Publications

2019

• Cavity-enhanced photodetachment of H$^-$as a means to produce energetic neutral beams for phasma Heating
  
  • Blondel Christophe
  • Bresteau David
  • Drag Cyril
  Atoms, MDPI, 2019, 7 (1), pp.32. Neutral beam injection, for plasma heating, will supposedly be achieved, in ITER, by collisional detachment of a pre-accelerated D− beam. Collisional detachment, however, makes use of a D2-filled neutralisation chamber, which has severe drawbacks, including the necessity to set the D− -ion source at −1 MV. Photodetachment, in contradistinction, would have several advantages as a neutralisation method, including the absence of gas injection, and the possibility to set the ion source close to the earth potential. Photodetachment, however, requires a very high laser flux. The presented work has consisted in implementing an optical cavity, with a finesse greater than 3000, to make such a high illumination possible with a state-of-the-art CW (continuous-wave) laser. A 1.2 keV 1H− -beam (only 20 times slower than the 1 MeV 2D− ion beams to be prepared for ITER) was photodetached with more-than-50% efficiency, with only 24 W of CW laser input. This experimental demonstration paves the way for developing real-size photoneutralizers, based on the implementation of refolded optical cavities around the ion beams of neutral beam injectors. Depending on whether the specifications of the laser power or the cavity finesse will be more difficult to achieve in real scale, different architectures can be considered, with greater or smaller numbers of optical refoldings or (inclusively) optical cavities in succession, on the beam to be neutralised. (10.3390/atoms7010032)
  DOI : 10.3390/atoms7010032
• Numerical simulations of high cross-helicity turbulence from 0.2 to 1 AU
  
  • Verdini Andrea
  • Grappin Roland
  • Montagud-Camps Victor
  • Landi Simone
  • Franci Luca
  • Papini Emanuele
  Il Nuovo cimento della Societa italiana di fisica. C, Springer-Verlag, 2019, 42, pp.17. Turbulence in the fast stream of the solar wind is maintained despitethe small compressibility and a dominance of outward-propagatingfluctuations ( z^+>z^- , in contrast to its rapid decay in imbalancedhomogenous MHD turbulence. We numerically study if the inhomogeneityintroduced by solar wind expansion can be an effective source of z^-that maintains turbulence. Starting at 0.2 AU with z^-=0 , we obtain adamping with distance of z^+ and a quasi-steady level of z^- . The z^+spectrum steepens with distance toward a -1.4 power-law at 1 AU, whilethe z^- spectrum has a -5/3 power-law index at all distances. Theseproperties are robust against variations of the input spectrum andexpansion rate and are in agreement with in-situ data, suggesting thatimbalanced turbulence can be maintained by expansion alone. (10.1393/ncc/i2019-19017-x)
  DOI : 10.1393/ncc/i2019-19017-x
• Three-dimensional local anisotropy of velocity fluctuations in the solar wind
  
  • Verdini Andrea
  • Grappin Roland
  • Alexandrova Olga
  • Franci L.
  • Landi S.
  • Matteini L.
  • Papini E.
  Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2019, 486, pp.3006-3018. We analyse velocity fluctuations in the solar wind at magneto-fluid scales in two data sets, extracted from Wind data in the period 2005-2015, that are characterized by strong or weak expansion. Expansion affects measurements of anisotropy because it breaks axisymmetry around the mean magnetic field. Indeed, the small-scale three-dimensional local anisotropy of magnetic fluctuations (deltaB) as measured by structure functions (SF<SUB>B</SUB>) is consistent with tube-like structures for strong expansion. When passing to weak expansion, structures become ribbon-like because of the flattening of SF<SUB>B</SUB> along one of the two perpendicular directions. The power-law index that is consistent with a spectral slope -5/3 for strong expansion now becomes closer to -3/2. This index is also characteristic of velocity fluctuations in the solar wind. We study velocity fluctuations (deltaV) to understand if the anisotropy of their structure functions (SF<SUB>V</SUB>) also changes with the strength of expansion and if the difference with the magnetic spectral index is washed out once anisotropy is accounted for. We find that SF<SUB>V</SUB> is generally flatter than SF<SUB>B</SUB>. When expansion passes from strong to weak, a further flattening of the perpendicular SF<SUB>V</SUB> occurs and the small-scale anisotropy switches from tube-like to ribbon-like structures. These two types of anisotropy, common to SF<SUB>V</SUB> and SF<SUB>B</SUB>, are associated with distinct large-scale variance anisotropies of deltaB in the strong- and weak-expansion data sets. We conclude that SF<SUB>V</SUB> show anisotropic three-dimensional scaling similar to SF<SUB>B</SUB>, with however systematic flatter scalings, reflecting the difference between global spectral slopes. (10.1093/mnras/stz1041)
  DOI : 10.1093/mnras/stz1041
• Whistler Waves' Propagation in Plasmas With Systems of Small-Scale Density Irregularities: Numerical Simulations and Theory
  
