Publications

2010

• Operation of a load current multiplier on a nanosecond mega-ampere pulse forming line generator
  
  • Chuvatin Alexandre S.
  • Kantsyrev Viktor L.
  • Rudakov Leonid I.
  • Cuneo Michael E.
  • Astanovitskiy A. L.
  • Presura Radu
  • Safronova Alla S.
  • Cline W.
  • Williamson Kenneth M.
  • Shrestha Ishor
  • Osborne Glenn C.
  • Le Galloudec B.
  • Nalajala Vidya
  • Pointon T. D.
  • Mikkelson K. A.
  Physical Review Special Topics: Accelerators and Beams, American Physical Society, 2010, 13 (1), pp.010401. We investigate the operation of a load current multiplier (LCM) on a pulse-forming-line nanosecond pulse-power generator. Potential benefits of using the LCM technique on such generators are studied analytically for a simplified case. A concrete LCM design on the Zebra accelerator (1.9 Ohm, ∼1  MA, 100 ns) is described. This design is demonstrated experimentally with high-voltage power pulses having a rise time of dozens of nanoseconds. Higher currents and magnetic energies were observed in constant-inductance solid-state loads when a better generator-to-load energy coupling was achieved. The load current on Zebra was increased from the nominal 0.80.9 MA up to about 1.6 MA. This result was obtained without modifying the generator energetics or architecture and it is in good agreement with the presented numerical simulations. Validation of the LCM technique at a nanosecond time scale is of importance for the high-energy-density physics research. (10.1103/PhysRevSTAB.13.010401)
  DOI : 10.1103/PhysRevSTAB.13.010401
• A simple model of intrinsic rotation in high confinement regime tokamak plasmas
  
  • Gürcan Özgür D.
  • Diamond P.H.
  • Mcdevitt C.J.
  • Hahm T.S.
  Physics of Plasmas, American Institute of Physics, 2010, 17, pp.032509. A simple unified model of intrinsic rotation and momentum transport in high confinement regime (H-mode) tokamak plasmas is presented. Motivated by the common dynamics of the onset of intrinsic rotation and the L-H transition, this simple model combines E×B shear-driven residual stress in the pedestal with a turbulent equipartition pinch to yield rotation profiles. The residual stress is the primary mechanism for buildup of intrinsic rotation in the H-mode pedestal, while the pinch drives on-axis peaking of rotation profiles. Analytical estimates for pedestal flow velocities are given in terms of the pedestal width, the pedestal height, and various model parameters. The predicted scaling of the toroidal flow speed with pedestal width is found to be consistent with the International Tokamak Physics Activity database global scaling of the flow speed on-axis with the total plasma stored energy. (10.1063/1.3339909)
  DOI : 10.1063/1.3339909
• Scientific objectives and instrumentation of Mercury Plasma Particle Experiment (MPPE) onboard MMO
  
  • Saito Y.
  • Sauvaud J.-A.
  • Hirahara M.
  • Barabash S.
  • Delcourt Dominique C.
  • Takashima T.
  • Asamura K.
  Planetary and Space Science, Elsevier, 2010, 58 (1-2), pp.182-200. Mercury is one of the least explored planets in our solar system. Until the recent flyby of Mercury by MESSENGER, no spacecraft had visited Mercury since Mariner 10 made three flybys: two in 1974 and one in 1975. In order to elucidate the detailed plasma structure and dynamics around Mercury, an orbiter BepiColombo MMO (Mercury Magnetospheric Orbiter) is planned to be launched in 2013 as a joint mission between ESA and ISAS/JAXA. Mercury Plasma Particle Experiment (MPPE) was proposed in order to investigate the plasma/particle environment around Mercury. MPPE is a comprehensive instrument package for plasma, high-energy particle and energetic neutral atom measurements. It consists of seven sensors: two Mercury electron analyzers (MEA1 and MEA2), Mercury ion analyzer (MIA), Mercury mass spectrum analyzer (MSA), high-energy particle instrument for electron (HEP-ele), high-energy particle instrument for ion (HEP-ion), and energetic neutrals analyzer (ENA). Since comprehensive full three-dimensional simultaneous measurements of low to high-energy ions and electrons around Mercury as well as measurements of energetic neutral atoms will not be realized before BepiColombo/MMO's arrival at Mercury, it is expected that many unresolved problems concerning the Mercury magnetosphere will be elucidated by the MPPE observation. (10.1016/j.pss.2008.06.003)
  DOI : 10.1016/j.pss.2008.06.003
• The Plasma Wave Investigation (PWI) onboard the BepiColombo/MMO: First measurement of electric fields, electromagnetic waves, and radio waves around Mercury
  
