Publications

2014

• Langmuir probe analysis in electronegative plasmas
  
  • Bredin Jérôme
  • Chabert Pascal
  • Aanesland Ane
  Physics of Plasmas, American Institute of Physics, 2014, 21 (12), pp.123502. This paper compares two methods to analyze Langmuir probe data obtained in electronegative plasmas. The techniques are developed to allow investigations in plasmas, where the electronegativity α0 = n/ne (the ratio between the negative ion and electron densities) varies strongly. The first technique uses an analytical model to express the Langmuir probe current-voltage (I-V) characteristic and its second derivative as a function of the electron and ion densities (ne, n , n), temperatures (Te, T , T), and masses (me, m , m). The analytical curves are fitted to the experimental data by adjusting these variables and parameters. To reduce the number of fitted parameters, the ion masses are assumed constant within the source volume, and quasi-neutrality is assumed everywhere. In this theory, Maxwellian distributions are assumed for all charged species. We show that this data analysis can predict the various plasma parameters within 510%, including the ion temperatures when α0 > 100. However, the method is tedious, time consuming, and requires a precise measurement of the energy distribution function. A second technique is therefore developed for easier access to the electron and ion densities, but does not give access to the ion temperatures. Here, only the measured I-V characteristic is needed. The electron density, temperature, and ion saturation current for positive ions are determined by classical probe techniques. The electronegativity α0 and the ion densities are deduced via an iterative method since these variables are coupled via the modified Bohm velocity. For both techniques, a Child-Law sheath model for cylindrical probes has been developed and is presented to emphasize the importance of this model for small cylindrical Langmuir probes. (10.1063/1.4903328)
  DOI : 10.1063/1.4903328
• Magnetic clouds' structure in the magnetosheath as observed by Cluster and Geotail: four case studies
  
  • Turc Lucile
  • Fontaine Dominique
  • Savoini Philippe
  • Kilpua E. K. J.
  Annales Geophysicae, European Geosciences Union, 2014, 32, pp.1247-1261. Magnetic clouds (MCs) are large-scale magnetic flux ropes ejected from the Sun into the interplanetary space. They play a central role in solar-terrestrial relations as they can efficiently drive magnetic activity in the near-Earth environment. Their impact on the Earth's magnetosphere is often attributed to the presence of southward magnetic fields inside the MC, as observed in the upstream solar wind. However, when they arrive in the vicinity of the Earth, MCs first encounter the bow shock, which is expected to modify their properties, including their magnetic field strength and direction. If these changes are significant, they can in turn affect the interaction of the MC with the magnetosphere. In this paper, we use data from the Cluster and Geotail spacecraft inside the magnetosheath and from the Advanced Composition Explorer (ACE) upstream of the Earth's environment to investigate the impact of the bow shock's crossing on the magnetic structure of MCs. Through four example MCs, we show that the evolution of the MC's structure from the solar wind to the magnetosheath differs largely from one event to another. The smooth rotation of the MC can either be preserved inside the magnetosheath, be modified, i.e. the magnetic field still rotates slowly but at different angles, or even disappear. The alteration of the magnetic field orientation across the bow shock can vary with time during the MC's passage and with the location inside the magnetosheath. We examine the conditions encountered at the bow shock from direct observations, when Cluster or Geotail cross it, or indirectly by applying a magnetosheath model. We obtain a good agreement between the observed and modelled magnetic field direction and shock configuration, which varies from quasi-perpendicular to quasi-parallel in our study. We find that the variations in the angle between the magnetic fields in the solar wind and in the magnetosheath are anti-correlated with the variations in the shock obliquity. When the shock is in a quasi-parallel regime, the magnetic field direction varies significantly from the solar wind to the magnetosheath. In such cases, the magnetic field reaching the magnetopause cannot be approximated by the upstream magnetic field. Therefore, it is important to take into account the conditions at the bow shock when estimating the impact of an MC with the Earth's environment because these conditions are crucial in determining the magnetosheath magnetic field, which then interacts with the magnetosphere. (10.5194/angeo-32-1247-2014)
  DOI : 10.5194/angeo-32-1247-2014
• Kinetic simulations of electric field structure within magnetic island during magnetic reconnection and their applications to the satellite observations
  