  • Zudin I. Yu.
  • Zaboronkova T. M.
  • Gushchin M. E.
  • Aidakina N. A.
  • Korobkov S. V.
  • Krafft Catherine
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2019, 124, pp.4739. The propagation of whistler waves in a magnetized plasma containingmultiple small-scale (100 m to 1 km) field-aligned irregularities ofenhanced electron density is considered analytically and by means ofnumerical simulations. Such systems of irregularities can develop in theupper ionosphere during the generation of density ducts by high-frequency heating facilities and other types of active experiments. Thesimulation parameters are close to those of an active experiment where awhistler wave of 18 kHz emitted by a ground-based very low frequency(VLF) transmitter was received onboard the DEMETER satellite at 700 kmabove the SURA heater. The study reveals a number of remarkableproperties of the VLF waves' propagation, including the existence ofspecific waveguide modes of the small-scale density structures and of acharacteristic transverse size d<SUB>0</SUB> of the irregularities.Irregularities with small density enhancements around 10-20% andtransverse sizes larger than d<SUB>0</SUB>?1 km can serve as separatewaveguides for VLF waves. In their turn, single irregularities narrowerthan d<SUB>0</SUB> cannot be considered as individual ductingstructures. Numerical simulations show that, for the analysis of theelectromagnetic whistlers' propagation, a system of closely spacedirregularities with scales narrower than d<SUB>0</SUB> can be modeled byan equivalent ducting structure with a smoothed density profile. Suchequivalent structure has the same ducting properties for whistlers andcan be produced by averaging with a sliding window of a scale aboutd<SUB>0</SUB> the original density distribution. (10.1029/2019JA026637)
  DOI : 10.1029/2019JA026637
• Plasma-surface interaction: dielectric and metallic targets and their influence on the electric field profile in a kHz AC-driven He plasma jet
  
  • Sobota Ana
  • Guaitella Olivier
  • Sretenović G. B.
  • Kovačević V. V.
  • Slikboer Elmar
  • Krstić I. B.
  • Obradović B. M.
  • Kuraica M. M.
  Plasma Sources Science and Technology, IOP Publishing, 2019, 28 (4), pp.045003. Plasma catalysis, biomedical applications or atomic layer deposition at atmospheric pressure all make use of non-thermal plasmas in contact with a wide variety of surfaces. As the presence of a target (substrate) has been shown to modify the plasma in addition to the plasma modifying the target, it is reasonable to describe and study the plasma-surface as one system. This work shows how the presence of dielectric and metallic targets influences a kHz AC-driven discharge in a He plasma jet. Next to bringing the absolute values of the axial electric field along the plume of the jet, the presence of the surface has been shown to significantly elongate both the plume and the electric field profile. In addition, when a dielectric target is placed closer than the maximum length of the freely expanding jet, the electric field profile is enhanced only in the vicinity of the dielectric, typically between 0.3 and 2 mm above the target surface. The maximum measured relative increase is 31%, for 1000 SCCM flow with the target at 7 mm distance, when the electric field increased from 14.1 kV cm&#8722;1 for the freely expanding jet to 32.6 kV cm&#8722;1 when the jet was impinging on glass. Finally, a grounded metallic target enhances the electric field compared to the glass target only within a very thin layer just above the surface, typically about 0.2 mm. The highest measured electric field was 40.1 kV cm&#8722;1 at a grounded metallic target 12 mm away from the nozzle, for 1000 SCCM of helium flow. The discussion on the effects of the flow on the electric field profile are supported by the visualization of the flow. The discussion brings, among other, the comparison of properties between the 30 kHz AC-driven system and the 5 kHz pulsed jet. (10.1088/1361-6595/ab0c6a)
  DOI : 10.1088/1361-6595/ab0c6a
• Physics research on the TCV tokamak facility: from conventional to alternative scenarios and beyond
  
