Publications

2024

• Curlometer and gradient techniques: past and future applications
  
  • Dunlop M.
  • Fu H.-S.
  • Shen C.
  • Tan X.
  • Dong X.-C.
  • Yang Y.-Y.
  • Robert P.
  • Escoubet C.
  Frontiers in Astronomy and Space Sciences, Frontiers Media, 2024, 11. We review the range of applications and use of multi spacecraft techniques, applicable to close formation arrays of spacecraft, focusing on spatial gradient based methods, and the curlometer in particular. The curlometer was originally applied to Cluster multi-spacecraft magnetic field data, but later was updated for different environments and measurement constraints such as the NASA MMS mission, small-scale formation of 4 spacecraft; the 3 spacecraft configurations of the NASA THEMIS mision, and derived 2-4 point measurements from the ESA Swarm mission. In general, spatial gradient based methods are adaptable to a range of multi-point and multi-scale arrays. We also review the range of other techniques based on the computation of magnetic field gradients and magnetic field topology in general, including: magnetic rotation analysis and various least squares approaches. We review Taylor expansion methodology (FOTE), in particular, which has also been applied to both Cluster and MMS constellations, as well as interpretation of simulations. Four-point estimates of magnetic gradients are limited by uncertainties in spacecraft separations and the magnetic field, as well as the presence of non-linear gradients and temporal evolution. Nevertheless, the techniques can be reliable in many magnetospheric regions where time stationarity is largely applicable, or when properties of the morphology can be assumed (for example, the expected orientation of underlying large-scale structure). Many magnetospheric regions have been investigated directly (illustrated here by the magnetopause, ring current and field-aligned currents at high and low altitudes), and options for variable numbers of spacecraft have been considered. The comparative use of plasma measurements and possible new methodology for arrays of spacecraft greater than four are also considered briefly. (10.3389/fspas.2024.1422341)
  DOI : 10.3389/fspas.2024.1422341
• High-resolution observational analysis of flare ribbon fine structures
  
  • Thoen Faber Jonas
  • Joshi Reetika
  • van der Voort Luc Rouppe
  • Wedemeyer Sven
  • Fletcher Lyndsay
  • Aulanier Guillaume
  • Nóbrega-Siverio Daniel
  Astronomy & Astrophysics - A&A, EDP Sciences, 2024, 693, pp.A8. Context. Since the mechanism of energy release from solar flares is still not fully understood, the study of fine-scale features developing during flares becomes important for progressing towards a consistent picture of the essential physical mechanisms. Aims. Solar flares release most of their luminous energy in the chromosphere. These luminous signatures, known as flare ribbons, act as the footpoints of the released energy and are crucial for the interpretation of reconnection processes causing these events. We aim to probe the fine structures in flare ribbons at the chromospheric level using high-resolution observations with imaging and spectral techniques. Methods. We present a GOES C2.4 class solar flare (SOL2022-06-26T08:12) observed with the Swedish 1-m Solar Telescope (SST), the Interface Region Imaging Spectrograph (IRIS), and the Atmospheric Imaging Assembly (AIA). Utilising imaging data from SST, IRIS, and AIA, we detail both the global and fine-structure evolution of the flare. The high-resolution SST observations offer spectroscopic data in the H α , Ca II 8542 Å, and H β lines, which we use to analyse the flare ribbon. Results. The flare was associated with a filament eruption. Fibrils and coronal loops were connected from a negative polarity region to two positive polarity regions. Within the eastern flare ribbon, chromospheric bright blobs were detected and analysed in Ca II 8542 Å, H α , and H β wavelengths. A comparison of blobs in H β observations and Si IV 1400 Å has also been performed. These blobs are observed as almost circular structures having widths from 140 km–200 km. The intensity profiles of the blobs show a red wing asymmetry. Conclusions. From the high spatial and temporal resolution H β observations, we conclude that the periodicity of the blobs in the flare ribbon, which are near-equally spaced in the range 330–550 km, is likely due to fragmented reconnection processes within a flare current sheet. This supports the theory of a direct link between fine-structure flare ribbons and current sheet tearing. We believe our observations represent the highest resolution evidence of fine-structure flare ribbons to date. (10.1051/0004-6361/202452370)
  DOI : 10.1051/0004-6361/202452370
• Modeling, development, and test of a 3D-printed plasma camera for in-situ measurements in space
  
  • Hénaff Gwendal
  , 2024. Key phenomena governing the dynamics of space plasmas - including charged particle acceleration, magnetic reconnection and the turbulent dissipation of electromagnetic energy - are multi-scale in nature. In order to understand their role in the Sun-Earth relationship, whether in the solar wind, at the magnetopause or in the Earth's magnetosphere, it is essential to develop instrumentation that is both compact and high-performance, enabling the deployment of satellite constellations. However, the reference instruments used to measure the energy distribution of charged particles have a limited field of view. Adding electrostatic deflection systems circumvents this limitation, with the disadvantage of making these instruments heavier, slowing down their measurement rate, and therefore reducing their performance. In this case, more sensors are needed to achieve the desired performance, impacting satellite size and, ultimately, the number of satellites that can be deployed. The characterization of charged particle fluxes for studying space weather, conducted using compact instruments with a limited field of view, faces the same limitations.The first step in this research project was to develop a method for designing a new range of plasma spectrometers that overcome these limitations. These spectrometers are based on an innovative toroidal topology, offering an instantaneous hemispherical field of view that eliminates the need for electrostatic deflectors. Their planar detection system makes them true plasma cameras. The methods developed have enabled the numerical generation and characterization by simulating a wide range of plasma cameras with different angular resolutions that could meet these various scientific needs.A model instrument was then designed to meet the challenges of space weather applications, with an energy range of up to 22 keV. It features dual ion/electron detection capability, avoiding the need for separate sensors for electron and ion measurements. Intended for nanosatellites, it has a mass of 1.8 kg and a diameter of 19 cm. A 3D-printing manufacturing process and functionalization of the material have been defined and implemented. An ion/electron conversion system using carbon foils, enabling dual use of this plasma camera, has also been developed. An instrument integrating the electrostatic optics and a simplified dual detection system has been tested under an electron beam to obtain precise experimental responses in terms of energy and angle. The beam tests showed behavior very close to the simulation, reinforcing confidence in the numerical modeling. The principle of the conversion system was tested under electron and ion beams. One of the short-term prospects of this thesis is the development, with the support of CNES, of a complete model of this plasma camera, with the aim to demonstrate in orbit the performances of this instrument dedicated to space weather applications.
• Edge turbulence self-organization in fusion plasmas
  
