Publications

2020

• Italian Solar Orbiter-SWA Working Group on "Multiscale Physics
  
  • d'Amicis R.
  • Alberti T.
  • Bruno R.
  • Califano F.
  • Carnevale G.
  • Catapano F.
  • Cerri S. S.
  • Coco I.
  • del Zanna L.
  • de Marco R.
  • Di Matteo S.
  • Franci L.
  • Greco A.
  • Jagarlamudi V. K.
  • Landi S.
  • Lepreti F.
  • Malara F.
  • Marcucci M. F.
  • Marino R.
  • Matteini L.
  • Nieves-Chinchilla T.
  • Nigro G.
  • Nisticò G.
  • Papini E.
  • Pecora F.
  • Perri S.
  • Pezzi O.
  • Perrone D.
  • Primavera L.
  • Qamili E.
  • Retino A.
  • Servidio S.
  • Sorriso-Valvo L.
  • Innocenti M. E.
  • Telloni D.
  • Tenerani A.
  • Trenchi L.
  • Valentini F.
  • Velli M. C. M.
  • Veltri P.
  • Verdini A.
  • Villante U.
  • Zimbardo G.
  , 2020, 2020. Despite more than a half-century of study, the basic physical processes responsible for heating and accelerating the solar wind are still not fully understood. These phenomena are at the center of a hot debate that is of great interest for the Solar Orbiter mission (as discussed in details in the Science Activity Plan, SAP) and are strictly linked to the turbulent nature of solar wind fluctuations which cover an extended range of spatial and temporal scales. So the identification of these physical processes is of primary importance for understanding the origins and evolution of the solar wind and its impact on the different bodies of the solar system. Moreover, in a broader context, it would allow also to achieve significant progress in our understanding of stellar astrophysics. <P />Within this context, the Italian Solar Orbiter-SWA Working Group (WGs) on `Multiscale Physics' was created in response to the interest manifested by scientists from several Italian and international institutions on some important topics such as radial evolution of turbulence and Alfvénicity and link between fluid and kinetic scales; solar wind origin; reconnection, intermittency and particle acceleration in the turbulent solar wind (just to cite some of them), with a particular attention to the synergies with other in-situ and remote sensing instruments on board Solar Orbiter and also with other ESA and NASA missions (e.g. L1 observatories and Parker Solar Probe). The `Multiscale Physics' WG involves scientists with an extensive experience in solar wind turbulence and reconnection processes including expertise in data analysis, simulations and modeling. <P />In this contribution, we present the activity developed so far with a particular focus on the scientific cases identified.
• Italian SWA-Solar Orbiter Working Group on "Kinetic Processes
  
  • Valentini F.
  • Califano F.
  • Camporeale E.
  • Carbone V.
  • Cerri S. S.
  • d'Amicis R.
  • del Sarto D.
  • Franci L.
  • Innocenti M. E.
  • Landi S.
  • Lepreti F.
  • Malara F.
  • Matteini L.
  • Nigro G.
  • Papini E.
  • Pegoraro F.
  • Perri S.
  • Pezzi O.
  • Pucci F.
  • Retino A.
  • Servidio S.
  • Settino A.
  • Sorriso-Valvo L.
  • Telloni D.
  • Vecchio A.
  • Veltri P.
  • Verdini A.
  • Perrone D.
  • Bruno R.
  • de Marco R.
  • Pecora F.
  • Trotta D.
  • Lapenta G.
  • Marcucci M. F.
  • Jagarlamudi V. K.
  • Carnevale G.
  , 2020, 2020. Since inter-particle collisions are to a large extent negligible, the solar wind plasma is typically observed in a state far from thermodynamic equilibrium, meaning that the velocity distributions of particles of different species display significant deviations from a single Maxwellian. These deviations retain the whole history of the interaction of particles with the turbulent electromagnetic fields. Simultaneous measurements of fields and particle velocity distributions at kinetic scales made at different radial distances can help in tracking this history backwards allowing one to identify the physical mechanisms at play to heat and accelerate particles in the interplanetary medium close to the Sun and all along solar wind expansion. <P />In this scenario, data from the Solar Wind Analyzer instrument onboard the ESA Solar Orbiter mission are very promising in addressing the kinetic physics of protons, ions and electrons in the solar wind close to the Sun. The Italian SWA-Solar Orbiter Working Group entitled "Kinetic Processes" has been formed in May 2020 and gathers scientists from several Italian and international institutions and with different expertises (theoretical modeling, observational data analysis and numerical simulations), who convene periodically to discuss these scientific subjects. <P />The main scientific goals of this working group are (i) to maximize the scientific return of Solar Orbiter data within the Heliospheric communities and (ii) to identify open scientific questions and relevant physical problems for which Solar Orbiter data could play a key role and help scientists to find answers and explanations. <P />Exploitation of combined measurements from the in-situ instruments (SWA, MAG, EPD and RPW) onboard Solar Orbiter will provide the unique opportunity of attacking fundamental plasma physics processes responsible for energy exchanges between fluctuations and particles. <P />In this Abstract, we will present some of the activities carried on within the Italian SWA-Solar Orbiter WG "Kinetic Processes" in the last few months, with the aim of engaging the broader scientific community into the discussion and in preparation of the release of the first Orbiter data.
• Performances and First Results from the RPW/Search Coil Magnetometer onboard Solar Orbiter
  
