Publications

2013

• Capacitively coupled radio-frequency plasmas excited by tailored voltage waveforms
  
  • Lafleur Trevor
  • Delattre Pierre-Alexandre
  • Johnson E.V.
  • Booth Jean-Paul
  Plasma Physics and Controlled Fusion, IOP Publishing, 2013, 55 (12), pp.124002. By applying certain types of ?tailored? voltage waveforms (TVWs) to capacitively coupled plasmas, a dc self-bias and an asymmetric plasma response can be produced, even in geometrically symmetric reactors. Furthermore, these arbitrary applied waveforms can produce a number of interesting phenomena that are not present in typical single-frequency sinusoidal discharges. This electrical asymmetry effect presents emerging possibilities for the improved control of the ion energy and ion flux in these systems; parameters of vital importance to both etching and deposition applications for materials processing. With a combined research approach utilizing both experimental measurements, and particle-in-cell simulations, we review and extend recent investigations that study a particular class of TVW. The waveforms used have a pulse-type shape and are composed of a varying number of harmonic frequencies. This allows a strong self-bias to be produced, and causes most of the applied voltage to be dropped across a single sheath. Additionally, decreasing the pulse width (by increasing the number of harmonics), allows the plasma density and ion flux to be increased. Simulation and experimental results both demonstrate that this type of waveform can be used to separately control the ion flux and ion energy, while still producing a uniform plasma over large area (50 cm diameter) rf electrodes. (10.1088/0741-3335/55/12/124002)
  DOI : 10.1088/0741-3335/55/12/124002
• Anomalous collisionality in low-pressure plasmas
  
  • Lafleur Trevor
  • Chabert Pascal
  • Turner M.M.
  • Booth Jean-Paul
  Physics of Plasmas, American Institute of Physics, 2013, 20 (12), pp.124503. Based on a theoretical argument from fundamental kinetic theory, by way of simple worked examples, and through the use of particle-in-cell simulations of capacitively coupled plasmas, we demonstrate that conventional methods for calculating the momentum transfer collision frequency in low-pressure plasmas can be seriously erroneous. This potentially plays an important and previously unconsidered role in many low-pressure discharges, and at least in part provides a possible explanation for anomalous behaviour often encountered in these plasmas. (10.1063/1.4859155)
  DOI : 10.1063/1.4859155
• Action diffusion and lifetimes of quasistationary states in the Hamiltonian Mean Field model
  
  • Ettoumi Wahb
  • Firpo Marie-Christine
  Physical Review E, American Physical Society (APS), 2013, 87, pp.030102(R). Out-of-equilibrium quasistationary states (QSSs) are one of the signatures of a broken ergodicity in long-range interacting systems. For the widely studied Hamiltonian Mean-Field model, the lifetime of some QSSs has been shown to diverge with the number N of degrees of freedom with a puzzling N¹.7 scaling law, contradicting the otherwise widespread N scaling law. It is shown here that this peculiar scaling arises from the locality properties of the dynamics captured through the computation of the diffusion coefficient in terms of the action variable. The use of a mean first passage time approach proves to be successful in explaining the non-trivial scaling at stake here, and sheds some light on another case, where lifetimes diverging as e^N above some critical energy have been reported. (10.1103/PhysRevE.87.030102)
  DOI : 10.1103/PhysRevE.87.030102
• Study of reconnection physics using laser-generated B fields
  
  • Smets Roch
  • Belmont Gérard
  • Aunai N.
  • Fuchs J. C.
  • Boniface C.
  • Aulanier G.
  , 2013.
• Commutation rapide déclenchée par filamentation laser femtoseconde
  
  • Larour Jean
  • Arantchouk Léonid
  • Houard Aurélien
  Flash X - La revue scientifique de l'Ecole polytechnique, Ecole polytechnique, 2013, 15, pp.55-57. La revue scientifique de l'Ecole polytechnique (parution annuelle) ISSN : 1775-0385 Le phénomène de filamentation laser dans lair a été décrit dans un précédent numéro (Flash X n° 12) par A. Houard et A. Mysyrowicz, chercheurs du LOA (Laboratoire dOptique appliquée). Rappelons ici quil apparaît spontanément le long dun faisceau laser de durée femtoseconde (fs) quand sa puissance dépasse un seuil critique de quelques GW. un équilibre dynamique entre la diffraction naturelle du faisceau, leffet Kerr, et la défocalisation par lair ionisé, maintient lintensité dans le coeur du faisceau sur une distance qui peut dépasser un mètre, entraînant la formation dun canal de plasma dans le sillage de limpulsion. Le point de démarrage, la longueur des canaux de plasma et leur distribution transverse sont contrôlables en jouant sur les paramètres de limpulsion laser initiale (énergie, durée, diamètre du faisceau).
• Interaction of plasma transport and turbulence on particle fuelling
  
