Publications

2016

• Controlling the shape of the ion energy distribution at constant ion flux and constant mean ion energy with tailored voltage waveforms
  
  • Bruneau Bastien
  • Lafleur Trevor
  • Booth Jean-Paul
  • Johnson Erik
  Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (2), pp.025006. In this paper, we investigate the excitation of a capacitively coupled plasma using a non-sinusoidal voltage waveform whose amplitude- and slope-asymmetry varies continuously with a period which is a multiple of the fundamental RF period. We call this period the ?beating? period. Through particle-in-cell (PIC) simulations, we show that such waveforms cause oscillation of the self-bias at this beating frequency, corresponding to the charging and discharging of the external capacitor. The amplitude of this self-bias oscillation depends on the beating period, the value of the external capacitor, and the ion flux to the electrodes. This self-bias oscillation causes temporal modulation of the ion flux distribution function (IFDF), albeit at a constant ion flux and constant mean ion energy, and allows the energy width of the IFDF (averaged over the beating period) to be varied in a controlled fashion. (10.1088/0963-0252/25/2/025006)
  DOI : 10.1088/0963-0252/25/2/025006
• Diffusion of Energetic Electrons in Turbulent Plasmas of the Solar Wind
  
  • Volokitin A. S.
  • Krafft C.
  The Astrophysical Journal, American Astronomical Society, 2016, 833 (2), pp.166. A method of calculation of the diffusion coefficients D (v) of particles in velocity space, based on the statistical analysis of the motion of a great number of test electrons, is proposed. In the case of Langmuir turbulence developing in plasmas with fluctuating density inhomogeneities such as the solar wind, simulations provide coefficients D (v) which mainly depend on the Langmuir wave spectra and agree well with the analytical predictions D <SUB>th</SUB>(v) of the quasilinear theory of weak turbulence. Nevertheless, some noticeable differences exist with this theory: in the range of phase velocity of the short waves where the main part of the wave energy is concentrated, D (v) is noticeably smaller than D <SUB>th</SUB>(v), due to the scattering, the reflection, and the focusing effects encountered by the Langmuir waves when they interact with the plasma density inhomogeneities. Moreover, the probability of large velocity jumps in the particles' trajectories essentially exceeds the probability of a Gaussian distribution. These large jumps, which are connected with the waves' transformation processes, modify the nature of the particle diffusion, which is no more classical. These higher order effects cause the discrepancies observed with the quasilinear theory, which does not take them into account in its perturbative approach. The solar wind plasmas, which present fluctuating density inhomogeneities of noticeable average levels, are a very good laboratory to study such diffusion processes, which can eventually influence significantly on the development of essential physical phenomena, as electromagnetic radio emissions by type III solar radio bursts, for example. (10.3847/1538-4357/833/2/166)
  DOI : 10.3847/1538-4357/833/2/166
• Electron Acceleration by Langmuir Waves Produced by a Decay Cascade
  
  • Krafft C.
  • Volokitin A. S.
  The Astrophysical Journal, American Astronomical Society, 2016, 821 (2), pp.99. It was recently reported that a significant part of the Langmuir waveforms observed by the STEREO satellite during type III solar radio bursts are likely consistent with the occurrence of electrostatic decay instabilities, when a Langmuir wave L resonantly interacts with another Langmuir wave L <SUP>\prime</SUP> and an ion sound wave S <SUP>\prime</SUP> through the decay channel L \to L <SUP>\prime</SUP> S <SUP>\prime</SUP> . Usually such wave-wave interactions occur in regions of the solar wind where the presence of electron beams can drive Langmuir turbulence to levels allowing waves L to decay. Moreover, such solar wind plasmas can present long-wavelength, randomly fluctuating density inhomogeneities or monotonic density gradients which can significantly modify the development of such resonant instabilities. If some conditions are met, the waves can encounter a second decay cascade (SDC) according to L <SUP>\prime</SUP> \to L <SUP>\prime\prime</SUP> S <SUP>\prime\prime</SUP> . Analytical estimates and observations based on numerical simulations show that the Langmuir waves L <SUP>\prime\prime</SUP> produced by this SDC can accelerate beam particles up to velocities and kinetic energies exceeding two times the beam drift velocity v<SUB>b</SUB> and half the initial beam energy, respectively. Moreover, this process can be particularly efficient if the scattering effects of waves on the background plasma inhomogeneities have already accelerated a sufficient amount of beam electrons up to the velocity range where the phase velocities of the L <SUP>\prime\prime</SUP> waves are lying. The paper shows that the conditions necessary for such process to occur can be easily met in solar wind plasmas if the beam velocities do not exceed around 35 times the plasma thermal velocity. (10.3847/0004-637X/821/2/99)
  DOI : 10.3847/0004-637X/821/2/99
• Transport of Solar Wind H<SUP>+</SUP> and He<SUP>++</SUP> Ions across Earth's Bow Shock
  
