Publications

2017

Geodesic acoustic modes with poloidal mode couplings ad infinitum
- Singh Rameswar
- Gürcan Özgür D.
Physics of Plasmas, American Institute of Physics, 2017, 24, pp.022507. Geodesic acoustic modes (GAMs) are studied including all poloidal mode (m) couplings within a drift reduced Braginskii framework. An exact analytical formula for GAM frequency is given within the toroidal Hasegawa Mima model with the full finite larmor radius effect and poloidal mode couplings ad infinitum using a scalar continued fraction formulation, which results from reduction of the semi-infinite chain of interactions that is obtained from the nearest neighbor coupling pattern due to geodesic curvature. This pattern can be described by a semi-infinite chain model of the GAM with the mode-mode coupling matrix elements proportional to the radial wave number kr. In the more general case of multi-field description of the GAM, the infinite chain can be reduced to a renormalized bi-nodal chain with a matrix continued fraction formulation. The convergence study of the linear GAM dispersion with respect to kr and the m-spectra confirms that the coupling beyond m = 1 is sustained only when kr ≠ 0 and the higher m couplings become important with increasing kr and increasing ion to electron temperature ratio τi. (10.1063/1.4976116)
DOI : 10.1063/1.4976116
The case for in situ resource utilisation for oxygen production on Mars by non-equilibrium plasmas
- Guerra V.
- Silva Tiago
- Ogloblina P.
- Grovulović Marija
- Terraz Loann
- Silva Mrio Lino Da
- Pintassilgo C.D.
- Alves Lus L
- Guaitella Olivier
Plasma Sources Science and Technology, IOP Publishing, 2017, 26 (11), pp.11LT01. Herein, it is argued that Mars has nearly ideal conditions for CO 2 decomposition by non-equilibrium plasmas. It is shown that the pressure and temperature ranges in the ##IMG## [http://ej.iop.org/images/0963-0252/26/11/11LT01/psstaa8dccieqn1.gif] ∼ 96 % CO 2 Martian atmosphere favour the vibrational excitation and subsequent up-pumping of the asymmetric stretching mode, which is believed to be a key factor for an efficient plasma dissociation, at the expense of the excitation of the other modes. Therefore, gas discharges operating at atmospheric pressure on Mars are extremely strong candidates to produce O 2 efficiently from the locally available resources. (10.1088/1361-6595/aa8dcc)
DOI : 10.1088/1361-6595/aa8dcc
Occurrence rate of whistler waves in the magnetotail reconnection region
- Huang S. Y.
- Yuan Z. G.
- Sahraoui Fouad
- Fu H.S.
- Pang Y.
- Zhou M.
- Fujimoto K.
- Deng X. H.
- Retinò Alessandro
- Wang D. D.
- Yu X. D.
- Li H. M.
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2017, 122 (7), pp.7188-7196. It has long been suggested that whistler waves play an active role during magnetic reconnection. However, all previous observations were based on case studies and could not give a detailed picture as to where the whistler waves occur in the reconnection region. In this paper, a statistical study by using the Cluster data is performed to investigate the spatial distribution and the occurrence rate of whistler waves in the magnetotail reconnection region. It is found that the occurrence rate of the whistler waves is large in the separatrix region (|Bx/B0| > 0.4) and in the pileup region (|Bx/B0| < 0.2, |theta| > 45°, where theta = arctan(Bz/Bx)), but is very small in the vicinity of the X-line. These statistical results are well consistent with recent kinetic simulations and with previous observational case studies. (10.1002/2016JA023670)
DOI : 10.1002/2016JA023670
Linear gyrokinetic investigation of the geodesic acoustic modes in realistic tokamak configurations
- Novikau I.
- Biancalani A.
- Bottino A.
- Conway G. D.
- Gürcan Özgür D.
- Manz P.
- Morel Pierre
- Poli E.
- Siena A. Di
- Team Asdex Upgrade
Physics of Plasmas, American Institute of Physics, 2017, 24 (12), pp.122117. In order to provide scaling formulae for the geodesic acoustic mode (GAM) frequency and damping rate, GAMs are studied by means of the gyrokinetic global particle-in-cell code ORB5. Linear electromagnetic simulations in the low-βe limit have been performed in order to separate acoustic and Alfvnic time scales and obtain more accurate measurements. The dependence of the frequency and damping rate on several parameters such as the safety factor, the GAM radial wavenumber, and the plasma elongation is studied. All simulations have been performed with kinetic electrons with a realistic electron/ion mass ratio. Interpolating formulae for the GAM frequency and damping rate, based on the results of the gyrokinetic simulations, have been derived. Using these expressions, the influence of the temperature gradient on the damping rate is also investigated. Finally, the results are applied to the study of a real discharge of the ASDEX Upgrade tokamak. (10.1063/1.5003784)
DOI : 10.1063/1.5003784
Emission Spectroscopy Study of the Microwave Discharge Igniter
- Shcherbanev S.A.