  • Tamain Patrick
  • Bonhomme Gérard
  • Brochard Frédéric
  • Clairet Frédéric
  • Gil C.
  • Gunn J.
  • Hennequin P.
  • Hornung G.
  • Segui J. L.
  • Vermare L.
  • Ghendrih Philippe
  • Team Tore Supra
  Journal of Nuclear Materials, Elsevier, 2013, 438 (S), pp.S148-S154. We report the results of an experimental investigation of the impact of Supersonic Molecular Beam Injection in the Tore Supra tokamak. Several diagnostics were synchronised with the injection to extract a global picture of the physics at play from the time scale of turbulence (similar to 10 mu s) to the full-recovery time (similar to 1 s). As previously reported, a strong impact of the injection on density and temperature profiles is observed. Both fields exhibit a complex dynamic response involving different phases and time scales. In particular, we show that the effective particle fuelling efficiency is determined by a period of degraded confinement that follows the injection, during which the edge density collapses, in some cases, lower than the initial one. This phase is characterised by a dramatic change in the turbulent transport, with a drop of the frequency spectrum and the observation of large coherent structures as opposed to small intermittent fluctuations before the injection. (10.1016/j.jnucmat.2013.01.023)
  DOI : 10.1016/j.jnucmat.2013.01.023
• On the supply of heavy planetary material to the magnetotail of Mercury
  
  • Delcourt Dominique C.
  Annales Geophysicae, European Geosciences Union, 2013, 31 (10), pp.1673-1679. We examine the transport of low-energy heavy ions of planetary origin (O + , Na + , Ca +) in the magneto-sphere of Mercury. We show that, in contrast to Earth, these ions are abruptly energized after ejection into the magneto-sphere due to enhanced curvature-related parallel acceleration. Regardless of their mass-to-charge ratio, the parallel speed of these ions is rapidly raised up to ∼ 2 V E×B (denoting by V E×B the magnitude of the local E × B drift speed), in a like manner to Fermi-type acceleration by a moving magnetic mirror. This parallel energization is such that ions with very low initial energies (a few tenths of eVs) can overcome gravity and, regardless of species or convection rate, are transported over comparable distances into the night-side magnetosphere. The region of space where these ions reach the magnetotail is found to extend over altitudes similar to those where enhanced densities are noticeable in the MESSENGER data, viz., from ∼ 1000 km up to ∼ 6000 km in the pre-midnight sector. The observed density enhancements may thus follow from E × B related focusing of planetary material of dayside origin into the magnetotail. Due to the planetary magnetic field offset, an asymmetry is found between drift paths anchored in the Northern and Southern hemispheres, which puts forward a predominant role of heavy material originating in the Northern Hemisphere in populating the innermost region of Mercury's magnetotail. (10.5194/angeo-31-1673-2013)
  DOI : 10.5194/angeo-31-1673-2013
• Plasma/surface interaction: example of air plasmas and plasma bullets
  
  • Guaitella Olivier
  , 2013.
• Theory of a Strip Loop Antenna Located on the Surface of an Axially Magnetized Plasma Column
  
