Publications

2020

• Negative ion source operation with deuterium
  
  • Bacal M.
  • Wada M
  Plasma Sources Science and Technology, IOP Publishing, 2020, 29 (3), pp.033001. When the working gas of a negative ion source is changed from hydrogen to its isotope, deuterium, an 'isotope effect' is observed; namely, several plasma characteristics such as the electron energy distribution, the atomic fraction and the spectra of rovibrationally excited molecules change. The understanding of the effect becomes more important, as research and development aiming at ITER power level operation is being challenged with feeding deuterium to the ion sources. As a historical review of the effort to develop hydrogen/deuterium negative ion sources, several types of negative ion sources designed for the neutral beam plasma heating are described: double charge exchange sources, volume sources and surface-plasma sources. The early results with volume sources operated with and without cesium are introduced. The characteristics of the source charged with deuterium are compared to those of the source charged with hydrogen. The isotope effect did not appear pronounced as the negative ion density was measured in a small source but became more pronounced when the plasma source size was enlarged and the discharge power density was increased to higher values. Surface plasma sources were optimized for deuterium operation but could not achieve the same performance as a source operated with hydrogen at the same power and pressure. The lower velocity of negative deuterium ions leaving the low work function surface seemed to limit the production efficiency. Fundamental processes causing these differences in negative ion source operation are summarized. After explaining the current status of negative ion source research and development, the acquired knowledge is utilized to the development of large negative ion sources for nuclear fusion research and to the development of compact negative ion sources for neutron source applications. (10.1088/1361-6595/ab6881)
  DOI : 10.1088/1361-6595/ab6881
• Influence of N<SUB>2</SUB> on the CO<SUB>2</SUB> vibrational distribution function and dissociation yield in non-equilibrium plasmas
  
  • Terraz L.
  • Silva T.
  • Morillo-Candas A.
  • Guaitella O.
  • Tejero-Del-Caz A.
  • Alves L. L.
  • Guerra V.
  Journal of Physics D: Applied Physics, IOP Publishing, 2020, 53, pp.094002. This work explores the effect of nitrogen addition on CO<SUB>2</SUB> dissociation under various non-equilibrium plasma conditions. Experiments are performed in non-thermal plasmas sustained by DC pulsed discharges, for pressure and current ranges of 1 to 5 Torr and 20 to 50 mA, respectively. A self-consistent model, previously validated for pure CO<SUB>2</SUB> discharges, is further extended to take into account e-V, V-T and V-V reactions involving N<SUB>2</SUB>. Both model predictions and experimental data reveal a maximum of the asymmetric vibrational temperature T<SUB>3</SUB> at 5 Torr during the discharge around 1 ms, while no such maximum is visible at 1 Torr before the saturation occurs. It is shown that V-T deactivation by O atoms can have a strong influence on the vibrational kinetics, by directly affecting the relaxation of N<SUB>2</SUB> vibrational excited states and, as a consequence, the very important energy transfers between vibrationally excited N<SUB>2</SUB> and CO<SUB>2</SUB> molecules. The experimental results show a twice as large CO<SUB>2</SUB>-conversion rate when N<SUB>2</SUB> gas is added to the plasma. The simulations suggest this effect cannot be the result of an increased dissociation by direct electron impact due to modifications in the reduced electric field, but rather of some other contribution to dissociation and/or inhibition of reactions giving back CO<SUB>2</SUB>. (10.1088/1361-6463/ab55fb)
  DOI : 10.1088/1361-6463/ab55fb
• Design of a variable frequency comb reflectometer system for the ASDEX Upgrade tokamak
  
  • Happel T.
  • Kasparek W.
  • Hennequin Pascale
  • Höfler K.
  • Honoré Cyrille
  • Team Asdex Upgrade
  Plasma Science and Technology, IOP Publishing, 2020, 22 (6), pp.064002. Comb reflectometers offer the advantage of measuring several radial positions in plasma simultaneously. This allows for the investigation of fast timescales during L-H transitions, I-phases, I-mode bursts, transients during heat wave propagation, etc. A drawback of many present-day systems is that they use a fixed frequency difference between the probing frequencies. Hence, although the central probing frequency can be varied, the probing frequency difference is usually fixed. The new design presented in this work uses an advanced microwave generation and detection scheme, which allows for arbitrary probing frequencies and probing frequency separations. (10.1088/2058-6272/ab618c)
  DOI : 10.1088/2058-6272/ab618c
• Early results from the Proton Alfa Sensor (PAS/SWA) onboard Solar Orbiter: the Solar Wind at different scales.
  