  • Kretzschmar Matthieu
  • Krasnoselskikh V.
  • Dudok de Wit Thierry
  • Froment C.
  • Jean-Yves B.
  • Jannet G.
  • Le Contel O.
  • Maksimovic M.
  • Chust T.
  • Soucek J.
  • Vecchio A.
  • Bale S. D.
  • Khotyaintsev Y.
  • Lorfevre E.
  • Plettemeier D.
  • Steller M.
  • Stverak S.
  • Travnicek P.
  • Vaivads A.
  , 2020, 2020, pp.18 pp.. The Search Coil Magnetometer (SCM) onboard Solar Orbiter is part of the Radio and Plasma Waves (RPW) experiment. It measures magnetic field fluctuations in the frequency range from a few Hz to 50 kHz on three axes and between 1 kHz and 1MHz in one axis. RPW has been working nearly continuously and SCM has recorded many interesting features, including whistler and other types of waves as well as local characteristics of turbulence. We will provide an overview of these observations as well as a description of the in flight performances of SCM.
• Cancer-associated fibroblasts in cholangiocarcinoma
  
  • Vaquero Javier
  • Aoudjehane Lynda
  • Fouassier Laura
  Current Opinion in Gastroenterology, Lippincott, Williams & Wilkins, 2020, 36 (2), pp.63-69. Purpose of review: To give a state-of-art knowledge regarding cancer-associated fibroblasts (CAF) in cholangiocarcinoma (CCA) based both on direct evidence and studies on other desmoplastic cancers. High contingency of CAF characterizes CCA, a tumor with a biliary epithelial phenotype that can emerge anywhere in the biliary tree. Current treatments are very limited, the surgical resection being the only effective treatment but restricted to a minority of patients, whereas the remaining patients undergo palliative chemotherapy regimens. In cancer, CAF shape the tumor microenvironment, drive cancer growth and progression, and contribute to drug resistance. All these functions are accomplished through an interplay network between CAF and surrounding cells including tumor and other stromal cells, i.e. immune and endothelial cells. Recent findings: Several studies have pointed out the existence of CAF sub-populations carrying out several and opposite functions, cancer-promoting or cancer-restraining as shown in pancreatic cancer, another prototypic desmoplastic tumor in which heterogeneity of CAF is well demonstrated. Summary: New CAF functions are now emerging in pancreatic and breast cancers like the modulation of immune responses or tumor metabolism, opening new area for treatments. (10.1097/MOG.0000000000000609)
  DOI : 10.1097/MOG.0000000000000609
• Interaction dust – plasma in Titan's ionosphere: An experimental simulation of aerosols erosion
  