  • Huang S. Y.
  • Zhou M.
  • Yuan Z. G.
  • Deng X. H.
  • Sahraoui Fouad
  • Pang Y.
  • Fu S. Y.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2014, 119, pp.7402-7412. islands are considered to play a crucial role in collisionless magnetic reconnection. We use particle-in-cell simulations to investigate electric field E<SUB>z</SUB> structure in the magnetic islands (including primary and secondary islands) with and without a guide field during magnetic reconnection. It is found that the electric field has multilayers in the primary island and a large bipolar structure in the secondary island in the absence of guide field. The electric field is provided by the Hall term (J × B)<SUB>z</SUB> (mainly), the divergence of electron pressure tensor, and the convective term (V<SUB>i</SUB> × B)<SUB>z</SUB> in the outer and the inner region of primary island, while the electric field is much smaller (~0) in the middle and the core region of primary island due to the cancelation of the three terms. The single bipolar electric field is primarily provided by the Hall term in the secondary island. In the presence of a guide field, the electric field has multiple layers in the primary island (similar to zero guide field case) and the secondary island. However, there still exists one single large sharp bipolar structure of electric field in the central region of the secondary island. The differences of electric field in the primary and secondary islands are essentially due to the variations of the current J<SUB>y</SUB>. These features can be used as the observational criteria to identify different types of magnetic islands in the magnetosphere using the data of future mission, such as the Magnetospheric Multiscale mission. (10.1002/2014JA020054)
  DOI : 10.1002/2014JA020054
• A model of the magnetosheath magnetic field during magnetic clouds
  
  • Turc Lucile
  • Fontaine Dominique
  • Savoini Philippe
  • Kilpua E.K.J.
  Annales Geophysicae, European Geosciences Union, 2014, 32 (2), pp.157-173. Magnetic clouds (MCs) are huge interplanetary structures which originate from the Sun and have a paramount importance in driving magnetospheric storms. Before reaching the magnetosphere, MCs interact with the Earth's bow shock. This may alter their structure and therefore modify their expected geoeffectivity. We develop a simple 3-D model of the magnetosheath adapted to MCs conditions. This model is the first to describe the interaction of MCs with the bow shock and their propagation inside the magnetosheath. We find that when the MC encounters the Earth centrally and with its axis perpendicular to the Sun–Earth line, the MC's magnetic structure remains mostly unchanged from the solar wind to the magnetosheath. In this case, the entire dayside magnetosheath is located downstream of a quasi-perpendicular bow shock. When the MC is encountered far from its centre, or when its axis has a large tilt towards the ecliptic plane, the MC's structure downstream of the bow shock differs significantly from that upstream. Moreover, the MC's structure also differs from one region of the magnetosheath to another and these differences vary with time and space as the MC passes by. In these cases, the bow shock configuration is mainly quasi-parallel. Strong magnetic field asymmetries arise in the mag-netosheath; the sign of the magnetic field north–south component may change from the solar wind to some parts of the magnetosheath. We stress the importance of the B x component. We estimate the regions where the magnetosheath and magnetospheric magnetic fields are anti-parallel at the mag-netopause (i.e. favourable to reconnection). We find that the location of anti-parallel fields varies with time as the MCs move past Earth's environment, and that they may be situated near the subsolar region even for an initially northward magnetic field upstream of the bow shock. Our results point out the major role played by the bow shock configuration in modifying or keeping the structure of the MCs unchanged. Note that this model is not restricted to MCs, it can be used to describe the magnetosheath magnetic field under an arbitrary slowly varying interplanetary magnetic field. (10.5194/angeo-32-157-2014)
  DOI : 10.5194/angeo-32-157-2014
• Quantified energy dissipation rates in the terrestrial bow shock: 2. Waves and dissipation
  
