Publications

2014

• Radiation from Ag high energy density Z-pinch plasmas and applications to lasing
  
  • Weller Michael E.
  • Safronova Alla S.
  • Kantsyrev Viktor L.
  • Esaulov A. A.
  • Shrestha Ishor
  • Apruzese J. P.
  • Giuliani J. L.
  • Chuvatin Alexandre S.
  • Stafford A.
  • Keim S. F.
  • Shlyaptseva V. V.
  • Osborne Glenn C.
  • Petkov E. E.
  Physics of Plasmas, American Institute of Physics, 2014, 21 (03), pp.031206. Silver (Ag) wire arrays were recently introduced as efficient x-ray radiators and have been shown to create L-shell plasmas that have the highest electron temperature (>1.8 keV) observed on the Zebra generator so far and upwards of 30 kJ of energy output. In this paper, results of single planar wire arrays and double planar wire arrays of Ag and mixed Ag and Al that were tested on the UNR Zebra generator are presented and compared. To further understand how L-shell Ag plasma evolves in time, a time-gated x-ray spectrometer was designed and fielded, which has a spectral range of approximately 3.55.0 Å. With this, L-shell Ag as well as cold Lα and Lβ Ag lines was captured and analyzed along with photoconducting diode (PCD) signals (>0.8 keV). Along with PCD signals, other signals, such as filtered XRD (>0.2 keV) and Si-diodes (SiD) (>9 keV), are analyzed covering a broad range of energies from a few eV to greater than 53 keV. The observation and analysis of cold Lα and Lβ lines show possible correlations with electron beams and SiD signals. Recently, an interesting issue regarding these Ag plasmas is whether lasing occurs in the Ne-like soft x-ray range, and if so, at what gains? To help answer this question, a non-local thermodynamic equilibrium (LTE) kinetic model was utilized to calculate theoretical lasing gains. It is shown that the Ag L-shell plasma conditions produced on the Zebra generator at 1.7 maximum current may be adequate to produce gains as high as 6 cm−1 for various 3p → 3s transitions. Other potential lasing transitions, including higher Rydberg states, are also included in detail. The overall importance of Ag wire arrays and plasmas is discussed. (10.1063/1.4865368)
  DOI : 10.1063/1.4865368
• CLUSTER-STAFF search coil magnetometer calibration - comparisons with FGM
  
  • Robert Patrick
  • Cornilleau-Wehrlin Nicole
  • Piberne Rodrigue
  • de Conchy Y.
  • Lacombe C.
  • Bouzid V.
  • Grison B.
  • Alison Dominique
  • Canu Patrick
  Geoscientific Instrumentation, Methods and Data Systems, European Geosciences Union, 2014, 3, pp.153-177. The main part of the Cluster Spatio-Temporal Analysis of Field Fluctuations (STAFF) experiment consists of triaxial search coils allowing the measurements of the three magnetic components of the waves from 0.1 Hz up to 4 kHz. Two sets of data are produced, one by a module to filter and transmit the corresponding waveform up to either 10 or 180 Hz (STAFF-SC), and the second by the onboard Spectrum Analyser (STAFF-SA) to compute the elements of the spectral matrix for five components of the waves, 3 × B and 2 × E (from the EFW experiment), in the frequency range 8 Hz to 4 kHz. In order to understand the way the output signals of the search coils are calibrated, the transfer functions of the different parts of the instrument are described as well as the way to transform telemetry data into physical units across various coordinate systems from the spinning sensors to a fixed and known frame. The instrument sensitivity is discussed. Cross-calibration inside STAFF (SC and SA) is presented. Results of cross-calibration between the STAFF search coils and the Cluster Fluxgate Magnetometer (FGM) data are discussed. It is shown that these cross-calibrations lead to an agreement between both data sets at low frequency within a 2% error. By means of statistics done over 10 yr, it is shown that the functionalities and characteristics of both instruments have not changed during this period. (10.5194/gi-3-153-2014)
  DOI : 10.5194/gi-3-153-2014
• Comment on "Micronewton electromagnetic thruster
  