- de Martino A.
- Khomenko A.Yu.
- Starikovskaia Svetlana
- Padala S.
- Ikeda Yuji
SAE Technical papers, SAE International, 2017, pp.2017-24-0153. Requirements for reducing consumption of hydrocarbon fuels, as well as reducing emissions force the scientific community to develop new ignition systems. One of possible solutions is an extension of the lean ignition limit of stable combustion. With the decrease of the stoichiometry of combustible mixture the minimal size of the ignition kernel (necessary for development of combustion) increases. Therefore, it is necessary to use some special techniques to extend the ignition kernel region. Pulsed microwave discharge allows the formation of the ignition kernels of larger diameters. Although the microwave discharge igniter (MDI) was already tested for initiation of combustion and demonstrated quite promising results, the parameters of plasma was not yet studied before. Present work demonstrates the results of the dynamics of spatial structure of the MDI plasma with nanosecond time resolution. It was shown that the size of the plasma region increases from 2-4 to 10-15 mm3 during the discharge development. Gas temperature and vibrational temperature were measured with optical emission spectroscopy. The electric field in the discharge was measured from the analysis of emission intensities of the first negative (N 2(B2Σ u) → N 2(X2Σ u)) and the second positive (N2(C3Πu) → N2(B3Πg)) systems of molecular nitrogen. The electron density was estimated from the atomic line broadening (10.4271/2017-24-0153)
DOI : 10.4271/2017-24-0153
The 2017 Plasma Roadmap: Low temperature plasma science and technology
- Adamovich I.
- Baalrud S D
- Bogaerts A.
- Bruggeman P J
- Cappelli M.
- Colombo V.
- Czarnetzki U.
- Ebert U.
- Eden J G
- Favia P.
- Graves D B
- Hamaguchi S.
- Hieftje G.
- Hori M.
- Kaganovich I D
- Kortshagen U.
- Kushner M.J.
- Mason N J
- Mazouffre S.
- Mededovic Thagard S.
- Metelmann H-R
- Mizuno A.
- Moreau Éric
- Murphy a B
- Niemira B A
- Oehrlein G S
- Petrovic Z Lj
- Pitchford L C
- Pu Y-K
- Rauf S.
- Sakai O.
- Samukawa S.
- Starikovskaia Svetlana
- Tennyson J.
- Terashima K.
- Turner M.M.
- Sanden M C M van De
- Vardelle A.
Journal of Physics D: Applied Physics, IOP Publishing, 2017, 50 (32), pp.323001 (46 p.). Journal of Physics D: Applied Physics published the first Plasma Roadmap in 2012 consisting of the individual perspectives of 16 leading experts in the various sub-fields of low temperature plasma science and technology. The 2017 Plasma Roadmap is the first update of a planned series of periodic updates of the Plasma Roadmap. The continuously growing interdisciplinary nature of the low temperature plasma field and its equally broad range of applications are making it increasingly difficult to identify major challenges that encompass all of the many sub-fields and applications. This intellectual diversity is ultimately a strength of the field. The current state of the art for the 19 sub-fields addressed in this roadmap demonstrates the enviable track record of the low temperature plasma field in the development of plasmas as an enabling technology for a vast range of technologies that underpin our modern society. At the same time, the many important scientific and technological challenges shared in this roadmap show that the path forward is not only scientifically rich but has the potential to make wide and far reaching contributions to many societal challenges. (10.1088/1361-6463/aa76f5)
DOI : 10.1088/1361-6463/aa76f5
Simultaneous Diagnostic of Temperature Distribution and Electric Field induced in Dielectric Target by Atmospheric Pressure Plasma Jet
- Slikboer Elmar
- Garcia-Caurel Enric
- Sobota Ana
- Guaitella Olivier
, 2017, 62.