  • Kasaba Y.
  • Bougeret J.-L.
  • Blomberg L. G.
  • Kojima H.
  • Yagitani S.
  • Moncuquet M.
  • Trotignon Jean-Gabriel
  • Chanteur Gérard
  • Kumamoto A.
  • Kasahara Y.
  • Lichtenberger J.
  • Omura Y.
  • Ishisaka K.
  • Matsumoto H.
  Planetary and Space Science, Elsevier, 2010, 58 (1-2), pp.238-278. The BepiColombo Mercury Magnetospheric Orbiter (MMO) spacecraft includes the plasma and radio wave observation system called Plasma Wave Investigation (PWI). Since the receivers for electric field, plasma waves, and radio waves are not installed in any of the preceding spacecraft to Mercury, the PWI will provide the first opportunity for conducting in-situ and remote-sensing observations of electric fields, plasma waves, and radio waves in the Hermean magnetosphere and exosphere. These observations are valuable in studying structure, dynamics, and energy exchange processes in the unique magnetosphere of Mercury. They are characterized by the key words of the non-MHD environment and the peculiar interaction between the relatively large planet without ionosphere and the solar wind with high dynamic pressure. The PWI consists of three sets of receivers (EWO, SORBET, and AM<SUP>2</SUP>P), connected to two sets of electric field sensors (MEFISTO and WPT) and two kinds of magnetic field sensors (LF-SC and DB-SC). The PWI will observe both waveforms and frequency spectra in the frequency range from DC to 10 MHz for the electric field and from 0.3 Hz to 640 kHz for the magnetic field. From 2008, we will start the development of the engineering model, which is conceptually consistent with the flight model design. The present paper discusses the significance and objectives of plasma/radio wave observations in the Hermean magnetosphere, and describes the PWI sensors, receivers and their performance as well as the onboard data processing. (10.1016/j.pss.2008.07.017)
  DOI : 10.1016/j.pss.2008.07.017
• Chorus source region localization in the Earth's outer magnetosphere using THEMIS measurements
  
  • Agapitov O
  • Krasnoselskikh V
  • Zaliznyak Yu
  • Angelopoulos V
  • Le Contel Olivier
  • Rolland G
  Annales Geophysicae, European Geosciences Union, 2010, 28, pp.1377–1386. Discrete ELF/VLF chorus emissions, the most intense electromagnetic plasma waves observed in the Earth's radiation belts and outer magnetosphere, are thought to propagate roughly along magnetic field lines from a localized source region near the magnetic equator towards the magnetic poles. THEMIS project Electric Field Instrument (EFI) and Search Coil Magnetometer (SCM) measurements were used to determine the spatial scale of the chorus source lo-calization region on the day side of the Earth's outer magne-tosphere. We present simultaneous observations of the same chorus elements registered onboard several THEMIS spacecraft in 2007 when all the spacecraft were in the same orbit. Discrete chorus elements were observed at 0.15–0.25 of the local electron gyrofrequency, which is typical for the outer magnetosphere. We evaluated the Poynting flux and wave vector distribution and obtained chorus wave packet quasi-parallel propagation to the local magnetic field. Amplitude and phase correlation data analysis allowed us to estimate the characteristic spatial correlation scale transverse to the local magnetic field to be in the 2800–3200 km range. Keywords. Electromagnetics (Random media and rough surfaces) – Magnetospheric physics (Plasma waves and in-stabilities) – Radio science (Remote sensing) (10.5194/angeo-28-1377-2010)
  DOI : 10.5194/angeo-28-1377-2010
• Formation of a sodium ring in Mercury's magnetosphere
  