  • Kudrin A. V.
  • Zaitseva Anna S
  • Zaboronkova T. M.
  • Krafft C.
  • Kyriacou George A
  Progress In Electromagnetics Research B, EMW Publishing, 2013, 51, pp.221--246. We study the current distribution and input impedance of a circular loop antenna in the form of an infinitesimally thin, perfectly conducting narrow strip coiled into a ring. The antenna is located on the surface of an axially magnetized plasma column surrounded by a homogeneous isotropic medium. The current in the antenna is excited by a time-harmonic voltage creating an electric field with the azimuthal component in a gap of small angular opening on the strip surface. The emphasis is placed on the solution of the integral equations for the azimuthal harmonics of the antenna current in the case where the magnetoplasma inside the column is nonresonant. The properties of the kernels of the integral equations are discussed and the current distribution in the antenna is obtained. It is shown that the presence of a magnetized plasma column can significantly influence the electrodynamic characteristics of the antenna compared with the case where it is located in the surrounding medium or a homogeneous plasma medium the parameters of which coincide with those inside the column. (10.2528/PIERB13032304)
  DOI : 10.2528/PIERB13032304
• Influence of the dissipation mechanism on collisionless magnetic reconnection in symmetric and asymmetric current layers
  
  • Aunai Nicolas
  • Hesse Michael
  • Black Carrie
  • Evans Rebekah
  • Kuznetsova M. M.
  Physics of Plasmas, American Institute of Physics, 2013, 20, pp.2901. Numerical studies implementing different versions of the collisionless Ohm's law have shown a reconnection rate insensitive to the nature of the non-ideal mechanism occurring at the X line, as soon as the Hall effect is operating. Consequently, the dissipation mechanism occurring in the vicinity of the reconnection site in collisionless systems is usually thought not to have a dynamical role beyond the violation of the frozen-in condition. The interpretation of recent studies has, however, led to the opposite conclusion that the electron scale dissipative processes play an important dynamical role in preventing an elongation of the electron layer from throttling the reconnection rate. This work re-visits this topic with a new approach. Instead of focusing on the extensively studied symmetric configuration, we aim to investigate whether the macroscopic properties of collisionless reconnection are affected by the dissipation physics in asymmetric configurations, for which the effect of the Hall physics is substantially modified. Because it includes all the physical scales a priori important for collisionless reconnection (Hall and ion kinetic physics) and also because it allows one to change the nature of the non-ideal electron scale physics, we use a (two dimensional) hybrid model. The effects of numerical, resistive, and hyper-resistive dissipation are studied. In a first part, we perform simulations of symmetric reconnection with different non-ideal electron physics. We show that the model captures the already known properties of collisionless reconnection. In a second part, we focus on an asymmetric configuration where the magnetic field strength and the density are both asymmetric. Our results show that contrary to symmetric reconnection, the asymmetric model evolution strongly depends on the nature of the mechanism which breaks the field line connectivity. The dissipation occurring at the X line plays an important role in preventing the electron current layer from elongating and forming plasmoids. (10.1063/1.4795727)
  DOI : 10.1063/1.4795727
• Space Research in Africa. Some Achievements from 2007 to 2012
  
  • Amory-Mazaudier Christine
  • Fleury Rolland
  Sun and Geosphere, BBC SWS Regional Network, 2013, 2 (8), pp.65-70. This article presents the results of a research network Europe Africa established in 1995 after the International Electrojet Equatorial Year (1992-1994). During the last decade, this research network has been involved in two international projects: the International Heliophysical Year (2007-2009) and International Space Weather Initiative (2010-2012).The participation in these international projects increased the number of PhD and multiplied the number of scientific papers. Many scientific results have been obtained. Teaching and working methods have been also developed. We emphasize in this article the last two points.
• Negative ion extraction from hydrogen plasma bulk
  
  • Oudini N.
  • Taccogna F.
  • Minelli P.
  • Aanesland Ane
  • Raimbault Jean-Luc
  Physics of Plasmas, American Institute of Physics, 2013, 20 (10), pp.103506. A two-dimensional particle-in-cell/Monte Carlo collision model has been developed and used to study low electronegative magnetized hydrogen plasma. A configuration characterized by four electrodes is used: the left electrode is biased at Vl = −100 V, the right electrode is grounded, while the upper and lower transversal electrodes are biased at an intermediate voltage Vud between 0 and −100 V. A constant and homogeneous magnetic field is applied parallel to the lateral (left/right) electrodes. It is shown that in the magnetized case, the bulk plasma potential is close to the transversal electrodes bias inducing then a reversed sheath in front of the right electrode. The potential drop within the reversed sheath is controlled by the transversal electrodes bias allowing extraction of negative ions with a significant reduction of co-extracted electron current. Furthermore, introducing plasma electrodes, between the transversal electrodes and the right electrode, biased with a voltage just above the plasma bulk potential, increases the negative ion extracted current and decreases significantly the co-extracted electron current. The physical mechanism on basis of this phenomenon has been discussed. (10.1063/1.4825246)
  DOI : 10.1063/1.4825246
• Isopropanol saturated TiO<SUB>2</SUB> surface regeneration by non-thermal plasma : Influence of air relative humidity
  
