Publications

2012

• Model of a low-pressure radio-frequency inductive discharge in Ar/O<SUB>2</SUB> used for plasma spray deposition
  
  • Lazzaroni Claudia
  • Baba K.
  • Nikravech M.
  • Chabert Pascal
  Journal of Physics D: Applied Physics, IOP Publishing, 2012, 45, pp.485207. A global (volume-averaged) model of a low-pressure radio-frequency (RF) inductive discharge used for nanostructured zinc oxide thin film deposition, the so-called spray-plasma device, is proposed. The plasma reactor is fed with an admixture of argon and oxygen and the pressure is typically several tens of mTorr. In the first step of the modelling, the injector and the substrate holder are not taken into account, and therefore zinc-containing species are not considered. The global model is based on the numerical integration of the particle balance equations and the electronic power balance equation. The model is first run until the steady state is reached to determine the equilibrium discharge parameters that are the species densities and the electron temperature. A parametric study is carried out varying the gas pressure, the RF power and the O2 fraction in the reactor. A parameter of great importance for the deposition process is the flux of the reactive species on the substrate holder and the model allows a fast exploration of this parameter. For continuous plasmas, the ratio of the reactive species flux to the total positive ion flux can be controlled varying the three basic parameters cited before (pressure, power and dilution). In the last part of the paper, we also investigate pulsed plasmas and the effect of the duty cycle variations on the neutral/ion flux ratio. (10.1088/0022-3727/45/48/485207)
  DOI : 10.1088/0022-3727/45/48/485207
• Control of the ion flux and ion energy in CCP discharges using non-sinusoidal voltage waveforms
  
  • Lafleur Trevor
  • Booth Jean-Paul
  Journal of Physics D: Applied Physics, IOP Publishing, 2012, 45, pp.395203. Using particle-in-cell simulations we perform a characterization of the ion flux and ion energy in a capacitively coupled rf plasma reactor excited with non-sinusoidal voltage waveforms. The waveforms used are positive Gaussian type pulses (with a repetition frequency of 13.56 MHz), and as the pulse width is decreased, three main effects are identified that are not present in typical symmetric sinusoidal discharges: (1) the ion flux (and plasma density) rapidly increases, (2) as the pressure increases a significant asymmetry in the ion fluxes to the powered and grounded electrodes develops and (3) the average ion energy on the grounded electrode cannot be made arbitrarily small, but in fact remains essentially constant (together with the bias voltage) for the pressures investigated (20500 mTorr). Effects (1) and (3) potentially offer a new form of control in these types of rf discharges, where the ion flux can be increased while keeping the average ion energy on the grounded electrode constant. This is in contrast with the opposite control mechanism recently identified in Donkó et al (2009 J. Phys. D: Appl. Phys. 42 025205), where by changing the phase angle between applied voltage harmonics the ion flux can be kept constant while the ion energy (and bias voltage) varies. (10.1088/0022-3727/45/39/395203)
  DOI : 10.1088/0022-3727/45/39/395203
• 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=n&#948;v is due to the localization of the electron-to-ion velocity difference &#948;v 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
• 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. ...
• 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. ...
• 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
• Solar wind charge exchange X-ray emission from Mars.
  
  • Koutroumpa Dimitra
  • Modolo Ronan
  • Chanteur Gérard
  • Chaufray Jean-Yves
  • Kharchenko V.
  • Lallement Rosine
  Astronomy & Astrophysics - A&A, EDP Sciences, 2012, 545, pp.A153. We study the soft X-ray emission induced by charge exchange (CX) collisions between solar-wind, highly charged ions and neutral atoms of the Martian exosphere. A 3D multi species hybrid simulation model with improved spatial resolution (130 km) is used to describe the interaction between the solar wind and the Martian neutrals. We calculated velocity and density distributions of the solar wind plasma in the Martian environment with realistic planetary ions description, using spherically symmetric exospheric H and O profiles. Following that, a 3D test-particle model was developed to compute the X-ray emission produced by CX collisions between neutrals and solar wind minor ions. The model results are compared to XMM-Newton observations of Mars. We calculate projected X-ray emission maps for the XMM-Newton observing conditions and demonstrate how the X-ray emission reflects the Martian electromagnetic structure in accordance with the observed X-ray images. Our maps confirm that X-ray images are a powerful tool for the study of solar wind - planetary interfaces. However, the simulation results reveal several quantitative discrepancies compared to the observations. Typical solar wind and neutral coronae conditions corresponding to the 2003 observation period of Mars cannot reproduce the high luminosity or the corresponding very extended halo observed with XMM-Newton. Potential explanations of these discrepancies are discussed. (10.1051/0004-6361/201219720)
  DOI : 10.1051/0004-6361/201219720
• 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 < omega<SUB>ci</SUB>, other studies suggest that the KAW mode cannot carry the turbulence cascade down to electron scales and that the whistler mode (i.e., omega > omega<SUB>ci</SUB>) is more relevant. Here, we study key properties of the short-wavelength plasma modes under limited, but realistic, SW conditions, typically beta<SUB> i </SUB> >~ beta<SUB> e </SUB> ~ 1 and for high oblique angles of propagation 80° <= Theta<SUB> kB </SUB> < 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<SUB> kB </SUB> < 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<SUB> i </SUB> >~ 1 to frequencies either larger or smaller than omega<SUB>ci</SUB>, depending on the anisotropy k <SUB>par</SUB>/k <SUB></SUB>. This extension into small scales is more readily called whistler (omega > omega<SUB>ci</SUB>) or KAW (omega < omega<SUB>ci</SUB>), 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
• 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
• 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
• Microcrystalline silicon solar cells deposited using a plasma process excited by tailored voltage waveforms
  