  • Wilson Iii L. B.
  • Sibeck David G.
  • Breneman A. W.
  • Le Contel Olivier
  • Cully C. M.
  • Turner D. L.
  • Angelopoulos V.
  • Malaspina D. M.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2014, 119 (8), pp.6475-6495. We present the first quantified measure of the energy dissipation rates, due to wave-particle interactions, in the transition region of the Earth's collisionless bow shock using data from the Time History of Events and Macroscale Interactions during Substorms spacecraft. Our results show that wave-particle interactions can regulate the global structure and dominate the energy dissipation of collisionless shocks. In every bow shock crossing examined, we observed both low-frequency (<10 Hz) and high-frequency (&#8819;10 Hz) electromagnetic waves throughout the entire transition region and into the magnetosheath. The low-frequency waves were consistent with magnetosonic-whistler waves. The high-frequency waves were combinations of ion-acoustic waves, electron cyclotron drift instability driven waves, electrostatic solitary waves, and whistler mode waves. The high-frequency waves had the following: (1) peak amplitudes exceeding deltaB 10 nT and deltaE 300 mV/m, though more typical values were deltaB 0.1-1.0 nT and deltaE 10-50 mV/m; (2) Poynting fluxes in excess of 2000 muW m<SUP>-2</SUP> (typical values were 1-10 muW m<SUP>-2</SUP>); (3) resistivities > 9000 Omega m; and (4) associated energy dissipation rates >10 muW m<SUP>-3</SUP>. The dissipation rates due to wave-particle interactions exceeded rates necessary to explain the increase in entropy across the shock ramps for 90% of the wave burst durations. For 22% of these times, the wave-particle interactions needed to only be <= 0.1% efficient to balance the nonlinear wave steepening that produced the shock waves. These results show that wave-particle interactions have the capacity to regulate the global structure and dominate the energy dissipation of collisionless shocks. (10.1002/2014JA019930)
  DOI : 10.1002/2014JA019930
• Finite ballooning angle effects on ion temperature gradient driven mode in gyrokinetic flux tube simulations
  
  • Singh Rameswar
  • Brunner Stephan
  • Ganesh R.
  • Jenko F.
  Physics of Plasmas, American Institute of Physics, 2014, 21 (3), pp.032115. This paper presents effects of finite ballooning angles on linear ion temperature gradient (ITG) driven mode and associated heat and momentum flux in Gyrokinetic flux tube simulation GENE. It is found that zero ballooning angle is not always the one at which the linear growth rate is maximum. The ITG mode acquires a short wavelength (SW) branch (k&#8869;&#961;i&#8201;>&#8201;1) when growth rates maximized over all ballooning angles are considered. However, the SW branch disappears on reducing temperature gradient showing characteristics of zero ballooning angle SWITG in case of extremely high temperature gradient. Associated heat flux is even with respect to ballooning angle and maximizes at nonzero ballooning angle while the parallel momentum flux is odd with respect to the ballooning angle. (10.1063/1.4868425)
  DOI : 10.1063/1.4868425
• Momentum transport in the vicinity of q<SUB>min</SUB> in reverse shear tokamaks due to ion temperature gradient turbulence
  
  • Singh Rameswar
  • Singh R.
  • Jhang Hogun
  • Diamond P.H.
  Physics of Plasmas, American Institute of Physics, 2014, 21 (1), pp.012302. This paper presents effects of finite ballooning angles on linear ion temperature gradient (ITG) driven mode and associated heat and momentum flux in Gyrokinetic flux tube simulation GENE. It is found that zero ballooning angle is not always the one at which the linear growth rate is maximum. The ITG mode acquires a short wavelength (SW) branch (k&#8869;&#961;i&#8201;>&#8201;1) when growth rates maximized over all ballooning angles are considered. However, the SW branch disappears on reducing temperature gradient showing characteristics of zero ballooning angle SWITG in case of extremely high temperature gradient. Associated heat flux is even with respect to ballooning angle and maximizes at nonzero ballooning angle while the parallel momentum flux is odd with respect to the ballooning angle. (10.1063/1.4861625)
  DOI : 10.1063/1.4861625
• Numerical computation of the modified plasma dispersion function with curvature
  
