Publications

2021

• 2D radial-azimuthal particle-in-cell benchmark for E × B discharges
  
  • Villafana W
  • Petronio Federico
  • Denig A
  • Jimenez M
  • Eremin D
  • Garrigues Laurent
  • Taccogna F
  • Alvarez-Laguna Alejandro
  • Boeuf Jean-Pierre
  • Bourdon Anne
  • Chabert Pascal
  • Charoy Thomas
  • Cuenot B
  • Hara K
  • Pechereau F
  • Smolyakov A
  • Sydorenko D
  • Tavant A
  • Vermorel O
  Plasma Sources Science and Technology, IOP Publishing, 2021, 30 (7), pp.075002. In this paper we propose a representative simulation test-case of E × B discharges accounting for plasma wall interactions with the presence of both the electron cyclotron drift instability and the modified-two-stream-instability. Seven independently developed particle-in-cell (PIC) codes have simulated this benchmark case, with the same specified conditions. The characteristics of the different codes and computing times are given. Results show that both instabilities were captured in a similar fashion and good agreement between the different PIC codes is reported as main plasma parameters were closely related within a 5% interval. The number of macroparticles per cell was also varied and statistical convergence was reached. Detailed outputs are given in the supplementary data, to be used by other similar groups in the perspective of code verification (10.1088/1361-6595/ac0a4a)
  DOI : 10.1088/1361-6595/ac0a4a
• Characteristics of Resonant Electrons Interacting With Whistler Waves in the Nearest Dipolarizing Magnetotail
  
  • Malykhin A.
  • Grigorenko E.
  • Shklyar D.
  • Panov E.
  • Le Contel O.
  • Avanov L.
  • Giles B.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2021, 126 (7). (10.1029/2021JA029440)
  DOI : 10.1029/2021JA029440
• Cold Atmospheric Plasma Promotes Killing of Staphylococcus aureus by Macrophages
  
  • Duchesne Constance
  • Frescaline Nadira
  • Blaise Océane
  • Lataillade Jean-Jacques
  • Banzet Sébastien
  • Dussurget Olivier
  • Rousseau Antoine
  MSphere, American Society for Microbiology., 2021, 6 (3), pp.e0021721. Macrophages are important immune cells that are involved in the elimination of microbial pathogens. Following host invasion, macrophages are recruited to the site of infection, where they launch antimicrobial defense mechanisms. Effective microbial clearance by macrophages depends on phagocytosis and phagolysosomal killing mediated by oxidative burst, acidification, and degradative enzymes. However, some pathogenic microorganisms, including some drug-resistant bacteria, have evolved sophisticated mechanisms to prevent phagocytosis or escape intracellular degradation. Cold atmospheric plasma (CAP) is an emerging technology with promising bactericidal effects. Here, we investigated the effect of CAP on Staphylococcus aureus phagocytosis by RAW 264.7 macrophage-like cells. We demonstrate that CAP treatment increases intracellular concentrations of reactive oxygen species (ROS) and nitric oxide and promotes the elimination of both antibiotic-sensitive and antibiotic-resistant S. aureus by RAW 264.7 cells. This effect was inhibited by antioxidants indicating that the bactericidal effect of CAP was mediated by oxidative killing of intracellular bacteria. Furthermore, we show that CAP promotes the association of S. aureus to lysosomal-associated membrane protein 1 (LAMP-1)-positive phagosomes, in which bacteria are exposed to low pH and cathepsin D hydrolase. Taken together, our results provide the first evidence that CAP activates defense mechanisms of macrophages, ultimately leading to bacterial elimination. IMPORTANCE Staphylococcus aureus is the most frequent cause of skin and soft tissue infections. Treatment failures are increasingly common due to antibiotic resistance and the emergence of resistant strains. Macrophages participate in the first line of immune defense and are critical for coordinated defense against pathogenic bacteria. However, S. aureus has evolved sophisticated mechanisms to escape macrophage killing. In the quest to identify novel antimicrobial therapeutic approaches, we investigated the activity of cold atmospheric plasma (CAP) on macrophages infected with S. aureus. Here, we show that CAP treatment promotes macrophage ability to eliminate internalized bacteria. Importantly, CAP could trigger killing of both antibiotic-sensitive and antibiotic-resistant strains of S. aureus. While CAP did not affect the internalization capacity of macrophages, it increased oxidative-dependent bactericidal activity and promoted the formation of degradative phagosomes. Our study shows that CAP has beneficial effects on macrophage defense mechanisms and may potentially be useful in adjuvant antimicrobial therapies. (10.1128/mSphere.00217-21)
  DOI : 10.1128/mSphere.00217-21
• Proof of the zeroth law of turbulence in one-dimensional compressible magnetohydrodynamics and shock heating
  