  • Chatain Audrey
  • Carrasco Nathalie
  • Ruscassier N.
  • Gautier Thomas
  • Vettier Ludovic
  • Guaitella Olivier
  Icarus, Elsevier, 2020, 345, pp.113741. Organic aerosols accumulated in Titan's orange haze start forming in its ionosphere. This upper part of the atmosphere is highly reactive and complex ion chemistry takes place at altitudes from 1200 to 900 km. The ionosphere is a nitrogen plasma with a few percent of methane and hydrogen. Carbon from methane enables the formation of macromolecules with long organic chains, finally leading to the organic aerosols. On the other hand, we suspect that hydrogen and the protonated ions have a different erosive effect on the aerosols.Here we experimentally studied the effect of hydrogen and protonated species on organic aerosols. Analogues of Titan's aerosols were formed in a radiofrequency capacitively coupled plasma (RF CCP) discharge in 95% N2 and 5% CH4. Thereafter, the aerosols were exposed to a DC plasma in 99% N2 and 1% H2. Samples were analysed by scanning electron microscopy and in situ infrared transmission spectroscopy. Two pellet techniques – KBr pressed pellets and thin metallic grids - were compared to confirm that modifications seen are not due to the material used to make the pellet.We observed that the spherical aerosols of ~500 nm in diameter were eroded under N2-H2 plasma exposure, with the formation of holes of ~10 nm at their surface. Aerosols were globally removed from the pellet by the plasma. IR spectra showed a faster disappearance of isonitriles and/or carbo-diimides compared to the global band of nitriles. The opposite effect was seen with β-unsaturated nitriles and/or cyanamides. Double bonds as CC and CN were more affected than amines and CH bonds. NH and CH absorption bands kept a similar ratio in intensity and their shape did not vary.Therefore, it seems that carbon and hydrogen play opposite roles in Titan's ionosphere: the carbon from methane leads to organic growth while hydrogen and protonated species erode the aerosols and react preferentially with unsaturated chemical functions. (10.1016/j.icarus.2020.113741)
  DOI : 10.1016/j.icarus.2020.113741
• Interaction of an atmospheric pressure plasma jet with grounded and floating metallic targets: simulations and experiments
  
  • Viegas Pedro
  • Hofmans Marlous
  • van Rooij Olivier
  • Obrusník Adam
  • L M Klarenaar Bart
  • Bonaventura Zdenek
  • Guaitella Olivier
  • Sobota Ana
  • Bourdon Anne
  Plasma Sources Science and Technology, IOP Publishing, 2020, 29 (9), pp.095011. The interaction of kHz μs-pulsed atmospheric pressure He jets with metallic targets is studied through simulations and experiments, focusing on the differences between floating and grounded targets. It is shown that the electric potential of the floating target is close to grounded in the instants after the impact of the discharge, but rises to a high voltage, potentially more than half of the applied voltage, at the end of the 1 μs pulse. As a result, a return stroke takes place after the discharge impact with both grounded and floating targets, as a redistribution between the high voltage electrode and the low voltage target. Electric field, electron temperature and electron density in the plasma plume are higher during the pulse with grounded target than with floating target, as gradients of electric potential progressively dissipate in the latter case. Finally, at the fall of the pulse, another electrical redistribution takes place, with higher intensity with the highly-charged floating target than with the grounded target. It is shown that this phenomenon can lead to an increase in electric field, electron temperature and electron density in the plume with floating target. (10.1088/1361-6595/aba7ec)
  DOI : 10.1088/1361-6595/aba7ec
• An Active Plume Eruption on Europa During Galileo Flyby E26 as Indicated by Energetic Proton Depletions
  
  • Huybrighs H. L. F.
  • Roussos E.
  • Blöcker A.
  • Krupp N.
  • Futaana Y.
  • Barabash S.
  • Hadid Lina
  • Holmberg M. K. G.
  • Lomax O.
  • Witasse O.
  Geophysical Research Letters, American Geophysical Union, 2020, 47 (10), pp.e2020GL087806. Strong depletions of energetic protons (115–244 keV) were observed during Galileo flyby E26 of Europa. We simulate the flux of energetic protons using a Monte Carlo particle backtracing code and show that energetic proton depletions during E26 are reproduced by taking into account the perturbations of the electromagnetic fields calculated by magnetohydrodynamic (MHD) simulations and charge exchange with a global atmosphere and plume. A depletion feature occurring shortly after closest approach is driven by plume associated charge exchange, or a combination with plume associated field perturbations. We therefore conclude, with a new method and independent data set, that Galileo could have encountered a plume during E26. (10.1029/2020GL087806)
  DOI : 10.1029/2020GL087806
• Corrigendum: Non-isothermal sheath model for low pressure plasmas (2019 Plasma Sources Sci. Technol. 28 075007)
  