  • Johnson E.V.
  • Delattre Pierre-Alexandre
  • Booth Jean-Paul
  Applied Physics Letters, American Institute of Physics, 2012, 100, pp.133504. Thin film solar cells in a p-i-n structure with an absorbing layer of intrinsic hydrogenated microcrystalline silicon (&#956;c-Si:H) deposited through plasma enhanced chemical vapour deposition excited by tailored voltage waveforms have been prepared. The use of an asymmetric voltage waveform decouples the ion-bombardment energy at the growth surface from the injected power and allows the growth of good quality &#956;c-Si:H at reasonable deposition rates (3&#8201;Å/s) using low pressure, powder-free conditions. Unoptimized photovoltaic devices with an efficiency of 6.1% are demonstrated using an i-layer deposited at 1.3&#8201;Å/s and a process pressure of 500 mTorr. (10.1063/1.3699222)
  DOI : 10.1063/1.3699222
• Spatio-temporal evolution of the L -> I -> H transition
  
  • Miki K.
  • Diamond P.H.
  • Gürcan Özgür D.
  • Tynan G.R.
  • Estrada T.
  • Schlitz L.
  • Xu G.S.
  Physics of Plasmas, American Institute of Physics, 2012, 19, pp.092306. We investigate the dynamics of the low(L)&#8201;&#8594;&#8201;high(H) transition using a time-dependent, one dimensional (in radius) model which self-consistently describes the time evolution of zonal flows (ZFs), mean flows (MFs), poloidal spin-up, and density and pressure profiles. The model represents the physics of ZF and MF competition, turbulence suppression via E×B shearing, and poloidal flows driven by turbulence. Numerical solutions of this model show that the L&#8594;H transition can occur via an intermediate phase (I-phase) which involves oscillations of profiles due to ZF and MF competition. The I-phase appears as a nonlinear transition wave originating at the edge boundary and propagates inward. Locally, I-phase exhibits the characteristics of a limit-cycle oscillation. All these observations are consistent with recent experimental results. We examine the trigger of the L&#8594;H transition, by defining a ratio of the rate of energy transfer from the turbulence to the zonal flow to the rate of energy input into the turbulence. When the ratio exceeds order unity, ZF shear gains energy, and a net decay of the turbulence is possible, thus triggering the L&#8594;H transition. Numerical calculations indicate that the L&#8594;H transition is triggered by this peak of the normalized ZF shearing. Zonal flows act as reservoir, in which to store increasing fluctuation energy without increasing transport, thus allowing the mean flow shear to increase and lock in the transition. A counterpart of the L &#8594; I&#8594;H transition, i.e., an L&#8594;H transition without I-phase, is obtained in a fast power ramp, for which I-phase is compressed into a single burst of ZF, which triggers the transition. Effects of neutral charge exchange on the L&#8594;H transition are studied by varying ZF damping and neoclassical viscosity. Results show that the predicted L&#8594;H transition power increases when either ZF damping or viscosity increase, suggesting a link between recycling, ZF damping, and the L&#8594;H threshold. Studies of fueling effects on the transition and pedestal structure with an emphasis on the particle pinch are reported. (10.1063/1.4753931)
  DOI : 10.1063/1.4753931
• Device convolution effects on the collective scattering signal of the E x B mode from Hall thruster experiments: 2D dispersion relation
  
