Publications

2018

The effects of secondary electron emission on plasma sheath characteristics and electron transport in an ExB discharge via kinetic simulations
- Tavant Antoine
- Croes Vivien
- Lucken Romain
- Lafleur Trevor
- Bourdon Anne
- Chabert Pascal
Plasma Sources Science and Technology, IOP Publishing, 2018, 27 (12), pp.124001. Hall-effect thrusters, which are electrostatic devices based on an E B ´ plasma discharge, have successfully been used as satellite propulsion systems for the last few decades. However, the presence of anomalous electron cross-field transport is still poorly understood, and involves complex and strongly coupled mechanisms such as azimuthal electron drift instabilities and intense secondary electron emission (SEE) from the thruster walls. The present work focuses on the relative importance of these two phenomena. We use a 2D particle-in-cell/Monte Carlo collision model configured to simulate the radial-azimuthal directions near the thruster exit plane. A constant radial magnetic field and axial electric field are imposed, and electron drift instabilities are observed in the azimuthal (E B ´) direction. A simplified SEE model is implemented and an extensive parametric study is performed to directly determine the effect on electron transport. It is found that, for the operating conditions used in our simulations, SEE enhances the near-wall electron mobility by a factor 2, while reducing the bulk plasma mobility by about 20% (due to electron cooling). However, the dominant contribution to anomalous electron transport is still observed to be caused by electron drift instabilities driven by the E B ´ discharge configuration. SEE modifies the electron mobility profile, but the spatially-averaged value remains relatively constant. Three different operating regimes are identified depending on the SEE rate value: two that are stable, and a third which shows an oscillatory behaviour. In addition to electron transport, the kinetic simulations give new insight into the plasma sheath formation at the radial walls, and comparison with typical analytical sheath models demonstrate important differences. (10.1088/1361-6595/aaeccd)
DOI : 10.1088/1361-6595/aaeccd
3D Anisotropy of Solar Wind Turbulence, Tubes, or Ribbons?
- Verdini Andrea
- Grappin Roland
- Alexandrova Olga
- Lion Sonny
The Astrophysical Journal, American Astronomical Society, 2018, 853 (1), pp.85. We study the anisotropy with respect to the local magnetic field of turbulent magnetic fluctuations at magnetofluid scales in the solar wind. Previous measurements in the fast solar wind obtained axisymmetric anisotropy, despite that the analysis method allows nonaxisymmetric structures. These results are probably contaminated by the wind expansion that introduces another symmetry axis, namely, the radial direction, as indicated by recent numerical simulations. These simulations also show that while the expansion is strong, the principal fluctuations are in the plane perpendicular to the radial direction. Using this property, we separate 11 yr of Wind spacecraft data into two subsets characterized by strong and weak expansion and determine the corresponding turbulence anisotropy. Under strong expansion, the small-scale anisotropy is consistent with the Goldreich & Sridhar critical balance. As in previous works, when the radial symmetry axis is not eliminated, the turbulent structures are field-aligned tubes. Under weak expansion, we find 3D anisotropy predicted by the Boldyrev model, that is, turbulent structures are ribbons and not tubes. However, the very basis of the Boldyrev phenomenology, namely, a cross-helicity increasing at small scales, is not observed in the solar wind: the origin of the ribbon formation is unknown. (10.3847/1538-4357/aaa433)
DOI : 10.3847/1538-4357/aaa433
Solar Wind Turbulence Studies Using MMS Fast Plasma Investigation Data
- Bandyopadhyay Riddhi
- Chasapis A.
- Chhiber R.
- Parashar T. N.
- Maruca B. A.
- Matthaeus W. H.
- Schwartz S. J.
- Eriksson S.
- Le Contel Olivier
- Breuillard Hugo
- Burch J. L.
- Moore T. E.
- Pollock C. J.
- Giles B. L.
- Paterson W. R.
- Dorelli J. C.
- Gershman D. J.
- Torbert R. B.
- Russell C. T.
- Strangeway R. J.