  • David Vincent
  • Galtier Sebastien
  Physical Review E, American Physical Society (APS), 2021, 103 (6), pp.063217. The zeroth law is one of the oldest conjectures in turbulence that is still unproven. Here, we consider weak solutions of one-dimensional compressible magnetohydrodynamics and demonstrate that the lack of smoothness of the fields introduces a dissipative term, named inertial dissipation, into the expression of energy conservation that is neither viscous nor resistive in nature. We propose exact solutions assuming that the kinematic viscosity and the magnetic diffusivity are equal, and we demonstrate that the associated inertial dissipation is positive and equal on average to the mean viscous dissipation rate in the limit of small viscosity, proving the conjecture of the zeroth law of turbulence and the existence of an anomalous dissipation. As an illustration, we evaluate the shock heating produced by discontinuities detected by Voyager in the solar wind around 5 AU. We deduce a heating rate of ∼10 −18 J m −3 s −1 , which is significantly higher than the value obtained from the turbulent fluctuations. This suggests that collisionless shocks can be a dominant source of heating in the outer solar wind. (10.1103/PhysRevE.103.063217)
  DOI : 10.1103/PhysRevE.103.063217
• A new look at oxygen plasmas : Quantitative VUV and IR absorption spectroscopy of reactive intermediates
  
  • Booth Jean-Paul
  , 2021. Despite many decades of study, models of discharges in molecular gases still lack accurate data on many key collisional processes, even for such “simple” and ubiquitous gases as O2. Good data is lacking for near-threshold electron-impact dissociation, surface recombination, the role of metastables, of gas heating, of vibrational excitation, of energy transfer and surface thermal accommodation. We present measurement in a DC positive column discharge in pure O2. Although it has been studied for a very long time, new experimental methods, including synchrotron Vacuum ultraviolet absorption spectroscopy and laser cavity ringdown absorption spectroscopy (CRDS), allow the densities of all the major species (atomic, molecular, in ground and excited states), as well as the gas translational temperature to be measured, with much-improved absolute accuracy, and with time resolution. Applied to (partially- and fully-) modulated discharges, these measurements provide unprecedented insight into the kinetic processes occurring (in the gas phase and at surfaces), and a profound test of the models.
• Operating near the L-H power threshold in WEST full tungsten environment
  
  • Goniche M
  • Bourdelle C
  • Ostuni V
  • Artaud J F
  • Bucalossi J
  • Clairet F
  • Colas L
  • Desgranges C
  • Delpech L
  • Devynck P
  • Dumont R
  • Ekedahl A
  • Fedorczak N
  • Garcia J
  • Gaspar J
  • Gil C
  • Guillemaut C
  • Guirlet R
  • Gunn J
  • Hillairet J
  • Klepper C
  • Król K
  • Lau C
  • Maget P
  • Manenc P
  • Martin E
  • Mazon D
  • Meyer O
  • Morales J
  • Moreau Ph
  • Nouailletas R
  • Peret M
  • Peysson Y
  • Regal-Mezin X
  • Vermare L
  • Vezinet D
  • Wallace G
  , 2021.
• Electric Field Measurements in Plasmas with E-FISH Using Focused Gaussian Beams.
  