  • Lafleur Trevor
  Applied Physics Letters, American Institute of Physics, 2014, 105, pp.146101. ... (10.1063/1.4897967)
  DOI : 10.1063/1.4897967
• Les débris spatiaux : le revers de l'ère spatiale
  
  • Aanesland Ane
  • Grondein Pascaline
  Flash X - La revue scientifique de l'Ecole polytechnique, Ecole polytechnique, 2014 (16), pp.12.
• Theory and Modeling for the Magnetospheric Multiscale Mission
  
  • Hesse Michael
  • Aunai Nicolas
  • Birn Joachim
  • Cassak P.
  • Denton R.~e.
  • Drake J. F.
  • Gombosi Tamas I.
  • Hoshino M.
  • Matthaeus W.
  • Sibeck David G.
  • Zenitani Seiji
  Space Science Reviews, Springer Verlag, 2014. The Magnetospheric Multiscale (MMS) mission will provide measurement capabilities, which will exceed those of earlier and even contemporary missions by orders of magnitude. MMS will, for the first time, be able to measure directly and with sufficient resolution key features of the magnetic reconnection process, down to the critical electron scales, which need to be resolved to understand how reconnection works. Owing to the complexity and extremely high spatial resolution required, no prior measurements exist, which could be employed to guide the definition of measurement requirements, and consequently set essential parameters for mission planning and execution. Insight into expected details of the reconnection process could hence only been obtained from theory and modern kinetic modeling. This situation was recognized early on by MMS leadership, which supported the formation of a fully integrated Theory and Modeling Team (TMT). The TMT participated in all aspects of mission planning, from the proposal stage to individual aspects of instrument performance characteristics. It provided and continues to provide to the mission the latest insights regarding the kinetic physics of magnetic reconnection, as well as associated particle acceleration and turbulence, assuring that, to the best of modern knowledge, the mission is prepared to resolve the inner workings of the magnetic reconnection process. The present paper provides a summary of key recent results or reconnection research by TMT members. (10.1007/s11214-014-0078-y)
  DOI : 10.1007/s11214-014-0078-y
• 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
• Observational evidence of electron pitch angle scattering driven by ECH waves
  
  • Kurita S.
  • Miyoshi Y.
  • Cully C. M.
  • Angelopoulos V.
  • Le Contel Olivier
  • Hikishima M.
  • Misawa H.
  Geophysical Research Letters, American Geophysical Union, 2014. Using the plasma wave and electron data obtained from Time History of Events and Macroscale Interactions during Substorms, we show a signature of electron pitch angle scattering driven by Electrostatic Cyclotron Harmonic (ECH) waves in the velocity distribution function (VDF). The diffusion curve of whistler mode waves is used as a proxy to identify changes in VDFs due to wave-particle interactions. We confirm that the shape of the VDF well agrees with the diffusion curve of whistler mode waves when whistler mode chorus alone is active. On the other hand, we find that the shape of the VDF deviates from the diffusion curves at low pitch angles when ECH waves are active following the inactivation of chorus waves. The result is observational support for electron pitch angle scattering caused by ECH waves and suggests that ECH waves can contribute to generation of diffuse auroras. (10.1002/2014GL061927)
  DOI : 10.1002/2014GL061927
• Quantified energy dissipation rates in the terrestrial bow shock: 1. Analysis techniques and methodology
  
  • 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.6455-6474. We present a detailed outline and discussion of the analysis techniques used to compare the relevance of different energy dissipation mechanisms at collisionless shock waves. We show that the low-frequency, quasi-static fields contribute less to ohmic energy dissipation, (-j·E), than their high-frequency counterparts. In fact, we found that high-frequency, large-amplitude (>100 mV/m and/or >1 nT) waves are ubiquitous in the transition region of collisionless shocks. We quantitatively show that their fields, through wave-particle interactions, cause enough energy dissipation to regulate the global structure of collisionless shocks. The purpose of this paper, part one of two, is to outline and describe in detail the background, analysis techniques, and theoretical motivation for our new results presented in the companion paper. The companion paper presents the results of our quantitative energy dissipation rate estimates and discusses the implications. Together, the two manuscripts present the first study quantifying the contribution that high-frequency waves provide, through wave-particle interactions, to the total energy dissipation budget of collisionless shock waves. (10.1002/2014JA019929)
  DOI : 10.1002/2014JA019929
• E x B shear pattern formation by radial propagation of heat flux waves
  