The Astrophysical Journal, American Astronomical Society, 2018, 866 (2), pp.81. Studies of solar wind turbulence traditionally employ high-resolution magnetic field data, but high-resolution measurements of ion and electron moments have been possible only recently. We report the first turbulence studies of ion and electron velocity moments accumulated in pristine solar wind by the Fast Plasma Investigation (FPI) instrument on board the Magnetospheric Multiscale Mission. Use of these data is made possible by a novel implementation of a frequency domain Hampel filter, described herein. After presenting procedures for processing of the data, we discuss statistical properties of solar wind turbulence extending into the kinetic range. Magnetic field fluctuations dominate electron and ion-velocity fluctuation spectra throughout the energy-containing and inertial ranges. However, a multispacecraft analysis indicates that at scales shorter than the ion inertial length, electron velocity fluctuations become larger than ion-velocity and magnetic field fluctuations. The kurtosis of ion-velocity peaks around a few ion inertial lengths and returns to a near Gaussian value at sub-ion scales. (10.3847/1538-4357/aade93)
DOI : 10.3847/1538-4357/aade93
Kinetic study of CO2 plasmas under non-equilibrium conditions. II. Input of vibrational energy
- Grofulovic Marija
- Silva Tiago
- Klarenaar Bart
- Morillo-Candas Ana-Sofia
- Guaitella Olivier
- Engeln Richard
- Pintassilgo C.D.
- Guerra V.
Plasma Sources Science and Technology, IOP Publishing, 2018. This is the second of two papers presenting the study of vibrational energy exchanges in non-equilibrium CO2 plasmas in low-excitation conditions. The companion paper addresses a theoretical and experimental investigation of the time relaxation of ~70 individual vibrational levels of ground-state CO2(X1Σ ) molecules during the afterglow of a pulsed DC glow discharge, operating at pressures of a few Torr and discharge currents around 50 mA, where the rate coefficients for vibration-translation (V-T) and vibration-vibration (V-V) energy transfers among these levels are validated. Herein the investigation focus the active discharge, by extending the model with the inclusion of electron impact processes for vibrational excitation and de-excitation (e-V). The time-dependent calculated densities of the different vibrational levels are compared with experimental data obtained from time-resolved in situ Fourier Transform Infrared spectroscopy. It is shown that the vibrational temperature of the asymmetric stretching mode is always larger than the vibrational temperatures of the bending and symmetric stretching modes along the discharge pulse, the latter two remaining very nearly the same and close to the gas temperature. The general good agreement between the model predictions and the experimental results validates the e-V rate coefficients used and gives confidence that the proposed kinetic scheme provides a solid basis to understand the vibrational energy exchanges occurring in CO2 plasmas. (10.1088/1361-6595/aadb60)
DOI : 10.1088/1361-6595/aadb60
Electron Power-Law Spectra in Solar and Space Plasmas
- Oka Mitsuo
- Birn J.
- Battaglia Marina
- Chaston C. C.
- Hatch S. M.
- Livadiotis G.
- Imada S.
- Miyoshi Y.
- Kuhar M.
- Effenberger F.
- Eriksson E.
- Khotyaintsev Yu. V.
- Retinò Alessandro
Space Science Reviews, Springer Verlag, 2018, 214. Particles are accelerated to very high, non-thermal energies in solar and space plasma environments. While energy spectra of accelerated electrons often exhibit a power law, it remains unclear how electrons are accelerated to high energies and what processes determine the power-law index delta . Here, we review previous observations of the power-law index delta in a variety of different plasma environments with a particular focus on sub-relativistic electrons. It appears that in regions more closely related to magnetic reconnection (such as the `above-the-looptop' solar hard X-ray source and the plasma sheet in Earth's magnetotail), the spectra are typically soft (delta &gsim;4). This is in contrast to the typically hard spectra (delta &lsim;4) that are observed in coincidence with shocks. The difference implies that shocks are more efficient in producing a larger non-thermal fraction of electron energies when compared to magnetic reconnection. A caveat is that during active times in Earth's magnetotail, delta values seem spatially uniform in the plasma sheet, while power-law distributions still exist even in quiet times. The role of magnetotail reconnection in the electron power-law formation could therefore be confounded with these background conditions. Because different regions have been studied with different instrumentations and methodologies, we point out a need for more systematic and coordinated studies of power-law distributions for a better understanding of possible scaling laws in particle acceleration as well as their universality. (10.1007/s11214-018-0515-4)
DOI : 10.1007/s11214-018-0515-4
Experimental benchmark of kinetic simulations of capacitively coupled plasmas in molecular gases
- Donkó Z.
- Derzsi A.
- Korolov Ihor
- Hartmann P.
- Brandt S.
- Schulze J.
- Berger B.
- Koepke M.
- Bruneau Bastien
- Johnson Erik
- Lafleur Trevor
- Booth Jean-Paul
- Gibson Andrew
- O'Connell D.
- Gans T.