  • Coda S.
  • Agostini M.
  • Albanese R.
  • Alberti S.
  • Alessi E.
  • Allan S.
  • Allcock J.
  • Ambrosino R.
  • Anand H.
  • Andrèbe Y.
  • Arnichand H.
  • Auriemma F.
  • Ayllon-Guerola J.M.
  • Bagnato F.
  • Ball J.
  • Baquero-Ruiz M.
  • Beletskii A.A.
  • Bernert M.
  • Bin W.
  • Blanchard P.
  • Blanken T.C.
  • Boedo J.A.
  • Bogar O.
  • Bolzonella T.
  • Bombarda F.
  • Bonanomi N.
  • Bouquey F.
  • Bowman C.
  • Brida D.
  • Bucalossi J.
  • Buermans J.
  • Bufferand H.
  • Buratti P.
  • Calabró G.
  • Calacci L.
  • Camenen Y.
  • Carnevale D.
  • Carpanese F.
  • Carr M.
  • Carraro L.
  • Casolari A.
  • Causa F.
  • Čeřovský J.
  • Chellaï O.
  • Chmielewski P.
  • Choi D.
  • Christen N.
  • Ciraolo G.
  • Cordaro L.
  • Costea S.
  • Cruz N.
  • Czarnecka A.
  • Molin A. Dal
  • David P.
  • Decker J.
  • Oliveira H. De
  • Douai D.
  • Dreval M.B.
  • Dudson B.
  • Dunne M.
  • Duval B.P.
  • Eich T.
  • Elmore S.
  • Embréus O.
  • Esposito B.
  • Faitsch M.
  • Farník M.
  • Fasoli A.
  • Fedorczak N.
  • Felici F.
  • Feng S.
  • Feng X.
  • Ferro G.
  • Février O.
  • Ficker O.
  • Fil Alexandre
  • Fontana M.
  • Frassinetti L.
  • Furno I.
  • Gahle D.S.
  • Galassi D.
  • Ga\lązka K.
  • Gallo A.
  • Galperti C.
  • Garavaglia S.
  • Garcia J.
  • Garcia-Muñoz M.
  • Garrido A.J.
  • Garrido I.
  • Gath J.
  • Geiger B.
  • Giruzzi G.
  • Gobbin M.
  • Goodman T.P.
  • Gorini G.
  • Gospodarczyk M.
  • Granucci G.
  • Graves J.P.
  • Gruca M.
  • Gyergyek T.
  • Hakola A.
  • Happel T.
  • Harrer G.F.
  • Harrison J.
  • Havlíčková E.
  • Hawke J.
  • Henderson S.
  • Hennequin P.
  • Hesslow L.
  • Hogeweij D.
  • Hogge J.-Ph.
  • Hopf C.
  • Hoppe M.
  • Horáček J.
  • Huang Z.
  • Hubbard A.
  • Iantchenko A.
  • Igochine V.
  • Innocente P.
  • Schrittwieser C. Ionita
  • Isliker H.
  • Jacquier R.
  • Jardin A.
  • Kappatou A.
  • Karpushov A.
  • Kazantzidis P.-V.
  • Keeling D.
  • Kirneva N.
  • Komm M.
  • Kong M.
  • Kovacic J.
  • Krawczyk N.
  • Kudlacek O.
  • Kurki-Suonio T.
  • Kwiatkowski R.
  • Labit B.
  • Lazzaro E.
  • Linehan B.
  • Lipschultz B.
  • Llobet X.
  • Lombroni R.
  • Loschiavo V.P.
  • Lunt T.
  • Macusova E.
  • Madsen J.
  • Maljaars E.
  • Mantica P.
  • Maraschek M.
  • Marchetto C.
  • Marco A.
  • Mariani A.
  • Marini C.
  • Martin Y.
  • Matos F.
  • Maurizio R.
  • Mavkov B.
  • Mazon D.
  • Mccarthy P.
  • Mcdermott R.
  • Menkovski V.
  • Merle A.
  • Meyer H.
  • Micheletti D.
  • Militello F.
  • Mitosinkova K.
  • Mlynář J.
  • Moiseenko V.
  • Cabrera P.A. Molina
  • Morales J.
  • Moret J.-M.
  • Moro A.
  • Mumgaard R.T.
  • Naulin V.
  • Nem R.D.
  • Nespoli F.
  • Nielsen A.H.
  • Nielsen S.K.
  • Nocente M.
  • Nowak S.
  • Offeddu N.
  • Orsitto F.P.
  • Paccagnella R.
  • Palha A.
  • Papp G.
  • Pau A.
  • Pavlichenko R.O.
  • Perek A.
  • Ridolfini V. Pericoli
  • Pesamosca F.
  • Piergotti V.
  • Pigatto L.
  • Piovesan P.
  • Piron C.
  • Plyusnin V.
  • Poli E.
  • Porte L.
  • Pucella G.
  • Puiatti M.E.
  • Pütterich T.
  • Rabinski M.
  • Rasmussen J. Juul
  • Ravensbergen T.
  • Reich M.
  • Reimerdes H.
  • Reimold F.
  • Reux C.
  • Ricci D.
  • Ricci P.
  • Rispoli N.
  • Rosato J.
  • Saarelma S.
  • Salewski M.
  • Salmi A.
  • Sauter O.
  • Scheffer M.
  • Schlatter Ch.
  • Schneider B.S.
  • Schrittwieser R.
  • Sharapov S.
  • Sheeba R.R.
  • Sheikh U.
  • Shousha R.
  • Silva M.
  • Sinha J.
  • Sozzi C.
  • Spolaore M.
  • Stipani L.
  • Strand P.
  • Tala T.
  • Tema Biwole A.S.
  • Teplukhina A.A.
  • Testa D.
  • Theiler C.
  • Thornton A.
  • Tomaž G.
  • Tomes M.
  • Tran M.Q.
  • Tsironis C.
  • Tsui C.K.
  • Urban Joanna M
  • Valisa M.
  • Valla M.
  • Vugt D. Van
  • Vartanian S.
  • Vasilovici O.
  • Verhaegh K.
  • Vermare L.
  • Vianello N.
  • Viezzer E.
  • Vijvers W.A.J.
  • Villone F.
  • Voitsekhovitch I.
  • Vu N.M.T.
  • Walkden N.
  • Wauters T.
  • Weiland M.
  • Weisen H.
  • Wensing M.
  • Wiesenberger M.
  • Wilkie G.
  • Wischmeier M.
  • Wu K.
  • Yoshida M.
  • Zagorski R.
  • Zanca P.
  • Zebrowski J.
  • Zisis A.
  • Zuin M.
  • Eurofusion Mst1 Team The
  Nuclear Fusion, IOP Publishing, 2019, 59 (11), pp.112023. The research program of the TCV tokamak ranges from conventional to advanced-tokamak scenarios and alternative divertor configurations, to exploratory plasmas driven by theoretical insight, exploiting the device’s unique shaping capabilities. Disruption avoidance by real-time locked mode prevention or unlocking with electron-cyclotron resonance heating (ECRH) was thoroughly documented, using magnetic and radiation triggers. Runaway generation with high-Z noble-gas injection and runaway dissipation by subsequent Ne or Ar injection were studied for model validation. The new 1 MW neutral beam injector has expanded the parameter range, now encompassing ELMy H-modes in an ITER-like shape and nearly non-inductive H-mode discharges sustained by electron cyclotron and neutral beam current drive. In the H-mode, the pedestal pressure increases modestly with nitrogen seeding while fueling moves the density pedestal outwards, but the plasma stored energy is largely uncorrelated to either seeding or fueling. High fueling at high triangularity is key to accessing the attractive small edge-localized mode (type-II) regime. Turbulence is reduced in the core at negative triangularity, consistent with increased confinement and in accord with global gyrokinetic simulations. The geodesic acoustic mode, possibly coupled with avalanche events, has been linked with particle flow to the wall in diverted plasmas. Detachment, scrape-off layer transport, and turbulence were studied in L- and H-modes in both standard and alternative configurations (snowflake, super-X, and beyond). The detachment process is caused by power ‘starvation’ reducing the ionization source, with volume recombination playing only a minor role. Partial detachment in the H-mode is obtained with impurity seeding and has shown little dependence on flux expansion in standard single-null geometry. In the attached L-mode phase, increasing the outer connection length reduces the in–out heat-flow asymmetry. A doublet plasma, featuring an internal X-point, was achieved successfully, and a transport barrier was observed in the mantle just outside the internal separatrix. In the near future variable-configuration baffles and possibly divertor pumping will be introduced to investigate the effect of divertor closure on exhaust and performance, and 3.5 MW ECRH and 1 MW neutral beam injection heating will be added. (10.1088/1741-4326/ab25cb)
  DOI : 10.1088/1741-4326/ab25cb
• Single&#8209;mode scannable nanosecond Ti:sapphire laser for&#57375;high&#8209;resolution two&#8209;photon absorption laser&#8209;induced fluorescence (TALIF)
  