  • Gürcan Özgür D.
  Journal of Computational Physics, Elsevier, 2014, 269, pp.156-167. A particular generalization of the plasma dispersion function, which is linked to the regular plasma dispersion function via recurrence relations is discussed. The generalization allows a fast numerical implementation of a certain two-dimensional integral that appears in the description of the plasma dispersion in curved geometry, by reducing it to a single integral over a function involving the generalized plasma dispersion function. The local dielectric function of the toroidal ion temperature gradient driven mode can be written in terms of these integral functions. A matrix method is proposed to combine the consecutive integrals as a single 1D integral over a single integrand. The method allows two orders of magnitude speed up over the 2D integral implementation. Using various optimizations and an efficient implementation of the regular plasma dispersion function, further speed up is obtained. (C) 2014 Elsevier Inc. All rights reserved. (10.1016/j.jcp.2014.03.017)
  DOI : 10.1016/j.jcp.2014.03.017
• Experimental characterization of plasma formation and shockwave propagation induced by high power pulsed underwater electrical discharge
  
  • Claverie Alain
  • Deroy Julien
  • Boustié Michel
  • Avrillaud Gilles
  • Chuvatin Alexandre S.
  • Mazanchenko Ekaterina
  • Demol G.
  • Dramane B.
  Review of Scientific Instruments, American Institute of Physics, 2014, 85 (06), pp.063701. High power pulsed electrical discharges into liquids are investigated for new industrial applications based on the efficiency of controlled shock waves. We present here new experimental data obtained by combination of detailed high speed imaging equipments. It allows the visualization of the very first instants of plasma discharge formation, and then the pulsations of the gaseous bubble with an accurate timing of events. The time history of the expansion/compression of this bubble leads to an estimation of the energy effectively transferred to water during the discharge. Finally, the consecutive shock generation driven by this pulsating bubble is optically monitored by shadowgraphs and schlieren setup. These data provide essential information about the geometrical pattern and chronometry associated with the shock wave generation and propagation. (10.1063/1.4879715)
  DOI : 10.1063/1.4879715
• Propagation of lower-band whistler-mode waves in the outer Van Allen belt: Systematic analysis of 11 years of multi-component data from the Cluster spacecraft
  
  • Santolík O.
  • Macusova E.
  • Kolmasova Ivana
  • Cornilleau-Wehrlin Nicole
  • Conchy Y.
  Geophysical Research Letters, American Geophysical Union, 2014, 41, pp.2729-2737. Lower-band whistler-mode emissions can influence the dynamics of the outer Van Allen radiation belts. We use 11&#8201;years of measurements of the STAFF-SA instruments onboard the four Cluster spacecraft to systematically build maps of wave propagation parameters as a function of position. We determine probability distributions of wave vector angle weighted by the wave intensity. The results show that wave vector directions of intense waves are close to a Gaussian-shaped peak centered on the local magnetic field line. The width of this peak is between 10 and 20 degrees. The cumulative percentage of oblique waves is below 1015%. This result is especially significant for an important class of whistler-mode emissions of lower-band chorus at higher latitudes, well outside their source region, where a simple ray tracing model fails and another mechanism is necessary to keep the wave vectors close to the field-aligned direction. (10.1002/2014GL059815)
  DOI : 10.1002/2014GL059815
• Oxidation of isopropanol and acetone adsorbed on TiO<SUB>2</SUB> under plasma generated ozone flow: Gas phase and adsorbed species monitoring
  
  • Barakat Christelle
  • Gravejat Paul
  • Guaitella Olivier
  • Thévenet Frédéric
  • Rousseau Antoine
  Applied Catalysis B: Environmental, Elsevier [1992-....], 2014, 147, pp.302-313. The regeneration of isopropanol (IPA) and/or acetone saturated TiO2 surface by ozone is investigated. TiO2 catalyst is placed downstream a dielectric barrier discharge and is subsequently exposed to ozone considered as the main oxidative species generated by non-thermal plasma and able to interact with the material surface at room temperature. The oxidation of isopropanol and/or acetone is monitored using two parallel and complementary infrared diagnostics: (1) Fourier Transform Infrared Spectroscopy for the analysis of the gas phase composition; and (2) Diffuse Reflectance Infrared Fourier Transform Spectroscopy for the in situ analysis of the adsorbent/catalyst surface. In this study, the pollutant is first adsorbed on the TiO2 surface, the plasma being switched off. The irreversibly adsorbed amounts of isopropanol and acetone have been respectively quantified as 5.3 &#956;mol/m2 and 1.9 &#956;mol/m2. In a second step, the plasma is switched on to regenerate the surface by mineralization of the adsorbed organic species. A 70-min plasma phase, with approximately 20 ppm of ozone constantly flowing through the adsorbent bed yields 8.5 nmol and 8.9 nmol of CO2 per injected joule of energy for isopropanol and acetone saturated surfaces, respectively. Acetone has been evidenced as the main oxidation intermediate of isopropanol on TiO2 surface. It has been proven that the complete oxidation of isopropanol and acetone is mainly limited by the acetone oxidation rate. Competitive adsorption on the surface of the catalyst between both compounds has been studied. Results obtained are compared with those observed in the photocatalytic oxidation of the same species. (10.1016/j.apcatb.2013.09.008)
  DOI : 10.1016/j.apcatb.2013.09.008
• A nanosecond surface dieletric barrier discharge in air at high pressure and different polarities of applied pulses. Transition to filamentary mode
  
