Dans ce nouveau podcast de la série « Dans les pas d'Archimède », partez à la rencontre de Christelle Reynard-Carette, Directrice Adjointe Recherche de l'Institut, professeure à AMU et chercheuse a
Laureat
"The purpose of my mobility project has been to develop the skills to perform, analyse and understand the PIV technique for flow fields in complex geometry. I have chosen the Thermo-fluid lab at George Washington University because of their deep knowledge in such topic. Furthermore, they have a facility similar to the one I am using during my PhD project.
The project began with the purpose of making and analysing PIV index match measures. Unfortunately, after the first few weeks of preparing the experimental setup, during a pressure test, so we decided to perform the analysis on experiments done previously on the same setup but never analysed. At the same time, I was able to learn how to perform PIV measurements on another type of setup, even more interesting but less related to my PhD subject: an entire sloshing swimming pool (yes, it was big enough you could actually swim inside!).
Beyond the professional aspect, this experience has also enriched me on a personal level. It was my first time in the United States and Washington, DC welcomed me with its heterogeneity of cultures that led me to change my point of view and challenge my beliefs. During the 3 months of the mobility project, I was able to travel and attend conferences and seminars in other cities as well, such as that of the American Physical Society in Phoenix and that of the American Nuclear Society in Washington, DC."
Laureat
"Le domaine de mon projet de mobilité qui s’intitulait SIM-IRRAD (Slovenian International Mobility at the Jožef Stefan Institute for an IRRADiation campaign preparation and radiation/matter simulations) concernait des travaux de recherche sur l'instrumentation nucléaire innovante pour la mesure en ligne de paramètres nucléaires clés dans les installations de recherche nucléaire tels que les flux de neutrons (thermiques et rapides). Cette période internationale s'est déroulée de fin Septembre à fin Décembre 2021 à l'Institut Jožef Stefan (JSI) de Ljubljana en Slovénie (Europe centrale). Ce projet de mobilité était composé de diverses activités de recherche telles que des travaux de simulation numérique 3D des interactions rayonnement/matière avec un code Monte-Carlo dans des détecteurs semiconducteurs neutroniques, la préparation de leur étude expérimentale et leur caractérisation lors d'une campagne d'irradiation dans le réacteur de type TRIGA Mark II du JSI. Plus précisément les capteurs étudiés étaient des diodes à jonction p+n en Carbure de Silicium (SiC) avec ou sans convertisseur de neutrons thermiques et un détecteur en diamant monocristallin (fonctionnant comme une chambre d'ionisation à l'état solide). Ci-dessous sont présentées quelques-unes des principales activités réalisées dans le cadre de cette mobilité :
"L'Institut Jožef Stefan, qui était l’organisme hôte, est le plus grand et le principal institut de recherche de Slovénie dans plusieurs domaines : Physique, Chimie et Biochimie, Electronique et Technologies de l'Information, Ingénierie des Réacteurs et Energétique. L'équipe qui m'a accueilli appartient au département de physique et se nomme Physique des Réacteurs (F8). De plus, cet institut est un partenaire clé et reconnu internationalement et possède un réacteur de recherche nucléaire : un réacteur de recherche de type TRIGA Mark II de puissance thermique 250 kW. Il s'agit d'un réacteur de type piscine à eau légère, refroidi par convection naturelle et doté de nombreux canaux d'irradiation en cœur et hors du cœur. De plus, en raison de l'absence de telles infrastructures en France et de la longue planification du processus (financement, études de sécurité, préparation, ...), il est assez rare au cours d'une thèse de doctorat d'obtenir une telle opportunité.
Cette mobilité m'a permis d'avoir une réelle expérience professionnelle à l'étranger (plus qu'une conférence conventionnelle) et me permettra d'avoir la possibilité de postuler au Label Européen qui contribuera à mettre en avant ma thèse. Cette mobilité m'a également permis d'accroître la collaboration avec les membres de l'équipe de Physique des Réacteurs et de développer mon réseau pour mes activités post thèse.