  • Panico Olivier
  , 2024. This PhD work is a step forward in the characterisation of turbulence self-organization in edge tokamak plasmas, key player in transport and confinement. Three main results are obtained: the plasma parameter regimes prone to turbulence self-organization are identified, some of the underlying physical mechanisms at work are unravelled, and some experimental evidence of self-organization is obtained by means of Doppler Back-scattering (DBS) measurements in tokamak plasmas.Tokamaks aim at confining hot plasmas by means of large magnetic fields. The last closed flux surface separates the confined inner region from the scrape-off layer where the plasma interactàs with materials. In the tokamak operational regime, cross-field transport – hence confinement – is governed by micro-scale turbulence. Understanding the mechanisms of its saturation would open the route towards its possible control. Plasma conditions at the edge transition region are key. In this region, turbulence generates avalanche transport events, which deteriorates the confinement, and zonal flows that efficiently contribute to turbulence saturation. Understanding and predicting turbulence self-organization – i.e. the self-consistent interplay between potentially radially structured ZFs, flux-surface averaged profiles and turbulent transport – in the various parameter regimes of edge tokamak plasmas constitutes the backbone of this work.To this aim, the reduced nonlinear model Tokam1D, developed on purpose, evolves the mean profiles and the fluctuations in a self-consistent manner in the flux-driven regime. Importantly, in view of studying turbulence self-organization at mesoscales, no scale separation is assumed. The model features two instabilities thought to be dominant at the edge, namely collisional drift waves CDW, originating from a finite phase shift between density and electric potential fluctuations, and interchange due to the magnetic field inhomogeneity (curvature). Their control parameters exhibit different dependencies with respect to plasma parameters, so that different regimes can be expected in edge tokamak plasmas. The model is reduced to 1-dimension by retaining a single parallel and poloidal mode for the fluctuations, in the spirit of a generalized quasilinear approach.A large scan of the instability control parameters, both at fixed source and fixed distance-to-threshold, allows one to pave the edge plasma parameter space. Large flow-to-turbulence energy ratios are predicted at low collisionality or large magnetic curvature. The latter favours interchange turbulence characterised by avalanche-like transport events and radially-structured ZFs leading to corrugated pressure profiles known as staircases. ZFs are driven by the electric and diamagnetic components of the Reynolds stress, whose phase alignment and relative amplitude vary with the turbulence regime. The system's freedom to store energy both in the pressure profile and mean flows is shown crucial to the staircase existence. Overall, the confinement is improved in regimes featuring staircases. Experimentally measurable signatures of avalanches are found in the form of a two slope radial correlation function, the smallest slope attributed to small-scale turbulent eddies, the largest capturing the avalanche typical propagation length.Experimental studies have been carried out on the Tore Supra (CEA Cadarache) and TCV (EPFL) tokamaks using a two-channel DBS for correlations. In Tore Supra long range correlations are observed when using poloidally and toroidally separated channels. When filtering out dominating geodesic acoustic modes, signatures of the elusive low frequency ZFs are found. In TCV, measurements using two co-located channels exhibit radial correlation functions with two slopes in certain regimes similarly to simulations. This result constitutes an additional indirect proof of the existence of avalanche events in tokamak plasmas, notoriously difficult to diagnose.
• The Radio & Plasma Wave Investigation (RPWI) for the JUpiter ICy moons Explorer (JUICE)
  
