Publications

2019

• 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 field 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 field 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 field 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 field in the wave front, 811 kV cm−1, has a fairly weak dependence on pressure, with the peak reduced electric field reaching ≈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 field 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
• 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.
• 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.
• 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
• 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
• 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
• 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
• 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
• 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
• 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
• 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
• 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^+&gt;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
• 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
• 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
• ViDA: a Vlasov-DArwin solver for plasma physics at electron scales
  
  • Pezzi Oreste
  • Cozzani Giulia
  • Califano Francesco
  • Valentini Francesco
  • Guarrasi Massimiliano
  • Camporeale Enrico
  • Brunetti Gianfranco
  • Retinò Alessandro
  • Veltri Pierluigi
  Journal of Plasma Physics, Cambridge University Press (CUP), 2019, 85 (5), pp.905850506. We present a VlasovDArwin numerical code (ViDA) specifically designed to address plasma physics problems, where small-scale high accuracy is requested even during the nonlinear regime to guarantee a clean description of the plasma dynamics at fine spatial scales. The algorithm provides a low-noise description of proton and electron kinetic dynamics, by splitting in time the multi-advection Vlasov equation in phase space. Maxwell equations for the electric and magnetic fields are reorganized according to the Darwin approximation to remove light waves. Several numerical tests show that ViDA successfully reproduces the propagation of linear and nonlinear waves and captures the physics of magnetic reconnection. We also discuss preliminary tests of the parallelization algorithm efficiency, performed at CINECA on the Marconi-KNL cluster. ViDA will allow the running of Eulerian simulations of a non-relativistic fully kinetic collisionless plasma and it is expected to provide relevant insights into important problems of plasma astrophysics such as, for instance, the development of the turbulent cascade at electron scales and the structure and dynamics of electron-scale magnetic reconnection, such as the electron diffusion region. (10.1017/S0022377819000631)
  DOI : 10.1017/S0022377819000631
• Crossing of Plasma Structures by spacecraft: a path calculator
  
  • Manuzzo Roberto
  • Belmont Gérard
  • Rezeau Laurence
  • Califano F.
  • Denton R E
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2019, 124 (12), pp.10119-10140. When spacecraft (s/c) missions probe plasma structures (PSs) the relative location of the s/c with respect to the PS is unknown. This information is, however, needed to measure the geometrical features of the PS (orientation and thickness) and to understand the physical processes underlying the PS dynamics. Methods to determine the s/c location exist, but they need strong assumptions to be satisfied (stationarity and special spatial dependencies). The number of cases for which these assumptions are likely to be valid for the entire PS seems to be limited, and even weak departures from these hypotheses may affect the results. For a quasi‐1‐D geometry in particular, the determination of the velocity component along the two quasi‐invariant directions is very inaccurate and the assumption of strict stationarity may lead these quantities to diverge. In this paper we present new methods to compute the s/c trajectory through a PS, without a priori assumption on its spatial geometry, and able to work even in the presence of weak nonstationarities. The methods are tested both on artificial and real data, the latter provided by the Magnetospheric Multiscale mission probing the Earth's magnetopause. The 1‐D and 2‐D trajectories of the Magnetospheric Multiscale are found that can be used as an initial step for future reconstruction studies. Advanced minimization procedures to optimize the results are discussed. (10.1029/2019JA026632)
  DOI : 10.1029/2019JA026632
• Energy Conversion and Electron Acceleration in the Magnetopause Reconnection Diffusion Region
  
  • Pritchard K. R.
  • Burch J. L.
  • Fuselier S. A.
  • Webster J. M.
  • Torbert R. B.
  • Argall M. R.
  • Broll J.
  • Genestreti K. J.
  • Giles B. L.
  • Le Contel Olivier
  • Mukherjee J.
  • Phan T. D.
  • Rager A. C.
  • Russell C. T.
  • Strangeway R. J.
  Geophysical Research Letters, American Geophysical Union, 2019, 46 (17-18), pp.10274-10282. Data are analyzed from a Magnetospheric Multiscale encounter with a dayside magnetopause reconnection region on 29 December 2016. The uniqueness of the event stems from the small ( 7 km) average spacecraft separation and the sequential sampling of an electron diffusion region with electron crescent distributions. We quantitatively investigate the earthward acceleration of magnetosheath electrons through the in-plane null by the polarization electric field EN that points radially outward from the magnetopause. The results compare favorably with previous plasma simulations with one important difference that the reconnection electric field (EM) extends throughout the region of strong EN so that both fields energize electrons in the same region. This acceleration is quantified here for the first time. As the spacecraft penetrate deeper into the region of enhanced EN, the magnetic reflection of lower-energy electrons produces a thinner crescent. (10.1029/2019GL084636)
  DOI : 10.1029/2019GL084636
• Plasma gun for medical applications: engineering an equivalent electrical target of human body and deciphering relevant electrical parameters
  
