Magnetorotational Instability in a Swirling Partially Ionized Gas

The magnetorotational instability (MRI) has been proposed as the method of angular momentum transport that enables accretion in astrophysical discs. However, for weakly-ionized discs, such as protoplanetary discs, it remains unclear whether the combined non-ideal magnetohydrodynamic (MHD) effects of Ohmic resistivity, ambipolar diffusion, and the Hall effect make these discs MRI-stable. While much effort has been made to simulate non-ideal MHD MRI, these simulations make simplifying assumptions and are not always in agreement with each other. Furthermore, it is difficult to directly observe the MRI astrophysically because it occurs on small scales. Here, we propose the concept of a swirling gas experiment of weakly-ionized argon gas between two concentric cylinders threaded with an axial magnetic field that can be used to study non-ideal MHD MRI. For our proposed experiment, we derive the hydrodynamic equilibrium flow and a dispersion relation for MRI that includes the three non-ideal effects. We solve this dispersion relation numerically for the parameters of our proposed experiment. We find it should be possible to produce non-ideal MRI in such an experiment because of the Hall effect, which increases the MRI growth rate when the vertical magnetic field is anti-aligned with the rotation axis. As a proof of concept, we also present experimental results for a hydrodynamic flow in an unmagnetized prototype. We find that our prototype has a small, but non-negligible, $\alpha$-parameter that could serve as a baseline for comparison to our proposed magnetized experiment, which could be subject to additional turbulence from the MRI.Comment: 14 pages, 13 figures, submitted to MNRA

Linear and nonlinear excitation of TAE modes by external electromagnetic perturbations using ORB5

The excitation of toroidicity induced Alfv{\'e}n eigenmodes (TAEs) using prescribed external electromagnetic perturbations (hereafter ``antenna") acting on a confined toroidal plasma as well as its nonlinear couplings to other modes in the system is studied. The antenna is described by an electrostatic potential resembling the target TAE mode structure along with its corresponding parallel electromagnetic potential computed from Ohm's law. Numerically stable long-time linear simulations are achieved by integrating the antenna within the framework of a mixed representation and pullback scheme [A. Mishchenko, et al., Comput. Phys. Commun. \textbf{238} (2019) 194]. By decomposing the plasma electromagnetic potential into symplectic and Hamiltonian parts and using Ohm's law, the destabilizing contribution of the potential gradient parallel to the magnetic field is canceled in the equations of motion. Besides evaluating the frequencies as well as growth/damping rates of excited modes compared to referenced TAEs, we study the interaction of antenna-driven modes with fast particles and indicate their margins of instability. Furthermore, we show first nonlinear simulations in the presence of a TAE-like antenna exciting other TAE modes, as well as Global Alfv\'en Eigenmodes (GAE) having different toroidal wave numbers from that of the antenna

Transport barrier onset and edge turbulence shortfall in fusion plasmas

Turbulent plasmas notably self-organize to higher energy states upon application of additional free energy sources or modification of edge operating conditions. Mechanisms whereby such bifurcations occur have been actively debated for decades. Enhanced confinement occurs at the plasma edge, where a shortfall of predicted turbulence intensity has been puzzling scientists for decades. We show, from the primitive kinetic equations that both problems are connected and that interplay of confined plasma turbulence with its material boundaries is essential to curing the shortfall of predicted turbulence and to triggering spontaneous transport barrier onset at the plasma edge. Both problems determine access to improved confinement and are central to fusion research. A comprehensive discussion of the underlying mechanisms is proposed. These results, highly relevant to the quest for magnetic fusion may also be generic to many problems in fluids and plasmas where turbulence self-advection is active

Triangularity effects on global flux-driven gyrokinetic simulations

On the road to fusion energy production, many alternative scenarios have been investigated in order to address certain well-known problems of tokamak devices; among which, anomalous transport, ELMs and disruptions. The studies on plasma shaping fall into this effort. In particular, it has been experimentally observed that when operating in L mode, negative triangularity (NT) features better confinement properties than positive triangularity (PT). However, even though the trend is quite clear, a complete and satisfying theoretical explanation for this experimental findings is still lacking. With the aim of understanding and describing these improvements starting from first principles, we present the first comparison between PT and NT with global flux-driven gyrokinetic simulations performed with the ORB5 code. The numerical setup includes: electrostatic turbulence, kinetic trapped electrons, non-linear collisional operator, ECRH source, limiter and wall as boundary conditions. The simulations have been performed on ideal MHD equilibria and kinetic profiles inspired by TCV experiments, in a mixed ITG-TEM regime. First analysis reveal a strong reduction of transport in NT; while at the edge PT shows superdiffusivity, NT does not. The limiter plays an important role that has to be further clarified