Vibrational excitation and temperature evolution in a pulsed CO2 discharge
- Guaitella Olivier
- Morillo-Candas Ana-Sofia
- Yap David
- Drag Cyril
- Booth Jean-Paul
- Klarenaar Bart
- Engeln Richard
- Grofulovic Marija
- Silva Tiago
- Guerra V.
, 2017, 62.
The I-mode confinement regime at ASDEX Upgrade: global properties and characterization of strongly intermittent density fluctuations
- Happel T.
- Manz P.
- Ryter F.
- Bernert M.
- Dunne M.
- Hennequin Pascale
- Hetzenecker A.
- Stroth U.
- Conway G. D.
- Guimarais L.
- Honoré Cyrille
- Viezzer E.
Plasma Physics and Controlled Fusion, IOP Publishing, 2017, 59 (1), pp.014004. Properties of the I-mode confinement regime on the ASDEX Upgrade tokamak are summarized. A weak dependence of the power threshold for the L-I transition on the toroidal magnetic field strength is found. During improved confinement, the edge radial electric field well deepens. Stability calculations show that the I-mode pedestal is peeling-ballooning stable. Turbulence investigations reveal strongly intermittent density fluctuations linked to the weakly coherent mode in the confined plasma, which become stronger as the confinement quality increases. Across all investigated structure sizes ( ##IMG## [http://ej.iop.org/images/0741-3335/59/1/014004/ppcfaa3b87ieqn001.gif] k_\bot≈ 5 ?12?cm ?1 , with ##IMG## [http://ej.iop.org/images/0741-3335/59/1/014004/ppcfaa3b87ieqn002.gif] k_\bot the perpendicular wavenumber of turbulent density fluctuations), the intermittent turbulence bursts are observed. Comparison with bolometry data shows that they move poloidally toward the X-point and finally end up in the divertor. This might be indicative that they play a role in inhibiting the density profile growth, such that no pedestal is formed in the edge density profile. (10.1088/0741-3335/59/1/014004)
DOI : 10.1088/0741-3335/59/1/014004
A statistical study of kinetic-size magnetic holes in turbulent magnetosheath: MMS observations
- Huang S. Y.
- Du J. W.
- Sahraoui Fouad
- Yuan Z. G.
- He J. S.
- Zhao J. S.
- Le Contel Olivier
- Breuillard Hugo
- Wang D. D.
- Yu X. D.
- Deng X. H.
- Fu H.S.
- Zhou M.
- Pollock C. J.
- Torbert R. B.
- Russell C. T.
- Burch J. L.
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2017, 122 (8), pp.8577-8588. Kinetic-size magnetic holes (KSMHs) in the turbulent magnetosheath are statistically investigated using high time resolution data from the MMS mission. The KSMHs with short duration (i.e., < 0.5 s) have their cross section smaller than the ion gyro-radius. Superposed epoch analysis of all events reveals that an increase in the electron density and total temperature, significantly increase (resp. decrease) the electron perpendicular (resp. parallel) temperature, and an electron vortex inside KSMHs. Electron fluxes at ~ 90° pitch angles with selective energies increase in the KSMHs, are trapped inside KSMHs and form the electron vortex due to their collective motion. All these features are consistent with the electron vortex magnetic holes obtained in 2D and 3D particle-in-cell simulations, indicating that the observed KSMHs seem to be best explained as electron vortex magnetic holes. It is furthermore shown that KSMHs are likely to heat and accelerate the electrons. (10.1002/2017JA024415)
DOI : 10.1002/2017JA024415
MMS Observations of Reconnection at Dayside Magnetopause Crossings During Transitions of the Solar Wind to Sub-Alfvénic Flow
- Farrugia C. J.
- Lugaz N.
- Alm L.
- Vasquez B.
- Argall M. R.
- Kucharek H.
- Matsui H.
- Torbert R. B.
- Lavraud B.
- Le Contel Olivier
- Cohen I. J.
- Burch J. L.
- Russell C. T.
- Strangeway R. J.
- Shuster J.
- Dorelli J. C.
- Eastwood Jonathan P.
- Ergun R. E.
- Fuselier S. A.
- Gershman D. J.
- Giles B. L.
- Khotyaintsev Y. V.
- Lindqvist P. A.
- Marklund G. T.
- Paulson K. W.
- Petrinec S. M.
- Phan T. D.
- Pollock C. J.