  • Yagi Manabu
  • Seki K.
  • Matsumoto Y.
  • Delcourt Dominique C.
  • Leblanc François
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2010, 115 (A10), pp.A10253. We have performed a statistical analysis of exospheric sodium ion paths in Mercury's magnetosphere under northward interplanetary magnetic field conditions. Electric and magnetic field models used in the simulation were obtained from a global MHD simulation model, whereas the initial conditions of test Na+ ions were derived from a sodium exosphere model. We observe the formation of a ring-shaped high-pressure region consisting of energetic sodium ions traveling around the planet close to the equatorial plane. The configuration of this "sodium ring" as well as the acceleration processes leading to its formation strongly depend on the solar wind conditions. When the dynamic pressure is low, most of the Na+ are picked up in the magnetosphere and accelerated by the large-scale convective electric field. In contrast, in the case of high dynamic pressure, ions that are picked up in the magnetosheath and penetrate into the magnetosphere significantly contribute to the sodium ring. The configuration of this ring also depends upon the intensity of the solar wind electric field. Our analysis reveals that the pressure built by the Na+ ions may be significant as compared to the MHD pressure around the planet. (10.1029/2009JA015226)
  DOI : 10.1029/2009JA015226
• Non adiabatic electron behavior through a supercritical perpendicular collisionless shock: Impact of the shock front turbulence
  
  • Savoini Philippe
  • Lembège Bertrand
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2010, 115 (A11), pp.A11103. Adiabatic and nonadiabatic electrons transmitted through a supercritical perpendicular shock wave are analyzed with the help of test particle simulations based on field components issued from 2 − D full-particle simulation. A previous analysis (Savoini et al., 2005) based on 1 − D shock profile, including mainly a ramp (no apparent foot) and defined at a fixed time, has identified three distinct electron populations: adiabatic, overadiabatic, and underadiabatic, respectively, identified by μds/μus ≈ 1, >1 and <1, where μus and μds are the magnetic momenta in the upstream and downstream regions. Presently, this study is extended by investigating the impact of the time evolution of 2 − D shock front dynamics on these three populations. Analysis of individual time particle trajectories is performed and completed by statistics based on the use of different upstream velocity distributions (spherical shell of radius vshell and a Maxwellian with thermal velocity vthe). In all statistics, the three electron populations are clearly recovered. Two types of shock front nonstationarity are analyzed. First, the impact of the nonstationarity along the shock normal (due to the front self-reformation only) strongly depends on the values of vshell or vthe. For low values, the percentages of adiabatic and overadiabatic electrons are almost comparable but become anticorrelated under the filtering impact of the self-reformation; the percentage of the underadiabatic population remains almost unchanged. In contrast, for large values, this impact becomes negligible and the adiabatic population alone becomes dominant. Second, when 2 − D nonstationarity effects along the shock front (moving rippling) are fully included, all three populations are strongly diffused, leading to a larger heating; the overadiabatic population becomes largely dominant (and even larger than the adiabatic one) and mainly contributes to the energy spectrum. (10.1029/2010JA015381)
  DOI : 10.1029/2010JA015381
• A comparison of global models for the solar wind interaction with Mars
  
  • Brain D.
  • Barabash S.
  • Boesswetter A.
  • Bougher S.
  • Brecht S.
  • Chanteur Gérard
  • Hurley D.
  • Dubinin Eduard
  • Fang X.
  • Fraenz M.
  • Halekas J.
  • Harnett E.
  • Holmstrom M.
  • Kallio E.
  • Lammer H.
  • Ledvina S.
  • Liemohn M.
  • Liu K.
  • Luhmann J.
  • Ma Y.
  • Modolo Ronan
  • Nagy A.
  • Motschmann U.
  • Nilsson Hans
  • Shinagawa H.
  • Simon Sunil
  • Terada N.
  Icarus, Elsevier, 2010, 206 (1), pp.139-151. We present initial results from the first community-wide effort to compare global plasma interaction model results for Mars. Seven modeling groups participated in this activity, using MHD, multi-fluid, and hybrid assumptions in their simulations. Moderate solar wind and solar EUV conditions were chosen, and the conditions were implemented in the models and run to steady state. Model output was compared in three ways to determine how pressure was partitioned and conserved in each model, the location and asymmetry of plasma boundaries and pathways for planetary ion escape, and the total escape flux of planetary oxygen ions. The two participating MHD models provided similar results, while the five sets of multi-fluid and hybrid results were different in many ways. All hybrid results, however, showed two main channels for oxygen ion escape (a pickup ion 'plume' in the hemisphere toward which the solar wind convection electric field is directed, and a channel in the opposite hemisphere of the central magnetotail), while the MHD models showed one (a roughly symmetric channel in the central magnetotail). Most models showed a transition from an upstream region dominated by plasma dynamic pressure to a magnetosheath region dominated by thermal pressure to a low altitude region dominated by magnetic pressure. However, calculated escape rates for a single ion species varied by roughly an order of magnitude for similar input conditions, suggesting that the uncertainties in both the current and integrated escape over martian history as determined by models are large. These uncertainties are in addition to those associated with the evolution of the Sun, the martian dynamo, and the early atmosphere, highlighting the challenges we face in constructing Mars' past using models. (10.1016/j.icarus.2009.06.030)
  DOI : 10.1016/j.icarus.2009.06.030
• Correction to 'Simulation of electric field and current during the 11 June 1993 disturbance dynamo event: Comparison with the observations
  