  • Wahlund J.-E.
  • Bergman J.
  • Åhlén L.
  • Puccio W.
  • Cecconi B.
  • Kasaba Y.
  • Müller-Wodarg I.
  • Rothkaehl H.
  • Morawski M.
  • Santolik O.
  • Soucek J.
  • Grygorczuk J.
  • Wisniewski Ł.
  • Henri P.
  • Rauch J.
  • Le Duff O.
  • Retinò A.
  • Mansour M.
  • Stverak S.
  • Laifr J.
  • Andrews D.
  • André M.
  • Benko I.
  • Berglund M.
  • Cripps V.
  • Cully C.
  • Davidsson J.
  • Dimmock A.
  • Edberg N.
  • Eriksson A.
  • Fredriksson J.
  • Gill R.
  • Gomis S.
  • Holback B.
  • Jansson S.-E.
  • Johansson F.
  • Johansson E.
  • Khotyaintsev Y.
  • Mårtensson B.
  • Morooka M.
  • Nilsson T.
  • Ohlsson D.
  • Pelikan D.
  • Richard L.
  • Shiwa F.
  • Vigren E.
  • Wong H.
  • Bonnin X.
  • Girard J.
  • Grosset L.
  • Henry F.
  • Lamy L.
  • Lebreton J.-P.
  • Zarka P.
  • Katoh Y.
  • Kita H.
  • Kumamoto A.
  • Misawa H.
  • Tsuchiya F.
  • Galand M.
  • Barcinski T.
  • Baran J.
  • Kowalski T.
  • Szewczyk P.
  • Grison B.
  • Jansky J.
  • Kolmasova I.
  • Lan R.
  • Pisa D.
  • Taubenschuss U.
  • Uhlir L.
  • Bochra K.
  • Borys M.
  • Duda M.
  • Kucinski T.
  • Ossowski M.
  • Palma P.
  • Tokarz M.
  • Colin F.
  • Dazzi P.
  • de Léon E.
  • Hachemi T.
  • Millet A.-L.
  • Randrianboarisson O.
  • Sene O.
  • Chust T.
  • Le Contel O.
  • Canu P.
  • Hadid L.
  • Sahraoui F.
  • Zouganelis Y.
  • Alison D.
  • Ba N.
  • Jeandet A.
  • Techer J.-D.
  • Mehrez F.
  • Varizat L.
  • Sumant A.
  • Sou Gérard
  • Hellinger P.
  • Travnicek P.
  • Bylander L.
  • Giono G.
  • Ivchenko N.
  • Kullen A.
  • Roth L.
  • Vaivads A.
  • Tanimoto K.
  • Mizuno H.
  • Sawamura A.
  • Suzuki T.
  • Namiki M.
  • Fujishima S.
  • Asai K.
  • Shimoyama T.
  • Fujii M.
  • Sato Y.
  • Birch J.
  • Bakhit B.
  • Greczynski G.
  • Gare P.
  • Landström S.
  • Leletty R.
  • Ryszawa E.
  • Torralba I.
  • Trescastro J.
  • Osipenco S.
  • Wiklund U.
  • Roos A.
  • Söderström J.
  • Björneholm O.
  • Fischer G.
  • Nyberg T.
  • Kovi K.
  • Balikhin M.
  • Yearby K.
  • Holmberg M.
  • Jackman C.
  • Louis C.
  • Rhouni A.
  • Leray V.
  • Geyskens N.
  • Berthod C.
  • Lemaire B.
  • Clémencon A.
  • Wattieaux G.
  • André N.
  • Garnier P.
  • Génot V.
  • Louarn P.
  • Marchaudon A.
  • Modolo Ronan
  • Baskevitch Claire
  • Hess Sébastien
  • Leclercq Ludivine
  • Saur J.
  • Kimura T.
  • Kojima H.
  • Yagitani S.
  • Miyoshi Y.
  Space Science Reviews, Springer Verlag, 2024, 221 (1), pp.1. The Radio & Plasma Wave Investigation (RPWI) onboard the ESA JUpiter ICy moons Explorer (JUICE) is described in detail. The RPWI provides an elaborate set of state-of-the-art electromagnetic fields and cold plasma instrumentation, including active sounding with the mutual impedance and Langmuir probe sweep techniques, where several different types of sensors will sample the thermal plasma properties, including electron and ion densities, electron temperature, plasma drift speed, the near DC electric fields, and electric and magnetic signals from various types of phenomena, e.g., radio and plasma waves, electrostatic acceleration structures, induction fields etc. A full wave vector, waveform, polarization, and Poynting flux determination will be achieved. RPWI will enable characterization of the Jovian radio emissions (including goniopolarimetry) up to 45 MHz, has the capability to carry out passive radio sounding of the ionospheric densities of icy moons and employ passive sub-surface radar measurements of the icy crust of these moons. RPWI can also detect micrometeorite impacts, estimate dust charging, monitor the spacecraft potential as well as the integrated EUV flux. The sensors consist of four 10 cm diameter Langmuir probes each mounted on the tip of 3 m long booms, a triaxial search coil magnetometer and a triaxial radio antenna system both mounted on the 10.6 m long MAG boom, each with radiation resistant pre-amplifiers near the sensors. There are three receiver boards, two Digital Processing Units (DPU) and two Low Voltage Power Supply (LVPS) boards in a box within a radiation vault at the centre of the JUICE spacecraft. Together, the integrated RPWI system can carry out an ambitious planetary science investigation in and around the Galilean icy moons and the Jovian space environment. Some of the most important science objectives and instrument capabilities are described here. RPWI focuses, apart from cold plasma studies, on the understanding of how, through electrodynamic and electromagnetic coupling, the momentum and energy transfer occur with the icy Galilean moons, their surfaces and salty conductive sub-surface oceans. The RPWI instrument is planned to be operational during most of the JUICE mission, during the cruise phase, in the Jovian magnetosphere, during the icy moon flybys, and in particular Ganymede orbit, and may deliver data from the near surface during the final crash orbit. (10.1007/s11214-024-01110-0)
  DOI : 10.1007/s11214-024-01110-0
• Kinetics of transient reactive species in oxygen plasmas at intermediate pressure
  
  • Zhang Shu
  , 2024. Low-temperature plasmas are extensively used in various processing applications, such as surface cleaning, sterilization of medical devices, plasma-based water treatment, chemical vapor deposition of metal oxide films, material etching, and resist stripping in microelectronics. However, unlike plasmas at low or atmospheric pressures, plasmas at intermediate pressures have been less studied and not fully understood due to the challenges associated with diagnostics and simulations in this pressure range. In this thesis, we investigate oxygen-argon plasmas using both experimental and modeling methods to gain a deeper understanding of the dynamics of neutral and charged particles in plasmas at this pressure range. The following research was conducted:1.Investigation of DC Glow Discharges at Intermediate Pressure: This study involved measuring the density of oxygen atoms and the gas temperature using the CRDS technique. The effects of plasma stabilization over time were examined, emphasizing the importance of achieving a stable plasma state before taking measurements. The impact of introducing argon gas, and the effects of small air leaks, were carefully analyzed. Recommendations for actinometry calibration involving argon gas were provided, highlighting the importance of maintaining good vacuum conditions for optimal plasma reproducibility. The study also discussed the mechanisms behind the surface loss of oxygen atoms and introduced a novel model that better aligns with our experimental observations. Additionally, the influence of varying wall temperatures on these processes was explored.2.Experimental Study of RF (Radiofrequency) CCP (Capacitively Coupled Plasma) in O2-Ar Mixtures at Intermediate Pressure: This study focused on the effect of different power, pressure, and gas mixtures. Various electrical and neutral properties were measured, including plasma impedance (using an Octiv and Tektronix probes), electron density (using a hairpin probe), and surface ion flux (using an electrostatic probe array). Additionally, oxygen atom density, gas temperature, O atom loss rate, and ozone concentration in the afterglow were obtained using the CRDS technique. The effects of pressure, power, and gas mixtures were discussed, and the mechanisms occurring in such plasmas for both charged particles and neutrals were demonstrated.3.Modeling of RF CCP at Intermediate Pressure: The thesis also presents modeling efforts using both Particle-in-Cell (PIC) and fluid simulations. The study discusses the costs and limitations of the PIC (Particle-in-Cell) model, comparing it with the fluid model, and explains the conditions under which the fluid model becomes invalid. Ultimately, the thesis offers suggestions for choosing the appropriate modeling method for different pressure ranges and proposes a hybrid model that combines the strengths of both PIC and fluid simulations.
• Role of FLR effects in magnetopause equilibrium
  