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2017, 122 (10), pp.9934-9951. We present MMS observations during two dayside magnetopause crossings under hitherto unexamined conditions: (i) when the bow shock is weakening and the solar wind transitioning to sub-Alfvénic flow and (ii) when it is reforming. Interplanetary conditions consist of a magnetic cloud with (i) a strong B (20 nT) pointing south and (ii) a density profile with episodic decreases to values of 0.3 cm-3 followed by moderate recovery. During the crossings the magnetosheath magnetic field is stronger than the magnetosphere field by a factor of 2.2. As a result, during the outbound crossing through the ion diffusion region, MMS observed an inversion of the relative positions of the X and stagnation (S) lines from that typically the case: the S line was closer to the magnetosheath side. The S line appears in the form of a slow expansion fan near which most of the energy dissipation is taking place. While in the magnetosphere between the crossings, MMS observed strong field and flow perturbations, which we argue to be due to kinetic Alfvén waves. During the reconnection interval, whistler mode waves generated by an electron temperature anisotropy (Te⊥>Te||) were observed. Another aim of the paper is to distinguish bow shock-induced field and flow perturbations from reconnection-related signatures. The high-resolution MMS data together with 2-D hybrid simulations of bow shock dynamics helped us to distinguish between the two sources. We show examples of bow shock-related effects (such as heating) and reconnection effects such as accelerated flows satisfying the Walén relation. (10.1002/2017JA024563)
DOI : 10.1002/2017JA024563
Lower Hybrid Drift Waves and Electromagnetic Electron Space-Phase Holes Associated With Dipolarization Fronts and Field-Aligned Currents Observed by the Magnetospheric Multiscale Mission During a Substorm
- Le Contel Olivier
- Nakamura R.
- Breuillard Hugo
- Argall M. R.
- Graham D. B.
- Fischer D.
- Retinò Alessandro
- Berthomier Matthieu
- Pottelette Raymond
- Mirioni Laurent
- Chust Thomas
- Wilder F. D.
- Gershman D. J.
- Varsani A.
- Lindqvist P.-A.
- Khotyaintsev Y. V.
- Norgren C.
- Ergun R. E.
- Goodrich K. A.
- Burch J. L.
- Torbert R. B.
- Needell J.
- Chutter M.
- Rau D.
- Dors I.
- Russell C. T.
- Magnes W.
- Strangeway R. J.
- Bromund K. R.
- Wei H. Y.
- Plaschke F.
- Anderson B. J.
- Le G.
- Moore T. E.
- Giles B. L.
- Paterson W. R.
- Pollock C. J.
- Dorelli J. C.
- Avanov L. A.
- Saito Y.
- Lavraud B.
- Fuselier S. A.
- Mauk B. H.
- Cohen I. J.
- Turner D. L.
- Fennell J. F.
- Leonard T.
- Jaynes A. N.
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2017, 122 (12), pp.236-257. We analyze two ion scale dipolarization fronts associated with field-aligned currents detected by the Magnetospheric Multiscale mission during a large substorm on 10 August 2016. The first event corresponds to a fast dawnward flow with an antiparallel current and could be generated by the wake of a previous fast earthward flow. It is associated with intense lower hybrid drift waves detected at the front and propagating dawnward with a perpendicular phase speed close to the electric drift and the ion thermal velocity. The second event corresponds to a flow reversal: from southwward/dawnward to northward/duskward associated with a parallel current consistent with a brief expansion of the plasma sheet before the front crossing and with a smaller lower hybrid drift wave activity. Electromagnetic electron phase-space holes are detected near these low-frequency drift waves during both events. The drift waves could accelerate electrons parallel to the magnetic field and produce the parallel electron drift needed to generate the electron holes. Yet we cannot rule out the possibility that the drift waves are produced by the antiparallel current associated with the fast flows, leaving the source for the electron holes unexplained. (10.1002/2017JA024550)
DOI : 10.1002/2017JA024550
Examining Coherency Scales, Substructure, and Propagation of Whistler Mode Chorus Elements With Magnetospheric Multiscale (MMS)
- Turner D. L.
- Lee J. H.
- Claudepierre S. G.
- Fennell J. F.
- Blake J. B.
- Jaynes A. N.
- Leonard T.
- Wilder F. D.
- Ergun R. E.
- Baker D. N.
- Cohen I. J.
- Mauk B. H.
- Strangeway R. J.
- Hartley D. P.
- Kletzing C. A.
- Breuillard Hugo
- Le Contel Olivier
- Khotyaintsev Y. V.
- Torbert R. B.
- Allen R. C.
- Burch J. L.