A travers les différentes activités que j'ai menées, j'ai amélioré mes compétences et mes connaissances dans le domaine de l'instrumentation nucléaire, par la réalisation de cours, de mesures et d'expériences dans un réacteur de recherche nucléaire. Ainsi, il s'agissait d'une possibilité réelle et concrète de mener à bien mon travail de thèse au-delà des objectifs initiaux en élargissant l’étude des détecteurs en SiC à d’autres conditions expérimentales plus extrêmes (réacteur nucléaire en complément de mesures en générateur de neutrons rapides mono-énergétiques Deutérium-Tritium). Enfin, durant ces trois mois, j'ai amélioré mon niveau d'anglais, tant sur le plan technique que sur le plan courant."
"Le point le plus pertinent de ma mobilité concernant les activités réalisées a été la durée de la campagne d'irradiation. En effet, cette dernière a duré trois semaines et a permis d'obtenir plus de résultats que prévu en réalisant des études paramétriques poussées. En effet, des mesures en cœur et hors du cœur ont été réalisées pour différentes puissances de réacteur et donc différents flux neutroniques. De plus, différentes tensions de polarisation du détecteur ont été testées afin de faire varier la zone sensible appelée Zone de Charge d’Espace des diodes p+n en SiC et ainsi jouer sur l'efficacité de collecte des charges pour les deux types de détecteurs. Différentes diodes p+n en SiC avec différentes surfaces actives ont également été testées. Une comparaison de la réponse impulsionnelle des diodes p+n en SiC et du diamant monocristallin avec convertisseur de neutrons thermiques (Bore-10 et Lithium-6 respectivement) ainsi qu'une comparaison de la réponse des détecteurs en SiC avec et sans convertisseur de neutrons thermiques en Bore-10 ont été effectuées. De plus, j'ai mis en place et utilisé une nouvelle chaîne d'acquisition avec des équipements plus performants tels que des amplificateurs de courant rapide et un oscilloscope numérique avec une fréquence d'échantillonnage de 20 GHz."
"C'était la première fois que je venais en Slovénie et, plus largement, c'était mon plus long séjour dans un pays étranger. La Slovénie est un pays magnifique, avec une nature vraiment préservée et des paysages très différents (forêt, lacs, montagnes, mer, grottes). Culturellement, ce pays est influencé par ses pays limitrophes (Italie, Autriche, Hongrie et Croatie), ce qui le rend d'autant plus intéressant de par l'architecture des bâtiments, le mode de vie des gens, la cuisine et les traditions locales. Un aspect très positif est la convivialité des Slovènes envers les étrangers et en particulier les Français. J'ai été vraiment impressionné par l’accueil qui m’a été réservé que ce soit par mes collègues scientifiques slovènes ou les gens dans la vie courante. En ce qui concerne la capitale, Ljubljana, c'est une belle ville chargée d'histoire avec beaucoup de sites à découvrir et de lieux à visiter. De plus, la ville est très bien desservie par les transports en commun, ce qui permet de se déplacer assez facilement."
Laureat
"Le but de ce projet était de développer un modèle théorique de la conductivité thermique dans les milieux poreux au CRCT (Centre de recherche en thermochimie computationnelle) de Polytechnique Montréal à partir de mesures effectuées à l'IUSTI et de données de la littérature. L'objectif était de mettre en évidence l'importance de la microstructure pour le transfert de chaleur dans les milieux poreux et ultra-poreux à phase solide non connectée, en profitant de l'expertise sur le sujet du chercheur qui m'a accueilli. Nous avons cherché à caractériser l'influence de la porosité sur l'évolution des propriétés thermiques, en considérant différentes échelles et la présence de gaz. Une attention particulière a été portée sur les paramètres microstructuraux de la porosité : taille moyenne, distribution de la porosité et les effets du couplage porosité-paramètres microstructuraux (taille des grains, distribution de la taille des grains, chimie des joints de grains, configuration de la microstructure). Les modèles théoriques du CRCT ont été à la base de notre modélisation de la matière à l'échelle microscopique de manière théorique, de l'échelle moléculaire, à l'échelle de la taille des grains et de la porosité.
Cette mobilité était un moyen d'élargir les perspectives sur mon sujet de thèse en ayant une approche différente, plus théorique. Elle m'a également permis d'avoir une vision plus large de ce que pouvait être la recherche, surtout dans un pays étranger et une culture différente, ce qui pourrait influencer mes choix pour le post-doctorat. C'était aussi ma première occasion de voyager en Amérique du Nord, où j'ai pu découvrir une culture commune entre les Américains et les Français, ce qui a été une expérience très enrichissante. Malgré le froid et les restrictions dues au Covid, j'ai pu découvrir les joies du patinage sur des lacs gelés et de la randonnée à -20°C dans des paysages magnifiques. Cette mobilité a été très enrichissante d'un point de vue professionnel et personnel et c'est pourquoi j'encourage tous les doctorants à postuler pour ce genre d'expériences et d'opportunités uniques."