Plasma Physics and Controlled Fusion, IOP Publishing, 2018, 60 (1), pp.014010. We discuss the origin of uncertainties in the results of numerical simulations of low-temperature plasma sources, focusing on capacitively coupled plasmas. These sources can be operated in various gases/gas mixtures, over a wide domain of excitation frequency, voltage, and gas pressure. At low pressures, the non-equilibrium character of the charged particle transport prevails and particle-based simulations become the primary tools for their numerical description. The particle-in-cell method, complemented with Monte Carlo type description of collision processes, is a well-established approach for this purpose. Codes based on this technique have been developed by several authors/groups, and have been benchmarked with each other in some cases. Such benchmarking demonstrates the correctness of the codes, but the underlying physical model remains unvalidated. This is a key point, as this model should ideally account for all important plasma chemical reactions as well as for the plasma-surface interaction via including specific surface reaction coefficients (electron yields, sticking coefficients, etc). In order to test the models rigorously, comparison with experimental ?benchmark data? is necessary. Examples will be given regarding the studies of electron power absorption modes in O 2 , and CF 4 ?Ar discharges, as well as on the effect of modifications of the parameters of certain elementary processes on the computed discharge characteristics in O 2 capacitively coupled plasmas. (10.1088/1361-6587/aa8378)
DOI : 10.1088/1361-6587/aa8378
Multi-variable comprehensive analysis of two great geomagnetic storms of 2015
- Kashcheyey Anton
- Migoya-Orué Yenca
- Amory-Mazaudier Christine
- Fleury Rolland
- Nava B.
- Alazo-Cuartas K.
- Radicella S.
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2018. During the year 2015 two great geomagnetic storms (Dst<-200nT) occurred on 17 March and 22 June. These two geomagnetic storms have similarities. They occurred during the same decreasing phase of the sunspot cycle 24. The interplanetary and magnetospheric environments were calm before the beginning of the storms. Both events were due to Coronal Mass Ejections (CME)and High Speed Solar Wind (HSSW). Variations of the solar wind velocity and the Bz component of the interplanetary magnetic field (IMF) were also similar. Two key features that are different for these storms are UT time of the beginning(04:45UT for 17March and 18:33UT for 22June) and season (equinox and solstice). The comparison of the impact of the storms on the Earth ionosphere and magnetosphere have been performed using diverse parameters including global ionospheric maps (GIMs) of vertical total electron content (VTEC), data from individual GNSS receivers, ionosondes, magnetometers and instruments from different space missions. Visualizing GIM data as the difference of VTEC between consecutive days allowed understanding better the effect of the storms as a function of time of the beginning of the storm and of the season. It is shown that the presence or absence of scintillations in GNSS signals during these 2 storms in African longitude sector is clearly related to the local time at a given station at the beginning of the storm. (10.1029/2017JA024900)
DOI : 10.1029/2017JA024900
SMILEI : A collaborative, open-source, multi-purpose particle-in-cell code for plasma simulation
- Derouillat J.
- Beck A.
- Pérez F.
- Vinci T.
- Chiaramello M.
- Grassi A.
- Flé M.
- Bouchard G.
- Plotnikov I.
- Aunai Nicolas
- Dargent Jérémy
- Riconda C.
- Grech M.
Computer Physics Communications, Elsevier, 2018, 222, pp.351-373. SMILEI is a collaborative, open-source, object-oriented (C ) particle-in-cell code. To benefit from the latest advances in high-performance computing (HPC), SMILEI is co-developed by both physicists and HPC experts. The code's structures, capabilities, parallelization strategy and performances are discussed. Additional modules (e.g. to treat ionization or collisions), benchmarks and physics highlights are also presented. Multi-purpose and evolutive, SMILEI is applied today to a wide range of physics studies, from relativistic laser-plasma interaction to astrophysical plasmas. (10.1016/j.cpc.2017.09.024)
DOI : 10.1016/j.cpc.2017.09.024
Wave Phenomena and Beam-Plasma Interactions at the Magnetopause Reconnection Region
- Burch J. L.
- Webster J. M.
- Genestreti K. J.
- Torbert R. B.
- Giles B. L.
- Fuselier S. A.
- Dorelli J. C.
- Rager A. C.
- Phan T. D.
- Allen R. C.
- Chen L.-J.
- Wang S.
- Le Contel Olivier
- Russell C. T.
- Strangeway R. J.
- Ergun R. E.
- Jaynes A. N.
- Lindqvist P.-A.
- Graham D. B.
- Wilder F. D.
- Hwang K.-J.