  • Chng Tat Loon
  • Starikovskaia Svetlana M
  • Schanne-Klein Marie-Claire
  , 2021. (10.1109/CLEO/Europe-EQEC52157.2021.9542435)
  DOI : 10.1109/CLEO/Europe-EQEC52157.2021.9542435
• Operating near the L-H power threshold in WEST full tungsten environment
  
  • Goniche M
  • Bourdelle C
  • Ostuni V
  • Artaud J F
  • Bucalossi J
  • Colas L
  • Desgranges C
  • Delpech L
  • Devynck P
  • Dumont R
  • Ekedahl A
  • Fedorczak N
  • Garcia J
  • Gaspar J
  • Gil C
  • Guillemaut C
  • Guirlet R
  • Gunn J
  • Hillairet J
  • Klepper C
  • Król K
  • Lau C
  • Maget P
  • Manenc P
  • Martin E
  • Mazon D
  • Meyer O
  • Morales J
  • Moreau Ph
  • Nouailletas R
  • Peret M
  • Peysson Y
  • Regal-Mezin X
  • Vermare L
  • Vezinet D
  • Wallace G
  , 2021.
• High-cadence measurements of electron pitch-angle distributions from Solar Orbiter SWA-EAS burst mode operations
  
  • Owen C.
  • Kataria D.
  • Bercic L.
  • Horbury T.
  • Berthomier Matthieu
  • Verscharen D.
  • Bruno R.
  • Livi S.
  • Louarn P.
  • Anekallu C.
  • Kelly C.
  • Lewis G.
  • Watson G.
  • Fortunato V.
  • Mele G.
  • Nicolaou G.
  • Wicks R.
  • O'Brien H.
  • Evans V.
  • Angelini V.
  Astronomy & Astrophysics - A&A, EDP Sciences, 2021. (10.1051/0004-6361/202140959)
  DOI : 10.1051/0004-6361/202140959
• The interaction between ion transit-time and electron drift instabilities and their effect on anomalous electron transport in Hall thrusters
  
  • Charoy Thomas
  • Lafleur Trevor
  • Alvarez-Laguna Alejandro
  • Bourdon Anne
  • Chabert Pascal
  Plasma Sources Science and Technology, IOP Publishing, 2021, 30 (6), pp.065017. (10.1088/1361-6595/ac02b3)
  DOI : 10.1088/1361-6595/ac02b3
• Mars in situ oxygen and propellant production by non-equilibrium plasmas
  
  • Ogloblina P
  • Morillo-Candas A
  • Silva A
  • Silva T
  • Tejero-Del-Caz A
  • Alves L
  • Guaitella O
  • Guerra V
  Plasma Sources Science and Technology, IOP Publishing, 2021, 30 (6), pp.065005. (10.1088/1361-6595/abec28)
  DOI : 10.1088/1361-6595/abec28
• Plasma-liquid interfacial layer detected by in situ Raman light sheet microspectroscopy
  
  • Pai David Z
  Journal of Physics D: Applied Physics, IOP Publishing, 2021, 54 (35), pp.355201. In situ Raman microscopy has been adapted to study the plasma-water interface by applying a light sheet technique. The Raman modes of the-OO stretch of H2O2, symmetric stretch (v1) of NO3-, and-OH bend of water were measured simultaneously. By modulating the volume of water under detection, both the bulk liquid and interface regions have been probed with micrometer depth resolution. The plasma was a DC glow discharge generated in atmospheric-pressure air with a water cathode. In the bulk liquid, the molar concentration of aqueous NO3increased at a linear rate of 48 µM/minute, whereas aqueous H2O2 growth stopped at about 5 mM. The concentrations of H2O2 and NO3both increased when measuring at depths less than about 20 µm from the interface. The depth profile of NO3concentration was reconstructed, showing that the interfacial layer of NO3has a depth of 28 µm. The shape of the meniscus may influence the interpretation of this depth. Previous models of plasma-water interfaces have predicted interfacial layers of similar depth for short-lived aqueous species such as OH but not for long-lived species such as NO3- . (10.1088/1361-6463/ac07e0)
  DOI : 10.1088/1361-6463/ac07e0
• Plasma-water interfaces for applications in nanomaterials synthesis
  