  • Lottigier Pierre
  • Jucha Alain
  • Cabaret Louis
  • Blondel Christophe
  • Drag Cyril
  Applied Physics B - Laser and Optics, Springer Verlag, 2019 (125), pp.14. A pulsed Ti:sapphire laser has been developed so as to operate over a wide range of frequencies, even far from the optimum wavelength (790 nm), as a narrow-band light source for TALIF experiments on O, Cl, N and H. The coupling of the optical cavity, both to its injection seeder and to the laser output beam, relies on a reflecting plate, which makes it fundamentally easier to control the coupling coefficient over a wider spectral range than with an ordinary transmission coupler. Two intra- cavity prisms are used to bring the green pumping light longitudinally coincident with the cavity axis, inside the Ti:sapphire crystal. Seeding by a CW Ti:sapphire laser has made it possible to obtain single-mode emission over the whole range of tunability, thanks to the spectral selection of the prisms and to a specifically developed digital/analog controller. Experiments carried out with the system on oxygen atoms inside an oxygen plasma show that the experimental bandwidth is limited essentially by the collisional dephasing rate and the finite pulse duration. (10.1007/s00340-018-7124-5)
  DOI : 10.1007/s00340-018-7124-5
• Statistics of incompressible hydrodynamic turbulence: An alternative approach
  
  • Andrés Nahuel
  • Banerjee Supratik
  Physical Review Fluids, American Physical Society, 2019, 4, pp.024603. Using a recent alternative form of the Kolmogorov-Monin exact relationfor fully developed hydrodynamics (HD) turbulence, the incompressibleenergy cascade rate ? is computed. Under this current theoreticalframework, for three-dimensional (3D) freely decaying homogeneousturbulence, the statistical properties of the fluid velocity (u ),vorticity (? =? ×u ), and Lamb vector (L =? ×u ) are numericallystudied. For different spatial resolutions, the numerical results showthat ? can be obtained directly as the simple products of two-pointincrements of u and L , without the assumption of isotropy. Finally, theresults for the largest spatial resolutions show a clear agreement withthe cascade rates computed from the classical four-thirds law forisotropic homogeneous HD turbulence. (10.1103/PhysRevFluids.4.024603)
  DOI : 10.1103/PhysRevFluids.4.024603
• Electron affinity of lead
  
  • Bresteau D.
  • Drag Cyril
  • Blondel Christophe
  Journal of Physics B: Atomic, Molecular and Optical Physics, IOP Publishing, 2019, 52, pp.065001. A beam of Pb&#8722; ions produced by a cesium sputtering ion source is photodetached in the presence of an electric field, inside a linear optical cavity. Amplification of the light flux by the resonant cavity makes it possible to record exploitable photoelectron interferograms, even though the Pb&#8722; current does not exceed a few pA. The laser wavenumber is set either just above the first 3P1 finestructure excited threshold of neutral Pb, or above the higher 3P2 threshold. The photoelectron kinetic energy is deduced from the electron interferograms with a precision high enough to provide a new experimental value of the electron affinity of lead, 8 times more precise and slightly lower than the one measured in 2016: eA(Pb) = 287 714.9(1.5) m-1 or 0.356 721(2) eV, instead of 287 733(13) m&#8722;1 or 0.356 743(16) eV. (10.1088/1361-6455/aaf685)
  DOI : 10.1088/1361-6455/aaf685
• ELM-induced cold pulse propagation in ASDEX Upgrade
  