  • Kosuga Y.
  • Diamond P.H.
  • Dif-Pradalier Guilhem
  • Gürcan Özgür D.
  Physics of Plasmas, American Institute of Physics, 2014, 21 (5). A novel theory to describe the formation of E x B flow patterns by radially propagating heat flux waves is presented. A model for heat avalanche dynamics is extended to include a finite delay time between the instantaneous heat flux and the mean flux, based on an analogy between heat avalanche dynamics and traffic flow dynamics. The response time introduced here is an analogue of the drivers' response time in traffic dynamics. The microscopic foundation for the time delay is the time for mixing of the phase space density. The inclusion of the finite response time changes the model equation for avalanche dynamics from Burgers equation to a nonlinear telegraph equation. Based on the telegraph equation, the formation of heat flux jams is predicted. The growth rate and typical interval of jams are calculated. The connection of the jam interval to the typical step size of the E x B staircase is discussed. (C) 2014 AIP Publishing LLC. (10.1063/1.4872018)
  DOI : 10.1063/1.4872018
• Numerical simulations used for a validity check on the laser induced photo-detachment diagnostic method in electronegative plasmas
  
  • Oudini N.
  • Taccogna F.
  • Bendib A.
  • Aanesland Ane
  Physics of Plasmas, American Institute of Physics, 2014, 21 (6), pp.063515. Laser photo-detachment is used as a method to measure or determine the negative ion density and temperature in electronegative plasmas. In essence, the method consists of producing an electropositive channel (negative ion free region) via pulsed laser photo-detachment within an electronegative plasma bulk. Electrostatic probes placed in this channel measure the change in the electron density. A second pulse might be used to track the negative ion recovery. From this, the negative ion density and temperature can be determined. We study the formation and relaxation of the electropositive channel via a two-dimensional Particle-In-Cell/Mote Carlo collision model. The simulation is mainly carried out in a Hydrogen plasma with an electronegativity of α = 1, with a parametric study for α up to 20. The temporal and spatial evolution of the plasma potential and the electron densities shows the formation of a double layer (DL) confining the photo-detached electrons within the electropositive channel. This DL evolves into two fronts that move in the opposite directions inside and outside of the laser spot region. As a consequence, within the laser spot region, the background and photo-detached electron energy distribution function relaxes/thermalizes via collisionless effects such as Fermi acceleration and Landau damping. Moreover, the simulations show that collisional effects and the DL electric field strength might play a non-negligible role in the negative ion recovery within the laser spot region, leading to a two-temperature negative ion distribution. The latter result might have important effects in the determination of the negative ion density and temperature from laser photo detachment diagnostic. (10.1063/1.4886144)
  DOI : 10.1063/1.4886144
• Radiation from mixed multi-planar wire arrays
  
  • Safronova Alla S.
  • Kantsyrev Viktor L.
  • Esaulov A. A.
  • Chuvatin Alexandre S.
  • Weller Michael E.
  • Shlyaptseva V. V.
  • Shrestha Ishor
  • Keim S. F.
  • Stafford A.
  • Coverdale C. A.
  • Apruzese J. P.
  • Ouart N. D.
  • Giuliani J. L.
  Physics of Plasmas, American Institute of Physics, 2014, 21 (03), pp.031205. The study of radiation from different wire materials in wire array Z-pinch plasma is a very challenging topic because it is almost impossible to separate different plasmas at the stagnation. A new approach is suggested based on planar wire array (PWA) loads to assess this problem. Multi-planar wire arrays are implemented that consist of few planes, each with the same number of wires and masses but from different wire materials, arranged in parallel rows. In particular, the experimental results obtained with triple PWAs (TPWAs) on the UNR Zebra generator are analyzed with Wire Ablation Dynamics Model, non-local thermodynamic equilibrium kinetic model, and 2D radiation magneto-hydrodynamic to illustrate this new approach. In TPWAs, two wire planes were from mid-atomic-number wire material and another plane was from alloyed Al, placed either in the middle or at the edge of the TPWA. Spatial and temporal properties of K-shell Al and L-shell Cu radiations were analyzed and compared from these two configurations of TPWAs. Advantages of the new approach are demonstrated and future work is discussed. (10.1063/1.4864335)
  DOI : 10.1063/1.4864335
• Equivalence of the hard-wall and kinetic-fluid models of collisionless electron heating in capacitively coupled discharges
  