  • Parks G. K.
  • Lee E.
  • Fu S. Y.
  • Kim H. E.
  • Ma Y. Q.
  • Yang Z. W.
  • Liu Y.
  • Lin N.
  • Hong J.
  • Canu Patrick
  • Dandouras I.
  • Rème H.
  • Goldstein M. L.
  The Astrophysical Journal Letters, Bristol : IOP Publishing, 2016, 825 (2), pp.L27. We have investigated the dependence of mass, energy, and charge of solar wind (SW) transport across Earth?s bow shock. An examination of 111 crossings during quiet SW in both quasi-perpendicular and quasi-parallel shock regions shows that 64 crossings had various degrees of heating and thermalization of SW. We found 22 crossings where the SW speed was <400 km s ?1 . The shock potential of a typical supercritical quasi-perpendicular shock estimated from deceleration of the SW and cutoff energy of electron flat top distribution is ?50 Volts. We find that the temperatures of H and He beams that penetrate the shock can sometimes be nearly the same in the upstream and downstream regions, indicating little or no heating had occurred crossing the bow shock. None of the models predict that the SW can cross the bow shock without heating. Our observations are important constraints for new models of collisionless shocks. (10.3847/2041-8205/825/2/L27)
  DOI : 10.3847/2041-8205/825/2/L27
• Improved Design of a Multistage Axial Vircator With Reflectors for Enhanced Performances
  
  • Champeaux Stéphanie
  • Gouard Philippe
  • Cousin Richard
  • Larour Jean
  IEEE Transactions on Plasma Science, Institute of Electrical and Electronics Engineers, 2016, 44 (1), pp.31-38. The basic design of an axial virtual cathode oscillator (vircator) with axial extraction operating in TM01 mode is modified by introducing thin conducting disks, also called reflectors, into the cylindrical waveguide. The operation principal of this novel type of device relies on the formation of a series of virtual cathodes, located at the center of adjacent quasi-cavities. The behavior of this new type of multistage vircator is numerically investigated using CST Particle Studio 3-D particle-in-cell code. Progressively decreasing the radii of the reflectors installed upstream in the tube allows the mitigation of spurious modes. Tapering the radii of the reflectors turns out to be crucial in focusing the electron beam on axis in the downstream region and maximizing the TM01 power conversion efficiency. This novel architecture enables a five-reflector vircator operating with an injected electron beam of 508-kV mean voltage and 19-kA mean current to deliver up to 2-GW mean power sustained only by the TM01 mode in the S-band with a power conversion efficiency close to 21%. (10.1109/TPS.2015.2502432)
  DOI : 10.1109/TPS.2015.2502432
• The 2π charged particles analyzer: All-sky camera concept and development for space missions
  
  • Vaisberg Oleg
  • Berthelier Jean-Jacques
  • Moore T.
  • Avanov L.
  • Leblanc François
  • Leblanc Frédéric
  • Moiseev Pavel P.
  • Moiseenko D.
  • Becker Joël
  • Collier Michael R.
  • Laky G.
  • Keller J.
  • Koynash G.
  • Lichtenneger H.
  • Leibov A. W.
  • Zhuravlev R.
  • Shestakov A.
  • Burch J.
  • Mccomas D.
  • Shuvalov S.
  • Chornay D.
  • Torkar K.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2016, 121 (12), pp.11,750–11,765. Increasing the temporal resolution and instant coverage of velocity space of space plasma measurements is one of the key issues for experimentalists. Today the top-hat plasma analyzer appears to be the favorite solution due to its relative simplicity and the possibility to extend its application by adding a mass-analysis section and an electrostatic angular scanner. Similarly, great success has been achieved in MMS mission using such multiple top-hat analyzers to achieve unprecedented temporal resolution. An instantaneous angular coverage of charged particles measurements is an alternative approach to pursuing the goal of high time resolution. This was done with FONEMA 4-D and, to a lesser extent, by DYMIO instruments for Mars-96 and with the FIPS instrument for MESSENGER mission. In this paper we describe, along with precursors, a plasma analyzer with a 2π electrostatic mirror that was developed originally for the Phobos-Soil mission with a follow-up in the frame of the BepiColombo mission, and is under development for future Russian missions. Different versions of instrument are discussed along with their advantages and drawbacks. (10.1002/2016JA022568)
  DOI : 10.1002/2016JA022568