  • Ballerini Giulio
  • Rezeau Laurence
  • Belmont Gerard
  • Califano Francesco
  Journal of Plasma Physics, Cambridge University Press (CUP), 2024, 90 (6), pp.905900612. The Earth magnetopause, when sufficiently plane and stationary at a local scale, can be considered as a "quasi-tangential" discontinuity, since the normal component of the magnetic field B n is typically very small but not zero. Contrary to observations, the "Classic Theory of Discontinuities" (CTD) predicts that rotational and compressional jumps should be mutually exclusive in the general case Bn diff 0, but allows only one exception: the tangential discontinuity provided that B n is strictly zero. Here we show that Finite Larmor Radius (FLR) effects play an important role in the quasi-tangential case, whenever the ion Larmor radius is not fully negligible with respect to the magnetopause thickness. By including FLR effects, the results suggest that a rotational discontinuity undergoes a change comparable to the change of a Shear Alfvén into a Kinetic Alfvén wave when considering linear modes. For this new kind of discontinuity, the co-existence of rotational and compressional variations at the magnetopause does no more imply that this boundary is a strict tangential discontinuity, even in 1D-like regions far from X-lines if any. This result may lead to important consequences concerning the oldest and most basic questions of magnetospheric physics: how can the magnetopause be open, where and when? The role of FLR being established theoretically, the paper then shows that it can be proved experimentally. For that, we make use of MMS data and process them with the most recent available 4 spacecraft tools. First, we present the different processing techniques that we use to estimate spatial derivatives, such as grad(B) and div(P), and the magnetopause normal direction.We point out why this normal direction must be determined with extremely high accuracy to make the conclusions unambiguous. Then, the results obtained by these techniques are presented in a detailed case study and on a statistical basis. (10.1017/S0022377824001089)
  DOI : 10.1017/S0022377824001089
• Targeting cholangiocarcinoma cells by cold piezoelectric plasmas: in vitro efficacy and cellular mechanisms
  
  • Soulier Manon
  • Lekbaby Bouchra
  • Houari Imane
  • Decauchy Henri
  • Pavy Allan
  • Coumes Alexia
  • Morichon Romain
  • Dufour Thierry
  • Fouassier Laura
  Scientific Reports, Nature Publishing Group, 2024, 14 (1), pp.30178. Cold piezoelectric plasma (CPP) is a novel approach in cancer therapy, enabling the development of portable treatment devices capable of triggering cancer cell death. While its effectiveness remains underexplored, this research focuses on its application against cholangiocarcinoma (CCA), an aggressive cancer of the biliary tract. A CPP device is utilized to generate either a corona discharge (Pz-CD) or a dielectric barrier discharge (Pz-DBD) for in vitro experiments. Notably, Pz-CD can deliver more power than Pz-DBD, although both sources produce significant levels of reactive species in plasma and liquid phases. This work shows that CPP causes a gradient increase in medium temperature from the center towards the edges of the culture well, especially for longer treatment times. Although Pz-CD heats more significantly, it cools quickly after plasma extinction. When applied to human CCA cells, CPP shows immediate and long-term effects, more localized for Pz-CD, while more uniform for Pz-DBD. Immediate effects result also in actin cytoskeleton remodeling without alteration of the cell membrane permeability. Long-term effects of CPP, although the antioxidant system is engaged, include activation of the DNA damage response pathway leading to cell death. In conclusion, CPP should be recognized as a promising antitumor therapy. (10.1038/s41598-024-81664-9)
  DOI : 10.1038/s41598-024-81664-9
• Photoelectric Feedback Mechanism for Acceleration of Runaway Electrons in Gas Discharges at High Overvoltages
  
  • Pasko Victor P
  • Celestin Sebastien
  • Bourdon Anne
  Physical Review Letters, American Physical Society, 2024, 133 (23), pp.235301. <div xmlns="http://www.tei-c.org/ns/1.0"><p>Energy conservation dictates that an electron with elementary charge e traversing a vacuum gap formed by electrodes maintained at potential difference U volts acquires maximum energy of eU. In many experiments electrons with energies as high as 3eU have been observed. The experimental discovery of this effect was made over 50 years ago and is still a subject of significant controversy in applications related to x-ray generation from high voltage discharges. Here we explain these observations by a sequential increase of energy of runaway electrons released from the cathode due to the photoelectric absorption of bremsstrahlung radiation generated by the previous cycle of runaway electrons bombarding the anode.</p></div> (10.1103/physrevlett.133.235301)
  DOI : 10.1103/physrevlett.133.235301
• Propagation of untwisting solar jets from the low-beta corona into the super-Alfvénic wind: Testing a solar origin scenario for switchbacks
  
  • Touresse J.
  • Pariat E.
  • Froment C.
  • Aslanyan V.
  • Wyper P. F.
  • Seyfritz L.
  Astronomy & Astrophysics - A&A, EDP Sciences, 2024, 692, pp.A71. Context. Parker Solar Probe’s (PSP) discovery of the prevalence of switchbacks (SBs), localised magnetic deflections in the nascent solar wind, has sparked interest in uncovering their origins. A prominent theory suggests these SBs originate in the lower corona through magnetic reconnection processes, closely linked to solar jet phenomena. Jets are impulsive phenomena, observed at various scales in different solar atmosphere layers, associated with the release of magnetic twist and helicity.Aims. This study examines whether self-consistent jets can form and propagate into the super-Alfvénic wind, assesses the impact of different Parker solar wind profiles on jet dynamics, and determines if jet-induced magnetic untwisting waves display signatures typical of SBs.Methods. We employed parametric 3D numerical magnetohydrodynamics (MHD) simulations using the Adaptively Refined Magnetohydrodynamics Solver (ARMS) code to model the self-consistent generation of solar jets. Our study focuses on the propagation of solar jets in distinct atmospheric plasma β and Alfvén velocity profiles, including a Parker solar wind. We explored the influence of different atmospheric properties thanks to analysis techniques such as radius-time diagrams and synthetic in situ velocity and magnetic field measurements, akin to those observed by PSP or Solar Orbiter.Results. Our findings demonstrate that self-consistent coronal jets can form and then propagate into the super-Alfvénic wind. Notable structures such as the leading Alfvénic wave and trailing dense-jet region were consistently observed across different plasma β atmospheres. The jet propagation dynamics are significantly influenced by atmospheric variations, with changes in Alfvén velocity profiles affecting the group velocity and propagation ratio of the leading and trailing structures. U-loops, which are prevalent at jet onset, do not persist in the low-β corona but magnetic untwisting waves associated with jets exhibit SB-like signatures. However, full-reversal SBs were not observed.Conclusions. These findings may explain the absence of full reversal SBs in the sub-Alfvénic wind and illustrate the propagation of magnetic deflections through jet-like events, shedding light on possible SB formation processes. (10.1051/0004-6361/202452019)
  DOI : 10.1051/0004-6361/202452019
• Plasma speed optimization for improved tokamak plasma confinement
  
