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Advancements in Langmuir Probe Diagnostic for Measurements in RF Sheath and in Modelling of the ICRF Slow Wave Mariia Usoltceva

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Advancements in Langmuir Probe Diagnostic for Measurements in RF Sheath and in Modelling of the ICRF Slow Wave Mariia Usoltceva Advancements in Langmuir probe diagnostic for measurements in RF sheath and in modelling of the ICRF slow wave Mariia Usoltceva To cite this version: Mariia Usoltceva. Advancements in Langmuir probe diagnostic for measurements in RF sheath and in modelling of the ICRF slow wave. Plasma Physics [physics.plasm-ph]. Université de Lorraine; Rijksuniversiteit te Gent, 2019. English. NNT : 2019LORR0061. tel-02970051 HAL Id: tel-02970051 https://hal.univ-lorraine.fr/tel-02970051 Submitted on 17 Oct 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. AVERTISSEMENT Ce document est le fruit d'un long travail approuvé par le jury de soutenance et mis à disposition de l'ensemble de la communauté universitaire élargie. Il est soumis à la propriété intellectuelle de l'auteur. Ceci implique une obligation de citation et de référencement lors de l’utilisation de ce document. D'autre part, toute contrefaçon, plagiat, reproduction illicite encourt une poursuite pénale. Contact : [email protected] LIENS Code de la Propriété Intellectuelle. articles L 122. 4 Code de la Propriété Intellectuelle. articles L 335.2- L 335.10 http://www.cfcopies.com/V2/leg/leg_droi.php http://www.culture.gouv.fr/culture/infos-pratiques/droits/protection.htm _________________________________________________________________________ École doctorale "C2MP" ED 606 Sciences et Technologies Thése pour l’obtention du titre de Docteur de l’Université de Lorraine présentée par: Mariia Usoltceva _________________________________________________________________________ Advancements in Langmuir probe diagnostic for measurements in RF sheath and in modelling of the ICRF slow wave _________________________________________________________________________ Thèse soutenue publiquement le 26 Avril 2019 à Nancy devant le jury composé de: Prof. Gérard Henrion Président du jury - ULorraine Prof. Daniel De Zutter Co-président du jury - UGent Prof. Hartmut Zohm Rapporteur - MPI Prof. Stéphane Bechu Rapporteur – CNRS Grenoble Prof. Hendrik Rogier Examinateur - UGent Prof. Jean-Marie Noterdaeme Examinateur - UGent/MPI Prof. Kristel Crombé Co-directeur de thése - UGent/ERM Prof. Stéphane Heuraux Co-directeur de thése - ULorraine Dr. Eric Faudot Co-directeur de thése - ULorraine IJL - Université de Lorraine, Ghent University i Acknowledgments “I've heard it said That people come into our lives for a reason, Bringing something we must learn.” “Wicked” the musical This work would not be possible without the people who supervised, directed and advised me during the 3.5 years: Dr. Roman Ochoukov (IPP Garching), Dr. Eric Faudot (IJL, Université de Lorraine), Prof. Stéphane Heuraux (IJL, Université de Lorraine), Prof. Kristel Crombé (Ghent University) and Prof. Jean-Marie Noterdaeme (IPP Garching/Ghent University). I had a chance to learn from them many aspects of physics and of scientific skills in general. Thank you all for the amount of work you have done to ensure my progress and achievement of the final result – this doctoral dissertation. I would also like to acknowledge the high quality of the Fusion-DC program and its great variety of the learning opportunities for young scientists, I am glad to be a part of this international program. Many other people also helped and supported me in my PhD work. I am grateful to the ICRF department of IPP and to the plasma physics group of ULorraine, as well as to external collaborators. I am pleased to acknowledge the help offered by Ana Kostic, Guillermo Suárez, Wouter Tierens, Jonathan Jacquot, Rodolphe D’Inca, Stéphane Devaux, Georgiy Zadvitskiy, Jordan Cavalier and others during the course of this work. We shared very interesting theoretical discussions, experimental time on the test stands, data analysis, exchange of ideas, etc. I hope to continue fruitful collaboration with many of them in the future. I am happy that I had friends who surrounded me in always nice and friendly atmosphere during the years in Garching. Thanks to Anton for bringing me to the river surfing and for guaranteeing me a black eye on my very first week at work. Serhiy, you have been always a very welcoming and fun friend to me and we won so many battles together. Dmytro, thank you for introducing me to the tasty food of