple scattering of electromagnetic waves by an array of parallel gyrotropic rods
- Es'Kin V. A.
- Kudrin A. V.
- Zaboronkova T. M.
- Krafft C.
Physical Review E, American Physical Society (APS), 2012, 86, pp.067601. We study multiple scattering of electromagnetic waves by an array of parallel gyrotropic circular rods and show that such an array can exhibit fairly unusual scattering properties and provide, under certain conditions, a giant enhancement of the scattered field. Among the scattering patterns of such an array at its resonant frequencies, the most interesting is the distribution of the total field in the form of a perfect self-similar structure of chessboard type. The scattering characteristics of the array are found to be essentially determined by the resonant properties of its gyrotropic elements and cannot be realized for arrays of nongyrotropic rods. It is expected that the results obtained can lead to a wide variety of practical applications. (10.1103/PhysRevE.86.067601)
DOI : 10.1103/PhysRevE.86.067601
Electron acceleration in the reconnection diffusion region: Cluster observations
- Huang S. Y.
- Vaivads A.
- Khotyaintsev Y. V.
- Zhou M.
- Fu H.S.
- Retinò Alessandro
- Deng X. H.
- André M.
- Cully C. M.
- He J. S.
- Sahraoui Fouad
- Yuan Z. G.
- Pang Y.
Geophysical Research Letters, American Geophysical Union, 2012, 39, pp.11103. We present one case study of magnetic islands and energetic electrons in the reconnection diffusion region observed by the Cluster spacecraft. The cores of the islands are characterized by strong core magnetic fields and density depletion. Intense currents, with the dominant component parallel to the ambient magnetic field, are detected inside the magnetic islands. A thin current sheet is observed in the close vicinity of one magnetic island. Energetic electron fluxes increase at the location of the thin current sheet, and further increase inside the magnetic island, with the highest fluxes located at the core region of the island. We suggest that these energetic electrons are firstly accelerated in the thin current sheet, and then trapped and further accelerated in the magnetic island by betatron and Fermi acceleration. (10.1029/2012GL051946)
DOI : 10.1029/2012GL051946
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 (λ = 1.315 μm) at PALS center. The shocks have been probed by XUV imaging using a Zn X-raylaser (λ = 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
Electron-cylotron maser radiation from electron holes: downward current region
- Treumann R. A.
- Baumjohann W.
- Pottelette Raymond
Annales Geophysicae, European Geosciences Union, 2012, 30, pp.119-130. The electron-cyclotron maser emission theory from electron holes is applied to holes generated in the downward current region of the aurora. It is argued that the main background auroral kilometric radiation source may still be located in the upward current region electron-ring (horseshoe) distribution while the fine structure is caused by electron holes predominantly in the downward current region. There the existence of electron holes is well established and electron densities are high enough for substantial maser growth rates. Trapping of radiation by the holes provides strong amplification. Upward motion of holes favours the escape of radiation both, from the holes and from the downward current region, into the upward current region. Since upward and downward current regions always exist simultaneously, they are acting in tandem in generating auroral kilometric radiation and its fine structure by the same mechanism though in different ways. This mechanism solves the long-standing problem of auroral kilometric radiation fine structure. (10.5194/angeo-30-119-2012)
DOI : 10.5194/angeo-30-119-2012
On the reactivity of C2H2 on plasma pretreated TiO2 surface
- Marinov Daniil
- Lopatik D.
- Hübner M.
- Guaitella Olivier
- Roepcke J.
- Rousseau Antoine
, 2012.
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

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