  • Tavant Antoine
  • Lucken Romain
  • Bourdon Anne
  • Chabert Pascal
  Plasma Sources Science and Technology, IOP Publishing, 2020, 29, pp.039501. (10.1088/1361-6595/ab74b7)
  DOI : 10.1088/1361-6595/ab74b7
• The RPW Time Domain Sampler (TDS) on Solar Orbiter: In-flight performance and first data
  
  • Soucek Jan
  • Uhlir Ludek
  • Lan Radek
  • Pisa David
  • Kolmasova Ivana
  • Santolik Ondrej
  • Kruparova Oksana
  • Maksimovic Milan
  • Kretzschmar Matthieu
  • Khotyaintsev Yuri
  • Chust Thomas
  , 2020. The Radio and Plasma Wave instrument (RPW) for Solar Orbiter includes a Time Domain Sampler sub-unit (TDS) designed to capture electromagnetic waveform measurements of high-frequency plasma waves and antenna voltage spikes associated with dust impacts. TDS will digitize three components of the electric field and one magnetic component at 524 kHz sampling rate and scan the obtained signal for plasma waves and dust impact signatures. The main science target of TDS are Langmuir waves observed in the solar wind in association with Type II and Type III solar bursts, interplanetary shocks, magnetic holes, and other phenomena. In this poster, we present the scientific data products provided by the TDS instrument and discuss the first data obtained during the commissioning phase. The first data will be used to evaluate the actual performance of the RPW TDS instrument. (10.5194/egusphere-egu2020-18888)
  DOI : 10.5194/egusphere-egu2020-18888
• Numerical Study of Jet-Target Interaction: Influence of Dielectric Permittivity on the Electric Field Experienced by the Target
  
  • Viegas Pedro
  • Bourdon Anne
  Plasma Chemistry and Plasma Processing, Springer Verlag, 2020, 40, pp.661–683. This work presents a study of the influence of dielectric permittivity on the interaction between a positive pulsed He plasma jet and a 0.5 mm-thick dielectric target, using a validated two-dimensional numerical model. Six different targets are studied: five targets at floating potential with relative permittivities &#1013;r= 1, 4, 20, 56 and 80; and one grounded target of permittivity &#1013;r=56. The temporal evolution of the charging of the target and of the electric field inside the target are described, during the pulse of applied voltage and after its fall. It is found that the order of magnitude of the electric field inside the dielectric targets is the same for all floating targets with &#1013;r&#8805;4. For all these targets, during the pulse of applied voltage, the electric field perpendicular to the target and averaged through the target thickness, at the point of discharge impact, is between 1 and 5 kV cm&#8722;1. For the two remaining targets (&#1013;r=1 and grounded target with &#1013;r=56), the field is significantly higher than for all the other floating targets. (10.1007/s11090-019-10033-6)
  DOI : 10.1007/s11090-019-10033-6
• Seed-packed dielectric barrier device for plasma agriculture: Understanding its electrical properties through an equivalent electrical model
  
  • Judée Florian
  • Dufour Thierry
  Journal of Applied Physics, American Institute of Physics, 2020, 128 (4), pp.044901. Seeds have been packed in a dielectric barrier device where cold atmospheric plasma has been generated to improve their germinative properties. A special attention has been paid on understanding the resulting plasma electrical properties through an equivalent electrical model whose experimental validity has been demonstrated here. In this model, the interelectrode gap is subdivided into 4 types of elementary domains, according to whether they contain electric charges (or not) and according to their type of medium (gas, seed or insulator). The model enables to study the influence of seeds on the plasma electrical properties by measuring and deducing several parameters (charge per filament, gas capacitance, plasma power, …) either in no-bed configuration (i.e. no seed in the reactor) or in packed-bed configuration (seeds in the reactor). In that second case, we have investigated how seeds can influence the plasma electrical parameters considering six specimens of seeds (beans, radishes, corianders, lentils, sunflowers and corns). The influence of molecular oxygen (0-100 sccm) mixed with a continuous flow rate of helium (2 slm) is also investigated, especially through filaments breakdown voltages, charge per filament and plasma power. It is demonstrated that such bed-packing drives to an increase in the gas capacitance (ξOFF), to a decrease in the β-parameter and to variations of the filaments' breakdown voltages in a seed-dependent manner. Finally, we show how the equivalent electrical model can be used to assess the total volume of the contact points, the capacitance of the seeds in the packed-bed configuration and we demonstrate that germinative effects can be induced by plasma on four of the six agronomical specimens. (10.1063/1.5139889)
  DOI : 10.1063/1.5139889
• The 2020 plasma catalysis roadmap
  