  • Zaka K. Z.
  • Kobea A. T.
  • Doumbia V.
  • Richmond A. D.
  • Maute A.
  • Mene N. M.
  • Obrou O. K.
  • Assamoi P.
  • Boka K.
  • Adohi J.-P.
  • Amory-Mazaudier Christine
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2010, 115 (A12), pp.A12314. Not Available (10.1029/2010JA016292)
  DOI : 10.1029/2010JA016292
• Scaling and anisotropy in magnetohydrodynamic turbulence in a strong mean magnetic field
  
  • Grappin Roland
  • Müller Wolf-Christian
  Physical Review E, American Physical Society (APS), 2010, 82 (2), pp.26406. We present an analysis of the anisotropic spectral energy distribution in incompressible magnetohydrodynamic turbulence permeated by a strong mean magnetic field. The turbulent flow is generated by high-resolution pseudospectral direct numerical simulations with large-scale isotropic forcing. Examining the radial energy distribution for various angles &#952; with respect to B0 reveals a specific structure which remains hidden when not taking axial symmetry with respect to B0 into account. For each direction, starting at the forced large scales, the spectrum first exhibits an amplitude drop around a wave number k0 which marks the start of a scaling range and goes on up to a dissipative wave number kd(&#952;). The three-dimensional spectrum for k&#8805;k0 is described by a single &#952;-independent functional form F(k/kd), with the scaling law being the same in every direction. The previous properties still hold when increasing the mean field from B0=5 up to B0=10brms, as well as when passing from resistive to ideal flows. We conjecture that at fixed B0 the direction-independent scaling regime is reached when increasing the Reynolds number above a threshold which raises with increasing B0. Below that threshold critically balanced turbulence is expected. (10.1103/PhysRevE.82.026406)
  DOI : 10.1103/PhysRevE.82.026406
• On the efficiency of intrinsic rotation generation in tokamaks
  
  • Kosuga Y.
  • Diamond P.H.
  • Gürcan Özgür D.
  Physics of Plasmas, American Institute of Physics, 2010, 17, pp.102313. A theory of the efficiency of the plasma flow generation process is presented. A measure of the efficiency of plasma self-acceleration of mesoscale and mean flows from the heat flux is introduced by analogy with engines, using the entropy budget defined by thermal relaxation and flow generation. The efficiency is defined as the ratio of the entropy destruction rate due to flow generation to the entropy production rate due to &#8711;T relaxation (i.e., related to turbulent heat flux). The efficiencies for two different cases, i.e., for the generation of turbulent driven E×B shear flow (zonal flow) and for toroidal intrinsic rotation, are considered for a stationary state, achieved by balancing entropy production rate and destruction rate order by order in O(k&#8741;/k&#8869;), where k is the wave number. The efficiency of intrinsic toroidal rotation is derived and shown to be eIR&#8764;(Mach)2th&#8764;0.01. The scaling of the efficiency of intrinsic rotation generation is also derived and shown to be &#961;2&#8727;(q2/s2)(R2/L2T)=&#961;2&#8727;(L2s/L2T), which suggests a machine size scaling and an unfavorable plasma current scaling which enters through the shear length. (10.1063/1.3496055)
  DOI : 10.1063/1.3496055
• Simulations of radical and ion fluxes on wafer in a Cl<SUB>2</SUB>/Ar ICP discharge : Confrontation with GaAs and GaN etch experiments
  