  • Stepanyan S.A.
  • Starikovskiy a Yu
  • Popov N.A.
  • Starikovskaia Svetlana
  Plasma Sources Science and Technology, IOP Publishing, 2014, 23 (4), pp.045003. The development of a nanosecond surface dielectric barrier discharge in air at pressures 16 bar is studied. At atmospheric pressure, the discharge develops as a set of streamers starting synchronously from the high-voltage electrode and propagating along the dielectric layer. Streamers cover the dielectric surface creating a 'quasi-uniform' plasma layer. At high pressures and high voltage amplitudes on the cathode, filamentation of the discharge is observed a few nanoseconds after the discharge starts. Parameters of the observed 'streamers-to-filaments' transition are measured; physics of transition is discussed on the basis of theoretical estimates and numerical modeling. Ionization-heating instability on the boundary of the cathode layer is suggested as a mechanism of filamentation. (10.1088/0963-0252/23/4/045003)
  DOI : 10.1088/0963-0252/23/4/045003
• Pulsed discharges in a wide density range: plasma development and media excitation
  
  • Starikovskaia Svetlana
  • Starikovskii A.Yu.
  , 2014. The Chapter demonstrates the peculiarities of pulsed nonequilibrium plasma generation in different media by nano- and picosecond high-voltage discharges. Fast ionization waves in long tubes at low pressures, streamers and dielectric barrier discharges at moderate and atmospheric pressures, surface dielectric barrier discharges at elevated pressures are considered. Special attention is given to start of picoseconds and nanosecond discharges in liquid media. The data used were consciously restricted to ICCD fast imaging of picoseconds and nanosecond discharges, to underline the dynamics of pulsed discharge development and the importance of spatial distribution of the discharge energy.
• Ionospheric disturbance dynamo associated to a coronal hole: Case study of 5-10 April 2010
  
  • Fathy Ibrahim
  • Amory-Mazaudier Christine
  • Fathy A.
  • Mahrous A. M.
  • Yumoto K.
  • Ghamry E.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2014, 119 (5), pp.4120–4133. In this paper we study the planetary magnetic disturbance during the magnetic storm occurring on 5 April 2010 associated with high-speed solar wind stream due to a coronal hole following a coronal mass ejection. We separate the magnetic disturbance associated to the ionospheric disturbance dynamo (Ddyn) from the magnetic disturbance associated to the prompt penetration of magnetospheric electric field (DP2). This event exhibits different responses of ionospheric disturbance dynamo in the different longitude sectors (European-African, Asian, and American). The strongest effect is observed in the European-African sector. The Ddyn disturbance reduces the amplitude of the daytime H component at low latitudes during four consecutive days in agreement with the Blanc and Richmond's model of ionospheric disturbance dynamo. The amplitude of Ddyn decreased with time during the 4 days. We discuss its diverse worldwide effects. The observed signature of magnetic disturbance process in specific longitude sector is strongly dependent on which Earth's side faces the magnetic storms (i.e., there is a different response depending on which longitude sector is at noon when the SSC hits). Finally, we determined an average period of 22 h for Ddyn using wavelet analysis. (10.1002/2013JA019510)
  DOI : 10.1002/2013JA019510
• Study of gas heating mechanisms in millisecond pulsed discharges and afterglows in air at low pressures
  