  • Trier Elisée
  • Wolfrum E.
  • Willensdorfer M.
  • Yu Q.
  • Hoelzl M.
  • Orain F.
  • Ryter F.
  • Angioni C.
  • Bernert M.
  • G Dunne M.
  • S Denk S.
  • C Fuchs J.
  • Fischer R.
  • Hennequin Pascale
  • Kurzan B.
  • Mink F.
  • Mlynek A.
  • Odstrcil T.
  • a Schneider P.
  • Stroth U.
  • Tardini G.
  • Vanovac B.
  • Asdex Upgrade Team The
  • Eurofusion Mst1 Team The
  Plasma Physics and Controlled Fusion, IOP Publishing, 2019, 61 (4), pp.045003. In ASDEX Upgrade, the propagation of cold pulses induced by type-I edge localized modes (ELMs) is studied using electron cyclotron emission measurements, in a dataset of plasmas with moderate triangularity. It is found that the edge safety factor or the plasma current are the main determining parameters for the inward penetration of the T e perturbations. With increasing plasma current the ELM penetration is more shallow in spite of the stronger ELMs. Estimates of the heat pulse diffusivity show that the corresponding transport is too large to be representative of the inter-ELM phase. Ergodization of the plasma edge during ELMs is a possible explanation for the observed properties of the cold pulse propagation, which is qualitatively consistent with non-linear magneto-hydro-dynamic simulations. (10.1088/1361-6587/aaf9c3)
  DOI : 10.1088/1361-6587/aaf9c3
• Properties of the singing comet waves in the 67P/Churyumov-Gerasimenko plasma environment as observed by the Rosetta mission
  
  • Breuillard Hugo
  • Henri Pierre
  • Bucciantini Luca
  • Volwerk M.
  • Karlsson T.
  • Eriksson A.
  • Johansson F.
  • Odelstad E.
  • Richter I.
  • Goetz C.
  • Vallieres Xavier
  • Hajra R.
  Astronomy & Astrophysics - A&A, EDP Sciences, 2019, 630, pp.A39. Using in situ measurements from different instruments on board the Rosetta spacecraft, we investigate the properties of the newly discovered low-frequency oscillations, known as singing comet waves, that sometimes dominate the close plasma environment of comet 67P/Churyumov-Gerasimenko. These waves are thought to be generated by a modified ion-Weibel instability that grows due to a beam of water ions created by water molecules that outgass from the comet. We take advantage of a cometary outburst event that occurred on 2016 February 19 to probe this generation mechanism. We analyze the 3D magnetic field waveforms to infer the properties of the magnetic oscillations of the cometary ion waves. They are observed in the typical frequency range (~50 mHz) before the cometary outburst, but at ~20 mHz during the outburst. They are also observed to be elliptically right-hand polarized and to propagate rather closely (~0−50°) to the background magnetic field. We also construct a density dataset with a high enough time resolution that allows us to study the plasma contribution to the ion cometary waves. The correlation between plasma and magnetic field variations associated with the waves indicates that they are mostly in phase before and during the outburst, which means that they are compressional waves. We therefore show that the measurements from multiple instruments are consistent with the modified ion-Weibel instability as the source of the singing comet wave activity. We also argue that the observed frequency of the singing comet waves could be a way to indirectly probe the strength of neutral plasma coupling in the 67P environment. (10.1051/0004-6361/201834876)
  DOI : 10.1051/0004-6361/201834876
• Experimental investigation of the tilt angle of turbulent structures in the core of fusion plasmas
  
  • Pinzon Javier
  • Happel T.
  • Hennequin Pascale
  • Angioni Clemente
  • Estrada Teresa
  • Lebschy Alexander
  • Stroth Ulrich
  • Asdex Upgrade Team The
  Nuclear Fusion, IOP Publishing, 2019, 59, pp.074002. The tilt angle of turbulent structures stands for the anisotropy of turbulence which is essential for understanding the dynamics of magnetized plasmas. It is a quantity predicted by theory and simulations, that provides information on the interplay between turbulence, micro-instabilities and plasma flows. A new method for measuring the tilt angle of turbulent structures in the core region of fusion plasmas using Doppler reflectometry is presented. First measurements of this type on the ASDEX Upgrade tokamak have shown a significant difference of tilt angle for different plasma conditions. The dominance of sheared flows in determining the structure tilt is experimentally demonstrated for different turbulence regimes. (10.1088/1741-4326/ab227e)
  DOI : 10.1088/1741-4326/ab227e
• The Radio and Plasma Waves (RPW) Instrument on Solar Orbiter : Capabilities and Performance
  
  • Maksimovic M.
  • Soucek J.
  • Bale S. D.
  • Bonnin X.
  • Chust Thomas
  • Khotyaintsev Y.
  • Kretzschmar Matthieu
  • Plettemeier D.
  • Steller M.
  • Štverák S.
  , 2019, 2019, pp.15 pp.. We will review the instrumental capabilities of the Radio and Plasma Waves (RPW) Instrument on Solar Orbiter. This instrument is designed to measure in-situ magnetic and electric fields and waves from 'DC' to a few hundreds of kHz. RPW will also observe solar radio emissions up to 16 MHz. The RPW instrument is of primary importance to the Solar Orbiter mission and science requirements, since it is essential to answer three of the four mission overarching science objectives. In addition, RPW will exchange on-board data with the other in-situ instruments, in order to process algorithms for interplanetary shocks and type III Langmuir waves detections.
• The RPW/Search Coil Magnetometer onboard Solar Orbiter
  