  • Despiau-Pujo Emilie
  • Chabert Pascal
  • Bansropun S.
  • Thenot D.
  • Plouhinec P.
  • Cassette S.
  Journal of Vacuum Science and Technology, American Vacuum Society (AVS), 2010, 28 (4), pp.693. A two-dimensional fluid model is used to study an industrial Ar/Cl<SUB>2</SUB> inductively coupled plasma discharge designed to etch III-V samples. The effect of rf power, gas pressure, and chlorine content on the fluxes of reactive species reaching the wafer is numerically investigated. To understand how the etch process is influenced by the discharge conditions, simulation results are confronted with GaAs and GaN etch experiments performed in the same reactor geometry. When the source power is increased, the measured etch rate increase is consistent with the Cl radical and ion fluxes increase shown in the simulation, as well as the ion energy decrease due to the constant value of the wafer-holder power. Increasing the gas pressure results in a moderate increase in the etch rate due to the lower magnitude, lower mean energy, and anisotropy of the ion flux at high pressure. When the chlorine content is increased, the total ion flux decreases while Cl and Cl2 neutral fluxes increase significantly. A good correlation is obtained between calculated fluxes and etch characteristics, analyzed with scanning electron microscope images of etch profiles. (10.1116/1.3437492)
  DOI : 10.1116/1.3437492
• Electron drift velocity in SF<SUB>6</SUB> in strong electric fields determined from rf breakdown curves
  
  • Lisovskiy V.
  • Yegorenkov V.
  • Booth Jean-Paul
  • Landry K.
  • Douai D.
  • Cassagne V.
  Journal of Physics D: Applied Physics, IOP Publishing, 2010, 43, pp.385203. This paper presents measurements of the electron drift velocity Vdr in SF6 gas for high reduced electric fields (E/N = 3305655&#8201;Td (1&#8201;Td = 10e&#8722;17&#8201;V&#8201;cm2)). The drift velocities were obtained using the method of Lisovskiy and Yegorenkov (1998 J. Phys. D: Appl. Phys. 31 3349) based on the determination of the pressure and voltage of the turning points of rf capacitive discharge breakdown curves for a range of electrode spacings. The Vdr values thus obtained were in good agreement with those calculated from the cross-sections of Phelps and Van Brunt (1988 J. Appl. Phys. 64 4269) using the BOLSIG code. The validity of the LisovskiyYegorenkov method is discussed and we show that it is applicable over the entire E/N range where rf discharge ignition at breakdown occurs for rf frequencies of 13.56&#8201;MHz or above. (10.1088/0022-3727/43/38/385203)
  DOI : 10.1088/0022-3727/43/38/385203
• STAFF Instrument Products Distributed Through the Cluster Active Archive
  
  • Cornilleau-Wehrlin Nicole
  • Mirioni Laurent
  • Robert Patrick
  • Bouzid V.
  • Maksimovic M.
  • de Conchy Y.
  • Harvey Cc
  • Santolík O.
  , 2010, pp.159--168.
• Strike-point splitting induced by external magnetic perturbations: Observations on JET and MAST and associated modelling
  
  • Nardon Eric
  • Cahyna P.
  • Devaux S.
  • Kirk A.
  • Alfier A.
  • Luna Elena de La
  • de Temmerman G.
  • Denner P.
  • Eich T.
  • Gerbaud Thomas
  • Harting D.
  • Jachmich S.
  • Koslowski H. R.
  • Liang Y.
  • Sun Y.
  Journal of Nuclear Materials, Elsevier, 2010, 1, pp.S914 - S917. Heat flux deposition profiles, calculated from infrared imaging, are presented for JET and MAST cases in the presence of external magnetic perturbations. In L-mode, a clear splitting of the strike-point is observed and well reproduced by modelling based on field line tracing with no plasma magnetic response included (vacuum field approximation). In H-mode, in contrast, the splitting is not clearly observed in spite of being expected from vacuum field modelling. As illustrated by modelling results, screening effects could explain the absence of a clear splitting in H-mode. (C) 2010 Elsevier B.V. All rights reserved.Assoc EURATOM IPP CR, Inst Plasma Phys AS CR Vvi, Prague, Czech Republic.Max Planck Inst Plasma Phys, D-85748 Garching, Germany.Culham Sci Ctr, EURATOM CCFE Fus Assoc, Abingdon OX14 3DB, Oxon, England.Assoc Euratom Enea Fus, Consorzio RFX, I-35127 Padua, Italy.Asociac EURATOM CIEMAT, Lab Nacl Fus, Madrid, Spain.FOM Inst Plasma Phys Rijnhuizen, NL-3439 MN Nieuwegein, Netherlands.Ecole Polytech, CNRS, UMR 7648, LPTP, F-91128 Palaiseau, France.Forschungszentrum Julich, Assoc EURATOM FZJ, D-52425 Julich, Germany.TEC, EURATOM Belgian State Assoc, LPP ERM KMS, Brussels, Belgium. (10.1016/j.jnucmat.2010.10.072)
  DOI : 10.1016/j.jnucmat.2010.10.072
• Entropy production rate in tokamaks with nonaxisymmetric magnetic fields
  