- Goldstein J.
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2018, 123 (2), pp.1118-1133. This paper reports on Magnetospheric Multiscale observations of whistler mode chorus and higher-frequency electrostatic waves near and within a reconnection diffusion region on 23 November 2016. The diffusion region is bounded by crescent-shaped electron distributions and associated dissipation just upstream of the X-line and by magnetic field-aligned currents and electric fields leading to dissipation near the electron stagnation point. Measurements were made southward of the X-line as determined by southward directed ion and electron jets. We show that electrostatic wave generation is due to magnetosheath electron beams formed by the electron jets as they interact with a cold background plasma and more energetic population of magnetospheric electrons. On the magnetosphere side of the X-line the electron beams are accompanied by a strong perpendicular electron temperature anisotropy, which is shown to be the source of an observed rising-tone whistler mode chorus event. We show that the apex of the chorus event and the onset of electrostatic waves coincide with the opening of magnetic field lines at the electron stagnation point. (10.1002/2017JA024789)
DOI : 10.1002/2017JA024789
Electron bulk acceleration and thermalization at Earth's quasi-perpendicular bow shock
- Chen L.-J.
- Wang S.
- Wilson Iii L. B.
- Schwartz S. J.
- Bessho N.
- Moore T. E.
- Gershman D. J.
- Giles B. L.
- Malaspina D. M.
- Wilder F. D.
- Ergun R. E.
- Hesse Michael
- Lai H.
- Russell C. T.
- Strangeway R. J.
- Torbert R. B.
- Viñas A.-F.
- Burch J. L.
- Lee S.
- Pollock C.
- Dorelli J. C.
- Paterson W. R.
- Ahmadi N.
- Goodrich K. A.
- Lavraud B.
- Le Contel Olivier
- Khotyaintsev Y. V.
- Lindqvist P.-A.
- Boardsen S.
- Wei H.
- Le A.
- Avanov L. A.
Physical Review Letters, American Physical Society, 2018, 120, pp.225101. Electron heating at Earth's quasiperpendicular bow shock has been surmised to be due to the combined effects of a quasistatic electric potential and scattering through wave-particle interaction. Here we report the observation of electron distribution functions indicating a new electron heating process occurring at the leading edge of the shock front. Incident solar wind electrons are accelerated parallel to the magnetic field toward downstream, reaching an electron-ion relative drift speed exceeding the electron thermal speed. The bulk acceleration is associated with an electric field pulse embedded in a whistler-mode wave. The high electron-ion relative drift is relaxed primarily through a nonlinear current-driven instability. The relaxed distributions contain a beam traveling toward the shock as a remnant of the accelerated electrons. Similar distribution functions prevail throughout the shock transition layer, suggesting that the observed acceleration and thermalization is essential to the cross-shock electron heating. (10.1103/PhysRevLett.120.225101)
DOI : 10.1103/PhysRevLett.120.225101
Fast gas heating and radial distribution of active species in nanosecond capillary discharge in pure nitrogen and N2:O2 mixtures
- Lepikhin N D
- Popov N.A.
- Starikovskaia Svetlana
Plasma Sources Science and Technology, IOP Publishing, 2018, 27, pp.055005. Fast gas heating is studied experimentally and numerically using pulsed nanosecond capillary discharge in pure nitrogen and N2:O2 mixtures under the conditions of high specific deposited energy (up to 1 eV/molecule) and high reduced electric fields (100300 Td). Deposited energy, electric field and gas temperature are measured as functions of time. The radial distribution of active species is analyzed experimentally. The roles of processes involving excited N2 molecules and ions and N(2D) excited nitrogen species leading to heat release are analyzed using numerical modeling in the framework of 1D axial approximation. (10.1088/1361-6595/aab74e)
DOI : 10.1088/1361-6595/aab74e
Poloidal asymmetries of flows in the Tore Supra tokamak
- Vermare Laure
- Hennequin Pascale
- Gürcan Özgür D.
- Garbet X.
- Honoré Cyrille
- Clairet F.
- Giacalone J-C.
- Morel Pierre
- Storelli A.
- Team Tore Supra
Physics of Plasmas, American Institute of Physics, 2018, 25 (2), pp.020704. Simultaneous measurements of binormal velocity of density fluctuations using two separate Doppler backscattering systems at the low field side and at the top of the plasma show significant poloidal asymmetry. The measurements are performed in the core region between the radii 0.7 < ρ  <  0.95, over a limited number of L-mode discharges covering a wide range of plasma conditions in the Tore Supra tokamak. A possible generation mechanism by the ballooned structure of the underlying turbulence, in the form of convective cells, is proposed for explaining the observation of these poloidally asymmetric mean flows. (10.1063/1.5022122)
DOI : 10.1063/1.5022122
Calculated electron impact dissociation cross sections for molecular chlorine (Cl 2 )
- Hamilton James R.