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Laureat
"Within the frame of the Ph.D. project being carried out at the University of Aix-Marseille by myself, Daniele Villa, in the PTM team (PIIM UMR 7345), a mobility period to the University of Saskatchewan, in Saskatoon (Canada), took place from the 1st of March to the 31st of May. During this time my supervisors (Olivier Agullo and Nicolas Dubuit) and I collaborated with the host researcher, Prof. Andrei Smolyakov, on a number of topics that had been agreed upon before the start of the collaboration.
In order to better explain the work done, here follow a few words on the topic of the Ph.D. In the field of plasma physics, specifically of thermonuclear fusion by magnetic confinement, an area where the knowledge of the community is still incomplete is that of the interaction between turbulence and magnetic islands. Turbulence refers to the small-scale fluctuations of the plasma that give rise to flows and diffusion in the fluid, affecting the performance and stability of a potential nuclear reactor, while magnetic islands are large scale phenomena linked to the modification of the magnetic field used to confine the plasma itself. Magnetic islands are known to be particularly problematic for a potential reactor as their presence increases the risk of abrupt ejections of heat from the plasma (a so-called disruption) that would damage the machine, while at the same time limiting the maximum performance achievable. The complex dynamics and high computational cost to investigate such interaction has caused a reduced number of studies to be carried out on the topic. The team in Marseille has, however, developed a fluid code named “AMON” that allows to study self-consistently the interactions on such a wide spectrum of spatial and temporal scales, making the study of such interactions possible, and indeed already happen in certain contexts (Dubuit, PoP, 2021; Muraglia, PRL, 2011; Agullo, PoP, 2017).
The original goal of the Ph.D. project was that of focusing specifically on this interaction, exploring different contexts and scenarios, using a model recently developed by the TPM team at the University of Aix-Marseille (Frank et al., PoP, 2020) to carry out numerical and analytical studies. This model is based on the more streamlined and computationally light models already in use in the team, that have been extended with the specific goal of carrying out studies like the one envisioned for the Ph.D. project, related to phenomena of relevance for real-world applications.
Over the first year of the project, however, it became clear that the model itself needed some refining and reviewing to better suit the task, especially when an interesting and never before observed phenomenon related to the plasma pressure increasing in the presence of magnetic islands appeared in simulations. This result is not only new and unforeseen, but potentially far-reaching in its consequences for the analysis of real-world discharges, as the state-of-the-art understanding of magnetic islands dynamics is that pressure should be flattened inside the region of the plasma contained in the island (Fitzpatrick, 1995), and not show the local increase we get in our simulations, thus a very interesting new venue for investigation could be opened by our results.
Since this heating phenomenon, that constitutes the subject of a paper soon to be submitted, was made possible by some specific terms in the model that the team in Marseille had never dealt with before, the help of an external expert was sought, and Prof. Andrei Smolyakov showed interest in the subject.
The University of Saskatchewan hosts not only very prominent scientists for the theory of plasma physics, but also a number of experiments in the field, including a small compact Tokamak, known as STOR-M. I had the chance to learn about this machine and meet the people who conduct experiments on it during my stay.
Indeed, the first month and a half of the mobility was dedicated to identifying, understanding and fixing certain issues existing within the model, which more or less led to the model being rebuilt from the ground up, with significant improvements in its physical basis and reliability. Comparisons of this model to other existing and similar models ( GBS as described in Giacomin, submitted, 2021 and GRILLIX as described in Appendix A of Zholobenko, PPCF, 2021 ) were carried out, finding that, in the appropriate limits, the equations matched those of these pre-existing models. A consequence of this is that the model is now capable of allowing more in depth and accurate studies of the non-linear dynamics of systems, that will be crucial going forward in the Ph.D. to study the aforementioned interaction between magnetic islands and turbulence. It is also the case that the team in Marseille can now, to a degree, replicate and validate studies done by the teams using the codes indicated above, that, to our knowledge, have not been used to study the dynamics of magnetic islands yet. The focus was then shifted to the implementation of the new model into the existing numerical framework AMON established at the TPM group at the University of Aix-Marseille. This required some time to implement all the new elements of the model and properly test the code, making sure that results were reliable and understanding the properties of the new model.