- Santolík O.
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2017, 122 (11), pp.201-226. Whistler mode chorus waves are a naturally occurring electromagnetic emission observed in Earth's magnetosphere. Here, for the first time, data from NASA's Magnetospheric Multiscale (MMS) mission were used to analyze chorus waves in detail, including the calculation of chorus wave normal vectors, fi>k. A case study was examined from a period of substorm activity around the time of a conjunction between the MMS constellation and NASA's Van Allen Probes mission on 07 April 2016. Chorus wave activity was simultaneously observed by all six spacecraft over a broad range of L shells (5.5 < L < 8.5), magnetic local time (06:00 < MLT < 09:00), and magnetic latitude (-32° < MLAT < -15°), implying a large chorus active region. Eight chorus elements and their substructure were analyzed in detail with MMS. These chorus elements were all lower band and rising tone emissions, right-handed and nearly circularly polarized, and propagating away from the magnetic equator when they were observed at MMS (MLAT -31°). Most of the elements had "hook"-like signatures on their wave power spectra, characterized by enhanced wave power at flat or falling frequency following the peak, and all the elements exhibited complex and well-organized substructure observed consistently at all four MMS spacecraft at separations up to 70 km (60 km perpendicular and 38 km parallel to the background magnetic field). The waveforms in field-aligned coordinates also demonstrated that these waves were all phase coherent, allowing for the direct calculation of fi>k. Error estimates on calculated fi>k revealed that the plane wave approximation was valid for six of the eight elements and most of the subelements. The wave normal vectors were within 20-30° from the direction antiparallel to the background field for all elements and changed from subelement to subelement through at least two of the eight elements. The azimuthal angle of fi>k in the perpendicular plane was oriented earthward and was oblique to that of the Poynting vector, which has implications for the validity of cold plasma theory. (10.1002/2017JA024474)
DOI : 10.1002/2017JA024474
Nanosecond surface dielectric barrier discharge in atmospheric pressure air: I. measurements and 2D modeling of morphology, propagation and hydrodynamic perturbations
- Zhu Yifei
- Shcherbanev S.A.
- Baron B.
- Starikovskaia Svetlana
Plasma Sources Science and Technology, IOP Publishing, 2017, 26 (12), pp.125004. A parallel 2D code for modeling nanosecond surface dielectric barrier discharge (nSDBD), combining a discharge description, detailed kinetics and hydrodynamics, is developed and validated. A series of experiments and numerical modeling for a single pulse nSDBD in atmospheric pressure air at a voltage amplitude of 24 kV have been performed. The measured and calculated velocity of the discharge front, electrical current, 2D map of N2 (\rmC^3\rmΠ _u)\to \rmN_2(\rmB^3\rmΠ _g) emission and hydrodynamic perturbations caused by the discharge on the time scale 0.2−5 μs are compared. The data are presented and analyzed for the negative and positive polarity of the streamers. A set of parametric calculations with different dielectric permittivities and different dielectric thicknesses is presented. (10.1088/1361-6595/aa9304)
DOI : 10.1088/1361-6595/aa9304
Transient propagation dynamics of flowing plasmas accelerated by radio-frequency electric fields
- Dedrick J.
- Gibson Andrew
- Rafalskyi D.V.
- Aanesland Ane
Physics of Plasmas, American Institute of Physics, 2017, 24, pp.050703. Flowing plasmas are of significant interest due to their role in astrophysical phenomena and potential applications in magnetic-confined fusion and spacecraft propulsion. The acceleration of a charge-neutral plasma beam using the radio-frequency self-bias concept could be particularly useful for the development of neutralizer-free propulsion sources. However, the mechanisms that lead to space-charge compensation of the exhaust beam are unclear. Here, we spatially and temporally resolve the propagation of electrons in an accelerated plasma beam that is generated using the self-bias concept with phase-resolved optical emission spectroscopy. When combined with measurements of the extraction-grid voltage, ion and electron currents, and plasma potential, the pulsed-periodic propagation of electrons during the interval of sheath collapse at the grids is found to enable the compensation of space charge. (10.1063/1.4983059)
DOI : 10.1063/1.4983059
Controlled production of atomic oxygen and nitrogen in a pulsed radio-frequency atmospheric-pressure plasma
- Dedrick J.
- Schröter Sandra
- Niemi K.
- Wijaikhum A.
- Wagenaars E.
- Oliveira N De
- Nahon L.