  • Pintassilgo C.D.
  • Guerra V.
  • Guaitella Olivier
  • Rousseau Antoine
  Plasma Sources Science and Technology, IOP Publishing, 2014, 23, pp.025006. A self-consistent model is developed to study the temporal variation of the gas temperature in millisecond single dc pulsed discharges and their afterglows in air-like mixtures (N220%O2) at low pressures. The model is based on the solutions to the time-dependent gas thermal balance equation, under the assumption of a parabolic gas temperature profile across the discharge tube, coupled to the electron, vibrational and chemical kinetics. Modelling results provide a satisfactory explanation for recently published time-resolved experimental data for the gas temperature in a 5 ms pulsed air plasma with a current of 150mA and the corresponding afterglow at a pressure of 133 Pa (1 Torr). It is shown that the main heating mechanisms during the first millisecond of the pulse come predominantly from O2 dissociation by electron impact through the pre-dissociative excited state O2(B 3&#8722; u ) and the quenching of nitrogen electronically excited states N2(A 3 u , B 3 g, a 1&#8722; u , a 1 g, w 1u) by O2, agreeing with other studies on fast gas heating in air plasmas. As the pulse duration increases, other gas heating sources become important, namely VT N2O energy exchanges, recombination of oxygen atoms at the wall, N2(A) quenching by O(3P) and reaction N(4S) NO(X) &#8594; N2(X, v &#8764;3) O, contributing altogether to an additional smooth increase in the gas temperature until the end of the pulse. In the first instants of the early afterglow, the gas temperature decreases very rapidly as a consequence of the minor role played by electronic collisions and due to a fast decay of N2 electronic states. For afterglow times up to 10 ms, the gas temperature continues to decrease, following the time-dependent kinetics of [N2(X,v)], [N(4S)], [O(3P)] and [NO(X)]. Sensitivity of the model to different input parameters such as thermal accommodation coefficient and probabilities for atomic recombination at the wall are reported. (10.1088/0963-0252/23/2/025006)
  DOI : 10.1088/0963-0252/23/2/025006
• A radio-frequency sheath model for complex waveforms
  
  • Turner M.M.
  • Chabert Pascal
  Applied Physics Letters, American Institute of Physics, 2014, 104 (16), pp.164102. Plasma sheaths driven by radio-frequency voltages occur in contexts ranging from plasma processing to magnetically confined fusion experiments. An analytical understanding of such sheaths is therefore important, both intrinsically and as an element in more elaborate theoretical structures. Radio-frequency sheaths are commonly excited by highly anharmonic waveforms, but no analytical model exists for this general case. We present a mathematically simple sheath model that is in good agreement with earlier models for single frequency excitation, yet can be solved for arbitrary excitation waveforms. As examples, we discuss dual-frequency and pulse-like waveforms. The model employs the ansatz that the time-averaged electron density is a constant fraction of the ion density. In the cases we discuss, the error introduced by this approximation is small, and in general it can be quantified through an internal consistency condition of the model. This simple and accurate model is likely to have wide application. (10.1063/1.4872172)
  DOI : 10.1063/1.4872172
• Electron heating in capacitively coupled plasmas revisited
  
  • Lafleur Trevor
  • Chabert Pascal
  • Booth Jean-Paul
  Plasma Sources Science and Technology, IOP Publishing, 2014, 23 (3), pp.035010. We revisit the problem of electron heating in capacitively coupled plasmas (CCPs), and propose a method for quantifying the level of collisionless and collisional heating in plasma simulations. The proposed procedure, based on the electron mechanical energy conservation equation, is demonstrated with particle-in-cell simulations of a number of single and multi-frequency CCPs operated in regimes of research and industrial interest. In almost all cases tested, the total electron heating is comprised of collisional (ohmic) and pressure heating parts. This latter collisionless component is in qualitative agreement with the mechanism of electron heating predicted from the recent re-evaluation of theoretical models. Finally, in very electrically asymmetric plasmas produced in multi-frequency discharges, we observe an additional collisionless heating mechanism associated with electron inertia. (10.1088/0963-0252/23/3/035010)
  DOI : 10.1088/0963-0252/23/3/035010
• Ambipolar and non-ambipolar diffusion in an rf plasma source containing a magnetic filter
  