  • Krupka Anna
  , 2024. Maximizing plasma confinement is essential to the performance of future magnetic fusion reactors. Playing with plasma speed can be a way to stabilize possible instabilities and control turbulence with a very beneficial impact on fusion yield. It is, therefore, essential to understand how a tokamak plasma can be rotated.Ideally, the tokamak should work in a stationary state as a fusion reactor. It is, therefore, reasonable to determine the steady states of a tokamak plasma in full generality without imposing the nullity of the plasma velocity field. In the visco-resistive magnetohydrodynamics (MHD) framework, this amounts in particular to retaining the non-linear term (v.grad)v in the stationary Navier-Stokes equation.Using the FreeFem++ open-source software for solving partial differential equations using the finite element method, we numerically determined the axisymmetric stationary states of a tokamak plasma in realistic JET.This thesis shows that the plasma velocity root-mean-square behaves as eta f(H) as long as the inertial term remains negligible, where H stands for the Hartmann number Hequiv (etau)^{-1/2}, and that f(H) exhibits power-law behaviours in the limits H ll 1 and H gg 1. In the latter limit, we establish that f(H) scales as H^{1/4}, which is consistent with numerical results.Additionally, this work establishes Poisson's equation governing the pressure profile. It is shown that the simplifying assumption of a toroidal current density component arising solely from Ohm's law in response to a time-independent, curl-free toroidal electric field fails to produce realistic pressure levels. To overcome this, we introduce additional non-inductive current drives, comparable to those from neutral beam injection, modeled as modifications to the toroidal current. The new model is validated using numerical simulations, showing significant pressure profile improvements. For the examples considered, the effect of these current drives on the velocity profiles is moderate except in the case where the drives induce some reversals in the total toroidal current density, producing non-nested flux surfaces with internal separatrices.Finally, the effect of fixed current density profiles is examined, revealing a new second regime, where toroidal and poloidal velocities scale with Hartmann number as H^2.
• Advancements in maize cultivation: synergistic effects of dry atmospheric plasma combined with plasma-activated water
  
  • Kamseu-Mogo Jean-Paul
  • Soulier Manon
  • Kamgang-Youbi Georges
  • Apala Mafouasson Hortense Noëlle
  • Dufour Thierry
  Journal of Physics D: Applied Physics, IOP Publishing, 2024, 58 (5), pp.055201. In this study, we investigate the effects of pre-germinative and post-germinative plasma treatments, applied separately or in combination, to improve maize germination and early seedling development. Pre-germinative treatment consists of priming the seeds with a dry atmospheric plasma (DAP) generated by a dielectric barrier device, characterized by minimal radiative emission, low electrical power (4 W) and high emissions of O, OH and NO radicals. Post-germinative treatment, known as plasma-activated water (PAW), uses a single pin electrode device (SPED) to generate a DC discharge that features a power of 126 W and produces large amounts of OH radicals. The resulting PAW, after 5 min of SPED treatment, induces a slight acidification and increased concentrations of nitrate ions (from 24 to 250 mg l −1 ), nitrite ions (from less than 0.1 to 56.1 mg l −1 ) and hydrogen peroxide (from 0.3 to 18.5 mg l −1 ). Results indicate that DAP applied on maize seeds for 20 min boosts their germination rate by up to 90% (versus only 65% for untreated seeds) while reducing the median germination time by 37.5%. Then, seedling growth monitoring is achieved on control, DAP, PAW and DAP + PAW groups to assess stem length, hypocotyl length, leaf count, collar diameter and fresh/dry mass. The DAP + PAW group shows the most robust growth, demonstrating a synergistic effect of the combined treatments, particularly with significantly longer stem lengths. Additionally, physiological analyses of seedling leaves indicate a decrease in chlorophyll content despite enhanced growth, while fluorescence microscopy reveals a reduction in stomatal density in leaves treated with DAP and PAW, especially in the combined treatment group, potentially impacting photosynthetic efficiency and water regulation. (10.1088/1361-6463/ad8acf)
  DOI : 10.1088/1361-6463/ad8acf
• High-resolution observations of recurrent jets from an arch filament system
  
  • Joshi Reetika
  • Rouppe van der Voort Luc
  • Schmieder Brigitte
  • Moreno-Insertis Fernando
  • Prasad Avijeet
  • Aulanier Guillaume
  • Nóbrega-Siverio Daniel
  Astronomy & Astrophysics - A&A, EDP Sciences, 2024, 691, pp.A198. Context. Solar jets are collimated plasma ejections along magnetic field lines observed in hot (extreme-ultraviolet (EUV) jets) and cool (chromospheric surges) temperature diagnostics. Their trigger mechanisms and the relationship between hot and cool jets are still not completely understood. Aims. We aim to investigate the generation of a sequence of active-region solar jets and their evolution from the photospheric to the coronal heights using multithermal observations from ground-based and space-borne instruments. Methods. Using the synergy of high-spatial-resolution and high-temporal-resolution observations by the Swedish 1-m Solar Telescope (SST), along with the Solar Dynamics Observatory (SDO), we analyzed a sequence of solar jets originating in a mixed-polarity region between the leading and following sunspots of an active region. We investigated the kinematics of these jets using the spectra from the SST observations. We used a non-force-free field (NFFF) extrapolation technique to derive the magnetic field topology of the active region. Results. A mixed-polarity region is formed over a long period (24 hours) with persistent magnetic flux emergence. This region has been observed as an arch filament system (AFS) in chromospheric SST observations. In this region, negative polarities surrounded by positive polarities create a fan surface with a null point at a height of 6 Mm detected in the NFFF extrapolation. SST observations in the H β spectral line reveal a large flux rope over the AFS moving from north to south, causing successive EUV and cool jets to move in the east–west direction and later towards the south along the long open loops. Conclusions. The high-resolution SST observations (0″.038 per pixel) resolve the dark area observed at the jet base and reveal the existence of an AFS with an extended cool jet, which may be the result of a peeling-like mechanism of the AFS. Based on the combined analysis of SST and AIA observations along with extrapolated magnetic topology, it is suggested that the magnetic reconnection site may move southward by approximately 20 Mm until it reaches a region where the open magnetic field lines are oriented north–south. (10.1051/0004-6361/202449715)
  DOI : 10.1051/0004-6361/202449715
• Atom recombination on surfaces in plasmas - an experimental study
  