  • Lafleur Trevor
  • Chabert Pascal
  • Turner Miles
  • Booth Jean-Paul
  Plasma Sources Science and Technology, IOP Publishing, 2014, 23 (1), pp.015016. By re-evaluating the hard-wall collisionless electron heating model from first principles, we show that despite previous criticisms (Gozadinos et al 2001 Phys. Rev. Lett. 87 [http://dx.doi.org/10.1103/PhysRevLett.87.135004] 135004 ), this model can in general be made consistent with the requirement of radio frequency (rf) current continuity at the sheath edge, while still producing a net heating effect. In addition, we demonstrate that the hard-wall and kinetic-fluid heating models stem from the same basic physical mechanism, and are in many ways the same theory; they differ only in the spatial region where electron heating is assumed to occur, and the way in which the effective electron distribution function is determined. Fundamentally, both models predict that collisionless heating occurs because of a non-isothermal compression and expansion of the plasma electrons by an oscillating rf sheath. (10.1088/0963-0252/23/1/015016)
  DOI : 10.1088/0963-0252/23/1/015016
• Dynamics of plasma evolution in a nanosecond underwater discharge
  
  • Marinov Ilya
  • Starikovskaia Svetlana
  • Rousseau Antoine
  Journal of Physics D: Applied Physics, IOP Publishing, 2014, 47 (22), pp.224017. A positive discharge in water is generated by applying a 30 ns high-voltage (HV) pulse on a micrometre scale electrode. The applied voltage ranges from 6 to 15 kV and a fast plasma propagating mode is launched with a velocity of up to 60 km s−1 . Time-resolved shadowgraphy and spectroscopy are performed to monitor the time evolution of the discharge structure and of the plasma emission spectra. By analysing the dynamics of the shock front velocity and the lateral expansion of the plasma channel, it is possible to estimate the pressure at the ignition of the plasma by two independent methods: very good agreement is found at 6 kV giving initial pressures of 0.4 GPa and 0.3 GPa, respectively. At 15 kV, only the shock front velocity method is applicable under our experimental conditions, giving an estimate of the initial pressure of 5.8 GPa. Such high initial pressures show that, under a nanosecond HV pulse, the plasma is ignited directly in the dense phase. Emission spectra show a strong continuum emission as well as a broad Balmer α line with a strong red shift, with an estimate of the initial plasma density of 1.3 × 1026 m−3. The relaxation of discharge pressure and plasma density is studied under a series of six successive pulses. (10.1088/0022-3727/47/22/224017)
  DOI : 10.1088/0022-3727/47/22/224017
• Time-resolved imaging of nanosecond-pulsed micro-discharges in heptane
  
  • Hamdan A.
  • Marinov Ilya
  • Belmonte Thierry
  • Rousseau A.
  Journal of Physics D: Applied Physics, IOP Publishing, 2014, 47 (5). Nanosecond-pulsed micro-discharges in heptane are studied by time-resolved imaging in pin-to-plate configuration. When a voltage of +5 kV is applied to the pin electrode, the discharge exhibits one maximum in light intensity. At +15 kV, filtered images show that up to three maxima can be identified. These maxima are associated with local electron-ion recombination and bremsstrahlung emission and attributed to the development of a complex space-charge field. In the post-discharge, the dynamics of the gas bubble can be simulated by the Gilmore model, and the pressure evolution in this bubble is predicted. From our results, it seems reasonable to think that the gas bubble develops from the post-discharge of the spark. Results obtained by using the double-pulse technique show that light emission during the post-discharge of the second discharge lasts 10 times longer than the post-discharge of the first spark. The pressure drop in the gas bubble, predicted by the Gilmore model, is used to explain this result and it provides a control method by optical diagnostics in liquids. (10.1088/0022-3727/47/5/055203)
  DOI : 10.1088/0022-3727/47/5/055203
• Weak turbulence theory for rotating magnetohydrodynamics and planetary flows
  