- Tennyson Jonathan
- Booth Jean-Paul
- Gans Timo
- Gibson Andrew
Plasma Sources Science and Technology, IOP Publishing, 2018, 27 (9), pp.095008. Electron impact dissociation of Cl 2 is a key process for the formation of Cl atoms in low-temperature plasmas used for industrial etching processes. Despite this, relatively little cross section data exist for this process. In this work, electron impact dissociation cross sections were calculated for Cl 2 molecules using the UK molecular R-matrix code in the low electron energy range and extended to high energies using a scaling depending on the specific nature of each transition. Our results are compared with both previous calculations and with experimental measurements, and the similarities and differences are discussed. In addition, the rate coefficients for electron impact dissociation of Cl 2 are calculated by integrating the cross sections derived in this (and previous) work, with electron energy distribution functions representative of those normally found in low-temperature plasmas used in industry. Depending on the shape and effective temperature of the distribution function, significant differences arise between the rate coefficients calculated from our cross sections and those calculated using previous data. Deviations between the two sets of rate coefficients are particularly pronounced at the low electron temperatures typical of electron beam and remote plasma sources of interest for atomic layer etching and deposition. These differences are principally caused by the higher energy resolution in the near-threshold region in this work, emphasising the importance of accurate, high-resolution cross sections in this energy range. (10.1088/1361-6595/aada32)
DOI : 10.1088/1361-6595/aada32
Geocatalytic uptake of ozone onto natural mineral dust
- Wang Xianjie
- Romanias Manolis
- Thevenet Frederic
- Rousseau Antoine
Catalysts, MDPI, 2018, 8 (7), pp.263. Beyond tailored and synthetic catalysts sought out for ozone decomposition, mineral dusts provide naturally mixed metal oxide materials. The steady-state uptake of O3 evidenced across a wide concentration range signifies the catalytic decomposition of O3. The geocatalytic properties of such natural mineral dust open up new perspectives in atmospheric chemistry and catalytic processes. (10.3390/catal8070263)
DOI : 10.3390/catal8070263
On the origin of the energy dissipation anomaly in (Hall) magnetohydrodynamics
- Galtier Sébastien
Journal of Physics A: Mathematical and General (1975 - 2006), IOP Publishing, 2018, 51. Incompressible Hall magnetohydrodynamics (MHD) may be the subject of energy dissipation anomaly which stems from the lack of smoothness of the velocity and magnetic fields. I derive the exact expression of which appears to be closely connected with the well-known 4/3 exact law of Hall MHD turbulence theory. This remarkable similitude suggests a deeper mathematical property of the fluid equations. In the MHD limit, the expression of differs from the one derived by Gao et al (2013 Acta Math. Sci. 33 865−71) which presents miscalculations. The energy dissipation anomaly can be used to better estimate the local heating in space plasmas where in situ measurements are accessible. (10.1088/1751-8121/aabbb5)
DOI : 10.1088/1751-8121/aabbb5
Low-Altitude Observations of Recurrent Short-Lived keV Ion Micro-Injections Inside the Diffuse Auroral Zone
- Sauvaud J.-A.
- Delcourt Dominique C.
- Parrot Michel
- Payan D.
- Raita T.
- Penou E.
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2018, 123, pp.2054–2063. The AMBRE experiment onboard the ocean topography mapper JASON‐3 aims at measuring the spacecraft potential as well as auroral particle precipitation using two top‐hat analyzers for electrons and ions in the 20 eV‐28 keV energy range. The JASON‐3 spacecraft has a nearly circular orbit at an altitude of 1336 km with an inclination of 66°, at times probing the equatorward part of the auroral oval in a nearly tangential manner upon leaving the outer radiation belt. In this region of space, during periods of enhanced geomagnetic activity with small or moderate storms, AMBRE detected recurrent “micro‐injections” of ions with energies in the 200 eV‐28 keV range and which exhibit clear time of flight dispersion. Ray tracing using single trajectory computations suggests that these ions are launched from a source located in the 8000‐12000 km altitudinal range and subsequently propagate downward toward the ionosphere. Such observations of quasi‐periodic dispersed downflowing ions are new and we argue that these structures could be produced by ion‐wave interactions at mid‐altitudes. (10.1002/2017JA025075)
DOI : 10.1002/2017JA025075
Local energy transfer rate and kinetic signatures in solar wind turbulence
- Sorriso-Valvo L.