  • Grésillon D.
  • Cavalier J.
  • Lemoine N.
  • Bonhomme Gildas
  • Tsikata Sedina
  • Honoré Cyrille
  Physics of Plasmas, American Institute of Physics, 2012, 19, pp.082117. The effect of the collective light scattering diagnostic transfer function is considered in the context of the dispersion relation of the unstable E×B mode previously reported. This transfer function is found to have a contribution to the measured frequencies and mode amplitudes which is more or less significant depending on the measurement wavenumbers and angles. After deconvolution, the experimental data are found to be possibly compatible with the idea that the mode frequency in the jet frame (after subtraction of the Doppler effect due to the plasma motion along the thruster axis) is independent of the orientation of the wave vector in the plane orthogonal to the local magnetic field. (10.1063/1.4748286)
  DOI : 10.1063/1.4748286
• A load current multiplier of the MIG terawatt generator
  
  • Chaikovsky S. A.
  • Chuvatin Alexandre S.
  • Oreshkin V. I.
  Instruments and Experimental Techniques, MAIK Nauka/Interperiodica, 2012, 55 (2), pp.209-217. The design of the load current multiplier with a 1.75-current enlargement factor, when the pulse amplitude of the current through a 3-MA load of the MIG terawatt pulse generator (multifunction pulse generator), is described, and its operation is demonstrated. The design of the multiplier is sufficiently simple, and it is easily demounted, allowing one to use the MIG generator in other operation modes with different-impedance loads. It is shown that it is expedient to use the multiplier for operation with static low-inductance loads, e.g., in studies of the skin electric explosion or nonlinear diffusion of megagauss magnetic fields. In this case, the multiplier application is intended to ensure a one-and-one-half increase in the current through the load as compared to the standard operation mode of the &#1052;&#1048;&#1043; generator.
• VOC elimination using plasma enhanced catalysis in a post situ configuration
  
  • Barakat Christelle
  • Gravejat Paul
  • Guaitella Olivier
  • Rousseau Antoine
  , 2012.
• Adsorption and reactivity of nitrogen atoms on dielectric surfaces under
  
  • Marinov Daniil
  • Guaitella Olivier
  • Rousseau Antoine
  , 2012.
• 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
• Coupling Between Whistler Waves and Ion-Scale Solitary Waves: Cluster Measurements in the Magnetotail During a Substorm
  
  • Tenerani Anna
  • Le Contel Olivier
  • Califano F.
  • Pegoraro F.
  • Robert Patrick
  • Cornilleau-Wehrlin Nicole
  • Sauvaud J.-A.
  Physical Review Letters, American Physical Society, 2012, 109, pp.155005. We present a new model of self-consistent coupling between low frequency, ion-scale coherent structures with high frequency whistler waves in order to interpret Cluster data. The idea relies on the possibility of trapping whistler waves by inhomogeneous external fields where they can be spatially confined and propagate for times much longer than their characteristic electronic time scale. Here we take the example of a slow magnetosonic soliton acting as a wave guide in analogy with the ducting properties of an inhomogeneous plasma. The soliton is characterized by a magnetic dip and density hump that traps and advects high frequency waves over many ion times. The model represents a new possible way of explaining space measurements often detecting the presence of whistler waves in correspondence to magnetic depressions and density humps. This approach, here given by means of slow solitons, but more general than that, is alternative to the standard approach of considering whistler wave packets as associated with nonpropagating magnetic holes resulting from a mirror-type instability. (10.1103/PhysRevLett.109.155005)
  DOI : 10.1103/PhysRevLett.109.155005
• 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
• A nanosecond surface dielectric barrier discharge at elevated pressures : time-resolved electric field and efficiency of initiation of combustion
  
  • Kosarev I.N.
  • Khorunzhenko V.I.
  • Mintoussov E.I.
  • Sagulenko P.N.
  • Popov N.A.
  • Starikovskaia Svetlana
  Plasma Sources Science and Technology, IOP Publishing, 2012, 21, pp.045012. We study a nanosecond surface dielectric barrier discharge (SDBD) initiated by negative or positive polarity pulses 1015 kV in amplitude in a cable, 2530 ns FWHM, 5 ns rise time, in the regime of a single shot or 3 Hz repetitive frequency. Discharge parameters, namely spatial structure of the discharge and time- and space-resolved electric field are studied in a N2&#8201;:&#8201;O2 = 4&#8201;:&#8201;1 mixture for P = 15 atm. The possibility of igniting a combustible mixture with the help of an SDBD is demonstrated using the example of a stoichiometric C2H6&#8201;:&#8201;O2 mixture at ambient initial temperature and at 1 atm pressure. Flame propagation and ignited volume as a function of time are compared experimentally for two discharge geometries: SDBD and pin-to-pin configurations at the same shape and amplitude of the incident pulse. It is shown that the SDBD can be considered as a multi-point ignition system with maximum energy release near the high-voltage electrode. Numerical modeling of the discharge and subsequent combustion kinetics for the SDBD conditions is performed. The discharge action leads to the production of atoms and radicals as well as to fast gas heating, due to the relaxation of electronic and vibrational degrees of freedom. The calculated ignition delay time is in reasonable agreement with the experimental results. (10.1088/0963-0252/21/4/045012)
  DOI : 10.1088/0963-0252/21/4/045012
• A global model of the self-pulsing regime of micro-hollow cathode discharges
  