  • Bogaerts Annemie
  • Tu Xin
  • Whitehead J Christopher
  • Centi Gabriele
  • Lefferts Leon
  • Guaitella Olivier
  • Azzolina-Jury Federico
  • Kim Hyun-Ha
  • Murphy Anthony
  • Schneider William
  • Nozaki Tomohiro
  • Hicks Jason
  • Rousseau Antoine
  • Thevenet Frederic
  • Khacef Ahmed
  • Carreon Maria
  Journal of Physics D: Applied Physics, IOP Publishing, 2020, 53 (44), pp.443001. Plasma catalysis is gaining increasing interest for various gas conversion applications, such as CO2 conversion into value-added chemicals and fuels, CH4 activation into hydrogen, higher hydrocarbons or oxygenates, and NH3 synthesis. Other applications are already more established, such as for air pollution control, e.g. volatile organic compound remediation, particulate matter and NOx removal. In addition, plasma is also very promising for catalyst synthesis and treatment. Plasma catalysis clearly has benefits over 'conventional' catalysis, as outlined in the Introduction. However, a better insight into the underlying physical and chemical processes is crucial. This can be obtained by experiments applying diagnostics, studying both the chemical processes at the catalyst surface and the physicochemical mechanisms of plasma-catalyst interactions, as well as by computer modeling. The key challenge is to design cost-effective, highly active and stable catalysts tailored to the plasma environment. Therefore, insight from thermal catalysis as well as electro- and photocatalysis is crucial. All these aspects are covered in this Roadmap paper, written by specialists in their field, presenting the state-of-the-art, the current and future challenges, as well as the advances in science and technology needed to meet these challenges. (10.1088/1361-6463/ab9048)
  DOI : 10.1088/1361-6463/ab9048
• Target Design for XUV Probing of Radiative Shock Experiments
  
  • Chaulagain U.
  • Stehlé C.
  • Barroso P.
  • Kozlova M.
  • Nejdl J.
  • Suzuki Vidal F.
  • Larour Jean
  Journal of Nepal Physical Society, Nepal Physical Society, 2020, 6 (1), pp.30-41. Radiative shocks are strong shocks characterized by plasma at a high temperature emitting an important fraction of its energy as radiation. Radiative shocks are commonly found in many astrophysical systems and are templates of radiative hydrodynamic flows, which can be studied experimentally using high-power lasers. This is not only important in the context of laboratory astrophysics but also to benchmark numerical studies. We present details on the design of experiments on radiative shocks in xenon gas performed at the kJ scale PALS laser facility. It includes technical specifications for the tube targets design and numerical studies with the 1-D radiative hydrodynamics code MULTI. Emphasis is given to the technical feasibility of an XUV imaging diagnostic with a 21 nm (~58 eV) probing beam, which allows to probe simultaneously the post-shock and the precursor region ahead of the shock. The novel design of the target together with the improved X-ray optics and XUV source allow to show both the dense post-shock structure and the precursor of the radiative shock. (10.3126/jnphyssoc.v6i1.30514)
  DOI : 10.3126/jnphyssoc.v6i1.30514
• Analysis of Turbulence Properties in the Mercury Plasma Environment Using MESSENGER Observations
  