  • Garbet X.
  • Abiteboul J.
  • Trier Elisée
  • Gürcan Özgür D.
  • Sarazin Y.
  • Smolyakov A.
  • Allfrey S.
  • Bourdelle C.
  • Fenzi C.
  • Grandgirard Virginie
  • Ghendrih Philippe
  • Hennequin Pascale
  Physics of Plasmas, American Institute of Physics, 2010, 17, pp.072505. A variational principle based on an extremum of entropy production rate is derived for an integrable Hamiltonian system in the presence of two perturbations, which are resonant on neighboring or identical resonant surfaces. This method can be used to calculate the neoclassical transport in a magnetic fusion device with a nonaxisymmetric magnetic field. When applied to a tokamak with ripple, it provides an efficient means to discriminate between various regimes, depending on collisionality and ripple amplitude. This technique provides a means to test simplified collision operators suitable for gyrokinetic codes. It is found that these operators need only satisfy modest requirements in order to recover the correct neoclassical equilibrium, at least in the large aspect ratio limit. Finally, it appears that the toroidal viscous damping rate due to ripple is usually not negligible, thus removing the degeneracy between the radial electric field and the toroidal velocity. (10.1063/1.3454365)
  DOI : 10.1063/1.3454365
• Error analysis of dimensionless scaling experiments with multiple points using linear regression
  
  • Gürcan Özgür D.
  • Vermare Laure
  • Hennequin Pascale
  • Bourdelle C.
  Nuclear Fusion, IOP Publishing, 2010, 50, pp.022003. A general method of error estimation in the case of multiple point dimensionless scaling experiments, using linear regression and standard error propagation, is proposed. The method reduces to the previous result of Cordey (2009 Nucl. Fusion 49 052001) in the case of a two-point scan. On the other hand, if the points follow a linear trend, it explains how the estimated error decreases as more points are added to the scan. Based on the analytical expression that is derived, it is argued that for a low number of points, adding points to the ends of the scanned range, rather than the middle, results in a smaller error estimate. (10.1088/0029-5515/50/2/022003)
  DOI : 10.1088/0029-5515/50/2/022003
• Identifying the Driver of Pulsating Aurora
  
  • Nishimura Y.
  • Bortnik J.
  • Li W.
  • Thorne R. M.
  • Lyons L.R.
  • Angelopoulos V.
  • Mende S. B.
  • Bonnell J. W.
  • Le Contel Olivier
  • Cully C. M.
  • Ergun R.
  • Auster U.
  Science, American Association for the Advancement of Science (AAAS), 2010, 330 (6000), pp.81-84. Pulsating aurora, a spectacular emission that appears as blinking of the upper atmosphere in the polar regions, is known to be excited by modulated, downward-streaming electrons. Despite its distinctive feature, identifying the driver of the electron precipitation has been a long-standing problem. Using coordinated satellite and ground-based all-sky imager observations from the THEMIS mission, we provide direct evidence that a naturally occurring electromagnetic wave, lower-band chorus, can drive pulsating aurora. Because the waves at a given equatorial location in space correlate with a single pulsating auroral patch in the upper atmosphere, our findings can also be used to constrain magnetic field models with much higher accuracy than has previously been possible. (10.1126/science.1193186)
  DOI : 10.1126/science.1193186
• Laboratory modeling of the interaction of electron beams with a magnetoplasma
  
  • Starodubtsev M. V.
  • Krafft C.
  Radiophysics and Quantum Electronics, Springer Verlag, 2010, 53 (7), pp.401-416. We present the results of laboratory experiments in which the mechanisms of interaction of electron beams with whistler waves in a magnetoplasma are studied. Different mechanisms of whistler generation during the injection of a modulated electron beam in the plasma are studied, and the mechanism of conversion of the beam kinetic energy to radiation is demonstrated. The processes of whistler wave generation by the modulated beam at the &#711; Cerenkov and Doppler resonances are analyzed in detail. The excitation of whistler waves by means of a nonresonant mechanism of the transition radiation is studied. (10.1007/s11141-010-9238-4)
  DOI : 10.1007/s11141-010-9238-4
• Analysis of equatorial noise using data from the Cluster and Themis missions
  