- Booth Jean-Paul
- O'Connell D.
- Gans T.
Journal of Physics D: Applied Physics, IOP Publishing, 2017, 50 (45), pp.455204. Radio-frequency driven atmospheric pressure plasmas are efficient sources for the production of reactive species at ambient pressure and close to room temperature. Pulsing the radio-frequency power input provides additional control over species production and gas temperature. Here, we demonstrate the controlled production of highly reactive atomic oxygen and nitrogen in a pulsed radio-frequency ( ##IMG## [http://ej.iop.org/images/0022-3727/50/45/455204/daa8da2ieqn001.gif] 13.56 MHz) atmospheric-pressure plasma, operated with a small ##IMG## [http://ej.iop.org/images/0022-3727/50/45/455204/daa8da2ieqn002.gif] 0.1 % air-like admixture ( ##IMG## [http://ej.iop.org/images/0022-3727/50/45/455204/daa8da2ieqn003.gif] \rm N_2 / ##IMG## [http://ej.iop.org/images/0022-3727/50/45/455204/daa8da2ieqn004.gif] \rm O_2 at ##IMG## [http://ej.iop.org/images/0022-3727/50/45/455204/daa8da2ieqn005.gif] 4:1 ) through variations in the duty cycle. Absolute densities of atomic oxygen and nitrogen are determined through vacuum-ultraviolet absorption spectroscopy using the DESIRS beamline at the SOLEIL synchrotron coupled with a high resolution Fourier-transform spectrometer. The neutral-gas temperature is measured using nitrogen molecular optical emission spectroscopy. For a fixed applied-voltage amplitude (234?V), varying the pulse duty cycle from 10% to 100% at a fixed 10?kHz pulse frequency enables us to regulate the densities of atomic oxygen and nitrogen over the ranges of ##IMG## [http://ej.iop.org/images/0022-3727/50/45/455204/daa8da2ieqn006.gif] (0.18±0.03) ? ##IMG## [http://ej.iop.org/images/0022-3727/50/45/455204/daa8da2ieqn007.gif] (3.7±0.1)× 10^20 ##IMG## [http://ej.iop.org/images/0022-3727/50/45/455204/daa8da2ieqn008.gif] \rm m^-3 and ##IMG## [http://ej.iop.org/images/0022-3727/50/45/455204/daa8da2ieqn009.gif] (0.2±0.06) ? ##IMG## [http://ej.iop.org/images/0022-3727/50/45/455204/daa8da2ieqn010.gif] (4.4±0.8) × 10^19 ##IMG## [http://ej.iop.org/images/0022-3727/50/45/455204/daa8da2ieqn011.gif] \rm m^-3 , respectively. The corresponding 11?K increase in the neutral-gas temperature with increased duty cycle, up to a maximum of ##IMG## [http://ej.iop.org/images/0022-3727/50/45/455204/daa8da2ieqn012.gif] (314±4) K, is relatively small. This additional degree of control, achieved through regulation of the pulse duty cycle and time-averaged power, could be of particular interest for prospective biomedical applications. (10.1088/1361-6463/aa8da2)
DOI : 10.1088/1361-6463/aa8da2
A model for tailored-waveform radiofrequency sheaths
- Chabert Pascal
- Turner Miles
Journal of Physics D: Applied Physics, IOP Publishing, 2017, 50 (23), pp.23LT02. The sheath physics of radiofrequency plasmas excited by a sinusoidal waveform is reasonably well understood, but the existing models are complicated and are not easily extended to the more complex waveforms recently introduced in applications. Turner and Chabert (2014 Appl. Phys. Lett . 104 164102) proposed a model for collisionless sheaths that can easily be solved for arbitrary waveforms. In this paper we extend this model to the case of collisional sheaths in the intermediate pressure regime. Analytical expressions are derived for the electric field, the electric potential and the density profiles in the sheath region. The collisionless and collisional models are compared for a pulsed-voltage waveform. (10.1088/1361-6463/aa6e42)
DOI : 10.1088/1361-6463/aa6e42
Multi-shell transport model for L-H transition
- Berionni Vincent
- Morel Pierre
- Gürcan Özgür D.