  • Galtier Sébastien
  Journal of Fluid Mechanics, Cambridge University Press (CUP), 2014, 757, pp.114-154. A weak turbulence theory is derived for magnetohydrodynamics (MHD) under rapid rotation and in the presence of a uniform large-scale magnetic field which is associated with a constant Alfvén velocity . The angular velocity is assumed to be uniform and parallel to . Such a system exhibits left and right circularly polarized waves which can be obtained by introducing the magneto-inertial length . In the large-scale limit ( , with being the wavenumber) the left- and right-handed waves tend to the inertial and magnetostrophic waves, respectively, whereas in the small-scale limit ( ) pure Alfvén waves are recovered. By using a complex helicity decomposition, the asymptotic weak turbulence equations are derived which describe the long-time behaviour of weakly dispersive interacting waves via three-wave interaction processes. It is shown that the nonlinear dynamics is mainly anisotropic, with a stronger transfer perpendicular than parallel to the rotation axis. The general theory may converge to pure weak inertial/magnetostrophic or Alfvén wave turbulence when the large- or small-scale limits are taken, respectively. Inertial wave turbulence is asymptotically dominated by the kinetic energy/helicity, whereas the magnetostrophic wave turbulence is dominated by the magnetic energy/helicity. For both regimes, families of exact solutions are found for the spectra, which do not correspond necessarily to a maximal helicity state. It is shown that the hybrid helicity exhibits a cascade whose direction may vary according to the scale at which the helicity flux is injected, with an inverse cascade if and a direct cascade otherwise. The theory is relevant to the magnetostrophic dynamo, whose main applications are the Earth and the giant planets, such as Jupiter and Saturn, for which a small ( ) Rossby number is expected. (10.1017/jfm.2014.490)
  DOI : 10.1017/jfm.2014.490
• Wave normal angles of whistler-mode chorus rising and falling tones
  
  • Taubenschuss U.
  • Khotyaintsev Y. V.
  • Santolík O.
  • Vaivads A.
  • Cully C. M.
  • Le Contel Olivier
  • Angelopoulos V.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2014. We present a study of wave normal angles (θk) of whistler mode chorus emission as observed by Time History of Events and Macroscale Interactions during Substorms (THEMIS) during the year 2008. The three inner THEMIS satellites THA, THD, and THE usually orbit Earth close to the dipole magnetic equator (±20°), covering a large range of L shells from the plasmasphere out to the magnetopause. Waveform measurements of electric and magnetic fields enable a detailed polarization analysis of chorus below 4 kHz. When displayed in a frequency-θk histogram, four characteristic regions of occurrence are evident. They are separated by gaps at f/fc,e≈0.5 (f is the chorus frequency, fc,e is the local electron cyclotron frequency) and at θk∼40°. Below θk∼40°, the average value for θk is predominantly field aligned, but slightly increasing with frequency toward half of fc,e (θk up to 20°). Above half of fc,e, the average θk is again decreasing with frequency. Above θk∼40°, wave normal angles are usually close to the resonance cone angle. Furthermore, we present a detailed comparison of electric and magnetic fields of chorus rising and falling tones. Falling tones exhibit peaks in occurrence solely for θk>40° and are propagating close to the resonance cone angle. Nevertheless, when comparing rising tones to falling tones at θk>40°, the ratio of magnetic to electric field shows no significant differences. Thus, we conclude that falling tones are generated under similar conditions as rising tones, with common source regions close to the magnetic equatorial plane. (10.1002/2014JA020575)
  DOI : 10.1002/2014JA020575
• In situ spatiotemporal measurements of the detailed azimuthal substructure of the substorm current wedge
  