  • Sivachandiran Loganathan
  • Thévenet Frédéric
  • Gravejat Paul
  • Rousseau Antoine
  Chemical Engineering Journal, Elsevier, 2013, 214, pp.17-26. Environmental regulation on air quality requires the development of energetic efficient volatile organic compounds (VOCs) abatement techniques. Adsorption, photocatalysis, non-thermal plasma and their combinations have been widely studied for VOC treatment. Even if the plasma material (sorbent or catalyst) association appears as one of the most efficient configuration for VOC removal, it mainly consists in operating continuously the discharge on the material surface as long as the effluent flows across the reactor. This work aims at investigating another approach of plasma material association for VOC removal: in a first step, the material is used as a sorbent until the complete coverage of adsorption sites; in a second step, once VOC saturation is achieved, the discharge is ignited on the material surface. During both steps, the influence of air relative humidity (RH) is investigated in order to evaluate its impact on the process. The objectives of our approach are: (i) the reduction of energy consumption; (ii) the increase of sorbent life-times by efficient regeneration; (iii) the investigation of plasma interaction with VOC saturated materials; (iv) the investigation of air RH influence on such VOC treatment process. A packed bed reactor coated with TiO2 has been designed. IPA is used as a model VOC. First, injected power in the packed-bed reactor is characterized as a function of air RH. Complete coverage of TiO2 surface over 35% RH is suggested as a significant parameter. Then, adsorption of IPA on TiO2 was monitored until IPA breakthrough. The amount of IPA adsorbed per TiO2 surface unit is compared to values reported by other authors. The influence of air RH on reversibly and irreversibly adsorbed IPA fractions is investigated. Over 35% RH irreversible adsorption is favored, adsorption modes are discussed. Plasma regeneration of IPA saturated TiO2 surface leads simultaneously to IPA desorption and mineralization. Increasing air RH favors IPA mineralization and diminishes acetone production. Carbon balance obtained after 1 h plasma treatment reaches 91% in the presence of 50% RH. A thermal treatment is performed after each plasma treatment in order to evidence plasma insensitive adsorbed species and to restore TiO2 initial surface state. 97% of the carbon balance is collected under 50% RH after thermal treatment. During the thermal step, acetone and CO2 are mainly produced, their formation pathways are discussed. (10.1016/j.cej.2012.10.022)
  DOI : 10.1016/j.cej.2012.10.022
• Cavitation in the vicinity of the high-voltage electrode as a key step of nanosecond breakdown in liquids
  
  • Marinov Ilya
  • Guaitella Olivier
  • Rousseau Antoine
  • Starikovskaia Svetlana
  Plasma Sources Science and Technology, IOP Publishing, 2013, 22, pp.042001. Fast shadowgraphy of nanosecond discharge in liquids with different dielectric permittivity, namely in water, ethanol and n-pentane, has been performed. Formation of a gas cavity at a nanosecond time scale was observed as a pre-breakdown phenomenon at amplitudes of the high-voltage pulse close to the breakdown threshold. This phenomenon is considered as a possible key step of high-voltage breakdown in polar liquids. (10.1088/0963-0252/22/4/042001)
  DOI : 10.1088/0963-0252/22/4/042001
• Direct observation of ozone formation on SiO<SUB>2</SUB> surfaces in O<SUB>2</SUB> discharges
  