  • Pai David Z
  , 2021.
• Negative ion sources
  
  • Bacal M.
  • Sasao M.
  • Wada M.
  Journal of Applied Physics, American Institute of Physics, 2021, 129 (22), pp.221101. (10.1063/5.0049289)
  DOI : 10.1063/5.0049289
• A Case for Electron-Astrophysics
  
  • Verscharen Daniel
  • Wicks Robert
  • Alexandrova Olga
  • Bruno Roberto
  • Burgess David
  • Chen Christopher
  • D’amicis Raffaella
  • de Keyser Johan
  • Dudok de Wit Thierry
  • Franci Luca
  • He Jiansen
  • Henri Pierre
  • Kasahara Satoshi
  • Khotyaintsev Yuri
  • Klein Kristopher
  • Lavraud Benoit
  • Maruca Bennett
  • Maksimovic Milan
  • Plaschke Ferdinand
  • Poedts Stefaan
  • Reynolds Christopher
  • Roberts Owen
  • Sahraoui Fouad
  • Saito Shinji
  • Salem Chadi
  • Saur Joachim
  • Servidio Sergio
  • Stawarz Julia
  • Štverák Štěpán
  • Told Daniel
  Experimental Astronomy, Springer Link, 2021. Abstract The smallest characteristic scales, at which electron dynamics determines the plasma behaviour, are the next frontier in space and astrophysical plasma research. The analysis of astrophysical processes at these scales lies at the heart of the research theme of electron-astrophysics. Electron scales are the ultimate bottleneck for dissipation of plasma turbulence, which is a fundamental process not understood in the electron-kinetic regime. In addition, plasma electrons often play an important role for the spatial transfer of thermal energy due to the high heat flux associated with their velocity distribution. The regulation of this electron heat flux is likewise not understood. By focussing on these and other fundamental electron processes, the research theme of electron-astrophysics links outstanding science questions of great importance to the fields of space physics, astrophysics, and laboratory plasma physics. In this White Paper, submitted to ESA in response to the Voyage 2050 call, we review a selection of these outstanding questions, discuss their importance, and present a roadmap for answering them through novel space-mission concepts. (10.1007/s10686-021-09761-5)
  DOI : 10.1007/s10686-021-09761-5
• Tumor stiffening reversion through collagen crosslinking inhibition improves T cell migration and anti-PD-1 treatment
  
  • Nicolás-Boluda Alba
  • Vaquero Javier
  • Vimeux Lene
  • Guilbert Thomas
  • Barrin Sarah
  • Kantari-Mimoun Chahrazade
  • Ponzo Matteo
  • Renault Gilles
  • Deptula Piotr
  • Pogoda Katarzyna
  • Bucki Robert
  • Cascone Ilaria
  • Courty José
  • Fouassier Laura
  • Gazeau Florence
  • Donnadieu Emmanuel
  eLife, eLife Sciences Publication, 2021, 10. Only a fraction of cancer patients benefits from immune checkpoint inhibitors. This may be partly due to the dense extracellular matrix (ECM) that forms a barrier for T cells. Comparing five preclinical mouse tumor models with heterogeneous tumor microenvironments, we aimed to relate the rate of tumor stiffening with the remodeling of ECM architecture and to determine how these features affect intratumoral T cell migration. An ECM-targeted strategy, based on the inhibition of lysyl oxidase, was used. In vivo stiffness measurements were found to be strongly correlated with tumor growth and ECM crosslinking but negatively correlated with T cell migration. Interfering with collagen stabilization reduces ECM content and tumor stiffness leading to improved T cell migration and increased efficacy of anti-PD-1 blockade. This study highlights the rationale of mechanical characterizations in solid tumors to understand resistance to immunotherapy and of combining treatment strategies targeting the ECM with anti-PD-1 therapy. (10.7554/eLife.58688)
  DOI : 10.7554/eLife.58688
• Solar Coronal activity combining observations and modelling
  