  • Lazzaroni Claudia
  • Chabert Pascal
  Journal of Applied Physics, American Institute of Physics, 2012, 111, pp.053305. A global (volume-averaged) model of the self-pulsing regime of micro-hollow cathode discharges working in argon gas is proposed. The power balance is done using an equivalent circuit model of the discharge that allows the current and voltage dynamics to be calculated. The fraction of the total power dissipated in the discharge that contributes to electron heating is deduced from a sheath model previously described. The particle balance is first done in a very simplified reaction scheme involving only electrons, argon atomic ions, and argon molecular ions. In a second step, the excited states (the metastable state Ar*(3P2) and the resonant state Ar*(3P1)) are included in the particle balance equations. The models are compared to experiments and several conclusions are drawn. The model without excited states underestimates the electron density and does not capture well the trends in pressure. The model with the excited states is in better agreement which shows that multi-step ionization plays a significant role. The time-evolution of the electron density follows closely that of the discharge current but the excited states density presents two peaks: (i) the first at the early stage of the current peak due to direct excitation with high electron temperature, (ii) the second at the end of the current (and electron density) peak due to large production of excited states by electron-ion recombination at very low electron temperature. (10.1063/1.3690943)
  DOI : 10.1063/1.3690943
• Plasma decay in air and O<SUB>2</SUB> after a high-voltage nanosecond discharge
  
  • Aleksandrov N.L.
  • Anokhin E.M.
  • Kindysheva S.V.
  • Kirpichnikov A.A.
  • Kosarev I.N.
  • Nudnova M.M.
  • Starikovskaia Svetlana
  • Starikovskii A.Yu.
  Journal of Physics D: Applied Physics, IOP Publishing, 2012, 45, pp.255202. This paper presents the results of experimental and theoretical studies of an afterglow in room temperature air and O2 excited by a high-voltage nanosecond discharge for pressures between 1 and 10 Torr. We measured time-resolved electron density by a microwave interferometer for initial electron densities in the range (23) × 1012 cm&#8722;3. Discharge uniformity was investigated by optical methods. The balance equations for charged particles and electron temperature were numerically solved to describe the temporal evolution of the densities of electrons and ions in the discharge afterglow. It was shown that the loss of electrons is governed by dissociative and three-body electron recombination with \rm O₂^ ions under the conditions considered. Good agreement between the calculated and measured electron density histories could be obtained only when the rate of three-body recombination was increased by an order of magnitude and when the dependence of the recombination rate on electron temperature was changed. This could testify that the well-understood mechanism of three-body electron recombination with atomic ions could be noticeably modified in the case of molecular ions. (10.1088/0022-3727/45/25/255202)
  DOI : 10.1088/0022-3727/45/25/255202
• Detection of geodesic acoustic mode oscillations, using multiple signal classification analysis of doppler backscattering signal on tore supra
  
  • Vermare Laure
  • Hennequin Pascale
  • Gürcan Özgür D.
  • Tore Supra Team
  Nuclear Fusion, IOP Publishing, 2012, 52, pp.063008. This paper presents the first observation of geodesic acoustic modes (GAMs) on Tore Supra plasmas. Using the Doppler backscattering system, the oscillations of the plasma flow velocity, localized between r/a = 0.85 and r/a = 0.95, and with a frequency, typically around 10 kHz, have been observed at the plasma edge in numerous discharges. When the additional heating power is varied, the frequency is found to scale with Cs/R. The MUltiple SIgnal Classification (MUSIC) algorithm is employed to access the temporal evolution of the perpendicular velocity of density fluctuations. The method is presented in some detail, and is validated and compared against standard methods, such as the conventional fast Fourier transform method, using a synthetic signal. It stands out as a powerful data analysis method to follow the Doppler frequency with a high temporal resolution, which is important in order to extract the dynamics of GAMs. (10.1088/0029-5515/52/6/063008)
  DOI : 10.1088/0029-5515/52/6/063008
• 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