- Catapano F.
- Retinò Alessandro
- Greco A.
- Perri S.
- Marino R.
- Pezzi O.
- Perrone D.
- Bruno Roberto
- Valentini F.
- Servidio S.
- Panebianco V.
, 2018, 20, pp.EGU2018-6552. The transfer of energy from large to small scales in space turbulence is an important ingredient of the longstanding question about the mechanism of the interplanetary plasma heating. Statistical analysis in the context of magnetohydrodynamic (MHD) turbulence provided evidence that the total transported energy is compatible with the observed heating of the solar wind as it expands in the heliosphere. However, in order to understand which processes contribute to the plasma heating, it is necessary to have a local description of the energy flux across scales. To this aim, we propose a proxy of the scale-dependent, local energy transfer that includes magnetic, velocity and cross-helicity terms, and is based on the third-order moment scaling law for MHD turbulence. Data from Helios2 are used to determine the statistical properties of such a proxy in comparison with the magnetic and velocity fields PVI, and the correlation with local solar wind heating is pointed out. MMS data are used to study the correlation with kinetic-scale features, as for example the temperature anisotropy, the heat flux, the agyrothropy of the pressure tensor, and the deviation form Maxwellian. A comparison with hybrid direct numerical simulations of the Vlasov-Maxwell system, including alpha particles, is also performed. The good correlation between the turbulent local energy flux and the indicators of kinetic processes found in the data and in the simulations suggests an important role played by this proxy in the study of plasma energy dissipation.
Multi frequency matching for voltage waveform tailoring
- Schmidt Frederik
- Schulze J.
- Johnson Erik
- Booth Jean-Paul
- Keil D.
- French David M
- Trieschmann Jan
- Mussenbrock Thomas
Plasma Sources Science and Technology, IOP Publishing, 2018, 27 (9), pp.095012. Customized voltage waveforms composed of a number of frequencies and used as the excitation of radio-frequency plasmas can control various plasma parameters such as energy distribution functions, homogeneity of the ion flux, or ionization dynamics. So far this technology, while being extensively studied in academia, has yet to be established in applications. One reason for this is the lack of a suitable multi frequency matching network that allows for maximum power absorption for each excitation frequency that is generated and transmitted via a single broadband amplifier. In this work, a method is introduced for designing such a network based on network theory and synthesis. Using this method, a circuit simulation is established that connects an exemplary matching network to an equivalent circuit plasma model of a capacitive radio-frequency discharge. It is found that for a range of gas pressures and number of excitation frequencies the matching conditions can be satisfied, which proves the functionality and feasibility of the proposed concept. Based on the proposed multi frequency impedance matching, tailored voltage waveforms can be used at an industrial level. (10.1088/1361-6595/aad2cd)
DOI : 10.1088/1361-6595/aad2cd
How dielectric, metallic and liquid targets influence the evolution of electron properties in a pulsed He jet measured by Thomson and Raman scattering
- Klarenaar Bart
- Guaitella Olivier
- Engeln Richard
- Sobota Ana
Plasma Sources Science and Technology, IOP Publishing, 2018, 27 (8), pp.085004. Thomson scattering using a Bragg grating notch filter is used to determine the electron properties of a pulsed, kHz-driven, non-thermal atmospheric pressure plasma jet in helium expanding in air. The plasma jet is allowed to freely expand or interact with targets with different electrical properties, i.e. glass, copper and water. With the same setup, Raman scattering is used to determine spatially- and time-resolved the densities and rotational temperatures of oxygen and nitrogen molecules entrained into the jet. Fast imaging is used to determine the development of the discharge in the plasma jet as well as its behavior in the plasma-target interaction zone. As the discharge approaches the target, the rise of electron density was followed by the fall of electron temperature. The discharge is influenced only over a few millimeters before it hits the target. The electron density and temperature during the spreading of the discharge on the low-permittivity target are measured to be resp. 2 × 1019 m−3 and ≈1 eV. During the return stroke on the high-permittivity and the metallic target the densities rise with a factor 1.5 resp. 2.2, and the temperature with a factor 2.5 for both cases. The discharges on the high- and low-permittivity targets extinguished soon after the initial impact of the ionization front, while the diffuse discharge on the metallic target extinguished only after the end of the voltage pulse (with a duration of 1 μs). In the diffuse discharge the electron temperature reaches 3.4 eV, the gas temperature increases by approximately 100 K and the electron density increases by approximately a factor three with respect to before its formation. (10.1088/1361-6595/aad4d7)
DOI : 10.1088/1361-6595/aad4d7
Nested polyhedra model of isotropic magnetohydrodynamic turbulence
- Gürcan Özgür D.