  • Judée Florian
  • Dufour Thierry
  Journal of Physics D: Applied Physics, IOP Publishing, 2019, 52 (16), pp.16 - 18. Simulations and experimental works have been carried out in a complementary way to engineer a basic material target mimicking the same dielectric properties of the human body. It includes a resistor in parallel with a capacitor, whose values (Rh=1500 Ω and Ch=100 pF) are estimated in regard of parameters commonly utilized upon in vivo campaigns (frequency=30 kHz, gap=10 mm, high voltage electrode surface=12.6 mm 2). This equivalent electrical human body (EEHB) circuit can be used as a reference and realistic target to calibrate electrical properties of therapeutic plasma sources before their utilization on patients. In this letter, we consider a configuration where this EEHB target interacts with a plasma gun (PG). Plasma power measurements performed in such configuration clearly indicate two operating modes depending on the value of the supplied voltage. Hence, the plasma gun generates pulsed atmospheric plasma streams likely to present therapeutic interest for voltages comprised between 3.0 and 8.5 kV while for higher values, transient arcs of thermal plasma are generated and represent substantial risks for the patient. (10.1088/1361-6463/ab03b8)
  DOI : 10.1088/1361-6463/ab03b8
• Comparison between ad-hoc and instability-induced electron anomalous transport in a 1D fluid simulation of Hall-effect thruster
  
  • Martorelli Roberto
  • Lafleur Trevor
  • Bourdon Anne
  • Chabert Pascal
  Physics of Plasmas, American Institute of Physics, 2019, 26 (8), pp.083502. Anomalous electron transport is a long-standing problem in the understanding of Hall-effect thrusters. Recent results have suggested as a possible cause a kinetic instability, but few attempts have succeeded in implementing such phenomena in a fluid simulation of the thruster. The common approach in this case relies on including an ad-hoc model of the anomalous transport and so to fit experimental results. We propose here a comparison between the friction force and the anomalous heating arising from the ad-hoc model, with the corresponding effects coming from the use of the instability-induced transport. The results are obtained through a one-dimensional fluid simulation of the Hall-effect thruster with ad-hoc anomalous transport. The comparison shows good agreement between the two approaches, suggesting indeed that the instability-induced anomalous transport is the good candidate for reproducing the ad-hoc simulations and paving the way for a full self-consistent implementation of the phenomena in a fluid simulation. (10.1063/1.5089008)
  DOI : 10.1063/1.5089008
• Non-thermal DBD plasma array on seed germination of different plant species
  
  • Liu Bo
  • Honnorat Bruno
  • Yang Hang
  • Arancibia Monreal J.
  • Rajjou Loic
  • Rousseau Antoine
  Journal of Physics D: Applied Physics, IOP Publishing, 2019, 52 (2), pp.025401. A dielectric barrier discharge (DBD) reactor producing cold plasma at atmospheric pressure has been used to treat seeds of eight different species and investigate their responses in term of germination. The device is made of nine cylindrical DBDs organized in a array and partially immersed in water. O2, N2, and air were flown in the device; the cold plasma from such gas is formed in the bubbles and touch liquid surface. Seeds were either located inside the water during plasma treatment process (direct treatment) or were watered by the water exposed to cold plasma beforehand (indirect treatment). Such plasma activated water contains reactive oxygen species and reactive nitrogen species. The statistical analysis shows that the probability of germinating of treated mung bean, mustard and radish is significantly higher than in control groups (p&#8201;&#8201;<&#8201;&#8201;0.05) for indirect treatments. A comparison of different treatment modalities (direct versus indirect treatment and gas composition) on germination boost has been completed on mung bean seeds. It is shown that direct plasma treatment using different gas (O2, N2, and air) give a strong enhancement of the mung bean germination probability compared to the control group; in the case of indirect treatment, only plasma air-treated water lead to a significant germination boost compared to the control group; this effect is still smaller than the one obtained using a direct treatment. (10.1088/1361-6463/aae771)
  DOI : 10.1088/1361-6463/aae771
• Training on GNSS and Space Weather in Africa in the framework of the North-South scientific network GIRGEA
  
  • Amory-Mazaudier Christine
  • Fleury Rolland
  • Masson F.
  • Gadimova S.
  • Anas Emran
  Sun and Geosphere, BBC SWS Regional Network, 2019, 1 (141), pp.71-79. This paper presents the successful setting up of a research and teaching network for space weather in developed and fragile countries. This development took nearly a quarter of a century with the help of international cooperation. Numerous studies have been developed in different domains of Space Weather concerning the impact of solar events on the ionosphere and the Earth's magnetic field, ionospheric electric currents and the induced currents in the ground (GIC) Other studies have also been conducted on climate change, lightning and the movement of tectonic plates. We underline the importance of Global Navigation Satellite Systems [GNSS] for the development of space weather research and capacity building during the last decades (10.31401/SunGeo.2019.01.10)
  DOI : 10.31401/SunGeo.2019.01.10
• Inverse cascade of hybrid helicity in B&#937;-MHD turbulence
  