  • Kretzschmar Matthieu
  • Krasnoselskikh V.
  • Jannet G.
  • Jean-Yves B.
  • Fergeau P.
  • Timofeeva M.
  • Dudok de Wit Thierry
  • Maksimovic M.
  • Chust Thomas
  • Le Contel O.
  • Soucek J.
  , 2019, 2019. Measuring the fluctuating magnetic fields associated to various phenomena such as waves, shocks, and turbulence is essential for the Solar Orbiter mission. These measurements rely on a tri-axial Search Coil Magnetometer (SCM) built at the Laboratoire de Physique et Chimie de l'Environnement et de l'Espace (LPC2E) in Orléans, France. Two antennas of SCM covers the 10Hz-50kHz frequency range, while the third antenna is a dual-band one that covers also the 1kHz-1MHz frequency range. The highest sensitivity is reached respectively at 3kHz and 10kHz for the low and high frequency channels, with a level of 10^-5 nT/sqrt(Hz). The SCM is allocated on the boom of the spacecraft and the LFR, TDS, and HFR analyzers of the RPW experiment will register and process its signal on-board. <P />Extensive calibrations have been performed at RPW system level with both SCM and the analyzers ; the final calibration procedure depends on temperature, RPW configuration, and takes into account the signal received by all three magnetic antennas simultaneously.
• Filamentary nanosecond surface dielectric barrier discharge. Experimental comparison of the streamer-to-filament transition for positive and negative polarities.
  
  • Ding Chenyang
  • Khomenko A.Yu.
  • Shcherbanev S.A.
  • Starikovskaia Svetlana
  Plasma Sources Science and Technology, IOP Publishing, 2019, 28 (8), pp.085005. Streamer-to-filament transition is a general feature of high pressure high voltage nanosecond surface dielectric barrier discharges (nSDBDs) for mixtures containing molecular gases. The transition is observed at high pressures and voltages in a single-shot experiment a few nanoseconds after the start of the discharge. A set of experimental results comparing streamer-to-filament transition and properties of plasma in the filaments for the identical high voltage pulses of negative and positive polarity is presented. The transition curves in voltage-pressure coordinates are obtained for N 2 :O 2 mixtures with different content of molecular oxygen, from 0 to 20%, at the pressure range 1-12 bar. Continuous optical spectra are compared for both polarities in 6 bar synthetic air. Electron density is calculated from Stark broadening of H α line at λ = 656.5 nm in the discharge and in early afterglow, 40 nanoseconds after the end of the high voltage pulse. Hydrodynamic perturbations are measured using schlieren imaging in 1-6 bar air for streamer and filamentary mode for both polarities. The review of common and distinctive features of the filamentary single-shot nSDBD for two polarities of the applied pulse is provided. (10.1088/1361-6595/ab2d7a)
  DOI : 10.1088/1361-6595/ab2d7a
• Electric field induced second harmonic (E-FISH) generation for characterization of fast ionization wave discharges at moderate and low pressures
  
  • Chng Tat Loon
  • Orel Inna
  • Starikovskaia Svetlana
  • Adamovich I.V.
  Plasma Sources Science and Technology, IOP Publishing, 2019, 28 (4), pp.045004 (8pp). The electric &#64257;eld in an ionization wave discharge in nitrogen at 20100 mbar, initiated by positive polarity, high-voltage, ns duration pulses, is measured by ps second harmonic generation. The axial electric &#64257;eld component is determined both during the propagation of the ionization wave along the discharge tube, and after the wave reaches the grounded electrode, spanning the entire discharge gap. The temporal resolution of the present measurements is 200 ps, with the spatial resolution in the axial direction of approximately 0.5 mm. The second harmonic signal exhibits a quadratic dependence on the Laplacian electric &#64257;eld but indicates that in this pressure range most of the signal is generated within the wall of the tube. Absolute calibration of the signal is obtained from the current shunt data, after the ionization wave has reached the grounded electrode. Comparison of the data taken at different pressures shows that the peak value of the axial electric &#64257;eld in the wave front, 811 kV cm&#8722;1, has a fairly weak dependence on pressure, with the peak reduced electric &#64257;eld reaching &#8776;2000 Td at 20 mbar. Reducing the pressure from 100 to 20 mbar, while keeping the discharge pulse voltage waveform the same, steepens the ionization wave front considerably, from 3.0 to 1.0 ns full width at half maximum. The results demonstrate that ps second harmonic generation may be employed for electric &#64257;eld measurements in low-pressure discharges, discharges sustained in small diameter capillary tubes, and discharges sustained in gas mixtures with low nonlinear susceptibility, at the conditions when the detection of the signal generated directly in the plasma is challenging. High temporal resolution of the present measurements indicates a possibility of detection of non-local electron kinetics effects induced by a rapidly va (10.1088/1361-6595/ab0b22)
  DOI : 10.1088/1361-6595/ab0b22
• Four-Spacecraft Measurements of the Shape and Dimensionality of Magnetic Structures in the Near-Earth Plasma Environment
  