Physical Review E, American Physical Society (APS), 2018, 97, pp.063111. A nested polyhedra model has been developed for magnetohydrodynamic turbulence. Driving only the velocity field at large scales with random, divergence-free forcing results in a clear, stationary k^-5/3 spectrum for both kinetic and magnetic energies. Since the model naturally effaces disparate scale interactions, does not have a guide field, and avoids injecting any sign of helicity by random forcing, the resulting three-dimensional k spectrum is statistically isotropic. The strengths and weaknesses of the model are demonstrated by considering large or small magnetic Prandtl numbers. It was also observed that the timescale for the equipartition offset with those of the smallest scales shows a k^-1/2 scaling. (10.1103/PhysRevE.97.063111)
DOI : 10.1103/PhysRevE.97.063111
Compressible Magnetohydrodynamic Turbulence in the Earth’s Magnetosheath: Estimation of the Energy Cascade Rate Using in situ Spacecraft Data
- Hadid Lina
- Sahraoui Fouad
- Galtier Sébastien
- Huang S.Y.
Physical Review Letters, American Physical Society, 2018, 120 (5), pp.055102. The first estimation of the energy cascade rate |εC| of magnetosheath turbulence is obtained using the CLUSTER and THEMIS spacecraft data and an exact law of compressible isothermal magnetohydrodynamics turbulence. |εC | is found to be of the order of 10−13J.m−3.s−1, at least two orders of magnitude larger than its value in the solar wind (order of 10−16 J.m−3.s−1 in the fast wind). Two types of turbulence are evidenced and shown to be dominated either by incompressible Alfénic or compressible magnetosonic-like fluctuations. Density fluctuations are shown to amplify the cascade rate and its spatial anisotropy in comparison with incompressible Alfv´enic turbulence. Furthermore, for compressible magnetosonic fluctuations, large cascade rates are found to lie mostly near the linear kinetic instability of the mirror mode. New empirical power-laws relating |C | to the turbulent Mach number and to the internal energy are evidenced. These new finding have potential applications in distant astrophysical plasmas that are not accessible to in situ measurements. (10.1103/PhysRevLett.120.055102)
DOI : 10.1103/PhysRevLett.120.055102
Magnetospheric Multiscale Observations of an Ion Diffusion Region With Large Guide Field at the Magnetopause: Current System, Electron Heating, and Plasma Waves
- Zhou M.
- Berchem J.
- Walker R. J.
- El-Alaoui M.
- Goldstein M. L.
- Lapenta G.
- Deng X.
- Li J.
- Le Contel Olivier
- Graham D. B.
- Lavraud B.
- Paterson W. R.
- Giles B. L.
- Burch J. L.
- Torbert R. B.
- Russell C. T.
- Strangeway R. J.
- Zhao C.
- Ergun R. E.
- Lindqvist P.-A.
- Marklund G.