  • Marinov Daniil
  • Guaitella Olivier
  • Booth Jean-Paul
  • Rousseau Antoine
  Journal of Physics D: Applied Physics, IOP Publishing, 2013, 46, pp.032001. Ozone production is studied in a pulsed O2 discharge at pressures in the range 1.36.7 mbar. Time-resolved absolute concentrations of O3 and O are measured in the post-discharge using UV absorption spectroscopy and two-photon absorption laser-induced fluorescence. In a bare silica discharge tube ozone is formed mainly by three-body gas-phase recombination. When the tube surface is covered by a high specific surface silica catalyst heterogeneous formation becomes the main source of ozone. The efficiency of this surface process increases with O2 pressure and is favoured by the presence of OH groups and adsorbed H2O on the surface. At p = 6.7 mbar ozone production accounts for up to 25% of the atomic oxygen losses on the surface. (10.1088/0022-3727/46/3/032001)
  DOI : 10.1088/0022-3727/46/3/032001
• Radio-frequency capacitively coupled plasmas excited by tailored voltage waveforms: comparison of experiment and particle-in-cell simulations
  
  • Delattre Pierre-Alexandre
  • Lafleur Trevor
  • Johnson Erik
  • Booth Jean-Paul
  Journal of Physics D: Applied Physics, IOP Publishing, 2013, 46, pp.235201. Using a range of different diagnostics we have performed a detailed experimental characterization of a capacitively coupled rf plasma discharge excited by tailored voltage waveforms in argon (3?13 Pa). The applied pulse-type tailored waveforms consist of between 1 and 5 harmonics (with a fundamental of 15 MHz), and are used to generate an electrically asymmetric plasma response, manifested by the formation of a strong dc bias in the geometrically symmetric reactor used. Experimental measurements of the dc bias, electron density, ion current density, ion-flux energy distributions at the electrodes and discharge current waveforms, are compared with a one-dimensional particle-in-cell simulation for the same operating conditions. The experimental and simulation results are found to be in good agreement over the range of parameters investigated, and demonstrate a number of unique features present with pulse-type tailored waveforms, including: increased plasma density and ion flux with the number of harmonics, and a broader control range of the ion bombarding energy. (10.1088/0022-3727/46/23/235201)
  DOI : 10.1088/0022-3727/46/23/235201
• Dynamics of tilted eddies in a transversal flow at the edge of tokamak plasmas and the consequences for L-H transition
  
  • Fedorczak N.
  • Ghendrih Philippe
  • Hennequin Pascale
  • Tynan G.R.
  • Diamond P.H.
  • Manz P.
  Plasma Physics and Controlled Fusion, IOP Publishing, 2013, 55 (12), pp.124024. The dynamical interaction between eddies and shear flow is investigated through a simplified model of vorticity conservation with tilted eddies. Energy is transferred either to the flow or to eddies, depending on the eddy tilt with respect to the flow shear. When eddies are tilted in the shear direction, the system is favorable to shear increase: tilt instability (TI) or the negative viscosity phenomenon. When eddies are tilted in the opposite direction, the shear flow is damped via a Kelvin-Helmholtz (KH) process. The TI generally dominates the interaction on the largest radial scale, but a fraction of the energy cascades to smaller radial scales through the alternation of tilting and KH dynamics. Within this eddy description, we show that the symmetry breaking required to generate a net residual stress is set by the intrinsic eddy tilt. We recall that magnetic shear can provide an intrinsic tilt to ballooning modes at the edge of tokamak plasmas, with an orientation which depends on flux surface geometry. In L-mode weak shear regimes, this residual stress can dominate the Reynolds stress. Coupled to momentum sources acting in the scrape-off layer, it can induce a significant difference of the edge radial electric field between lower single null and upper single null geometries. A comparison with experimental profiles measured across the edge of Tore Supra L-mode plasmas is discussed. (10.1088/0741-3335/55/12/124024)
  DOI : 10.1088/0741-3335/55/12/124024
• ECH effects on toroidal rotation: KSTAR experiments, intrinsic torque modelling and gyrokinetic stability analyses
  