Transport d’impuretés dans les plasmas de tokamak: étude gyrocinétique du transport néoclassique et turbulent

Impurity transport is an issue of utmost importance for tokamaks. One reason is the choice of tungsten for ITER divertor. Indeed high-Z materials are only partially ionized in the plasma core, so that they can lead to prohibitive radiative losses even at low concentrations, and impact dramatically plasma performance and stability. On-axis accumulation of tungsten has been widely observed in tokamaks. While the very core impurity peaking is generally attributed to neoclassical effects, turbulent transport could well dominate in the gradient region at ITER relevant collisionality. The transport of low and medium-Z impurities also results from both neoclassical and turbulent transport. Up to recently, first principles simulations of corresponding fluxes were performed with different dedicated codes, implicitly assuming that both transport channels are separable and therefore additive. The validity of this assumption can be questionned.Preliminary simulations obtained with the gyrokinetic codeGYSELA have shown clear evidences of a neoclassical-turbulence synergy for impurity transport. However no clear theoretical explanation was given. New simulations have been done using a new and more accurate collision operator, improved boundary conditions and more flexible sources. The new simulations confirm the neoclassical turbulence synergy and allow identification of a mechanism that underly this synergy.Turbulence can induce poloidal asymmetries. An analytical work performed during this thesis allows to compute the level and the structure of the axisymmetric part of the electric potential knowing the turbulence intensity. Two mechanisms are found for the generation of poloidal asymmetries of the electric potential: at large frequencies, flow compressibility is a key player for the generation of poloidal asymmetries. In this case a sine structure is predicted as in the case of GAMs. For lower frequencies, the ballooning of the turbulence is instrumental in the generation of the poloidal asymmetries. In this case, a cosine structure is predicted.A new prediction for the neoclassical impurity flux in presence of large poloidal asymmetries and pressure anisotropies has been derived during this thesis. It turns out that both banana/plateau and Pfirsch-Schlüter contributions are significantly impacted by the presence of large poloidal asymmetries and pressure anisotropies. A fair agreement has been found between the new theoretical prediction for neoclassical impurity flux and the results of a GYSELA simulation displaying large poloidal asymmetries and pressure anisotropies induced by the presence of turbulence.La compréhension du transport d’impuretés dans les tokamaks est cruciale. Une des raisons vient du choix d’utiliser du tungstène pour le divertor d’ITER. En effet, les noyaux lourds ne sont que partiellement ionisés dans le coeur du plasma, ils peuvent alors fortement rayonner et entrainer une diminution importante de la qualité du plasma. Une accumulation des impuretés au coeur du plasma est souvent observée au sein des tokamaks. Cette accumulation est souvent attribuée à la physique néoclassique mais le transport turbulent pourrait bien dominer dans la zone de gradient dans ITER. Dans le cas des impuretés légères, les transports néoclassique et turbulent sont du même ordre de grandeur dans les machines actuelles. Jusqu’à récemment, le calcul des flux néoclassique et turbulent étaient réalisés de façon distincte, supposant implicitement que les deux canaux de transport sont indépendants. On peut se demander si cette hypothèse est valide.En effet, des simulations préliminaires obtenues avec le code gyrocinétique GYSELA ont montré l’existence d’une synergie entre transports néoclassique et turbulent dans le cas des impuretés. Mais la compréhension théorique de cette synergie était manquante. Des simulations utilisant un nouvel opérateur de collision, des conditions aux limites plus réalistes et des sources plus flexibles ont été réalisées. Ces simulations ont permis de confirmer l’existence d’une synergie et un mécanisme permettant sa compréhension a été trouvé.La turbulence peut générer des asymétries poloidales. Un travail analytique réalisé pendant cette thèse permet de prédire le niveau et la structure de la partie axisymétrique du potentiel électrique. Deux mécanismes sont à l’origine des asymétries poloidales du potentiel électrique: à haute fréquence, la compressibilité du flot est à l’origine de l’asymétrie et la théorie prédit une structure ensinus comme dans le cas des GAMs. Pour les fréquences plus basses, le ballonnement de la turbulence engendre l’asymétrie poloidale. Dans ce cas une structure en cosinus est prédite par le modèle analytique.Une nouvelle prédiction du flux d’impureté néoclassique en présence d’asymétries poloidales et d’anisotropie de la pression a été réalisée. Il s’avère que les contributions banane/plateau et Pfirsch-Schlüter sont fortement impactées par la présence d’asymétries poloidales et d’anisotropie de la pression. Un bon accord a été trouvé entre la nouvelle prédiction et une simulation réalisée avec GYSELA pour laquelle la turbulence est à l’origine des asymétries poloidales et de l’anisotropie de la pression