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2018, 123 (3), pp.1834-1852. We report Magnetospheric Multiscale (MMS) observations of a reconnecting current sheet in the presence of a weak density asymmetry with large guide field at the dayside magnetopause. An ion diffusion region (IDR) was detected associated with this current sheet. Parallel current dominated over the perpendicular current in the IDR, as found in previous studies of component reconnection. Electrons were preferentially heated parallel to the magnetic field within the IDR. The heating was manifested as a flattop distribution below 400eV. Two types of electromagnetic electron whistler waves were observed within the regions where electrons were heated. One type of whistler wave was associated with nonlinear structures in E-|| with amplitudes up to 20mV/m. The other type was not associated with any structures in E-||. Poynting fluxes of these two types of whistler waves were directed away from the X-line. We suggest that the nonlinear evolution of the oblique whistler waves gave rise to the solitary structures in E-||. There was a perpendicular super-Alfvenic outflow jet that was carried by magnetized electrons. Intense electrostatic lower hybrid drift waves were localized in the current sheet center and were probably driven by the super-Alfvenic electron jet, the velocity of which was approximately equal to the diamagnetic drift of demagnetized ions. Our observations suggest that the guide field significantly modified the structures (Hall electromagnetic fields and current system) and wave properties in the IDR. (10.1002/2017JA024517)
DOI : 10.1002/2017JA024517
The effect of liquid target on a nonthermal plasma jet−imaging, electric fields, visualization of gas flow and optical emission spectroscopy
- Kovačević Vv
- Sretenović Gb
- Slikboer Elmar
- Guaitella Olivier
- Sobota Ana
- Kuraica Mm
Journal of Physics D: Applied Physics, IOP Publishing, 2018, 51 (6), pp.065202. The article describes the complex study of the interaction of a helium plasma jet with distilled water and saline. The discharge development, spatial distribution of the excited species, electric field measurement results and the results of the Schlieren imaging are presented. The results of the experiments showed that the plasmaliquid interaction could be prolonged with the proper choice of the gas composition between the jet nozzle and the target. This depends on the gas flow and the target distance. Increased conductivity of the liquid does not affect the discharge properties significantly. An increase of the gas flow enables an extension of the plasma duration on the liquid surface up to 10 µs, but with a moderate electric field strength in the ionization wave. In contrast, there is a significant enhancement of the electric field on the liquid surface, up to 30 kV cm−1 for low flows, but with a shorter time of the overall plasma liquid interaction. Ignition of the plasma jet induces a gas flow modification and may cause turbulences in the gas flow. A significant influence of the plasma jet causing a mixing in the liquid is also recorded and it is found that the plasma jet ignition changes the direction of the liquid circulation. (10.1088/1361-6463/aaa288)
DOI : 10.1088/1361-6463/aaa288
The role of instability-enhanced friction on anomalous electron and ion transport in Hall-effect thrusters
- Lafleur Trevor
- Chabert Pascal
Plasma Sources Science and Technology, IOP Publishing, 2018, 27 (1), pp.015003. Using a self-consistent 2D particle-in-cell (PIC) simulation, we investigate the electron transport in Hall-effect thrusters. The PIC simulation is explicit in time and models the axial and azimuthal directions of a thruster without using any artificial parametric or geometric scaling factors. The applied discharge voltage and external magnetic field causes electrons to drift in the azimuthal direction, and this drives an instability in the plasma that produces large amplitude oscillations in both the plasma density and azimuthal electric field. A Fourier transform in time and space shows that the oscillations follow a dispersion relation similiar to that for an ion acoustic instability (in agreement with a recent kinetic theory). Correlated with the presence of this instability is an enhanced electron cross-field transport; even in the absence of electron-wall collisions and secondary electron emission. The amplitude of plasma density oscillations (but not electric field oscillations) is found to decrease significantly in a region just downstream of the thruster exit (before then increasing again), and reaches levels similar to those measured experimentally with collective light scattering techniques. By taking relevant velocity moments of the electron distribution function in the PIC simulations, we reconstruct each term in the electron momentum conservation equation and demonstrate that anomalous electron transport can be explained entirely due to an instability-enhanced friction force between electrons and ions. This friction force acts as an additional momentum loss allowing electrons to cross the magnetic field, and as an accelerating force causing ions to rotate azimuthally in the same direction as the electrons. Clear evidence of ion-wave trapping in the instability electric field is observed. (10.1088/1361-6595/aa9efe)
DOI : 10.1088/1361-6595/aa9efe
Production of hydrogen negative ions in an ECR volume source: balance between vibrational excitation and ionization
- Aleiferis S.
- Svarnas P.
- Béchu S.
- Tarvainen O.
- Bacal M.
Plasma Sources Science and Technology, IOP Publishing, 2018, 27 (7), pp.075015. The operation of an ECR-driven (2.45 GHz) hydrogen negative ion source is studied. Electron densities and temperatures are investigated with electrostatic probes and negative ion densities are measured with laser photodetachment. Vacuum ultraviolet irradiance measurements are focused on molecular transitions to the ground state while high-resolution visible emission spectroscopy is used to study the transitions between excited states for both molecules and atoms. The standalone operation of the source is found to be more efficient in higher pressures (12 mTorr) where negative ion densities are as high as 4×109 cm−3. Further investigation on the operation of the source reveals a rich vibrational spectrum. On the other hand, a limitation on the production of negative ions which is attributed to a lack of low-energy electrons becomes apparent. The underlying mechanisms that lead to this behavior are discussed along with possible solutions to this issue. Finally, the rates of different negative ion destruction processes are estimated and compared. (10.1088/1361-6595/aabf1b)
DOI : 10.1088/1361-6595/aabf1b

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