  • Louarn P.
  • Fedorov A.
  • Prech L.
  • Lavraud B.
  • Rouillard A. P.
  • Genot V. N.
  • Plotnikov I.
  • Penou E.
  • Barthe A.
  • Owen C. J.
  • Berthomier M.
  • Kataria D. O.
  • Bruno R.
  • Livi S. A.
  • Evans V.
  • Raines J. M.
  • Horbury T. S.
  • O'Brien H.
  • Angelini V.
  • Maksimovic M.
  • Bale S. D.
  • Chust T.
  • Khotyaintsev Y.
  • Krasnoselskikh V.
  • Kretzschmar Matthieu
  • Plettemeier D.
  • Soucek J.
  • Steller M.
  • Stverak S.
  • Travnicek P.
  • Vaivads A.
  • Vecchio A.
  , 2020, 2020. Solar Orbiter is designed to discover the fundamental connections between the rapidly varying solar atmosphere and the solar wind. In operations since mid-2020, the Solar Wind Analyzer (SWA) plasma package is now providing comprehensive in-situ measurements of the solar wind. In particular, the Proton-Alpha Sensor (PAS) is determining the properties of the dominant solar wind ion populations through the measurements of the 3D distribution functions, density, bulk velocities, temperatures, and heat fluxes, at temporal cadences ranging from 4 s to ~0.1 s. This offers the possibility of analyzing the solar wind dynamics at vastly different scales, from days to fractions of seconds. Specifically, kinetic structures can thus be described at unprecedented resolution, both in time or spatial scales and phase space organization of the associated ion populations. We will report on observations of various structures that propagate in the solar wind (large amplitude waves, solitary structures, discontinuities between different types of solar winds...) with a focus on the formation of non-thermal distribution functions, the associated wave activity and magnetic perturbations.
• Italian Solar Orbiter-SWA Working Group on Machine Learning and Artificial Intelligence
  
  • de Marco R.
  • Alberti T.
  • Amaya J.
  • Bruno R.
  • Califano F.
  • Camporeale E.
  • Consolini G.
  • Foldes R.
  • d'Amicis R.
  • Dupuis R.
  • Franci L.
  • Guedes dos Santos L. F.
  • Innocenti M. E.
  • Jagarlamudi V. K.
  • Lapenta G.
  • Laurenza M.
  • Marcucci M. F.
  • Narock A.
  • Papini E.
  • Perri S.
  • Perrone D.
  • Retino A.
  • Servidio S.
  • Sisti M.
  • Sorriso-Valvo L.
  • Valentini F.
  , 2020, 2020. The exponential growth of data volume experienced by astronomy and astrophysics causes new disciplines like machine learning (ML) and data mining (DM) to gain more and more ground in these fields. Applications like clustering, feature selection, automatic classification of events are proving to be a valuable aid in exploiting space data in the era of the synergy between "pure" science and "data-driven" science. <P />The Italian Solar Orbiter-SWA Working Group on Machine Learning and Artificial Intelligence<SUP>1</SUP> together with the European Commission Horizon 2020 project AIDA<SUP>2</SUP>, has the scope of applying ML and DM analysis techniques to the Solar Orbiter data. The implementations are numerous. First of all these new techniques can be used to discover unexpected relations between data, can automate tasks so that they can be carried out without human intervention, and can help to forecast physical properties and events. A non-exhaustive list of these activities includes automatic detection of coronal holes in images, automatic recognition of plasma regions, prediction of solar wind properties at 1 AU, classification of solar wind type based on new indicators, analysis of particle velocity distribution functions. <P />In addition, this Working Group will integrate the existing software developed in the context of the various heliospheric missions with the parts regarding Solar Orbiter. These packages are able to handle complex data set with ease and provide statistical analysis and visualization tools. Catalogs of scientific data are also produced, which report, among others, magnetic reconnection and particle acceleration events, detected by routines trained to browse data and select physical processes and features of interest. <P />Here we present the project overview along with the ML and DM tools which will be used to handle and analyse Solar Orbiter data. <P />1.https://sites.google.com/view/italian-solar-orbiter-swa/ 2.http://www.aida-space.eu
• Microphysics of Magnetic Reconnection in Near-Earth Space
  