With this new model the analysis done about the heating effect was carried out a-new, recovering the previous results, along with some new interesting elements, with a more robust model and a better understanding, on my part at least, of the physical processes involved, thus leading to the revision of the paper that is going to be submitted in the coming weeks.
The remaining time in Canada was spent, on the one hand, further working on the model to recover, with this more rigorous approach, certain features present in Frank et al., PoP, 2020, that had been put aside in the re-derivation of the new model, which will be useful for the continuation of the Ph.D. project. Such elements include the neoclassical terms, that allow to study a group of phenomena of great interest for modern fusion experiments, like Neoclassical Tearing Modes, a particular type of magnetic island that arises in large scale experiments limiting their operational possibilities, and that we plan to start including in our analysis in the near future. These neoclassical terms are not often included in fluid codes, but including them, in the appropriate regime, might provide further insight into the dynamics of real-world plasmas. On the other hand, during the remaining time in Saskatoon, the topic of the properties of turbulence itself in thermonuclear plasmas was expanded upon, in particular the effect of the plasma pressure on turbulence, as the turbulence transitions from electrostatic to electromagnetic. This is the second element that I’ll need to study in detail in the remainder of my project to then move to the analysis of the interaction of this with magnetic islands.
On this latter point, the work in the paper by Hirose, PoP, 2000, was re-analyzed from a numerical as well as analytical point of view, which is of particular interest going forward in the Ph.D. as the interaction between magnetic islands and turbulence we aim to study is of particular interest when it comes to Ion-Temperature-Gradient turbulence, studied in the above mentioned paper, that is particularly active in the core region of thermonuclear plasmas. Indeed, going forward, we aim to study more in depth the generation of magnetic islands by this kind of turbulence, the way the turbulence interacts with a pre-existing island and, finally, how this extends to the case of Neoclassical Tearing Modes, which will be of great interest for real-world applications.
The paper is also carried out, for a good portion, with a kinetic model, which is a much more fundamental description of the dynamics of plasmas compared to the fluid model described above. This approach to the study of plasma physics had not been explored in depth in the Ph.D. project up to this point, and this allowed me to obtain meaningful insight and skills on this topic.
The work done on the paper by Hirose was also a chance to develop code that could be applied to answer concrete questions, like finding eigenvalues for differential equations, and was also a chance to go hands-on with programming techniques such as multi-process parallelization with MPI and GPU computing, of which, up to this point, I had mostly been a user, never a developer.
As can be seen the mobility period lead to concrete and significant results already, that are in line with the initial goals we had set for ourselves, and has provided a lot of interesting material to work on for time to come.
On a more personal note, the possibility of dedicating myself to the task of analyzing and re-building the model from a first principle perspective has provided me with very precious insight, that I felt I was lacking with the more numerically-centered approach we were using in Marseille, and I thus feel like I have acquired a much more solid knowledge and understanding of the fluid description of plasmas. This will be invaluable going forward in the Ph.D. project and in my research career.
Also invaluable is the lesson learned when looking back on the work done thus far in the Ph.D. project of truly applying critical reasoning when approaching a problem and being presented with results, going in depth until all questions are answered.
More generally, the possibility of shifting perspective and getting input from new and different sources has proven very beneficial.
While I was not able to create a lot connections with researches working in a field similar to mine, since, like in Marseille, not many people work on a topic similar to mine at the University of Saskatchewan, I still had the chance to learn a lot about other fields of research by meeting colleagues and other students, as well as by learning about the research being carried out at the University of Saskatchewan. This was also a useful lesson in carrying out work independently, as not having frequent contact with my supervisors forced me to deal with problems on my own and “learn by doing”.
Still, at the University of Saskatchewan there are numerous interesting fields of research being explored, not only in Physics, so I got to discuss with people working on cold/dusty plasmas, veterinarians, cancer researchers and many others, in a stimulating and dynamic environment. Learning about the realities of the education system in North-American countries has also been extremely eye-opening and invaluable even for future life choices.