  • Forsyth C.
  • Fazakerley A.
  • Rae I. J.
  • Watt C. E. J.
  • Murphy K.
  • Wild James A.
  • Karlsson T.
  • Mutel R. L.
  • Owen C. J.
  • Ergun R.
  • Masson A.
  • Berthomier Matthieu
  • Donovan E.
  • Frey H.~u.
  • Matzka J.
  • Stolle C.
  • Zhang Y.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2014, 119 (2), pp.927-946. The substorm current wedge (SCW) is a fundamental component of geomagnetic substorms. Models tend to describe the SCW as a simple line current flowing into the ionosphere toward dawn and out of the ionosphere toward dusk, linked by a westward electrojet. We use multispacecraft observations from perigee passes of the Cluster 1 and 4 spacecraft during a substorm on 15 January 2010, in conjunction with ground-based observations, to examine the spatial structuring and temporal variability of the SCW. At this time, the spacecraft traveled east-west azimuthally above the auroral region. We show that the SCW has significant azimuthal substructure on scales of 100 km at altitudes of 40007000 km. We identify 26 individual current sheets in the Cluster 4 data and 34 individual current sheets in the Cluster 1 data, with Cluster 1 passing through the SCW 120240 s after Cluster 4 at 13002000 km higher altitude. Both spacecraft observed large-scale regions of net upward and downward field-aligned current, consistent with the large-scale characteristics of the SCW, although sheets of oppositely directed currents were observed within both regions. We show that the majority of these current sheets were closely aligned to a north-south direction, in contrast to the expected east-west orientation of the preonset aurora. Comparing our results with observations of the field-aligned current associated with bursty bulk flows (BBFs), we conclude that significant questions remain for the explanation of SCW structuring by BBF-driven wedgelets. Our results therefore represent constraints on future modeling and theoretical frameworks on the generation of the SCW. (10.1002/2013JA019302)
  DOI : 10.1002/2013JA019302
• Kelvin-Helmholtz Vortices and Double Mid-Latitude Reconnection at the Earth's Magnetopause: comparison between observations and simulations
  
  • Faganello Matteo
  • Califano F.
  • Pegoraro F.
  • Retinò Alessandro
  EPL - Europhysics Letters, European Physical Society / EDP Sciences / Società Italiana di Fisica / IOP Publishing, 2014. Observational signatures of Kelvin-Helmholtz (K-H) vortices and of double mid-latitude reconnection are highlighted in satellite data of the THEMIS mission. It is shown that the plasma fluid quantities at the low-latitude flank of the Earth's magnetosphere are compatible with K-H vortices, as described by three-dimensional simulations. At the same time it is shown that the particle fluxes are compatible with the presence of magnetic field lines, embedded in the K-H vortices, that close on Earth but are connected to the solar wind at low-latitude. These field lines are generated during the K-H evolution by magnetic reconnection proceeding spontaneously in both hemispheres at mid-latitudes, allowing the solar wind plasma to enter the Earth's magnetosphere directly.
• Ground state bromine atom density measurements by two-photon absorption laser-induced fluorescence
  
  • Sirse Nishant
  • Foucher Mickaël
  • Chabert Pascal
  • Booth Jean-Paul
  Plasma Sources Science and Technology, IOP Publishing, 2014, 23 (6), pp.062003. Ground state bromine atom detection by two-photon absorption laser-induced fluorescence (TALIF) is demonstrated. The bromine atoms are excited by two-photon absorption at 252.594 nm to the state and detected by 635.25 nm fluorescence to the (5s) 4 P 5/2 state. The atoms are generated in a radio-frequency inductively-coupled plasma in pure HBr. The excitation laser also causes some photodissociation of HBr molecules, but this can be minimized by not focussing the laser beam, still giving adequate signal levels. We determined the natural lifetime of the emitting state, and the rate constant for quenching of this state by collision with HBr molecules, ... (10.1088/0963-0252/23/6/062003)
  DOI : 10.1088/0963-0252/23/6/062003
• Fourier spectrum and phases for a signal in a finite interval
  