  • Hrbackova Z.
  • Santolík O.
  • Pickett J. S.
  • Gurnett D. A.
  • Cornilleau-Wehrlin Nicole
  • Le Contel Olivier
  • Krupar Vratislav
  , 2010, 38, pp.4040. We report the results of the analysis of equatorial noise (EN) using data from the Cluster and Themis spacecraft missions. EN is an intense electromagnetic wave emission that propagates close to the geomagnetic equator between the local proton cyclotron and local lower hybrid frequencies. Recent studies have shown that these waves might play a significant role in the acceleration of electrons to relativistic energies in the outer Van Allen radiation belt. The orbit of the Cluster mission has changed over the last two years, providing us with a larger and statistically more meaningful database from which to carry out our study of EN occurrence. We use onboard analyzed data from the STAFF-SA instrument and high-time resolution waveform data from the WBD instrument collected between 2002 and 2009. We present the results obtained by a systematic analysis of the fine spectral structures of the EN emissions observed by WBD. The frequencies of emission peaks have been visually selected from high-resolution spectrograms. We show histograms of the positions of the source regions of EN. The five spacecraft of the Themis mission have search coil magnetometers onboard which measure wave fluctuations in the frequency bandwidth from 0.1 Hz to 4 kHz. We present the results of the fine spectral analysis of these measurements.
• Overview of Emic Triggered Chorus Emissions in Cluster Data
  
  • Grison B.
  • Pickett J. S.
  • Omura Y.
  • Santolik O.
  • Engebretson M. J.
  • Dandouras I. S.
  • Masson A.
  • Decreau P. M.
  • Adrian M. L.
  • Cornilleau Wehrlin N.
  , 2010, 2010, pp.pp. 1542-1550. Electromagnetic ion cyclotron (EMIC) triggered emissions have been recently observed onboard the Cluster spacecraft close to the plasmapause in the equatorial region of the magnetosphere (Pickett et al., 2010). The nonlinear mechanism of the wave amplification is the same as for the well known whistler-mode chorus emissions (Omura et al., 2010). The EMIC triggered emissions appear as risers: electromagnetic structures that have a positive frequency drift with time. They can thus be considered as the EMIC analogue of rising frequency whistler-mode chorus emissions. In addition, they propagate away from the magnetic equator. These EMIC risers are not common in Cluster data. We present an overview of the properties of all the identified cases. Risers can be sorted out in two groups: in the first one the starting frequency of EMIC emissions is close to one half of the local proton gyrofrequency and the risers have a clear left-hand polarization. In the second group the risers have an opposite polarization with a starting frequency close to one half of the He+ gyrofrequency. Most of the cases have been detected close to 22 MLT (magnetic local time). This dependence will be investigated to determine if it is linked to the orbit effects or if there is a physical cause.
• Low energy Ar<SUP>+</SUP> bombardment of GaN surfaces : A statistical study of ion reflection and sputtering
  
  • Despiau-Pujo Emilie
  • Chabert Pascal
  Journal of Vacuum Science & Technology A, American Vacuum Society, 2010, 28 (5), pp.1263. Statistical molecular dynamics simulations are performed to analyze the sputtering of w-GaN (wurtzite) and z-GaN (zinc blende) surfaces under 100 eV Ar ion bombardment. Ion reflection and physical sputtering mechanisms are investigated as a function of the ion impact angle and the crystalline nature of samples. The probability of ion reflection is lower for the w-GaN phase and increases with the angle of incidence &#952;i. As &#952;i becomes more glancing, the reflected ions become more energetic and their angular distribution tends to narrow. The sputtering yields of w-GaN and z-GaN surfaces are maximum for &#952;i = 45°. For near-normal incidence, the probability of sputtering is smaller for the w-GaN phase, suggesting that the atomic arrangement in the pristine state modifies the characteristics of the momentum transfer occurring between the ion and the surface atoms during the collision cascade. Atomic nitrogen sputters preferentially and represents 87% to 100% of sputtered species due to its lower mass. These statistical results differ from the predictions of continuous ion bombardment simulations since the surfaces are not allowed to evolve self-consistently during the gathering of impact statistics. (10.1116/1.3480344)
  DOI : 10.1116/1.3480344
• Extraction and acceleration of ions from an ion-ion plasma
  