  • Fadanelli S
  • Lavraud B.
  • Califano F.
  • Jacquey C.
  • Kacem I.
  • Vernisse Y.
  • Penou E.
  • Gershman D J
  • Dorelli J
  • Pollock C.
  • Giles B L
  • Avanov L.
  • Burch J.
  • Chandler M. O.
  • Coffey V N
  • Eastwood J P
  • Ergun R
  • Farrugia C J
  • Fuselier S A
  • Genot V N
  • Grigorenko E
  • Hasegawa H
  • Khotyaintsev Y
  • Le Contel Olivier
  • Marchaudon Aurélie
  • Moore T E
  • Nakamura R
  • Paterson W R
  • Phan T
  • Rager A. C.
  • Russell C T
  • Saito Y
  • Sauvaud J.-A
  • Schiff C
  • Smith S E
  • Toledo Redondo S
  • Torbert R B
  • Wang S
  • Yokota S
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2019, 124 (8), pp.6850-6868. We present a new method for determining the main relevant features of the local magnetic field configuration, based entirely on the knowledge of the magnetic field gradient four-spacecraft measurements. The method, named "magnetic configuration analysis" (MCA), estimates the spatial scales on which the magnetic field varies locally. While it directly derives from the well-known magnetic directional derivative and magnetic rotational analysis procedures (Shi et al., 2005, htpps://doi.org/10.1029/ 2005GL022454; Shen et al., 2007, https://doi.org/10.1029/2005JA011584), MCA was specifically designed to address the actual magnetic field geometry. By applying MCA to multispacecraft data from the Magnetospheric Multiscale (MMS) satellites, we perform both case and statistical analyses of local magnetic field shape and dimensionality at very high cadence and small scales. We apply this technique to different near-Earth environments and define a classification scheme for the type of configuration observed. While our case studies allow us to benchmark the method with those used in past works, our statistical analysis unveils the typical shape of magnetic configurations and their statistical distributions. We show that small-scale magnetic configurations are generally elongated, displaying forms of cigar and blade shapes, but occasionally being planar in shape like thin pancakes (mostly inside current sheets). Magnetic configurations, however, rarely show isotropy in their magnetic variance. The planar nature of magnetic configurations and, most importantly, their scale lengths strongly depend on the plasma β parameter. Finally, the most invariant direction is statistically aligned with the electric current, reminiscent of the importance of electromagnetic forces in shaping the local magnetic configuration. (10.1029/2019JA026747)
  DOI : 10.1029/2019JA026747
• Introduction to plasma physics
  
  • Belmont Gérard
  • Rezeau Laurence
  • Riconda C.
  • Zaslavsky A.
  , 2019.
• Non-Isothermal Sheath Model for Low Pressure Plasmas
  
  • Tavant Antoine
  • Lucken Romain
  • Bourdon Anne
  • Chabert Pascal
  Plasma Sources Science and Technology, IOP Publishing, 2019, 28 (7), pp.075007. The evolution of the electron mean energy in the pre-sheath and the sheath of a low pressure plasma bounded by two planes is investigated with 1D particle in cell simulations. We observed that the electron mean energy is not constant in the sheath, but instead decreases significantly from the bulk towards the wall. From the simulations, a polytropic state law is proposed, allowing us to close the fluid equations for the electrons without the isothermal hypothesis. A comparison between the fluid model and the simulations show that the non-isothermal sheath model is more accurate than the isothermal model. The impact of the electron mean energy variation on the potential sheath drop and the electron particle and heat flux is evaluated. (10.1088/1361-6595/ab279b)
  DOI : 10.1088/1361-6595/ab279b
• High-Frequency Wave Generation in Magnetotail Reconnection: Linear Dispersion Analysis
  
  • Burch J. L.
  • Dokgo K.
  • Hwang K.-J.
  • Torbert R. B.
  • Graham D. B.
  • Webster J. M.
  • Ergun R. E.
  • Giles B. L.
  • Allen R. C.
  • Chen L.-J.
  • Wang S.
  • Genestreti K. J.
  • Russell C. T.
  • Strangeway R. J.
  • Le Contel Olivier
  Geophysical Research Letters, American Geophysical Union, 2019, 46 (8), pp.4089-4097. Plasma and wave measurements from the NASA Magnetospheric Multiscale mission are presented for magnetotail reconnection events on 3 July and 11 July 2017. Linear dispersion analyses were performed using distribution functions comprising up to six drifting bi-Maxwellian distributions. In both events electron crescent-shaped distributions are shown to be responsible for upper hybrid waves near the X-line. In an adjacent location within the 3 July event a monodirectional field-aligned electron beam drove parallel-propagating beam-mode waves. In the 11 July event an electron distribution consisting of a drifting core and two crescents was shown to generate upper-hybrid and beam-mode waves at three different frequencies, explaining the observed broadband waves. Multiple harmonics of the upper hybrid waves were observed but cannot be explained by the linear dispersion analysis since they result from nonlinear beam interactions. (10.1029/2019GL082471)
  DOI : 10.1029/2019GL082471
• Turbulent Heating in the Accelerating Region Using a Multishell Model
  
  • Verdini Andrea
  • Grappin Roland
  • Montagud-Camps Victor
  Solar Physics, Springer Verlag, 2019, 294. Recent studies of turbulence-driven solar winds indicate that fast winds are obtained only at the price of unrealistic bottom boundary conditions: too large wave amplitudes and small frequencies. In this work, the incompressible turbulent dissipation is modeled with a large-scale von Karman-Howarth-Kolmogorov-like phenomenological expression (Q_K41<SUP>0</SUP>). An evaluation of the phenomenology is thus necessary to understand if unrealistic boundary conditions result from physical or model limitations. To assess the validity of the Kolmogorov-like expression, Q_K41<SUP>0</SUP>, one needs to compare it to exact heating, which requires describing the cascade in detail. This has been done in the case of homogeneous MHD turbulence, including expansion, but not in the critical accelerating region. To assess the standard incompressible turbulent heating in the accelerating region, we use a reduced MHD model (multishell model) in which the perpendicular turbulent cascade is described by a shell model, allowing to reach a Reynolds number of 10<SUP>6</SUP>. We first consider the homogeneous and expanding cases, and find that primitive MHD and multishell equations give remarkably similar results. We thus feel free to use the multishell model in the accelerating region. The results indicate that the large-scale phenomenology is inaccurate and it overestimates the heating by a factor at least 20, thus invalidating earlier studies of winds driven by incompressible turbulence. We conclude that realistic 1D wind models cannot be based solely on incompressible turbulence, but probably need an addition of compressible turbulence and shocks to increase the wave reflection and thus the heating. (10.1007/s11207-019-1458-y)
  DOI : 10.1007/s11207-019-1458-y
• Nonlinear Diffusion Models for Gravitational Wave Turbulence
  