  • Dorville Nicolas
  • Belmont Gérard
  • Sahraoui Fouad
  • Rezeau Laurence
  , 2014, 13, pp.SH13B-4086. When investigating the physics of turbulent media, as the solar wind or the magnetosheath plasmas, obtaining accurate Fourier spectra and phases is a crucial issue. For the different fields, the spectra allow in particular verifying whether one or several power laws can be determined in different frequency ranges. Accurate phases are necessary as well for all the "higher order statistics" studies in Fourier space, the coherence ones and for the polarization studies. Unfortunately, the Fourier analysis is not unique for a finite time interval of duration T: the frequencies lower than 1/T have a large influence on the result, which can hardly be controlled. This unknown "trend" has in particular the effect of superposing jumps at the edges of the interval, for the function under study itself, as well as for all its derivatives. The Fourier transform obtained directly by FFT (Fast Fourier Transform) is generally much influenced by these effects and cannot be used without care for wide band signals. The interferences between the jumps and the signal itself also provide "hairs" on the spectrum, which are clearly visible fluctuations with df≈1/T. These fluctuations are usually eliminated by smoothing the spectrum, or by averaging several successive spectra. Nevertheless, such smoothing introduces uncertainties on the spectral laws and it makes the phases lost. Windowing is also a method currently used to suppress the jumps, but it modifies the signal (the windowed trend has a spectrum, which is convolved with the searched one) and the phases are also lost to a large extent. Here, we present a new data processing technique to circumvent these difficulties. It takes advantage of the fact that the signal is generally not unknown out of the interval under study: the complete signal is tapered to this interval of interest thanks to a new kind of window, sharp but not square. This kind of window is such that the spectrum obtained can then be deconvolved almost exactly, through a minimization procedure based on the weak- hypothesis that it is smooth at the scale of a few successive spectral points. Then, a later step allows reconstructing the phases. Tests with synthetic data are presented, that demonstrate the efficiency of the method, and first results from Cluster data are also shown.
• Rotational/compressional nature of the magnetopause: Application of the BV technique on a magnetopause case study
  
  • Dorville Nicolas
  • Belmont Gérard
  • Rezeau Laurence
  • Grappin Roland
  • Retinò Alessandro
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2014, 119, pp.1898-1908. The magnetopause boundary implies two main kinds of variations: a density/temperature gradient and a magnetic field rotation. Other variations are associated with these two, concerning in particular the flow velocity, the electric field, and the plasma composition. Compressional and rotational variations are always observed in a close vicinity of each other, if not inseparably mixed. We present a case study from the Cluster data where the two are clearly separated and investigate the natures of both layers, using the new BV method for discontinuity analysis. We evidence that the first one is a slow shock while the second is a rotational discontinuity. The interaction between these two kinds of discontinuities is then studied with the help of 1.5-D magnetohydrodynamics simulations. The comparison with the data is quite positive and, associated with general theoretical arguments, leads to think that most of the generic properties of the magnetopause may be interpreted in this sense. Our results suggest that a shaken magnetopause is made of the sum of several discontinuities: slow shocks and rotational discontinuities. A statistical study on a larger set of data will be necessary to check this conjecture. (10.1002/2013JA018927)
  DOI : 10.1002/2013JA018927
• Adsorption and reactivity of nitrogen atoms on silica surface under plasma exposure
  
