Publications

2015

• Magnetized retarding field energy analyzer measuring the particle flux and ion energy distribution of both positive and negative ions
  
  • Rafalskyi D.V.
  • Dudin S.V.
  • Aanesland Ane
  Review of Scientific Instruments, American Institute of Physics, 2015, 86 (5), pp.053302. This paper presents the development of a magnetized retarding field energy analyzer (MRFEA) used for positive and negative ion analysis. The two-stage analyzer combines a magnetic electron barrier and an electrostatic ion energy barrier allowing both positive and negative ions to be analyzed without the influence of electrons (co-extracted or created downstream). An optimal design of the MRFEA for ion-ion beams has been achieved by a comparative study of three different MRFEA configurations, and from this, scaling laws of an optimal magnetic field strength and topology have been deduced. The optimal design consists of a uniform magnetic field barrier created in a rectangular channel and an electrostatic barrier consisting of a single grid and a collector placed behind the magnetic field. The magnetic barrier alone provides an electron suppression ratio inside the analyzer of up to 6000, while keeping the ion energy resolution below 5 eV. The effective ion transparency combining the magnetic and electrostatic sections of the MRFEA is measured as a function of the ion energy. It is found that the ion transparency of the magnetic barrier increases almost linearly with increasing ion energy in the low-energy range (below 200 eV) and saturates at high ion energies. The ion transparency of the electrostatic section is almost constant and close to the optical transparency of the entrance grid. We show here that the MRFEA can provide both accurate ion flux and ion energy distribution measurements in various experimental setups with ion beams or plasmas run at low pressure and with ion energies above 10 eV. (10.1063/1.4919730)
  DOI : 10.1063/1.4919730
• Charge and energy transferred from a plasma jet to liquid and dielectric surfaces
  
  • Dang van Sung Mussard Marguerite
  • Foucher Emeric
  • Rousseau Antoine
  Journal of Physics D: Applied Physics, IOP Publishing, 2015, 48 (42), pp.424003. A key parameter in using plasma jets for biomedical applications is the transferred energy to the living tissues. The objective of this paper is to understand which parameters control the energy transfer from the plasma jet to a liquid or a dielectric surface. The plasma jet is own with helium and ignited by a 600 Hz ac high voltage (up to 15 kV). Capacitors are connected to two measurement electrodes placed in the plasma source region, and under the sample. Charge and energy transferred are estimated by plotting Lissajous cycles; the number of bullets and the charge probability density function are also calculated. It is shown that the applied voltage and the gap (distance between the end of the tube and the sample) have a dramatic in uence on the energy deposition on the sample as well as on the charge probability density function. Surprisingly, both gap distance and voltage have very little in uence on the number of bullets reaching the sample per cycle. It is also shown that the conductivity of the liquid sample has almost no in uence on the energy deposition and charge probability density function. (10.1088/0022-3727/48/42/424003)
  DOI : 10.1088/0022-3727/48/42/424003
• E × B probe measurements in molecular and electronegative plasmas
  
  • Renaud D.
  • Gerst D.
  • Mazouffre S.
  • Aanesland Ane
  Review of Scientific Instruments, American Institute of Physics, 2015, 86 (12), pp.123507. This paper reports on the design, the building, the calibration, and the use of a compact E × B probe that acts as a velocity filter or a mass filter for ion species. A series of measurements has been performed in the discharge and in the beam of the PEGASES (Plasma Propulsion with Electronegative GASES) ion source. PEGASES is a unique inductively coupled radio-frequency source able to generate a beam of positive and negative ions when operated with an electronegative gas. In this study, experiments have been carried out with SF6. Calibrated E × B probe spectra indicate that the diagnostic tool can be used to determine the ion velocity and the plasma composition even when many molecular fragments are present. In addition, the probe is able to detect both positive and negative ions. Measurements show a large variety of positively charged ions coming from SF6. Conversely, the beam is solely composed of F− and SF−6 negative ions in compliance with computer simulations. (10.1063/1.4937604)
  DOI : 10.1063/1.4937604
• Fourier spectrum and phases for a signal in a finite interval
  