  • Masson Sophie
  , 2021.
• In situ Raman spectroscopy at plasma-water interfaces
  
  • Pai David Z
  , 2021. The plasma-water interfacial region is believed to play an important role in applications involving plasma-activated water (PAW), such as graphene quantum dot synthesis in aqueous solution using a DC microplasma electrochemical reactor [1,2]. As a simplified test case, we experimentally investigated the interfacial region using in-situ Raman microspectroscopy with a spatial resolution of several microns. The plasma reactor of choice is a DC glow generated in ambient air using a pin electrode placed above an optical cell filled with deionized water. First, we examined the solvent itself, tracking the-OH stretch band of water. This band experiences increasing intensity of the fundamental frequency distribution around 3400-3500 cm-1 , which is weakly coupled to other hydrogen bonds. The peak around 3200-3300 cm-1 , resulting from intermolecular coupling and strong hydrogen bonding, experiences decreasing intensity. We will discuss both rapid and gradual changes to the Raman spectra over the course of plasma treatment, as well as the reversibility of these changes once the plasma is switched off. The effect of depth below the plasma-water interface will also be discussed: the changes to the Raman spectra become more pronounced as the detection volume approaches the interface. Second, we track the concentration of PAW species via their calibrated Raman spectra. The Raman modes of the-OO stretch of H2O2, symmetric stretch of NO3-, and-OH bend of water were measured simultaneously. In the bulk liquid, the molar concentration of aqueous NO3-increased at a linear rate of 48 µM/minute, whereas aqueous H2O2 growth stopped at about 5 mM. The concentrations of H2O2 and NO3-both increased when measuring at depths less than about 20 µm from the interface. Previous models of plasma-water interfaces have predicted interfacial layers of similar depth for short-lived aqueous species such as OH but not for long-lived species such as NO3- .
• Electron heat flux in the near-Sun environment
  
  • Halekas J.
  • Whittlesey P.
  • Larson D.
  • Mcginnis D.
  • Bale S.
  • Berthomier Matthieu
  • Case A.
  • Chandran B.
  • Kasper J.
  • Klein K.
  • Korreck K.
  • Livi R.
  • Macdowall R.
  • Maksimovic M.
  • Malaspina D.
  • Matteini L.
  • Pulupa M.
  • Stevens M.
  Astronomy & Astrophysics - A&A, EDP Sciences, 2021, 650, pp.A15. Aims. We survey the electron heat flux observed by the Parker Solar Probe (PSP) in the near-Sun environment at heliocentric distances of 0.125–0.25 AU. Methods. We utilized measurements from the Solar Wind Electrons Alphas and Protons and FIELDS experiments to compute the solar wind electron heat flux and its components and to place these in context. Results. The PSP observations reveal a number of trends in the electron heat flux signatures near the Sun. The magnitude of the heat flux is anticorrelated with solar wind speed, likely as a result of the lower saturation heat flux in the higher-speed wind. When divided by the saturation heat flux, the resulting normalized net heat flux is anticorrelated with plasma beta on all PSP orbits, which is consistent with the operation of collisionless heat flux regulation mechanisms. The net heat flux also decreases in very high beta regions in the vicinity of the heliospheric current sheet, but in most cases of this type the omnidirectional suprathermal electron flux remains at a comparable level or even increases, seemingly inconsistent with disconnection from the Sun. The measured heat flux values appear inconsistent with regulation primarily by collisional mechanisms near the Sun. Instead, the observed heat flux dependence on plasma beta and the distribution of suprathermal electron parameters are both consistent with theoretical instability thresholds associated with oblique whistler and magnetosonic modes. (10.1051/0004-6361/202039256)
  DOI : 10.1051/0004-6361/202039256
• Discontinuity analysis of the leading switchback transition regions
  