  • Marinov Daniil
  • Guaitella Olivier
  • Arcos T. de Los
  • von Keudell A.
  • Rousseau Antoine
  Journal of Physics D: Applied Physics, IOP Publishing, 2014, 47 (47), pp.475204. The kinetics of adsorption, desorption and recombination of nitrogen atoms on a silica surface is investigated. Stable nitrogen atoms are grafted to the inner surface of a fused silica discharge tube by a discharge in N 2 at 0.53?mbar. After the pre-treatment, the surface is analysed using x-ray photoelectron spectroscopy and an isotopic exchange technique. The latter consists of the exposure of the pre-treated surface with a discharge in the heavy nitrogen isotope 30 N 2 . Nitrogen isotopologues 29 N 2 and 28 N 2 produced on the surface are detected using a mass spectrometer and provide information about the coverage and reactivity of adsorbed 14 N atoms. It is found that during the pre-treatment, a silicon oxynitride (SiO x N y ) layer is formed on the initially clean SiO 2 surface. The coverage of N on the surface increases from 5? × ?10 13 to 5? × ?10 15? cm ?2 for a pre-treatment duration in the range of 10 ?2 ? 10 4 ?s. Atoms on the surface demonstrate a distribution of reactivity, which is attributed to a distribution of their binding energies and configurations on the surface. We demonstrate that stable chemisorbed N ads are not the main recombination sites for N atoms on the surface contrary to previous studies. We conclude that recombination takes place mainly on weakly bonding active sites with the binding energy smaller than 1?eV. (10.1088/0022-3727/47/47/475204)
  DOI : 10.1088/0022-3727/47/47/475204
• A review on ion-ion plasmas created in weakly magnetized electronegative plasmas
  
  • Aanesland Ane
  • Bredin Jérôme
  • Chabert Pascal
  Plasma Sources Science and Technology, IOP Publishing, 2014, 23 (4), pp.044003. IonIon plasmas are electronegative plasmas where the electron density is several orders of magnitude lower than the negative ion density. These plasmas have been scarcely observed and investigated since the 1960s and are formed as a transient state of pulsed plasmas or in separate regions in magnetized plasmas. In this review we focus on the latter case of continuous formation of ionion plasmas created at the periphery of magnetized plasma columns or downstream localized magnetic barriers. We bring together and review experimental results already published elsewhere and complement them with new results to illustrate the physics important in ionion plasma formation and highlight in particular unanswered questions. We show that with a good design the density in the ionion region is dropping only by a factor of 23 from the initial plasma density. These plasmas can therefore be well suited for various ion source applications when both fluxes or beams of positive and negative ions are desired, and when electrons can cause harmful effects. (10.1088/0963-0252/23/4/044003)
  DOI : 10.1088/0963-0252/23/4/044003
• Neutralizer-free gridded ion thruster
  
  • Rafalskyi D.V.
  • Aanesland Ane
  American Institute of Aeronautics and Astronautics paper, 2014, Propulsion and Energy Forum. (10.2514/6.2014-3423)
  DOI : 10.2514/6.2014-3423
• Characteristics of the flank magnetopause: Cluster observations
  
  • Haaland S.
  • Reistad J.
  • Tenfjord P.
  • Gjerloev J.
  • Maes Lukas
  • de Keyser J.
  • Maggiolo R.
  • Anekallu C.
  • Dorville Nicolas
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2014, 119, pp.9019-9037. The magnetopause is a current sheet forming the boundary between the geomagnetic field on one side and the shocked solar wind on the other side. This paper discusses properties of the low-latitude dawn and dusk flanks of the magnetopause. The reported results are based on a large number of measurements obtained by the Cluster satellites during magnetopause traversals. Using a combination of single-spacecraft and multispacecraft techniques, we calculated macroscopic features such as thickness, location, and motion of the magnetopause. The results show that the typical flank magnetopause is significantly thicker than the dayside magnetopause and also possesses a pronounced and persistent dawn-dusk asymmetry. Thicknesses vary from 150 to 5000 km, with an median thickness of around 1400 km at dawn and around 1150 km at dusk. Current densities are on average higher on dusk, suggesting that the total current at dawn and dusk are similar. Solar wind conditions and the interplanetary magnetic field cannot fully explain the observed dawn-dusk asymmetry. For a number of crossings we were also able to derive detailed current density profiles. The profiles show that the magnetopause often consists of two or more adjacent current sheets, each current sheet typically several ion gyroradii thick and often with different current direction. This demonstrates that the flank magnetopause has a structure that is more complex than the thin, one-dimensional current sheet described by a Chapman-Ferraro layer. (10.1002/2014JA020539)
  DOI : 10.1002/2014JA020539