  • Shi Y. J.
  • Ko W. H.
  • Kwon J.M.
  • Diamond P.H.
  • Lee S. G.
  • Ko S. H.
  • Wang L.
  • Yi Sukyoung
  • Ida K.
  • Terzolo L.
  • Yoon S. W.
  • Lee K. D.
  • Lee J. H.
  • Nam U. N.
  • Bae Y. S.
  • Oh Y. K.
  • Kwak J. G.
  • Bitter M.
  • Hill K.
  • Gürcan Özgür D.
  • Hahm T.S.
  Nuclear Fusion, IOP Publishing, 2013, 53 (11). Toroidal rotation profiles have been investigated in KSTAR H-mode plasma using combined auxiliary heating by co-neutral beam injection (NBI) and electron cyclotron resonance heating (ECH). The ion temperature and toroidal rotation are measured with x-ray imaging crystal spectroscopy and charge exchange recombination spectroscopy. H-mode plasma is achieved using co-current 1.3MW NBI, and a 0.35MW ECH pulse is added to the flat-top of H-mode. The core rotation profiles, which are centrally peaked in the pure NBI heating phase, flatten when ECH is injected, while the edge pedestal is unchanged. Dramatic decreases in the core toroidal rotation values (Delta V-tor/V-tor similar to -30%) are observed when on-axis ECH is added to H-mode. The experimental data show that the decrease of core rotation velocity and its gradient are correlated with the increase of core electron temperature and its gradient, and also with the likely steepening of the density gradient. We thus explore the viability of a hypothesized ITG (ITG ion temperature gradient instability) -> TEM (trapped electron mode instability) transition as the explanation of the observed counter-current flow induced by ECH. However, the results of linear microstability analyses using inferred profiles suggest that the TEM is excited only in the deep core, so the viability of the hypothesized explanation is not yet clear. (10.1088/0029-5515/53/11/113031)
  DOI : 10.1088/0029-5515/53/11/113031
• Remote sensing of a NTC radio source from a Cluster tilted spacecraft pair
  
  • Décréau Pierrette
  • Kougblénou S.
  • Lointier G.
  • Rauch Jean-Louis
  • Trotignon Jean-Gabriel
  • Vallières X.
  • Canu Patrick
  • Rochel Grimald S.
  • El-Lemdani Mazouz Farida
  • Darrouzet F.
  Annales Geophysicae, European Geosciences Union, 2013, 31 (11), pp.2097-2121. The Cluster mission operated a "tilt campaign" during the month of May 2008. Two of the four identical Cluster spacecraft were placed at a close distance (~50 km) from each other and the spin axis of one of the spacecraft pair was tilted by an angle of ~46°. This gave the opportunity, for the first time in space, to measure global characteristics of AC electric field, at the sensitivity available with long boom (88 m) antennas, simultaneously from the specific configuration of the tilted pair of satellites and from the available base of three satellites placed at a large characteristic separation (~1 RE). This paper describes how global characteristics of radio waves, in this case the configuration of the electric field polarization ellipse in 3-D-space, are identified from in situ measurements of spin modulation features by the tilted pair, validating a novel experimental concept. In the event selected for analysis, non-thermal continuum (NTC) waves in the 15-25 kHz frequency range are observed from the Cluster constellation placed above the polar cap. The observed intensity variations with spin angle are those of plane waves, with an electric field polarization close to circular, at an ellipticity ratio e = 0.87. We derive the source position in 3-D by two different methods. The first one uses ray path orientation (measured by the tilted pair) combined with spectral signature of magnetic field magnitude at source. The second one is obtained via triangulation from the three spacecraft baseline, using estimation of directivity angles under assumption of circular polarization. The two results are not compatible, placing sources widely apart. We present a general study of the level of systematic errors due to the assumption of circular polarization, linked to the second approach, and show how this approach can lead to poor triangulation and wrong source positioning. The estimation derived from the first method places the NTC source region in the dawn sector, at a large L value (L ~ 10) and a medium geomagnetic latitude (35° S). We discuss these untypical results within the frame of the geophysical conditions prevailing that day, i.e. a particularly quiet long time interval, followed by a short increase of magnetic activity. (10.5194/angeo-31-2097-2013)
  DOI : 10.5194/angeo-31-2097-2013
• Chlorine atom densities in the (3p<SUP>5</SUP>)<SUP>2</SUP> P<SUP>0</SUP><SUB>1/2</SUB> excited spin-orbit state measured by two-photon absorption laser-induced fluorescence in a chlorine inductively coupled plasma
  