  • Galtier Sébastien
  • Nazarenko Sergey V.
  • Buchlin Éric
  • Thalabard Simon
  Physica D: Nonlinear Phenomena, Elsevier, 2019, 390, pp.84-88. A fourth-order and a second-order nonlinear diffusion model in spectral space are proposed to describe gravitational wave turbulence in the approximation of strongly local interactions. We show analytically that the model equations satisfy the conservation of energy and wave action, and reproduce the power law solutions previously derived from the kinetic equations with a direct cascade of energy and an explosive inverse cascade of wave action. In the latter case, we show numerically by computing the second-order diffusion model that the non-stationary regime exhibits an anomalous scaling which is understood as a self-similar solution of the second kind with a front propagation following the law kf∼(t∗−t)3.296 , with t<t∗ . These results are relevant to better understand the dynamics of the primordial universe where potent sources of gravitational waves may produce space–time turbulence. (10.1016/j.physd.2019.01.007)
  DOI : 10.1016/j.physd.2019.01.007
• Waves in Kinetic-Scale Magnetic Dips: MMS Observations in the Magnetosheath
  
  • Yao S. T.
  • Shi Q. Q.
  • Yao Z. H.
  • Li J. X.
  • Yue C.
  • Tao X.
  • Degeling A. W.
  • Zong Q. G.
  • Wang X. G.
  • Tian A. M.
  • Russell C. T.
  • Zhou X. Z.
  • Guo R. L.
  • Rae I. J.
  • Fu H.S.
  • Zhang H.
  • Li L.
  • Le Contel Olivier
  • Torbert R. B.
  • Ergun R. E.
  • Lindqvist P.-A.
  • Pollock C. J.
  • Giles B. L.
  Geophysical Research Letters, American Geophysical Union, 2019, 46 (2), pp.523-533. Kinetic-scale magnetic dips (KSMDs), with a significant depression in magnetic field strength, and scale length close to and less than one proton gyroradius, were reported in the turbulent plasmas both in recent observation and numerical simulation studies. These KSMDs likely play important roles in energy conversion and dissipation. In this study, we present observations of the KSMDs that are labeled whistler mode waves, electrostatic solitary waves, and electron cyclotron waves in the magnetosheath. The observations suggest that electron temperature anisotropy or beams within KSMD structures provide free energy to generate these waves. In addition, the occurrence rates of the waves are higher in the center of the magnetic dips than at their edges, implying that the KSMDs might be the origin of various kinds of waves. We suggest that the KSMDs could provide favorable conditions for the generation of waves and transfer energy to the waves in turbulent magnetosheath plasmas. (10.1029/2018GL080696)
  DOI : 10.1029/2018GL080696
• Experimental study of pulsed microwave discharges at pressures ranging over five orders of magnitude
  
  • Shcherbanev S.A.
  • Ali Cherif Mhedine
  • Starikovskaia Svetlana
  • Ikeda Yuji
  Plasma Sources Science and Technology, 2019, 28, pp.045009 (10pp). Microwave discharge igniter (MDI) is a discharge system developed to initiate combustion in automotive engines. The MDI uses a sequence of N = 700 microwave (2.45 GHz) pulses 100 ns in duration separated by 1 &#956;s. The initial breakdown is provided by the &#64257;rst microwave pulse, 5 &#956;s in duration. The aim of pulsing the microwave signal is to keep an optimal combination of parameters when, even at elevated pressures, (i) the discharge propagates over the largest possible volume; (ii) the plasma is non-equilibrium. Properties of plasma produced by MDI igniter in non-combustible gas mixtures at ambient gas temperature and gas pressure in the range between 0.2 mbar and 8 bar were studied experimentally. Discharge spatial structure was analyzed with the help of time-resolved ICCD imaging. Near-UV optical emission spectra taken in different pulses provided the information about rotational and vibrational temperatures. The electric &#64257;eld was estimated on the basis of ratio of emission of the second positive and the &#64257;rst negative systems of molecular nitrogen. (10.1088/1361-6595/aae765)
  DOI : 10.1088/1361-6595/aae765
• Nonlinear interaction of whistler waves in a magnetized plasma with density ducts
  
  • Zaboronkova T. M.
  • Krafft Catherine
  • Yashina N. F.
  Physics of Plasmas, American Institute of Physics, 2019, 26, pp.102104. The nonlinear resonant interactions between whistler waves guided bydensity ducts surrounded by a uniform magnetized plasma are studied. Itis shown that, under specific conditions that are determined, a time-harmonic external electromagnetic field can drive the parametricinstability of guided whistlers. Both cases of cylindrical and planarducts are considered, of either decreased or increased plasma density.The frequency interval where the magnetized plasma may be resonant isanalyzed. The growth rate and the threshold of the parametricinstability are determined. Numerical calculations are presented forparameters typical of "space plasmas and" modeling laboratoryexperiments where guided whistler propagation was observed. (10.1063/1.5110958)
  DOI : 10.1063/1.5110958