  • Booth Jean-Paul
  , 2024. Surface-catalysed recombination of atoms and free radicals play a key role in the plasma equilibrium for molecular plasmas in enclosed reactors. This process is routinely characterized by a surface reaction probability,  , (between 0 and 1), which for simplicity is often assumed to be a constant. However, this parameter is nearly always poorly characterized, or indeed used as the adjustable variable to fit models to experimental data. Ab-initio theories are not currently able to predict surface reaction probabilities reliably, so they must be determined by in-situ measurements (either through the measurement of spatial mole-fraction gradients adjacent to the surface, or (more commonly) by time-resolved measurements in modulated plasmas).
• On the Response of Protons to Dynamical Reconfigurations of Mercury's Magnetosphere
  
  • Delcourt D.
  • Hadid L Z
  • Aizawa S.
  Geophysical Research Letters, American Geophysical Union, 2024, 51 (21). <div><p>We examine the dynamics of protons during tail-like to dipole-like reconfigurations of Mercury's magnetosphere. Such reconfigurations that frequently occur in the highly dynamical Hermean environment are accompanied by induced electric fields leading to short-lived convection enhancements. Using test particle calculations, we show that, under the effect of such induced electric fields, protons may be subjected to prominent energization while being injected into the inner magnetosphere. We demonstrate that this energization occurs in a nonadiabatic manner and can reach several tens of keV, possibly leading to particle trapping around the planet. Recent observations from BepiColombo during Mercury's third flyby provide evidences of energetic protons drifting in the vicinity of the planet. The present impulsive energization process is a possible mechanism for the build-up of such populations.</p></div> (10.1029/2024gl110351)
  DOI : 10.1029/2024gl110351
• Venusian ion escape under extreme conditions: A dynamic pressure and temperature simulation study
  
  • Katrougkalou M C
  • Persson M
  • Aizawa S
  • André N
  • Modolo Ronan
  • Jariel E
  • Kullen A
  • Karlsson T
  Astronomy & Astrophysics - A&A, EDP Sciences, 2024, 691 (November), pp.A206. We investigated the response of the Venusian atmospheric ion escape under the effect of interplanetary coronal mass ejections (ICMEs) using the Latmos Hybrid Simulation (LatHyS). Aims. In particular, we focused on the influence of extreme ICME dynamic pressures and temperatures, with the temperature being a parameter that has not been extensively studied in the past. Methods. Simulations were performed for two different dynamic pressures and three different temperatures. For the case of the dynamic pressure simulations, a density and a velocity enhancement event were studied separately. The H + and O + ion escape was then examined and compared for different escape channels. Results. In both dynamic pressure enhancement cases, we find that there is no clear dependence of the O + ion escape on the dynamic pressure, which is consistent with observations. On the other hand, the temperature of the incoming solar wind positively influences the H + and O + ion escape. This is attributed in part to the enhanced gyroradius of the particles, which allows them to penetrate deeper into the planet's atmosphere. (10.1051/0004-6361/202449326)
  DOI : 10.1051/0004-6361/202449326
• Internally Driven β-plane Plasma Turbulence Using the Hasegawa-Wakatani System
  
  • Gürcan Özgür
  , 2024. <div><p>General problem of plasma turbulence can be formulated as advection of potential vorticity (PV), which handles flow self-organization, coupled to a number of other fields, whose gradients provide free energy sources. Therefore, focusing on PV evolution separates the underlying linear instability from the flow self-organization, and clarifies key spatial scales in terms of balances between various time scales. Considering the Hasegawa-Wakatani model as a minimal, nontrivial model of plasma turbulence where the energy is injected internally by a linear instability, we find that the critical wavenumber kc = C/κ where C is the adiabaticity parameter and κ is the normalized density gradient separates the adiabatic (or highly zonostrophic) behavior for large scales from the hydrodynamic behavior at small scales. In the adiabatic range the non-zonal part of the wave-number spectrum goes from</p><p>in the "inertial" range, where γ k and ω k are the linear growth and frequency and U is the rms zonal velocity. This proposed spectrum fits very well for the large kc case, where the bulk of the spectrum is in the adiabatic range. In contrast for small kc, we get the usual forward enstrophy cascade with</p><p>where ϵW is the enstrophy dissipation. In contrast for kc ≈ 1, the system transitions to hydrodynamic forward enstrophy cascade right after the injection range, with zonal flows at large scales and forward enstrophy cascade at small scales. Note that kc, can also be used as a proxy for the scale at which the system switches from wave-dominated (i.e. E (k) ∝ ω 2 k k -3 ) to hydrodynamic (i.e. E (k) ∝ ϵ 2/3 W k -3 ) spectra usually denoted by k β in geophysical fluid dynamics. It is argued that the ratio R β ≡ k β /k peak ≈ kc/k peak where k peak is the peak wave-number can be defined as the zonostrophy parameter, and that the abundance of zonal flows vs. eddies in near and far from "marginality" that is commonly formulated in terms of the Kubo number in plasma problems can also be understood in terms of the zonostrophy parameter, since R β increases as we approach marginality.</p></div> (10.48550/arXiv.2403.09911)
  DOI : 10.48550/arXiv.2403.09911
• Analysis and control of Hall effect thruster using optical emission spectroscopy and artificial neural network
  
  • Ben Slimane Tarek
  • Leduc Alexandre
  • Schiesko Loïc
  • Bourdon Anne
  • Chabert Pascal
  Journal of Applied Physics, American Institute of Physics, 2024, 136 (15). This study presents a proof-of-principle for using optical emission spectroscopy and artificial neural networks for real-time monitoring and control of the operational parameters of a Hall effect thruster: the anode voltage, the anode xenon injection, the discharge current, and the coil current. In that regard, we build an optical database of 26 spectral lines across 6469 operating conditions to train and test the neural network. We then reduced the learning lines from 26 to 15 based on their statistical correlation with the target parameters. After tuning the hyperparameters of the network, the network predicted the thruster’s parameters with notable accuracies: 95% for the anode voltage, 84% for the coil current, and 99% for both the anode flow rate and the discharge current. The estimated uncertainty of predictions, at 3σ, is ±51V for voltage, ±1A for coil current, ±0.15A for discharge current, and ±0.15mgs−1 for anode flow rate. The prediction calculations were within milliseconds and enabled real-time monitoring of the thruster parameters. Therefore, a proportional-integrator-derivative controller (PID) controller was implemented to regulate the anode voltage and flow rate based on the optical emission of the plume. The PID showcased short settling times from 0.1 to 0.4 s and overshoot levels up to 3% of the target value for the voltage and 10% of the target value for the flow rate. These results were for a fixed coil current at 4A. The study showed that changing the coil current may necessitate more sophisticated prediction models and control strategies. Future work will expand the model’s generalizability to different thruster types, propellants, and magnetic field configurations. (10.1063/5.0214760)
  DOI : 10.1063/5.0214760
• Observation of super-Alfvénic slippage of reconnecting magnetic field lines on the Sun
  