  • Sirse Nishant
  • Booth Jean-Paul
  • Chabert Pascal
  • Surzhykov A.
  • Indelicato P.
  Journal of Physics D: Applied Physics, IOP Publishing, 2013, 46 (29), pp.295203. Chlorine atom densities in the spinorbit excited state were measured by two-photon absorption laser-induced fluorescence (TALIF) in an inductively coupled plasma discharge in pure Cl2. The atoms were excited by two photons at 235.702 nm to the state and detected by fluorescence to the (4s) 4P5/2 state at 726 nm. The population of this state relative to that in the ground state, was determined from the relative TALIF signal intensity from the two states, combined with new calculations of the two-photon absorption cross-sections. was found to increase continuously with radio-frequency power (50500 W), whereas with Cl2 pressure (590 mTorr) it passes through a maximum at 10 mTorr, reaching ~30% at 500 W. This maximum corresponds to the maximum of electron density in the discharge. Combining this density ratio measurement with previous measurements of the absolute ground state chlorine atom density [1] allows the absolute spin-orbit excited state density to be estimated. A significant fraction of the total chlorine atom density is in this excited state which should be included in plasma chemistry models. (10.1088/0022-3727/46/29/295203)
  DOI : 10.1088/0022-3727/46/29/295203
• Spatio-temporal evolution of the H -> L back transition
  
  • Miki K.
  • Diamond P.H.
  • Schmitz L.
  • Mcdonald D. C.
  • Estrada T.
  • Gürcan Özgür D.
  • Tynan G.R.
  Physics of Plasmas, American Institute of Physics, 2013, 20 (6), pp.062304. Since ITER will operate close to threshold and with limited control, the H&#8201;&#8594;&#8201;L back transition is a topic important for machine operations as well as physics. Using a reduced mesoscale model [Miki et al., Phys. Plasmas 19, 092306 (2012)], we investigate ELM-free H&#8201;&#8594;&#8201;L back transition dynamics in order to isolate transport physics effects. Model studies indicate that turbulence spreading is the key process which triggers the back transition. The transition involves a feedback loop linking turbulence and profiles. The I-phase appears during the back transition following a slow power ramp down, while fast ramp-downs reveal a single burst of zonal flow during the back transition. The I-phase nucleates at the pedestal shoulder, as this is the site of the residual turbulence in H-mode. Hysteresis in the profile gradient scale length is characterized by the Nusselt number, where Nu = &#967;i,turb/&#967;i,neo. Relative hysteresis of temperature gradient vs density gradient is sensitive to the pedestal Prandtl number, where Prped = Dped/&#967;i,neo. We expect the H-mode to be somewhat more resilient in density than in temperature. (10.1063/1.4812555)
  DOI : 10.1063/1.4812555
• Microsecond ramp compression of a metallic liner driven by a 5 MA current on the SPHINX machine using a dynamic load current multiplier pulse shaping
  
  • d'Almeida Thierry
  • Lassalle Francis
  • Morell Alain
  • Grunenwald Julien
  • Zucchini Frédéric
  • Loyen Arnaud
  • Maysonnave Thomas
  • Chuvatin Alexandre S.
  Physics of Plasmas, American Institute of Physics, 2013, 20 (09), pp.092512. SPHINX is a 6 MA, 1-&#956;s Linear Transformer Driver (LTD) operated by the CEA Gramat (France) and primarily used for imploding Z-pinch loads for radiation effects studies. Among the options that are currently being evaluated to improve the generator performances are an upgrade to a 20&#8201;MA, 1-&#956;s LTD machine and various power amplification schemes, including a compact Dynamic Load Current Multiplier (DLCM). A method for performing magnetic ramp compression experiments, without modifying the generator operation scheme, was developed using the DLCM to shape the initial current pulse in order to obtain the desired load current profile. In this paper, we discuss the overall configuration that was selected for these experiments, including the choice of a coaxial cylindrical geometry for the load and its return current electrode. We present both 3-D Magneto-hydrodynamic and 1D Lagrangian hydrodynamic simulations which helped guide the design of the experimental configuration. Initial results obtained over a set of experiments on an aluminium cylindrical liner, ramp-compressed to a peak pressure of 23&#8201;GPa, are presented and analyzed. Details of the electrical and laser Doppler interferometer setups used to monitor and diagnose the ramp compression experiments are provided. In particular, the configuration used to field both homodyne and heterodyne velocimetry diagnostics in the reduced access available within the liner's interior is described. Current profiles measured at various critical locations across the system, particularly the load current, enabled a comprehensive tracking of the current circulation and demonstrate adequate pulse shaping by the DLCM. The liner inner free surface velocity measurements obtained from the heterodyne velocimeter agree with the hydrocode results obtained using the measured load current as the input. An extensive hydrodynamic analysis is carried out to examine information such as pressure and particle velocity history profiles or magnetic diffusion across the liner. The potential of the technique in terms of applications and achievable ramp pressure levels lies in the prospects for improving the DLCM efficiency through the use of a closing switch (currently under development), reducing the load dimensions and optimizing the diagnostics. (10.1063/1.4823720)
  DOI : 10.1063/1.4823720
• Autocalibration Method for Anisotropic Magnetoresistive Sensors Using Offset Coils
  