  • Lafleur Trevor
  • Aanesland Ane
  Physics of Plasmas, American Institute of Physics, 2014, 21, pp.063510. By placing a magnetic filter across a rectangular plasma source (closed at one end with a ceramic plate and an rf antenna, and terminated at the opposite end by a grounded grid), we experimentally investigate the effect of conducting and insulating source walls on the nature of the plasma diffusion phenomena. The use of a magnetic filter creates a unique plasma, characterized by a high upstream electron temperature (Teu&#8764;5 eV) near the rf antenna and a low downstream electron temperature (Ted&#8764;1 eV) near the grid, which more clearly demonstrates the role of the source wall materials. For conducting walls a net ion current to ground is measured on the grid, and the plasma potential is determined by a mean electron temperature within the source. For insulating walls the plasma potential is determined by the downstream electron temperature (i.e., Vp&#8764;5.2Ted in argon), and the net current to the grid is exactly zero. Furthermore, by inserting a small additional upstream conductor (that can be made floating or grounded through an external circuit switch), we demonstrate that the plasma potential can be controlled and set to a low (Vp&#8764;5.2Ted), or high (Vp&#8764;5.2Teu) value. (10.1063/1.4885109)
  DOI : 10.1063/1.4885109
• Surface deactivation of vibrationally excited N<SUB>2</SUB> studied using infrared titration combined with quantum cascade laser absorption spectroscopy
  
  • Marinov Daniil
  • Lopatik D.
  • Guaitella Olivier
  • Ionikh Y.
  • Röpcke J.
  • Rousseau Antoine
  Journal of Physics D: Applied Physics, IOP Publishing, 2014, 47 (1), pp.015203. The wall de-excitation probability of vibrationally excited nitrogen molecules was determined using infrared (IR) titration with CO, CO 2 and N 2 O. Gas mixtures of N 2 with 0.05?0.5% of CO (CO 2 or N 2 O) were excited by a pulsed dc discharge at p = 133 Pa in a cylindrical discharge tube. During the afterglow, the vibrational relaxation of titrating molecules was monitored in situ with quantum cascade laser absorption spectroscopy. The value of was deduced from measured vibrational relaxation times using a model of vibrational kinetics in N 2 . It was found that adsorption of IR tracers on the surface may increase the value of by a factor up to two, depending on the molecule and the surface material. It was demonstrated that N 2 O is the most inert and reliable tracer and it was used for the determination of on silica, Pyrex, TiO 2 , Al 2 O 3 and anodized aluminum. Pretreatment of the silica surface by low-pressure plasma was found to have a strong effect on the vibrational de-excitation. Values of measured after O 2 , Ar and N 2 plasma pretreatment of the same silica discharge tube were 5.7 × 10 ?4 , 8.2 × 10 ?4 and 11 × 10 ?4 , respectively. This study clearly demonstrates that the presence of adsorbed atoms and molecules on the surface may significantly alter the value of . (10.1088/0022-3727/47/1/015203)
  DOI : 10.1088/0022-3727/47/1/015203
• Theory for helical turbulence under fast rotation
  
  • Galtier Sébastien
  Physical Review E, American Physical Society (APS), 2014, 89, pp.41001. Recent numerical simulations have shown the strong impact of helicity on homogeneous rotating hydrodynamic turbulence. The main effect can be summarized through the law n ñ=-4, where n and ñ are the power law indices of the one-dimensional energy and helicity spectra, respectively. We investigate this rotating turbulence problem in the small Rossby number limit by using the asymptotic weak turbulence theory derived previously. We show that the empirical law is an exact solution of the helicity equation where the power law indices correspond to perpendicular (to the rotation axis) wave number spectra. It is proposed that when the cascade towards small scales tends to be dominated by the helicity flux the solution tends to ñ=-2, whereas it is ñ=-3/2 when the energy flux dominates. The latter is compatible with the solution previously observed numerically and derived theoretically in the weak turbulence regime when only the energy equation is used, whereas the former solution is constrained by a locality condition. (10.1103/PhysRevE.89.041001)
  DOI : 10.1103/PhysRevE.89.041001
• Investigation of Switch Designs for the Dynamic Load Current Multiplier Scheme on the SPHYNX Microsecond Linear Transformer Driver
  