Physics of Plasmas, American Institute of Physics, 2017, 24 (12), pp.122310. A coupled model of transport, turbulence, and mesoscale flows is proposed, including turbulence spreading. The model consists of transport equations for plasma density and pressure coupled to a shell model of drift wave turbulence, which incorporates coupling to mesoscale flows via disparate scale interactions. The model can describe the turbulent cascade and its dynamical interplay with zonal and mean shear flows as well as the profile evolution (including the profiles of turbulence intensity itself) due to these self-consistent turbulent fluxes. This simple system of equations is shown to capture the low to high confinement (L-H) transition. It is also observed that as the heating is increased, the system goes through an intermediate phase that displays oscillations between zonal flows and turbulence. The transition towards the H mode, which is characterized by the presence of a strong mean shear flow at the edge, is triggered by the mesoscale dynamics due to the action of zonal flows, with turbulence spreading playing an important role in the H to L back transition. (10.1063/1.4998569)
DOI : 10.1063/1.4998569
Energy Cascade Rate in Compressible Fast and Slow Solar Wind Turbulence
- Hadid Lina
- Sahraoui Fouad
- Galtier Sébastien
The Astrophysical Journal, American Astronomical Society, 2017, 838 (1), pp.9. Estimation of the energy cascade rate in the inertial range of solar wind turbulence has been done so far mostly within incompressible magnetohydrodynamics (MHD) theory. Here, we go beyond that approximation to include plasma compressibility using a reduced form of a recently derived exact law for compressible, isothermal MHD turbulence. Using in situ data from the THEMIS/ARTEMIS spacecraft in the fast and slow solar wind, we investigate in detail the role of the compressible fluctuations in modifying the energy cascade rate with respect to the prediction of the incompressible MHD model. In particular, we found that the energy cascade rate (1) is amplified particularly in the slow solar wind; (2) exhibits weaker fluctuations in spatial scales, which leads to a broader inertial range than the previous reported ones; (3) has a power-law scaling with the turbulent Mach number; (4) has a lower level of spatial anisotropy. Other features of solar wind turbulence are discussed along with their comparison with previous studies that used incompressible or heuristic (nonexact) compressible MHD models. (10.3847/1538-4357/aa603f)
DOI : 10.3847/1538-4357/aa603f
Evidence for Quasi-adiabatic Motion of Charged Particles in Strong Current Sheets in the Solar Wind
- Malova H. V.
- Popov V. Y.
- Grigorenko E. E.
- Petrukovich A. A.
- Delcourt Dominique C.
- Sharma A. S.
- Khabarova O. V.
- Zelenyi L. M.
The Astrophysical Journal, American Astronomical Society, 2017, 834, pp.34. We investigate quasi-adiabatic dynamics of charged particles in strong current sheets (SCSs) in the solar wind, including the heliospheric current sheet (HCS), both theoretically and observationally. A self-consistent hybrid model of an SCS is developed in which ion dynamics is described at the quasi-adiabatic approximation, while the electrons are assumed to be magnetized, and their motion is described in the guiding center approximation. The model shows that the SCS profile is determined by the relative contribution of two currents: (i) the current supported by demagnetized protons that move along open quasi-adiabatic orbits, and (ii) the electron drift current. The simplest modeled SCS is found to be a multi-layered structure that consists of a thin current sheet embedded into a much thicker analog of a plasma sheet. This result is in good agreement with observations of SCSs at 1 au. The analysis of fine structure of different SCSs, including the HCS, shows that an SCS represents a narrow current layer (with a thickness of 104 km) embedded into a wider region of about 105 km, independently of the SCS origin. Therefore, multi-scale structuring is very likely an intrinsic feature of SCSs in the solar wind. (10.3847/1538-4357/834/1/34)
DOI : 10.3847/1538-4357/834/1/34
Concepts, Capabilities, and Limitations of Global Models: A Review
- Hurlbatt Andrew
- Gibson Andrew
- Schröter Sandra
- Bredin Jérôme
- Foote Alexander Paul Stuart
- Grondein Pascaline
- O'Connell D.
- Gans T.
Plasma Processes and Polymers, Wiley-VCH Verlag, 2017, 14 (1-2), pp.1600138. For researchers wishing to generate an understanding of complex plasma systems, global models often present an attractive first step, mainly due to their ease of development and use. These volume averaged models are able to give descriptions of plasmas with complex chemical kinetics, and without the computationally intensive numerical methods required for spatially resolved models. This paper gives a tutorial on global modeling, including development and techniques, and provides a discussion on the issues and pitfalls that researchers should be aware of. Further discussion is provided in the form of two reviews on methods of extending global modeling techniques to encompass variations in either time or space. (10.1002/ppap.201600138)
DOI : 10.1002/ppap.201600138
Understanding of wavenumber spectra measured by Doppler reflectometry through simulation and related estimation of wave-plasma interaction regimes.