  • Mohamadabadi K.
  • Jeandet Alexis
  • Hillion M.
  • Coillot Christophe
  IEEE Sensors Journal, Institute of Electrical and Electronics Engineers, 2013, 13 (2), pp.772-776. In this paper, we present a zero-cost indoor calibration method for anisotropic magnetoresistive (AMR) sensors. The implemented circuit is designed to calibrate AMR sensors using integrated coils. A microcontroller is used to generate an artificial three-dimensional magnetic field by injecting three separate currents into three offset coils. We show the similarity of the results for residual calibration norm by using this method compared with the calibration of the sensor in free Earth's magnetic field. Furthermore, this method does not need any other instruments such as Helmholtz coils or a platform for rotating the sensor. Here the sensor is placed inside a mu-metal box during calibration, and the calibration process is completely autonomous. (10.1109/JSEN.2012.2227595)
  DOI : 10.1109/JSEN.2012.2227595
• Energetic electron acceleration by unsteady magnetic reconnection
  
  • Fu H.S.
  • Khotyaintsev Y. V.
  • Vaivads A.
  • Retinò Alessandro
  • André M.
  Nature Physics, Nature Publishing Group [2005-....], 2013, 9, pp.426-430. The mechanism that produces energetic electrons during magnetic reconnection is poorly understood. This is a fundamental process responsible for stellar flares, substorms, and disruptions in fusion experiments. Observations in the solar chromosphere and the Earth's magnetosphere indicate significant electron acceleration during reconnection, whereas in the solar wind, energetic electrons are absent. Here we show that energetic electron acceleration is caused by unsteady reconnection. In the Earth's magnetosphere and the solar chromosphere, reconnection is unsteady, so energetic electrons are produced; in the solar wind, reconnection is steady, so energetic electrons are absent. The acceleration mechanism is quasi-adiabatic: betatron and Fermi acceleration in outflow jets are two processes contributing to electron energization during unsteady reconnection. The localized betatron acceleration in the outflow is responsible for at least half of the energy gain for the peak observed fluxes. (10.1038/nphys2664)
  DOI : 10.1038/nphys2664
• Inner radiation belt particle acceleration and energy structuring by drift resonance with ULF waves during geomagnetic storms
  
  • Delcourt Dominique C.
  • Benoist C.
  • Penou E.
  • Chen Y.
  • Russell C. T.
  • Sauvaud J.-A.
  • Walt M.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2013, 118 (4), pp.1723-1736. Geomagnetic storms are frequently associated with the formation of multiple bands of energetic electrons inside the inner radiation belt at L = 1.1-1.9 and with prominent energy structures of protons inside the slot region at L = 2.2-3.5. These structures typically from 100 keV up to the MeV range result from coherent interactions of energetic particles with quasi-monochromatic ultra-low frequency (ULF) waves. These waves are induced by magnetospheric changes due to the arrival of dense solar material and related nightside injections of particles from the outer magnetosphere that destabilize field lines in the inner magnetosphere down to L = 1.1. Using low-altitude data from the polar orbiting Demeter spacecraft, we perform case and statistical studies of these structures. We show that with such a spacecraft, these structures are best seen near the South Atlantic Anomaly because of lowering of the belt particle mirror point. As evidenced from ground measurements, energy bands are associated with quasi-sinusoidal ULF Pc5 and Pc4 waves with periods in the 1000 s range for L = 1.1-1.9 and in the 60 s range for L = 2.2-3.5. Numerical simulations of the coherent drift resonance of energetic particles with ultra-low frequency waves show how the particles are accelerated and how the observed structures build up. (10.1002/jgra.50125)
  DOI : 10.1002/jgra.50125