  • Huang S.
  • Wang Q. A.
  • Sahraoui F.
  • Yuan Z.
  • Liu Y.
  • Deng X.
  • Sun W.
  • Jiang K.
  • Xu S.
  • Yu X.
  • Wei Y.
  • Zhang J.
  The Astrophysical Journal, American Astronomical Society, 2020, 891 (2), pp.159. Turbulence is ubiquitous in space and astrophysical plasmas, such as the solar wind, planetary magnetospheres, and the interstellar medium. It plays a key role in converting electric and magnetic energies into kinetic energy of the plasma particles. Here, the properties of MHD and kinetic-scale magnetic fluctuations in the Mercury environment are investigated using data collected by the MESSENGER spacecraft from 2011 March 23 to 2015 April 28. It is found that spectral indices at MHD scales vary from ∼−5/3 in the near-Planet solar wind (possibly the foreshock) to ∼−1.3 within the magnetosheath close to bow shock. The spectra steepen further in the magnetosheath close to magnetopause, and reach ∼−2.2 within the magnetosphere. Only 15% of events were found to have the Kolmogorov scaling ∼−5/3 in the magnetosheath. The high variability of the spectral indices implies that the scaling of turbulent fluctuations in the magnetosheath is not universal, and it emphasizes the role of the bow shock on the turbulence dynamics, at least at the largest scales. Analysis of the magnetic compressibility shows that only ∼30% of events with Kolmogorov inertial range in the magnetosheath are dominated by (shear) Alfvénic fluctuations, which contrasts with well-known features of solar wind turbulence. At kinetic scales, the steepest spectra (slopes ∼−2.8) occur in the solar wind, before flattening to ∼−2 near the bow shock, then steepening again to ∼−2.8 in the magnetosheath. The spectral indices at kinetic scales are close to the ones at large scales in the magnetosphere, which may be caused by the presence of heavy ions in the latter. The statistical results are compared with previous observations reported in other planetary plasma environments. (10.3847/1538-4357/ab7349)
  DOI : 10.3847/1538-4357/ab7349
• MMS observations of intense whistler waves within Earth's supercritical bow shock: Source mechanism and impact on shock structure and plasma transport
  
  • Hull A.
  • Muschietti Laurent
  • Le Contel O.
  • Dorelli J.
  • Lindqvist P.-E.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2020, 125 (7), pp.e2019JA027290. The properties of whistler waves near lower‐hybrid frequencies within Earth's quasiperpendicular bow shock are examined using data from the Magnetospheric Multiscale (MMS) mission. These waves appear as right‐hand polarized wave packets propagating upstream obliquely to the magnetic field and shock normal with phase speeds from a few hundred up to 1600 km/s. The wavelengths are near the ion inertial length scale (λ~0.3‐1.3 λi). Detailed analysis finds characteristics consistent with the modified two‐stream instability mechanism driven by the reflected ion and electron drift. Correlations between wave and electron anisotropy variations reveal that the whistlers are affecting electron dynamics and thus their perpendicular and parallel temperatures. The electron signatures are explainable via the interaction of magnetized electrons in the whistler induced non‐monotonic magnetic fields. These waves have intense magnetic fields (δB/Bo~0.1‐1.) and carry sizable currents that are a significant fraction of the thermal current (|J/Jvte|~0.1‐0.5). The whistler induced currents and the electron anisotropies are sufficiently large to respectively excite high‐frequency (HF) electrostatic (> 100 Hz) and HF whistler waves (f~0.1‐0.5 fce). Energy dissipation J•E from whistlers at 30 Hz and below range from a few thousandths to few hundredths of μW/m3. Comparisons reveal that plasma energy is converted to wave energy in the foot, whereas wave energy gets dissipated into the plasma in the ramp, where irreversible heating occurs. These observed features are indicative of an intricate coupling between small‐scale interaction processes and larger‐scale structure transpiring within the layer. Such a characterization is only made possible now with the MMS high‐time‐resolution measurements. (10.1029/2019JA027290)
  DOI : 10.1029/2019JA027290
• 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
• Electron Heating by Debye-Scale Turbulence in Guide-Field Reconnection
  
  • Khotyaintsev Yu. v.
  • Graham D. b.
  • Steinvall K.
  • Alm L.
  • Vaivads A.
  • Johlander A.
  • Norgren C.
  • Li W.
  • Divin A.
  • Fu H. s.
  • Hwang K.-J.
  • Burch J. l.
  • Ahmadi N.
  • Le Contel O.
  • Gershman D. j.
  • Russell C. t.
  • Torbert R. b.
  Physical Review Letters, American Physical Society, 2020, 124 (4). (10.1103/PhysRevLett.124.045101)
  DOI : 10.1103/PhysRevLett.124.045101
• Turbulence as a Network of Fourier Modes
  