  • Akhavan-Tafti M.
  • Kasper J.
  • Huang J.
  • Bale S.
  Astronomy & Astrophysics - A&A, EDP Sciences, 2021, 650, pp.A4. Context. Magnetic switchbacks are magnetic structures characterized as intervals of sudden reversal in the radial component of the pristine solar wind’s magnetic field. Switchbacks comprise of magnetic spikes that are preceded and succeeded by switchback transition regions within which the radial magnetic field reverses. Determining switchback generation and evolution mechanisms will further our understanding of the global circulation and transportation of the Sun’s open magnetic flux. Aims. The present study juxtaposes near-Sun switchback transition regions’ characteristics with similar magnetic discontinuities observed at greater radial distances with the goal of determining local mechanism(s) through which switchback transition regions may evolve. Methods. Measurements from fields and plasma suites aboard the Parker Solar Probe were utilized to characterize switchback transition regions. Minimum variance analysis (MVA) was applied on the magnetic signatures of the leading switchback transition regions. The leading switchback transition regions with robust MVA solutions were identified and categorized based on their magnetic discontinuity characteristics. Results. It is found that 78% of the leading switchback transition regions are rotational discontinuities (RD). Another 21% of the leading switchback transition regions are categorized as “either” discontinuity (ED), defined as small relative changes in both magnitude and the normal component of the magnetic field. The RD-to-ED event count ratio is found to reduce with increasing distance from the Sun. The proton radial temperature sharply increases (+ 29.31%) at the leading RD-type switchback transition regions, resulting in an enhanced thermal pressure gradient. Magnetic curvature at the leading RD-type switchback transition regions is often negligible. Magnetic curvature and the thermal pressure gradient are parallel (i.e., “bad” curvature) in 74% of the leading RD-type switchback transition regions. Conclusions. The leading switchback transition regions may evolve from RD-type into ED-type magnetic discontinuities while propagating away from the Sun. Local magnetic reconnection is likely not the main driver of this evolution. Other drivers, such as plasma instabilities, need to be investigated to explain the observed significant jump in proton temperature and the prevalence of bad curvature at the leading RD-type switchback transition regions. (10.1051/0004-6361/202039508)
  DOI : 10.1051/0004-6361/202039508
• Experimental study of plasma parameters in nanosecond surface dielectric barrier filamentary discharge
  