  • Cozzani Giulia
  , 2020. (10.1007/978-3-030-56142-0)
  DOI : 10.1007/978-3-030-56142-0
• Solar Orbiter: early in situ measurements
  
  • Horbury T. S.
  • Rodriguez-Pacheco J.
  • Maksimovic M.
  • Owen C. J.
  • Angelini V.
  • Bale S. D.
  • Bruno R.
  • Chust T.
  • Evans V.
  • Gomez-Herrero R.
  • Ho G. C.
  • Khotyaintsev Y.
  • Krasnoselskikh V.
  • Kretzschmar Matthieu
  • Livi R.
  • Lorfevre E.
  • Louarn P.
  • O'Brien H.
  • Plettemeier D.
  • Soucek J.
  • Steller M.
  • Stverak S.
  • Travnicek P.
  • Vaivads A.
  • Vecchio A.
  • Wimmer-Schweingruber R. F.
  , 2020, 2020. Solar Orbiter was launched in February 2020 and carries four in situ instruments: the Energetic Particle Detector (EPD); the magnetometer (MAG); the Radio and Plasma Wave experiment (RPW); and the Solar Wind Analyser (SWA). Following commissioning, all instruments are operating well and taking excellent data. We give a brief overview of the in situ measurements through Orbiter's early operations, including the first perihelion at 0.52 AU. These first data reveal a rich range of phenomena including: solar energetic particle events at 0.6 AU; a coronal mass ejection measured at 0.8 AU upstream of the Earth; the first heavy ion measurements from the inner heliosphere; dust and waves throughout the inner heliosphere, including from comet ATLAS; and switchbacks from polar coronal holes at 0.5 AU. We discuss the operational strategies of the in situ payload, the plans for coordination with other missions, and the prospects for science during the coming years.
• A deep insight into the ion foreshock with the help of test particle two-dimensional simulations
  
  • Savoini Philippe
  • Lembège Bertrand
  Annales Geophysicae, European Geosciences Union, 2020, 38 (6), pp.1217-1235. Two-dimensional (2D) test particle simulations based on shock profiles issued from 2D full particle-in-cell (PIC) simulations are used in order to analyze the formation processes of ions back streaming within the upstream region after interacting with a quasi-perpendicular curved shock front. Two different types of simulations have been performed based on (i) a fully consistent expansion (FCE) model, which includes all self-consistent shock profiles at different times, and (ii) a homothetic expansion (HE) model in which shock profiles are fixed at certain times and artificially expanded in space. The comparison of both configurations allows one to analyze the impact of the front nonstationarity on the back-streaming population. Moreover, the role of the space charge electric field El is analyzed by either including or canceling the El component in the simulations. A detailed comparison of these last two different configurations allows one to show that this El component plays a key role in the ion reflection process within the whole quasi-perpendicular propagation range. Simulations provide evidence that the different field-aligned beam (FAB) and gyro-phase bunched (GPB) populations observed in situ are essentially formed by a Et×B drift in the velocity space involving the convective electric field Et. Simultaneously, the study emphasizes (i) the essential action of the magnetic field component on the GPB population (i.e., mirror reflection) and (ii) the leading role of the convective field Et in the FAB energy gain. In addition, the electrostatic field component El is essential for reflecting ions at high θBn angles and, in particular, at the edge of the ion foreshock around 70∘. Moreover, the HE model shows that the rate BI% of back-streaming ions is strongly dependent on the shock front profile, which varies because of the shock front nonstationarity. In particular, reflected ions appear to escape periodically from the shock front as bursts with an occurrence time period associated to the self-reformation of the shock front. (10.5194/angeo-38-1217-2020)
  DOI : 10.5194/angeo-38-1217-2020
• A Murine Model of a Burn Wound Reconstructed with an Allogeneic Skin Graft
  