  • Popelier Lara
  • Aanesland Ane
  • Chabert Pascal
  AIP Conference Proceedings, American Institute of Physics, 2010, 1390, pp.688-674. Extraction and acceleration of positive and negative ions from a strong electronegative plasma and from an ion&#8208;ion plasma is investigated in the PEGASES thruster, working with SF6. The plasma is generated in a cylindrical quartz tube terminated by metallic endplates. The electrons are confined by a static magnetic field along the axis of the cylinder. The electron mobility along the field is high and the electrons are determining the sheaths in front of the endplates. The core plasma potential can therefore be controlled by the bias applied to the endplates. An ion&#8208;ion plasma forms at the periphery as a result of electron confinement and ions can freely diffuse along the perpendicular direction or extraction axis. Langmuir probe and RFEA measurements are carried out along this axis. The measured ion energy distributions shows a single peak centered around a potential consistent with the plasma potential and the peak position could be controlled with a positive voltage applied to the endplates. When the endplates are biased negatively, the plasma potential saturates and remained close to 15 V. A beam of negatively charged particles can be observed under certain conditions when the endplates were biased negatively. (10.1063/1.3637439)
  DOI : 10.1063/1.3637439
• Dual-frequency capacitive radiofrequency discharges: effect of low-frequency power on electron density and ion flux
  
  • Booth Jean-Paul
  • Curley G.
  • Marić D.
  • Chabert Pascal
  Plasma Sources Science and Technology, IOP Publishing, 2010, 19, pp.015005. The dependence of electron density and ion flux on radiofrequency (RF) power has been measured in a 2 27&#8201;MHz dual-frequency capacitive discharge with silicon electrodes at 6.7&#8201;Pa gas pressure. In Ar/O2 mixtures the electron density and the ion flux vary in a very similar way (i.e. their ratio, &#965;, is constant), in good agreement with the simple electropositive transport theory. Both 27 and 2&#8201;MHz RF powers have a significant effect on the plasma density and the ion flux. The effect of the 2&#8201;MHz power is likely a combination of enhanced plasma heating by dual-frequency excitation and ionization caused by secondary electron beams, which are known to be produced efficiently at oxidized silicon surfaces. In contrast, in Ar/C4F8/O2 mixtures such as those used for industrial dielectric etching, &#965; is always bigger than the theoretical electropositive value, and becomes very high when the ratio of 2 to 27&#8201;MHz power is high. Under these conditions the electron density is very small, whereas the ion flux remains considerable. We attribute the increased plasma transport to the presence of a significant density of F&#8722; negative ions, combined with increased penetration of the 2&#8201;MHz electric field into the plasma bulk at high 2/27&#8201;MHz power ratios. (10.1088/0963-0252/19/1/015005)
  DOI : 10.1088/0963-0252/19/1/015005
• Performance analysis of magnetic flux compression by plasma liner
  
  • Gasilov V. A.
  • d'Yatchenko S. V.
  • Chuvatin Alexandre S.
  • Ol'Khovskaya O. G.
  • Boldarev A. S.
  • Kartasheva E. L.
  • Bagdasarov G. A.
  Mathematical Models and Computer Simulations, 2010, 2 (3), pp.375-387. in english. The paper presents the results of the theoretical and numerical performance analysis of the experimental scheme for amplification of magnetic flux intensity via its compression by plasma liner. 0D estimations and 2D computations results are compared. The simulations were carried out with the use of RMHD code MARPLE (IMM RAS). The scheme performance affected by the Rayleigh-Taylor instability, developed in the case of initially disturbed plasma shell density, is studied. The possible penetration of the compressor shell plasma from the discharge chamber into the load area results in the nonuniformity of magnetic pressure in it. The simulation proves the possibility of elimination of this unwanted effect by proper selection of the experiments parameters. The correlation of the numerical results for this kind of problems using a simplified 0D model and 2D RMHD simulation with the MARPLE code are demonstrated. The prospects of the plasma magnetic flux compression scheme are discussed. Original Russian Text © V.A. Gasilov, S.V. Dyachenko, A.S. Chuvatin, O.G. Olkhovskaya, A.S. Boldarev, E.L. Kartasheva, G.A. Bagdasarov, 2009, published in Matematicheskoe Modelirovanie, 2009, Vol. 21, No. 11, pp. 5773. (10.1134/S2070048210030117)
  DOI : 10.1134/S2070048210030117