  • Belmont Gérard
  • Dorville Nicolas
  • Sahraoui Fouad
  • Rezeau Laurence
  , 2015, 17, pp.5320. 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 introducing 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 interference between the jumps and the signal itself provide in particular characteristic "hairs" on the spectrum, which are clearly visible on it with df≈1/T. These fluctuations are usually eliminated by smoothing the spectrum, or by averaging several successive spectra. Nevertheless, such treatments introduce uncertainties on the spectral laws (the phases being anyway completely lost). Windowing is also a method currently used to suppress or decrease the jumps, but it modifies the signal (the windowed trend has a spectrum, which is convolved with the searched one) and the phases are generally much altered. 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 window function, sharp but not square. This window function is chosen such that the spectrum obtained can 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 and first applications to Cluster data are presented, which demonstrate the capability of the method to better estimate the Fourier spectra.
• Mid-Atomic-Number Cylindrical Wire Array Precursor Plasma Studies on Zebra
  
  • Stafford A.
  • Safronova Alla S.
  • Kantsyrev Viktor L.
  • Weller Michael E.
  • Shrestha Ishor
  • Shlyaptseva V. V.
  • Coverdale C. A.
  • Chuvatin Alexandre S.
  IEEE Transactions on Plasma Science, Institute of Electrical and Electronics Engineers, 2015, 43 (8), pp.2497 - 2502. Precursor plasmas from low wire number cylindrical wire arrays (CWAs) were previously shown to radiate at temperatures >300 eV for Ni-60 (94% Cu and 6% Ni) wires in experiments on the 1-MA Zebra generator. Continued research into precursor plasmas has studied additional midatomic-number materials including Cu and Alumel (95% Ni, 2% Al, 2% Mn, and 1% Si) to determine if the >300 eV temperatures are common for midatomic-number materials. In addition, current scaling effects were observed by performing CWA precursor experiments at an increased current of 1.5 MA using a load current multiplier. The results show an increase in a linear radiation yield of ~50% (16 versus 10 kJ/cm) for the experiments at increased current. However, plasma conditions inferred through the modeling of X-ray time-gated spectra are very similar for the precursor plasma in both current conditions. (10.1109/TPS.2014.2382072)
  DOI : 10.1109/TPS.2014.2382072
• ICAN: High power neutral beam generation
  
  • Moustaizis Stavros
  • Lalousis P.
  • Perrakis K.
  • Auvray Philippe
  • Larour Jean
  • Ducret J.-E.
  • Balcou Philippe
  The European Physical Journal. Special Topics, EDP Sciences / Springer Verlag, 2015, 224, pp.2639 - 2643. During the last few years there is an increasing interest on the development of alternative high power new negative ion source for Tokamak applications. The proposed new neutral beam device presents a number of advantages with respect to: the density current, the acceleration voltage, the relative compact dimension of the negative ion source, and the coupling of a high power laser beam for photo-neutralization of the negative ion beam. Here we numerically investigate, using a multifluid 1-D code, the acceleration and the extraction of high power ion beam from a Magnetically Insulated Diode (MID). The diode configuration will be coupled to a high power device capable of extracting a current up to few kA with an accelerating voltage up to MeV. An efficiency of up to 92% of the coupling of the laser beam, is required in order to obtain a high power, up to GW, neutral beam. The new high energy, high average power, high efficiency (up to 30%) ICAN fiber laser is proposed for both the plasma generation and the photo-neutralizer configuration [1]. (10.1140/epjst/e2015-02576-6)
  DOI : 10.1140/epjst/e2015-02576-6
• Scaling and anisotropy of magnetohydrodynamic turbulence in a strong mean magnetic field
  