  • Blaise Océane
  • Duchesne Constance
  • Banzet Sébastien
  • Rousseau A.
  • Frescaline Nadira
  Journal of visualized experiments : JoVE, JoVE, 2020 (162). (10.3791/61339)
  DOI : 10.3791/61339
• Revealing Plasma-Surface Interaction at Atmospheric Pressure: Imaging of Electric Field and Temperature inside the Targeted Material
  
  • Slikboer Elmar
  • Acharya Kishor
  • Sobota Ana
  • Garcia-Caurel Enric
  • Guaitella Olivier
  Scientific Reports, Nature Publishing Group, 2020, 10, pp.2712. The plasma-surface interaction is studied for a low temperature helium plasma jet generated at atmospheric pressure using Mueller polarimetry on an electro-optic target. The influence of the AC kHz operating frequency is examined by simultaneously obtaining images of the induced electric field and temperature of the target. The technique offers high sensitivity in the determination of the temperature variation on the level of single degrees. Simultaneously, the evolution of the electric field in the target caused by plasma-driven charge accumulation can be measured with the threshold of the order of 10<SUP>5</SUP> V/m. Even though a specific electro-optic crystal is used to obtain the results, they are generally applicable to dielectric targets under exposure of a plasma jet when they are of 0.5 mm thickness, have a dielectric constant greater than 4 and are at floating potential. Other techniques to examine the induced electric field in a target do not exist to the best of our knowledge, making this technique unique and necessary. The influence of the AC kHz operating frequency is important because many plasma jet designs used throughout the world operate at different frequency which changes the time between the ionization waves and hence the leftover species densities and stability of the plasma. Results for our jet show a linear operating regime between 20 and 50 kHz where the ionization waves are stable and the temperature increases linearly by 25 K. The charge deposition and induced electric fields do not increase significantly but the surface area does increase due to an extended surface propagation. Additionally, temperature mapping using a 100 ?m GaAs probe of the plasma plume area has revealed a mild heat exchange causing a heating of several degrees of the helium core while the surrounding air slightly cools. This peculiarity is also observed without plasma in the gas plume. (10.1038/s41598-020-59345-0)
  DOI : 10.1038/s41598-020-59345-0
• Comparative Analysis of the Vlasiator Simulations and MMS Observations of Multiple X-Line Reconnection and Flux Transfer Events
  
  • Akhavan-Tafti M.
  • Palmroth M.
  • Slavin James A.
  • Battarbee M.
  • Ganse U.
  • Grandin M.
  • Le G.
  • Gershman D. J.
  • Eastwood Jonathan P.
  • Stawarz J. E.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2020, 125, pp.e27410. The Vlasiator hybrid-Vlasov code was developed to investigate global magnetospheric dynamics at ion-kinetic scales. Here we focus on the role of magnetic reconnection in the formation and evolution of magnetic islands at the low-latitude magnetopause, under southward interplanetary magnetic field conditions. The simulation results indicate that (1) the magnetic reconnection ion kinetics, including the Earthward pointing Larmor electric field on the magnetospheric side of an X-point and anisotropic ion distributions, are well-captured by Vlasiator, thus enabling the study of reconnection-driven magnetic island evolution processes, (2) magnetic islands evolve due to continuous reconnection at adjacent X-points, "coalescence" which refers to the merging of neighboring islands to create a larger island, "erosion" during which an island loses magnetic flux due to reconnection, and "division" which involves the splitting of an island into smaller islands, and (3) continuous reconnection at adjacent X-points is the dominant source of magnetic flux and plasma to the outer layers of magnetic islands resulting in cross-sectional growth rates up to + 0.3 R<SUB>E</SUB><SUP>2</SUP>/min. The simulation results are compared to the Magnetospheric Multiscale (MMS) measurements of a chain of ion-scale flux transfer events (FTEs) sandwiched between two dominant X-lines. The MMS measurements similarly reveal (1) anisotropic ion populations and (2) normalized reconnection rate ~0.18, in agreement with theory and the Vlasiator predictions. Based on the simulation results and the MMS measurements, it is estimated that the observed ion-scale FTEs may grow Earth-sized within ~10 min, which is comparable to the average transport time for FTEs formed in the subsolar region to the high-latitude magnetopause. Future simulations shall revisit reconnection-driven island evolution processes with improved spatial resolutions. (10.1029/2019JA027410)
  DOI : 10.1029/2019JA027410
• On the growth of the thermally modified non-resonant streaming instability
  