  • Ding Chenyang
  , 2021. Nanosecond surface dielectric barrier discharges (nSDBDs) at atmospheric pressure have been studied extensively over the last two decades for flow control. About ten years ago, the nSDBD at high pressures was suggested as a source for plasma assisted ignition of combustible mixtures. During last six years, it was found that a severe transformation of a single-shot nanosecond surface streamer discharge is observed at increasing gas pressure and/or voltage.The present thesis is devoted to study of streamer-to-filament transition in a single shot high pressure surface nanosecond barrier discharge in non-reactive gases (nitrogen, oxygen and their mixtures). Literature review presents detailed analysis of streamer discharges and of transitory nanosecond sparks widely studied during last 4 years.The results are presented in three parts. The first part shows the parameters of streamer-to-filament transition in the high pressure nSDBD for different gas mixture composition. For both negative and positive polarities, the transition is a function of pressure and of the voltage amplitude. For positive polarity, the effect of molecular oxygen addition on the transition is extremely strong. The influence of different dielectrics and different electrode materials of the start and development of the filaments is studied experimentally. The micro-images of discharge propagation on three electrode configurations at three different stages – streamer, transition to filament and filamentary regime – are compared.In the second part, plasma properties in the filaments are studied with the help of the energy measurements, optical emission spectroscopy (OES) and Particle Image Velocimetry (PIV). In the streamer and transition regime, the OES spectra mainly contains second positive system of molecular nitrogen while in the filamentary regime continuum (CW) emission and a few atomic lines are observed. The results of measured plasma parameters, namely synchronized in time specific deposited energy, electron temperature and electron density are included. The value of specific deposited energy in the filaments is as high as 6-8 eV/particle; the electron density is in the range of 10 to 18-19 power per cubic centimeter, and the electron temperature stays at the level of 1.5-2 eV in the near afterglow. Plasma at this stage is found to be close to the LTE demonstrating slow (tens of nanoseconds) electron density decay linked to the temperature relaxation. The results of the measurements are compared with the results of numerical modeling explaining the main experimentally observed features. In the model, stepwise ionization and dissociation from electronically excited states of molecular nitrogen leads to fast increase of the electron density, dissociation degree and gas heating at 6 bar on the time scale of parts of nanoseconds.The third part is devoted to detailed study of streamer-to-filament transition in the micro-scale with the spatial resolution 7.6 µm/px. At a given time instant, the surface ionization wave front, composed from the merged streamers, is broken by a few plasma channels, moving with a higher velocity (we call them “protrusions”). Their radii are 10-20 times smaller comparing to a typical streamer radius; they form, within a few nanoseconds, a regular structure of plasma channels around the high voltage electrode. Inside each of these channels, a backward emission of the second positive system of N2 propagates from the “protrusion” head back to the high-voltage electrode. Continuous spectra, atomic lines and high electron density first appear when the backward emission approaches the high-voltage electrode, at the distance about 500 µm from the edge. The calculated results of emission intensity, electron density and electric field are given for studying the influence of oxygen admixtures on the transition.
• Laser Cavity Ringdown Spectroscopy of Oxygen Plasmas: Direct Measurement of the Densities of Oxygen Atoms, Ozone and Negative Ions and Gas Temperature
  
  • Booth Jean-Paul
  , 2021. Plasmas in oxygen, or oxygen-containing gas mixtures, are ubiquitous in naturally-occurring and man-made electrical discharges. Reliable modelling of these systems requires accurate cross-section and rate constant data, which ultimately depend on accurate measurements of the absolute density of the transient species present, of which O (3 P) atoms are one of the most important. Various methods have been proposed to measure oxygen atom densities, including optical emission actinometry and Two-photon Laser-induced fluorescence (TALIF), calibrated against Xe TALIF. However, both techniques depend on poorly-known cross-section data for their absolute calibration. Absorption spectroscopy has many advantages for absolute density measurements, since the Beer-Lambert law is inherently self-calibrating, and the only uncertainty comes from the accuracy of the transition strength (and knowledge of the absorber density profile along the beam path, if this is inhomogeneous). For oxygen atoms, the resonance transitions at 130nm have long been used, despite the difficulty of working with vacuum ultraviolet light. However, these allowed transitions are far too strong to be useful except for low atom densities or short absorption path lengths. An alternative is to measure the weak, forbidden forbidden 3 P2 à 1 D2 transition at 630nm. In this case, the single-pass absorption will be weak, of the order 10-5 , which cannot be measured directly. However, the use of cavity ring-down spectroscopy, using a cw single-mode diode laser and mirrors with reflectivity of the order 99.99%, allows such absorptions to be measured routinely. We have made measurements in a DC positive column discharge in pure O2. This system provides a long column (>50cm) of uniform plasma with known reduced electric field, ideal for model validation. The gas temperature is also easily determined, from the Doppler profile of the absorption peak. The oxygen atom mole-fraction reaches up to 30%, due to the low recombination probability of the borosilicate glass walls. Time-resolved CRDS was developed to measure the oxygen atom kinetics in pulse-modulated discharges, since the atom lifetime is slow compared to the cavity time (~20µs). These measurements also showed a time-varying continuum absorption below the O atom peak, which can be attributed to absorption by both Ophotodetachment and the Chappuis bands of ozone. The very different kinetics of these two species allows their two contributions to be separated, allowing the densities and kinetics of all three species to be determined.
• Axisymmetric steady-state flows in tokamak plasmas under the visco-resistive MHD setting
  