- Happel T.
- Görler T.
- Hennequin Pascale
- Lechte C.
- Pinzón J R
- Bernert M.
- Conway G. D.
- Freethy Simon
- Honoré Cyrille
- Stroth U.
- Asdex Upgrade Team The
, 2017.
Contribution à l'étude de la scintillation ionosphérique équatoriale sur la crête sud de l'Afrique
- Kahindo Bruno
- Kazadi Mukenga Bantu Albert
- Fleury Rolland
- Tondozi Keto Franck
- Zana Ndontoni André
- Kakule Kaniki Marc
- Amory-Mazaudier Christine
- Groves K.
Journal des sciences, Université Cheikh Anta Diop, 2017, 17 (2), pp.27-47. Cet article présente une étude sur la scintillation ionosphérique associée aux irrégularités équatoriales sur la crête sud de l'anomalie équatoriale. Les données utilisées sont le contenu électronique total (TEC) et l'indice de scintillation d'amplitude S4 enregistrées par le GPS SCINDA de Kinshasa (République Démocratique du Congo). Du fait de cette position géographique bien particulière, ces mesures constituent une opportunité de caractériser l'ionosphère sensible à de nombreux phénomènes spécifiques (forts gradients d'ionisation, scintillation, …). Le travail couvre la période qui va de mai 2011 à octobre 2012 situé dans la phase ascendante du cycle solaire numéro 24. Les résultats montrent que la scintillation est très intense avec des valeurs S4 voisines de 1 et au-delà pendant de nombreux jours de l'année. La répartition temporelle montre que les fortes valeurs de l'indice sont observées entre 19h à 23h TL (Temps Local). Ce résultat confirme que la scintillation est un phénomène présent dès le coucher du Soleil et peut se prolonger sur les quelques heures après minuit. On a aussi observé un fort effet saisonnier qui s'est manifesté par des valeurs intenses de scintillation aux mois d'équinoxe comparés aux périodes de solstice. Afin de mieux caractériser le degré d'ionisation dans la région équatoriale sud, nous avons utilisé les mesures GPS (pseudo-distances, bi-fréquences) effectuées par la station NKLG à Libreville au Gabon. C'est la seule station dans cette partie de l'Afrique qui possède une série historique de mesures sous forme de fichiers Rinex journaliers. Elle est rattachée au réseau IGS, ce qui garantit un suivi opérationnel très sérieux. Nous avons exploité ces mesures pour calculer le VTEC (Contenu Total Electronique vertical) et suivre le comportement de l'ionisation pendant la période de nos mesures. Le VTEC montre une variation diurne fonction de l'angle solaire zénithal (rayonnement solaire) et du rayonnement solaire ionisant. Ce dernier est quantifié par le nombre glissant de taches solaires (R12) permettant de situer notre période de mesures dans le cycle solaire en cours. Le VTEC est compris entre quelques unités d'unités de TEC (1 TECU=1016 el/m2) la nuit et 80-100 tecu de jour. Le maximum diurne est observé aux environs de 15 TL.
Overview of progress in European medium sized tokamaks towards an integrated plasma-edge/wall solution
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Nuclear Fusion, IOP Publishing, 2017, 57. Integrating the plasma core performance with an edge and scrape-off layer (SOL) that leads to tolerable heat and particle loads on the wall is a major challenge. The new European medium size tokamak task force (EU-MST) coordinates research on ASDEX Upgrade (AUG), MAST and TCV. This multi-machine approach within EU-MST, covering a wide parameter range, is instrumental to progress in the field, as ITER and DEMO core/pedestal and SOL parameters are not achievable simultaneously in present day devices. A two prong approach is adopted. On the one hand, scenarios with tolerable transient heat and particle loads, including active edge localised mode (ELM) control are developed. On the other hand, divertor solutions including advanced magnetic configurations are studied. Considerable progress has been made on both approaches, in particular in the fields of: ELM control with resonant magnetic perturbations (RMP), small ELM regimes, detachment onset and control, as well as filamentary scrape-off-layer transport. For example full ELM suppression has now been achieved on AUG at low collisionality with n = 2 RMP maintaining good confinement {{H}\text{H (10.1088/1741-4326/aa6084)
DOI : 10.1088/1741-4326/aa6084

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