  • Marret A.
  • Ciardi A.
  • Smets R.
  • Fuchs J.
  Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2020, 500 (2), pp.2302-2315. The cosmic rays non-resonant streaming instability is believed to be the source of substantial magnetic field amplification. In this work, we investigate the effects of the ambient plasma temperature on the instability and derive analytical expressions of its growth rate in the hot, demagnetized regime of interaction. To study its non-linear evolution, we perform hybrid-PIC simulations for a wide range of temperatures. We find that in the cold limit, about two-thirds of the cosmic rays drift kinetic energy is converted into magnetic energy. Increasing the temperature of the ambient plasma can substantially reduce the growth rate and the magnitude of the saturated magnetic field. (10.1093/mnras/staa3465)
  DOI : 10.1093/mnras/staa3465
• DC/LF electric field and spacecraft potential measurements in the solar wind by RPW/BIAS on Solar Orbiter
  
  • Khotyaintsev Y. V.
  • Vaivads A.
  • Graham D.
  • Edberg N. J. T.
  • Johansson Erik P. G.
  • Eriksson A. I.
  • Maksimovic M.
  • Bale S. D.
  • Chust T.
  • Krasnoselskikh V.
  • Kretzschmar Matthieu
  • Lorfevre E.
  • Plettemeier D.
  • Soucek J.
  • Steller M.
  • Travnicek P.
  • Vecchio A.
  • Horbury T. S.
  • O'Brien H.
  • Angelini V.
  • Evans V.
  • Owen C. J.
  • Louarn P.
  • Fedorov A.
  , 2020, 2020, pp.18 pp.. The BIAS subsystem is a part of the Radio and Plasma Waves (RPW) instrument on the ESA Solar Orbiter mission. It allows sending bias current to each of the three RPW antennas. By setting the appropriate bias current the antenna potential can be shifted closer to the local plasma potential. This allows us to measure the floating potential of the spacecraft, as well as the electric field in the DC/LF frequency range with higher accuracy and lower noise level. Here we present the first results on RPW/BIAS in-flight performance based on the operations during the instrument commissioning and first months of science operations.
• The Radio and Plasma Waves (RPW) Instrument on Solar Orbiter: First results.
  
  • Maksimovic M.
  • Soucek J.
  • Bale S. D.
  • Bonnin X.
  • Chust T.
  • Khotyaintsev Y.
  • Kretzschmar Matthieu
  • Lorfevre E.
  • Plettemeier D.
  • Steller M.
  • Stverak S.
  • Vecchio A.
  • Vaivads A.
  • Krasnoselskikh V.
  • Krupar V.
  • Alexandrova O.
  • Travnicek P.
  • Rucker H. O.
  • Angelini V.
  • Evans V.
  • Fedorov A.
  • Horbury T. S.
  • Louarn P.
  • O'Brien H.
  • Owen C. J.
  • Rodriguez-Pacheco J.
  • Wimmer-Schweingruber R. F.
  • Zouganelis Y.
  , 2020, 2020. We will review the first observations and results obtained by the Radio and Plasma Waves (RPW) Instrument on the recently launched Solar Orbiter mission. RPW is designed to measure in-situ magnetic and electric fields and waves from 'DC' to a few hundreds of kHz. RPW is also capable of measuring solar radio emissions up to 16 MHz and link them to solar flares observed by the onboard remote sensing instruments. The first results concern a wide range of phenomena including: low frequency Doppler shifted ion-acoustic waves, Whistler Waves, dust impacts, Langmuir waves, shock crossings, Type III bursts, including events observed simultaneously by the Solar Orbiter Energetic Particle Detector (EPD).