  • Firpo Marie-Christine
  , 2021.
• Developing high performance RF heating scenarios on the WEST tokamak
  
  • Goniche Marc
  • Bourdelle Clarisse
  • Artaud J.F.
  • Bernard Jean-Michel
  • Bucalossi Jérôme
  • Colas Laurent
  • Desgranges Corinne
  • Léna Delpech
  • Devynck P
  • Bobkov V.
  • Gaspar J.
  • Klepper C.C.
  • Lerche E
  • Shiraiwa S.
  • Vermare L.
  • Wallace G.M.
  , 2021. High power experiments, up to 9.2 MW with LHCD and ICRH, have been carried out in the full tungsten tokamak WEST. Quasi non inductive discharges have been achieved allowing to extend the plasma duration to 53s at medium density (ne=3.7×10 19 m-3). Apart few pulses post-boronization, the plasma radiation is rather high (Prad/Ptot~50%) and is dominated by W. This fraction does not vary as the RF power is ramped up and, against expectations, it is quite similar in ICRH and/or LHCD heated plasmas. Hot L mode plasmas (Te(0)>3keV) with a confinement time following the ITER L96 scaling are routinely obtained, confirming the weak aspect ratio dependence of this scaling law. Tungsten accumulation is generally not an operational issue on WEST. Nonetheless, 25% of the of the discharges are affected by a rapid collapse of the central electron temperature which occurs when a slight decrease of Te leads to enhanced radiation causing flat or hollow current profiles. To this respect LHCD-only discharges are compared to ICRH and ICRH/LHCD discharges.
• Formation of the radial electric field profile in WEST tokamak
  
  • Vermare Laure
  • Dif-Pradalier Guilhem
  • Hennequin Pascale
  • Gunn James Paul
  • Garbet Xavier
  • Artaud Jean-Francois
  • Gurcan Ozgur
  • Bourdelle Clarisse
  • Clairet Frédéric
  • Fedorczak Nicolas
  • Honoré Cyrille
  • Morales Jorge
  • Sarazin Yanick
  • Grandgirard Virginie
  • Varennes Robin
  • Vezinet Didier
  • Peret Mathieu
  • Dumont Rémi
  • Goniche Marc
  • Maget Patrick
  , 2021. The shear of the radial electric field at the edge is widely accepted to be responsible for turbulence reduction in edge transport barriers and thought as a key ingredient of the improved confinement of H-mode plasmas. Nevertheless, a full understanding of how its profile builds up at the edge is still lacking. It can either be formulated as the result of a competition between several mechanisms that generate and damp flows or as the result of a non-ambipolar particle flux, which enhances radial charge separation imposing a radial electric field profile. Among possible mechanisms, one can think of turbulence generated flow via Reynolds stress, ion orbit losses, toroidal magnetic ripple and the effect of neutral friction at the edge. Based on previous results obtained on Tore Supra, the radial profile of the perpendicular flow expected in WEST can be separated in three spatial areas. Inside ρ = 0.8, the radial electric field is dominated by losses of thermal ions in the magnetic ripple while between 0.7 < ρ < 0.95, a competition between this latter and the generation of large scale flows by turbulence appears as a possible explanation of the measured poloidal asymmetry of the mean perpendicular velocity. In addition, edge conditions such as contact points and parallel dynamics in the scrape-off-layer (SOL) influence the edge profiles beyond ρ = 0.9 (3). This contribution presents unexpected differences in the radial electric field profile observed in WEST between Lower Single Null (LSN) and Upper Single Null (USN) configurations and a study of the competition between specific mechanisms playing a role in the formation of this profile.