  • Mueller W.
  • Grappin Roland
  AGU Fall Meeting Abstracts, AGU, 2010, 51, pp.03. We present a new analysis of the anisotropic spectral energy distribution in incompressible magnetohydrodynamic (MHD) turbulence permeated by a strong mean magnetic field. The turbulent flow is generated by high-resolution pseudo-spectral direct numerical simulations with large-scale isotropic forcing. Examining the radial energy distribution for various angles theta with respect to B 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 wavenumber k0 which marks the start of a scaling range and goes on up to a dissipative wavenumber kd(theta). The 3D spectrum for k >= k0 is described by a single theta-independent functional form F(k/kd), 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=10 (in units of brms), 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.
• Une énergie, des énergies. Comment fonctionne le monde ?
  
  • Aanesland Ane
  • Rezeau Laurence
  , 2015. Se chauffer, se déplacer, communiquer, créer? Toutes nos actions « consomment » de l'énergie. Omniprésente dans les débats géopolitiques, galvaudée par les slogans publicitaires, l'énergie hante notre société. Elle se conserve, mais s'épuise. Elle se manifeste partout, dans l'eau qui coule, dans les atomes qui se désintègrent ou fusionnent, mais pas toujours sous la forme qui conviendrait au moment voulu. Et pire, la voilà désormais associée aux pollutions ! La première partie de cet ouvrage fait le tour des signes perceptibles de l'énergie dans notre vie quotidienne (feu, vapeur?), puis ceux qui se produisent en nous et autour de nous, du vivant végétal jusqu'aux confins de l'Univers. La deuxième montre la manière dont les humains utilisent ces transformations énergétiques pour se nourrir, se déplacer, se loger, fabriquer des objets, alimenter leurs industries, créer ou communiquer. La troisième et dernière partie, plus théorique, donne les clés de l'énergie en se référant à l'histoire des sciences, l'épistémologie et la science contemporaine. À l'écart des jugements de valeur, ce livre rassemble les connaissances les plus actuelles, émanant de nombreux chercheurs de toutes disciplines.
• Anisotropy of Third-order Structure Functions in MHD Turbulence
  
  • Verdini Andrea
  • Grappin Roland
  • Hellinger P.
  • Landi Simone
  • Müller Wolf-Christian
  The Astrophysical Journal, American Astronomical Society, 2015, 804, pp.119. The measure of the third-order structure function, \boldsymbolY , is employed in the solar wind to compute the cascade rate of turbulence. In the absence of a mean field B<SUB>0</SUB>=0, \boldsymbolY is expected to be isotropic (radial) and independent of the direction of increments, so its measure yields directly the cascade rate. For turbulence with mean field, as in the solar wind, \boldsymbolY is expected to become more two-dimensional (2D), that is, to have larger perpendicular components, losing the above simple symmetry. To get the cascade rate, one should compute the flux of \boldsymbolY , which is not feasible with single-spacecraft data thus, measurements rely on assumptions about the unknown symmetry. We use direct numerical simulations (DNSs) of magnetohydrodynamic (MHD) turbulence to characterize the anisotropy of \boldsymbolY . We find that for strong guide field B<SUB>0</SUB>=5 the degree of two-dimensionalization depends on the relative importance of shear-Alfvén and pseudo-Alfvén polarizations (the two components of an Alfvén mode in incompressible MHD). The anisotropy also shows up in the inertial range. The more \boldsymbolY is 2D, the more the inertial range extent differs along parallel and perpendicular directions. We finally test the two methods employed in observations and find that the so-obtained cascade rate may depend on the angle between B<SUB>0</SUB> and the direction of increments. Both methods yield a vanishing cascade rate along the parallel direction, contrary to observations, suggesting a weaker anisotropy of solar wind turbulence compared to our DNSs. This could be due to a weaker mean field and/or to solar wind expansion. (10.1088/0004-637X/804/2/119)
  DOI : 10.1088/0004-637X/804/2/119