P-27 Subcellular distribution of carboxypeptidase O affected by nutrient availability

Carboxypeptidase O (CPO) is a member of the M14 family of proteolytic enzymes. Many members of this family of enzymes are secreted from cells and are involved in degradation of extracellular peptides; other carboxypeptidases remain within the secretory pathway of the cell where they are involved in the maturation of bioactive peptides. In order to determine the function of CPO, the subcellular localization of CPO was determined by immunofluorescence analysis. CPO, stably expressed in Madin-Darby canine kidney cells, was found in a punctate pattern partially associated with lipid droplets. When cells were serum-starved the structures associated with CPO became more numerous and often clustered. Addition of oleic acid to the medium caused CPO to be redistributed in a diffuse pattern with some concentration on the nuclear envelope. Western blot analysis of CPO following oleate incubation indicated an increase in the amount of a CPO isoform with lower mobility as seen by SDS-PAGE, suggestive of a post-translational modification. These changes in CPO size and distribution, dependent on nutrient availability, suggest a possible role for CPO in autophagy-related processes

Kinetic plasma-sheath self-organization

The interaction between a plasma and a solid surface is studied in a (1D-1V) kinetic framework using a localized particle and convective energy source. Matching the quasineutral plasma region and sheath horizon is addressed in the fluid framework with a zero heat flux closure. It highlights non-polytropic nature of the physics of parallel transport. Shortfalls of this approach compared to a reference kinetic simulation highlight the importance of the heat flux as the measure of kinetic effects. Non-collisional closure and higher moment closure are used to determine the sound velocity. Within these frameworks, no gain in the fluid predictive capability is obtained. The kinetic constraint at the sheath horizon is discussed and modified to account for conditions that are actually met in simulations, namely quasineutrality with a small but finite charge density. Analyzing the distribution functions shows that collisional transfer is mandatory to achieve steady-state self-organization on the open field lines

Drift-wave observation in a toroidal magnetized plasma and comparison with a modified Hasegawa-Wakatani model

International audienceThis paper presents the results of fluctuation measurements in a toroidal magnetized plasma, using Langmuir probes, and comparisons between the observed frequency modes and a Hasegawa-Wakatani model including curvature, adapted to the specifics of the toroidal device. More precisely, two kinds of signals are detected in the presence of an additional vertical magnetic field in the region of significant density and potential gradients. A high frequency, propagating component, corresponding to dissipative drift-waves in the curved magnetic field, is observed and the frequency and typical wavelengths are found to be in good agreement with the linear Hasegawa-Wakatani model including curvature effects. A second, low frequency component is observed at lower frequencies and is shown to correspond to large scale vertical electrostatic field structures. A significantly high level of cross correlation is observed between these two signals, with an identifiable time delay, which suggests an analogy to the time delayed quasi-periodic dynamics in predator-prey systems, and a similar phenomenon is observed between zonal flows and microturbulence in tokamaks and other magnetised plasma systems

P-23 Carboxypeptidase O Is Able to Cleave both Acidic and Polar Amino Acids from Substrate Proteins within the Early Secretory Pathway

Carboxypeptidase O (CPO) is a member of the M14 family of metallocarboxypeptidases with a preference for the cleavage of C-terminal acidic amino acids. CPO is largely expressed in the small intestine, although it has been detected in other tissues such as the brain and ovaries. CPO does not contain a prodomain, nor is it strongly regulated by pH, and hence appears to exist as a constitutively active enzyme. The goal of this study was to investigate the intracellular distribution and activity of CPO in order to predict physiological substrates and hence function. The intracellular distribution of CPO, expressed in MDCK cells, was analyzed by immunofluorescence microscopy. CPO was detected on the endoplasmic reticulum in a punctate, circular, or reticular pattern that was poorly associated with lipid raft markers, although modulated by cholesterol. Cholesterol also modified the enzymatic activity of CPO in vitro; no effect was observed in vivo. The ability of CPO to cleave C-terminal amino acids within the early secretory pathway was examined using Gaussia luciferase as a substrate, C-terminally tagged with variants of an ER retention signal. These data show that CPO functions in the ER or Golgi to remove C-terminal glutamates and aspartates, as well as a number of polar amino acids. Following bioinformatics analysis to determine candidate substrates, Torsin 1A, an ER protein known for its role in early-onset torsion dystonia, was confirmed as a substrate of CPO