Visiting Canada has been an extremely exciting experience, that allowed me to meet people from many various, and different to my own, backgrounds, discover what life is like in a North American country and travel to places that I might not have had the chance to visit as easily otherwise, both cities and wild areas. Coming from two years of Covid-19 pandemic, the possibility of traveling across the ocean and working with people face-to-face has been a very welcome change.
In conclusion, the experience has been very positive and formative, and I consider periods of mobility like these to be fundamental in the education of young researchers, and young people more in general. I am very grateful for the chance I was given by the University, and for the funds dedicated to this project by the ISFIN, as well as to my supervisors for according me the time and to the host researcher for taking on the project.
This is an experience I’ll definitely support and encourage others to undertake in the future."
Laureat
"A collaboration between the IRFM, CEA research institute in France and the DIII-D National Fusion Facility, operated by General Atomics (San Diego) was made possible thanks to the ISFIN mobility project (mobilité international pour les étudiants doctorants). The objective was to verify and validate the predictive capabilities of codes currently used for estimating W sources at divertors and impurity transport in SOL plasma through an experiment. Five biased samples were inserted into DIII-D lower divertor using the Divertor Material Evaluation System (DiMES) manipulator and exposed to constant L-mode attached plasma conditions. The samples were manually coated with Carbon microsphere in the weeks prior to the experiment. Plasma conditions at the lower divertor were optimized to obtain the best compromise between emissivity of eroded material and high screening above the target. W sources were monitored with in situ visible spectroscopy, imaging, VUV, LP, and Thomson scattering. The in situ measurements were then complemented by post-mortem analysis for measuring the net erosion of W. The experimental erosion data were finally compared with simulations done with the ERO2.0 code. The experience was enriching not only from a technical point of view but also from a personal point of view, offering the opportunity to visit and learn about a similar but at the same time distant culture as that of Western America.
The PhD topic is mainly concerned with the study of erosion and redeposition of W using quasi-analytical models. The main objective is to understand what are the main drivers underlying erosion in order to implement them in reduced models. In particular, the focus is on the effect of sheath potential drop, ionization coefficients, and incident distributions in the final estimation of erosion and redeposition. The experiment performed in the project is a key step in the thesis. In fact, this fits perfectly into the study plan, enriching it with the possibility of planning and co-conducting an experiment. The DIII-D tokamak was particularly suitable for the purpose. In fact, the uniqueness of DIII-D is that it is equipped with the DiMES system through which it is possible to insert and remove W surfaces at each shot at the lower divertor. Moreover, DIII-D is a reactor that does not normally employ W hence it is easy to isolate the contribution of local sources. To give an idea, in the WEST tokamak currently in use at CEA by IRFM this would not have been possible. In fact, since the machine is completely covered in W and without systems such as DiMES, figuring out the cause of erosion at the divertor is a very complex challenge linked to integrated effects.
The project lasted about six weeks. The timeline observed during the tenure is proposed in this section. In the first week an erosion study was devised based on sample design, the purpose was to predict where the W might be most eroded/redeposited. In the second and third weeks, a so-called mini-proposal was drafted in which the reasons for requesting the experiment were justified and the required plasma conditions were engineered (such as Power in input, which heating system to use, which probes, etc.). At the same time, once the samples were received, they were analyzed under an optical microscope and covered with microspheres with an average diameter of 6 μm, by hand. The morphology of the samples was analyzed by electron microscope (SEM) before erosion. In the fourth week, some preliminary meetings were made and the experiment was finally carried out. In the last two weeks, the post-exposure sample morphology was compared with the pre-exposure morphology, at the same time a comparison study was made between temperature and electron density data measured with Langmuir probes and Thomson scattering. Finally, a set of simulations with ERO2 was produced by taking as input the experimental electronic density and temperature measures.
America has amazed me on a personal level. The work culture is full of optimism, positivity, and team building. It really pushed me to give my best and enjoy the work there. At the same time, San Diego special microclimate makes it an oasis in the desert. Temperatures are almost always pleasant and thanks to the lack of rainfall it seems always summer. However, there are not only positive sides, in particular I noticed an extreme use of private vehicles (a problem also present in Europe), one must always go by car, both for safety and for lack of public transportation. Finally, there are obvious problems of homelessness especially around downtown. Still though, to have a research experience I think it is an excellent place."