  • Menu Mélissa
  • Galtier Sébastien
  • Petitdemange Ludovic
  Physical Review Fluids, American Physical Society, 2019, 4, pp.073701. We investigate the impact of a solid-body rotation &#937;0 on the large-scale dynamics of an incompressible magnetohydrodynamic turbulent flow in presence of a background magnetic field B0 and at low Rossby number. Three-dimensional direct numerical simulations are performed in a periodic box, at unit magnetic Prandtl number and with a forcing at intermediate wave number kf=20. When &#937;0 is aligned with B0 (i.e., &#952;&#8801;(&#937;0,B0)=0), inverse transfer is found for the magnetic spectrum at k<kf. This transfer is stronger when the forcing excites preferentially right-handed (rather than left-handed) fluctuations; it is smaller when &#952;>0 and becomes weak when &#952;&#8805;35&#8728;. These properties are understood as the consequence of an inverse cascade of hybrid helicity which is an inviscid/ideal invariant of this system when &#952;=0. Hybrid helicity emerges, therefore, as a key element for understanding rotating dynamos. Implication of these findings on the origin of the alignment of the magnetic dipole with the rotation axis in planets and stars is discussed. (10.1103/PhysRevFluids.4.073701)
  DOI : 10.1103/PhysRevFluids.4.073701
• Experimental and numerical investigation of the transient charging of a dielectric surface exposed to a plasma jet
  
  • Slikboer Elmar
  • Viegas Pedro
  • Bonaventura Z.
  • Garcia-Caurel Enric
  • Sobota Ana
  • Bourdon Anne
  • Guaitella Olivier
  Plasma Sources Science and Technology, IOP Publishing, 2019, 28 (9), pp.095016. This work investigates the dynamical charging of a surface under exposure of a non-equilibrium plasma jet at atmospheric pressure through a quantitative comparison between modeling and experiments. We show using mono-polar pulses with variable pulse duration and amplitude that the charging time (i.e. the time from impact of the ionization wave till the fall of the high voltage pulse) is a crucial element determining the plasma-surface interaction. This is done through direct measurements of the electric field induced inside the target using the optical diagnostic technique called Mueller polarimetry and comparison with the electric field calculated using a 2D fluid model of the plasma jet interaction with the target in the same conditions as in the experiments. When the charging time is kept relatively short (less than 100 ns), the surface spreading of the discharge and consequent surface charge deposition are limited. When it is relatively long (up to microseconds), the increased surface spreading and charge deposition significantly change the electric field to which the target is exposed during the charging time and when the applied voltage returns to zero. (10.1088/1361-6595/ab3c27)
  DOI : 10.1088/1361-6595/ab3c27
• Dependence on plasma shape and plasma fueling for small edge-localized mode regimes in TCV and ASDEX Upgrade
  
  • Labit B.
  • Eich T.
  • Harrer G.F.
  • Wolfrum E.
  • Bernert M.
  • Dunne M.G.
  • Frassinetti L.
  • Hennequin Pascale
  • Maurizio R.
  • Merle A.
  • Meyer H.
  • Saarelma S.
  • Sheikh U.
  • Eurofusion Mst1 Team The
  Nuclear Fusion, IOP Publishing, 2019, 59 (8), pp.086020. Within the EUROfusion MST1 work package, a series of experiments has been conducted on AUG and TCV devices to disentangle the role of plasma fueling and plasma shape for the onset of small ELM regimes. On both devices, small ELM regimes with high confinement are achieved if and only if two conditions are fulfilled at the same time. Firstly, the plasma density at the separatrix must be large enough (), leading to a pressure profile flattening at the separatrix, which stabilizes type-I ELMs. Secondly, the magnetic configuration has to be close to a double null (DN), leading to a reduction of the magnetic shear in the extreme vicinity of the separatrix. As a consequence, its stabilizing effect on ballooning modes is weakened. (10.1088/1741-4326/ab2211)
  DOI : 10.1088/1741-4326/ab2211
• [Plasma 2020 Decadal] Multipoint Measurements of the Solar Wind: A Proposed Advance for Studying Magnetized Turbulence of the Solar Wind: A Proposed Advance for Studying Magnetized Turbulence
  
  • G. Klein Kristopher
  • Alexandrova Olga
  • Bookbinder Jay
  • Caprioli D.
  • W. Case A.
  • D. G. Chandran B.
  • J. Chen L.
  • Horbury T.
  • C. Kasper J.
  • Le Contel Olivier
  • A. Maruca B.
  • Matthaeus W. H.
  • Retinò Alessandro
  • Roberts O.
  • Schekochihin A.
  • Skoug Ruth M.
  • W. Smith C.
  • Steinberg John T.
  • Spence H. E.
  • Vasquez B.
  • M. Tenbarge J.
  • Verscharen Daniel
  • Whittlesey P.
  • Jian L.
  , 2019.