  • Maysonnave Thomas
  • Bayol Frédéric
  • Demol Gauthier
  • d'Almeida Thierry
  • Lassalle Francis
  • Morell Alain
  • Grunenwald Julien
  • Chuvatin Alexandre S.
  • Pecastaing Laurent
  • de Ferron Antoine Silvestre
  IEEE Transactions on Plasma Science, Institute of Electrical and Electronics Engineers, 2014, 42 (10), pp.2974-2980. SPHINX is a microsecond linear transformer driver LTD, used essentially for implosion of Z-pinch loads in direct drive mode. It can deliver a 6-MA current pulse within 800 ns into a Z-pinch load. The dynamic load current multiplier concept enables the current pulse to be modified by increasing its amplitude while reducing its rise time before being delivered to the load. This compact system is made up of concentric electrodes (autotransformer), a dynamic flux extruder (cylindrical wire array), a vacuum convolute (eight postholes), and a vacuum closing switch, which is the key component of the system. Several different schemes are investigated for designing a vacuum switch suitable for operating the dynamic load current multiplier on the SPHINX generator for various applications, including isentropic compression experiments and Z-pinch radiation effects studies. In particular, the design of a compact vacuum surface switch and a multichannel vacuum switch, located upstream of the load are studied. Electrostatic simulations supporting the switch designs are presented along with test bed experiments. Initial results from shots on the SPHINX driver are also presented. (10.1109/TPS.2014.2313372)
  DOI : 10.1109/TPS.2014.2313372
• Plasma composition and ion acceleration in the PEGASES thruster
  
  • Renaud D.
  • Mazouffre S.
  • Aanesland Ane
  Space Propulsion, 2014 (2969109).
• Investigation of drift velocity effects on the EDGE and SOL transport.
  
  • Leybros Robin
  • Bufferand H.
  • Ciraolo G.
  • Fedorczak N.
  • Ghendrih Ph.
  • Hennequin Pascale
  • Marandet Y.
  • Serre E.
  • Schwander F.
  • Tamain P.
  , 2014.
• The double well mass filter
  
  • Gueroult R
  • Fisch Nathaniel J.
  • Rax Jean-Marcel
  Physics of Plasmas, American Institute of Physics, 2014, 21 (2), pp.020701. Various mass filter concepts based on rotating plasmas have been suggested with the specific purpose of nuclear waste remediation. We report on a new rotating mass filter combining radial separation with axial extraction. The radial separation of the masses is the result of a “double-well” in effective radial potential in rotating plasma with a sheared rotation profile. (10.1063/1.4864325)
  DOI : 10.1063/1.4864325
• Electron energy distributions in a magnetized inductively coupled plasma
  
  • Song Sang-Heon
  • Yang Yang
  • Chabert Pascal
  • Kushner M.J.
  Physics of Plasmas, American Institute of Physics, 2014, 21 (9), pp.093512. Optimizing and controlling electron energy distributions (EEDs) is a continuing goal in plasma materials processing as EEDs determine the rate coefficients for electron impact processes. There are many strategies to customize EEDs in low pressure inductively coupled plasmas (ICPs), for example, pulsing and choice of frequency, to produce the desired plasma properties. Recent experiments have shown that EEDs in low pressure ICPs can be manipulated through the use of static magnetic fields of sufficient magnitudes to magnetize the electrons and confine them to the electromagnetic skin depth. The EED is then a function of the local magnetic field as opposed to having non-local properties in the absence of the magnetic field. In this paper, EEDs in a magnetized inductively coupled plasma (mICP) sustained in Ar are discussed with results from a two-dimensional plasma hydrodynamics model. Results are compared with experimental measurements. We found that the character of the EED transitions from non-local to local with application of the static magnetic field. The reduction in cross-field mobility increases local electron heating in the skin depth and decreases the transport of these hot electrons to larger radii. The tail of the EED is therefore enhanced in the skin depth and depressed at large radii. Plasmas densities are non-monotonic with increasing pressure with the external magnetic field due to transitions between local and non-local kinetics. (10.1063/1.4896711)
  DOI : 10.1063/1.4896711