  • Lörinčík Juraj
  • Dudík Jaroslav
  • Sainz Dalda Alberto
  • Aulanier Guillaume
  • Polito Vanessa
  • de Pontieu Bart
  Nature Astronomy, Nature Publishing Group, 2024, 9 (1), pp.45-54. Abstract Slipping motions of magnetic field lines are a distinct signature of three-dimensional magnetic reconnection, a fundamental process driving solar and stellar flares. While being a key prediction of numerical experiments, the rapid super-Alfvénic field line slippage driven by the ‘slip-running’ reconnection has remained elusive in previous observations. New frontiers into exploring transient flare phenomena were introduced by recently designed high cadence observing programs of the Interface Region Imaging Spectrograph (IRIS). By exploiting high temporal resolution imagery (~2 s) of IRIS, here we reveal slipping motions of flare kernels at speeds reaching thousands of kilometres per second. The fast kernel motions are direct evidence of slip-running reconnection in quasi-separatrix layers, regions where magnetic field strongly changes its connectivity. Our results provide observational proof of theoretical predictions unaddressed for nearly two decades and extend the range of magnetic field configurations where reconnection-related phenomena can occur. (10.1038/s41550-024-02396-4)
  DOI : 10.1038/s41550-024-02396-4
• Decay of magnetohydrodynamic turbulence in the expanding solar wind: WIND observations
  
  • Verdini Andrea
  • Hellinger Petr
  • Landi Simone
  • Grappin Roland
  • Montagud-Camps Victor
  • Papini Emanuele
  Astronomy & Astrophysics - A&A, EDP Sciences, 2024, 690, pp.A265. We have studied the decay of turbulence in the solar wind. Fluctuations carried by the expanding wind are naturally damped because of flux conservation, slowing down the development of a turbulent cascade. The latter also damps fluctuations but results in plasma heating. We analyzed time series of the velocity and magnetic field ( v and B , respectively) obtained by the WIND spacecraft at 1 au. Fluctuations were recast in terms of the Elsasser variables, z ± = v ± B /√4 πρ , with ρ being the average density, and their second- and third-order structure functions were used to evaluate the Politano-Pouquet relation, modified to account for the effect of expansion. We find that expansion plays a major role in the Alfvénic stream, those for which z + ≫ z − . In such a stream, expansion damping and turbulence damping act, respectively, on large and small scales for z + , and also balance each other. Instead, z − is only subject to a weak turbulent damping because expansion is a negligible loss at large scales and a weak source at inertial range scales. These properties are in qualitative agreement with the observed evolution of energy spectra that is described by a double power law separated by a break that sweeps toward lower frequencies for increasing heliocentric distances. However, the data at 1 au indicate that injection by sweeping is not enough to sustain the turbulent cascade. We derived approximate decay laws of energy with distance that suggest possible solutions for the inconsistency: in our analysis, we either overestimated the cascade of z ± or missed an additional injection mechanism; for example, velocity shear among streams. (10.1051/0004-6361/202450811)
  DOI : 10.1051/0004-6361/202450811
• Infected wound repair correlates with collagen I induction and NOX2 activation by cold atmospheric plasma
  
  • Blaise Océane
  • Duchesne Constance
  • Capuzzo Elena
  • Nahori Marie-Anne
  • Fernandes Julien
  • Connor Michael G
  • Hamon Mélanie A
  • Pizarro-Cerda Javier
  • Lataillade Jean-Jacques
  • Mcguckin Colin
  • Rousseau Antoine
  • Banzet Sébastien
  • Dussurget Olivier
  • Frescaline Nadira
  NPJ Regenerative medicine, Springer Nature, 2024, 9 (1), pp.28. Cold atmospheric plasma (CAP) is a promising complement to tissue repair and regenerative medicine approaches. CAP has therapeutic potential in infected cutaneous wounds by mechanisms which remain enigmatic. Here, CAP is shown to activate phagocyte NADPH oxidase complex NOX2. CAP induced increased intracellular reactive oxygen species, alleviated by NOX2 inhibitors. Genetic and pharmacological inhibitions of NOX2 in macrophages and bioengineered skin infected with Staphylococcus aureus and treated with CAP reduced intracellular oxidants and increased bacterial survival. CAP triggered Rac activation and phosphorylation of p40 phox and p47 phox required for NOX2 assembly and activity. Furthermore, CAP induced collagen I expression by fibroblasts. Infection and healing kinetics showed that murine skin wounds infected with S. aureus and treated with CAP are characterized by decreased bacterial burden, increased length of neoepidermis and extracellular matrix formation. Collectively, our findings identify mechanisms triggered by CAP that subdue infection and result in enhanced repair following skin injury. (10.1038/s41536-024-00372-0)
  DOI : 10.1038/s41536-024-00372-0
• Plasma-enhanced detonability: experimental and calculated reduction of the detonation cell size
  
  • Cherif Mhedine Ali
  • Masuda Ryu
  • Claverie Alain
  • Vidal Pierre
  • Starikovskaia Svetlana
  Combustion and Flame, Elsevier, 2024, 268, pp.113639. This work analyzes the interaction between non-equilibrium plasma and detonation. The aim is to enhance the detonability of gaseous mixtures by reducing the detonation cell width through dissociation of a fresh gas mixture by plasma action. The experiments were performed in a square-section detonation tube, and the diagnostic tools used were ICCD chemiluminescence imaging, soot-plate recording, dynamic pressure sensors, and back current shunt technique. The results show that the application of a nanosecond plasma ahead of a self-sustained detonation reduces the cell width by a factor of about 2 in H₂:O₂:Ar, H₂:O₂, CH₄:H₂:O₂:Ar and CH₄:O₂:Ar mixtures for initial pressures between 100 and 200 mbar. A parametric study of plasma properties focused on the effect of the initial pressure on the deposited energy and homogeneity. A kinetic mechanism was proposed to estimate the dissociation effect of plasma chemistry on the fresh combustible mixture. The obtained densities of atoms produced in the plasma were used as input parameters to calculate the thermicity and temperature profiles of the detonation reaction zone according to the Zel’dovich-von Neumann-D¨oring model. The reduction factor of the ZND characteristic chemical length is about the same as the experimental cell widths, i.e. 2. This combination of experiments and calculations substantiates the relationship between plasma parameters, ZND chemical lengths, and detonation cell widths and, thus, demonstrates the possibility of controlling detonability using a nanosecond discharge. (10.1016/j.combustflame.2024.113639)
  DOI : 10.1016/j.combustflame.2024.113639
• Simulation and Characterization of Capacitively-Coupled Argon Plasma at Intermediate Pressure
  