  • Gürcan Özgür D.
  • Li Yang
  • Morel Pierre
  Mathematics, MDPI, 2020, 8 (4), pp.530. Turbulence is the duality of chaotic dynamics and hierarchical organization of a field over a large range of scales due to advective nonlinearities. Quadratic nonlinearities (e.g., advection) in real space, translates into triadic interactions in Fourier space. Those interactions can be computed using fast Fourier transforms, or other methods of computing convolution integrals. However, more generally, they can be interpreted as a network of interacting nodes, where each interaction is between a node and a pair. In this formulation, each node interacts with a list of pairs that satisfy the triadic interaction condition with that node, and the convolution becomes a sum over this list. A regular wavenumber space mesh can be written in the form of such a network. Reducing the resolution of a regular mesh and combining the nearby nodes in order to obtain the reduced network corresponding to the low resolution mesh, we can deduce the reduction rules for such a network. This perspective allows us to develop network models as approximations of various types of turbulent dynamics. Various examples, such as shell models, nested polyhedra models, or predator-prey models, are briefly discussed. A prescription for setting up a small world variants of these models are given. (10.3390/math8040530)
  DOI : 10.3390/math8040530
• 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
• 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.
• The study of low-frequency waves in the solar wind by the RPW/TDS instrument onboard Solar Orbiter
  
  • Pisa D.
  • Soucek J.
  • Santolik O.
  • Maksimovic M.
  • Bale S. D.
  • Chust T.
  • Khotyaintsev Y.
  • Krasnoselskikh V.
  • Kretzschmar Matthieu
  • Lorfevre E.
  • Plettemeier D.
  • Steller M.
  • Štverák Š.
  • Travnicek P.
  • Vaivads A.
  • Vecchio A.
  • Horbury T. S.
  • Angelini V.
  • O'Brien H.
  • Evans V.
  , 2020, 2020, pp.18 pp.. The Time Domain Sampler (TDS) receiver is a part of the Radio and Plasma Wave instrument (RPW) onboard Solar Orbiter, sampling three electric and one magnetic field components at frequencies up to 524 kHz. The RPW instrument has been operating almost continuously during the commissioning phase of the mission and through the first perihelion in June 2020, covering an interval of heliocentric distances between 0.5 AU to 1 AU. The most common plasma phenomena observed by TDS are low-frequency electrostatic waves at frequencies between hundreds of Hertz and 10 kHz. These are interpreted as strongly Doppler-shifted ion-acoustic waves, generated by solar wind ion beams and often accompanying large scale solar wind structures. In this initial study, we performed a statistical investigation of the waves, studying their modulation and polarization.
• 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
• 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
• On the deviation from Maxwellian of the ion velocity distribution functions in the turbulent magnetosheath
  
  • Perri S.
  • Perrone D.
  • Yordanova E.
  • Sorriso-Valvo L.
  • Paterson W. R.
  • Gershman D. J.
  • Giles B. L.
  • Pollock C. J.
  • Dorelli J. C.
  • Avanov L. A.
  • Lavraud B.
  • Saito Y.
  • Nakamura R.
  • Fischer D.
  • Baumjohann W.
  • Plaschke F.
  • Narita Y.
  • Magnes W.
  • Russell C. T.
  • Strangeway R. J.
  • Contel O. Le
  • Khotyaintsev Y.
  • Valentini F.
  Journal of Plasma Physics, Cambridge University Press (CUP), 2020, 86. The deviation from thermodynamic equilibrium of the ion velocity distribution functions (VDFs), as measured by the Magnetospheric Multiscale (MMS) mission in the Earth's turbulent magnetosheath, is quantitatively investigated. Making use of the unprecedented high-resolution MMS ion data, and together with Vlasov-Maxwell simulations, this analysis aims at investigating the relationship between deviation from Maxwellian equilibrium and typical plasma parameters. Correlations of the non-Maxwellian features with plasma quantities such as electric fields, ion temperature, current density and ion vorticity are found to be similar in magnetosheath data and numerical experiments, with a poor correlation between distortions of ion VDFs and current density, evidence that questions the occurrence of VDF departure from Maxwellian at the current density peaks. Moreover, strong correlation has been observed with the magnitude of the electric field in the turbulent magnetosheath, while a certain degree of correlation has been found in the numerical simulations and during a magnetopause crossing by MMS. This work could help shed light on the influence of electrostatic waves on the distortion of the ion VDFs in space turbulent plasmas. (10.1017/S0022377820000021)
  DOI : 10.1017/S0022377820000021
• 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