Laureat
" The purpose of my mobility was to conduct an experimental campaign on the RAID device at Swiss Plasma Centre in order to investigate the occurrence of coherent fluctuations and characteristics and to apply the analytical model currently being developed to investigate the stability of MISTRAL (a linear plasma device at PIIM laboratory to study cross field plasmas) plasmas to RAID plasma conditions. The key distinction between the two devices is that MISTRAL employs a DC discharge to produce plasma from a hot electron beam whereas RAID uses high-power helicon sources. This provided me an opportunity to work on a linear plasma device having a different source configuration and thus different characteristics from that of MISTRAL.
I measured the plasma characteristics (number density, electron temperature, plasma potential, and floating potential) using the Langmuir probe diagnostic available on the RAID device. We find out that in order to apply our model to the RAID plasma, one of the fundamental assumptions used in the model needs to be removed which can be one of the future perspectives of my current work. It was a great experience to work in a completely new environment. Additionally, it gave me the opportunity to see research from a wider perspective, especially when conducted in a foreign nation with a distinct culture.
However, since it was the summer period (June-August) and I lost a few days of work since some of the individuals I was meant to work with were on vacation, the timing of the mobility was not ideal. Apart from that, everything goes well, and it was an entirely different sort of experience.
Since EPFL is housed in Lausanne, Switzerland, I got the opportunity to explore this picturesque region, which was a visual feast. The people in the laboratory were very welcoming and helpful. It took very little time to feel at ease in the new surroundings. "
Laureat
"As a third-year Ph.D. candidate, I am working on the experimental characterization of the irradiation channels of the CNESTEN1’s TRIGA Mark II research reactor located in Rabat, Morocco. This type of reactor is specially designed to effectively implement the various fields of nuclear research such as Neutron Activation Analysis, education and training, Neutron Radiography, Detectors testing and radioisotopes production. During my PhD first year, I developed a complete computational model of the TRIGA reactor using the 3-D continuous energy Monte-Carlo code TRIPOLI4® in order to support planning, design and implementation of new experiments within and beyond the reactor core. Moreover, during my first Ph.D. year, measurements based on neutron activation technique, were carried out in order to characterize the neutron flux in different irradiation channels of the CNESTEN’s TRIGA reactor. The latter study was carried out as part of the bilateral collaboration between the French Atomic Energy and Alternative Energies Commission (CEA) and the CNESTEN. This collaboration has been established in order to accurately characterize the irradiation and instrumentation channels of the CNESTEN’s TRIGA reactor. Therefore, the work carried out during my thesis should make it possible to extend the experimental validation base of the CNESTEN TRIGA calculation scheme, by proposing, carrying out and analyzing experiments to characterize the neutron and gamma fields at different locations. A critical analysis of this computational model will then be carried out in order to improve its performance with regard to the important parameters during the qualification of nuclear instrumentation.
In January 2022, ISFIN has agreed to finance my mobility project to the CNESTEN. The project as built and presented in the proposal will allow me, on the one hand, to improve and strengthen my scientific and technical skills by carrying out and follow up closely new experiments in the reactor (neutron activation technique, ionization chamber measurements and nuclear heating measurements). On the other hand, spending 6 weeks interacting and scientifically exchanging with the nuclear experimental reactor team/community at the CNESTEN will be a great opportunity for me to develop disciplinary and inter-disciplinary skills in the nuclear instrumentation and it will inevitably have an eminent impact, a capital gain and a benefit on my PhD work.
Through this mobility project, I was able to conduct an experimental campaign combining different nuclear measurement methods and techniques that will enable me to strengthen the knowledge of neutron and photon flux within and around the reactor core. Eventually, the characterized irradiation positions will be used, firstly, to validate the computational model of the reactor and, secondly, to both test and calibrate innovative nuclear instrumentations before their implementation in nuclear power plants, and to carry out experiments allowing the improvement of existing knowledge on fundamental parameters in nuclear physics.
I would like to thank the Institut Sciences de la Fusion et de l'Instrumentation en environnements Nucléaires (ISFIN) for funding this mobility project. I would also like to thank the members of the CNESTEN’s TRIGA reactor, for providing suitable working conditions and for their valuable technical support to carry on the experiments in the reactor."
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