  • Sahu Rupali
  • Nuwal Nakul
  • Bera Kallol
  • Rauf Shahid
  • Booth Jean-Paul
  • Zhang Shu
  • Curley Garrett
  , 2024. Radiofrequency (RF) capacitively-couple plasmas (CCP) at intermediate pressure (a few Torr) are widely used in the semiconductor industry. However, the plasma behavior in this pressure regime is not well characterized. Argon plasma physics is investigated using a one-dimensional fluid-MCS hybrid plasma model in this pressure regime, and compared with experimental data. Owing to the simple chemistry of Argon plasma, validated results in this study can provide insights into plasma physics at intermediate pressures. Our model includes continuity equations for charged and neutral species, drift-diffusion approximation for electron flux, the momentum conservation equation for ions, energy conservation for electrons, and the Poisson equation for electric potential. While a fluid model is valid for bulk species in this pressure regime, secondary electrons emitted from the surface can exhibit kinetic behavior as they accelerate through the sheath. Therefore, a Monte Carlo model is used for secondary electron collisions to compute production rates of species, which are coupled to the fluid plasma model. The voltage-current characteristics of the plasma model are compared with the electrical measurements in a symmetric CCP chamber for model validation. Electron energy distribution function (EEDF) and the gas temperature are varied in the model to achieve good agreement between the simulations and experiments. The plasma simulations are performed at different pressures and powers. Validated results show that the EEDF transitions from a Maxwellian distribution at lower pressures of 1 Torr, to a Druyvesteyn distribution at higher pressures. It is also seen that at pressures of around 8 Torr, ionization source is concentrated in the presheath region, decaying exponentially in the bulk. At low pressures, the secondary electrons from the electrodes penetrate into the bulk plasma enhancing ionization in that region. The gas temperature is assumed to be uniform and increases from 350K to 600K as the pressure increases from 1 to 8 Torr, while the bulk electron temperature decreases. Our simulation results compare well with the experimental trends that can be useful in CCP based semiconductor processing.
• Experimental study of intermediate-Pressure RF-CCP in Ar-O2 mixtures
  
  • Zhang Shu
  • Curley Garrett
  • Booth Jean-Paul
  , 2024. Radio frequency (RF) capacitively coupled plasma (CCP) sources at pressures below a few 100 mTorr are widely used for industrial etching thin films [1] and have been extensively studied experimentally and modelled. However, RF-CCP’s at higher pressures (up to 10 Torr), widely used for thin film deposition, have been much less studied, because many of the diagnostic and modelling techniques developed at low pressure become problematic. We present an experimental study of a highly-symmetric CCP (Ø 50cm gap 2,.5cm), excited at 13.56 MHz (100-900W), at 1-6 Torr pressure in gas mixtures varying from pure Ar to pure O2. We used cavity ringdown spectroscopy (CRDS) at 630 nm [2] to measure oxygen atom densities and temperatures, as well as the O- negative ion density and ozone generation in the afterglow. Additionally we estimated electron density using a microwave hairpin probe and determined the RF current, voltage and phase at the electrodes. Even small (5%) addition of O2 to Ar strongly reduces the electron density and drastically changes the discharge impedance. Further oxygen addition (up to 100%) continues these trends, but more weakly. At high pressures the plasma becomes much more resistive. This can be attributed to the strong inelastic electron collision processes in oxygen, as well as the generation of negative ions. In some conditions (certain pressure and power), the O atom density passes through a maximum as the Ar % is increased, due to the competition between decreased O2 feedstock and increased electron density/dissociation rate. While in most conditions, the O density decreases monotonically with Ar addition, although the O2 dissociation fraction increases consistently, approaching 70%. *We thankApplied Materials and Fédération de Recherche PLAS@PAR for financial support
• Oxygen atom kinetics in pulsed Radiofrequency Capacitively-coupled plasmas in pure O2 at intermediate pressures
  
  • Booth Jean-Paul
  • Zhang Shu
  • Curley Garrett
  , 2024. We present an experimental study of a highly-symmetric CCP (Ø 50cm gap 2,.5cm), excited at 13.56 MHz (100-900W), operating in 0.5-6 Torr pure O2. We used monomode laser cavity ringdown spectroscopy (CRDS) at 630 nm in cw and pulsed plasmas to measure oxygen atom densities and kinetics, as well as the gas temperature, the O- negative ion density and ozone generation in the afterglow. Electron densities were estimated using a microwave hairpin probe. Above 2 Torr the oxygen atom density increases with pressure and RF power, reaching a mole-fraction of 15%. However, at lower pressures the oxygen atom density passes through a maximum with power. The electron density (and therefore O2 dissociation rate) increases with power, and at low-pressure shows a sharp increase, indicating a transition into a high density, low (bulk) electron energy mode. The O atom trend indicates that the oxygen atom loss rate (principally due to recombination at the electrode surfaces) increases with power at lower pressures. Time-resolved O atoms measurements in the afterglow confirm that the O atom loss rate increases with the RF power. This corresponds to conditions of increasing energy and flux of ion surface bombardment, suggesting that surface recombination is activated by energetic ion bombardment. Nevertheless, the decays are surprisingly slow, corresponding to surface reaction coefficients of the order several x10-4. These small values are rather surprising on bare metallic (aluminium) surfaces, and very comparable to the values observed on borosilicate glass, showing the importance of surface oxidation. At higher O2 pressures the oxygen atom decays become faster (and non-exponential), indicating the onset of gas-phase reactions. However, kinetic models with existing reaction sets are unable to explain our observations, implying that these models may need to be significantly revised