France and to many other nice things from the French life, as well as for the discussions of the plasma physics theory. My big gratitude to Wanja for wonderful photos, for your patience, kindness and for the inspiration. With Filip we have had a great time enjoying the nature, mountains, sheep, fish, swimming and skiing. Ana has been a very close person to me, not only because her table was next to mine in the office, but because we shared so many discussions and even some adventures. Thanks to all great people in Garching, so many awesome things happened, that I cannot recall them all. Thanks to Georgiy for the best time in Nancy, for making my dreams come true and just for being a very good friend. Jordan, I appreciate that you controlled the quality of the tea. I am very grateful to Sophie and Eric with whom I had luck to live under the same roof. They showed incredible hospitality ii and made me feel at home in a new country. Thanks to all amazing people who I met in France and had very nice time with. As going through the years of the PhD research also means going through the years of constantly changing and challenging life, thanks to my family for supporting me through it and to Ramon, I am who I am today because I know you and because of all I learned from you, for good. iii Summary of the thesis Coupling power to the plasma with ion cyclotron range of frequencies (ICRF) waves is a promising method for heating tokamak plasmas to fusion relevant temperatures. To ensure good ICRF wave accessibility to the plasma center, the ICRF antenna must be placed close to the plasma. In experiments on multiple tokamaks, active ICRF antennas modify key plasma edge parameters (electron temperature, plasma density, and plasma potential), as well as enhance plasma-wall interactions, i.e. changes in the edge plasma density profiles, increased heat loads and hot spot formations on limiters and enhanced sputtering of plasma facing components. The plasma ions accelerated in the radio-frequency (RF) sheath have been found to be the main cause of the observed deleterious effects. RF sheath is an additional sheath that exceeds standard Bohm sheath in voltage and is present during active ICRF operation. ICRF parallel electric field accelerates electrons towards the wall along the magnetic field lines; it leads to a negative charge on the surfaces and a positive charge of the bulk edge plasma where slow ions remain. At the end a DC potential is formed between the wall and the plasma and such a process, called RF sheath rectification, leads to further strong acceleration of ions into the plasma facing components. No unified guideline exists on optimal ICRF antenna design, as observed effects and found solutions differ from one machine to another. Conclusions are usually empirical and based on consequential parameters like impurities concentration, RF currents or temperature of antenna limiting structures, but not on the direct measurements of the RF-induced parallel electric field. Such measurements are hard to perform in a tokamak. One of possible approaches to study parallel electric field of an ICRF antenna directly is to move to a simple specially designed experiment. This thesis presents work carried out on two dedicated devices, Aline (A LINear Experiment) and IShTAR (Ion cyclotron Sheath Test Arrangement), testbeds that are fully focused on ICRF studies and exhibit plasma conditions that are similar to the tokamak edge but quite simplified in order to provide easier accessibility for dedicated diagnostics and to decouple ICRF physics from many other phenomena existing in a complex plasma machine. Aline is a linear low-temperature plasma device with a capacitive radio-frequency discharge created by a simple antenna of a disk shape and it focuses on plasma characterization with the Langmuir probe diagnostic. The diagnostic setup has numerous advantages over common Langmuir probes setups in tokamaks. The probe inside the plasma vessel can be exchanged or modified quickly and at any time, even during the experimental day. A Langmuir probe is fixed on a 3D movable manipulator. The movement accuracy is 10 µm which is required to achieve precise spatial resolution for measurements in a sheath, the later itself being of the order of µm-mm size. The probe can reach the whole space around the RF antenna which is centered in the vacuum chamber. Very long (hours) and stable plasma discharges guarantee reliable measurements and absence of transient events changing the density profile globally or locally. iv To be able to do measurements for the RF sheath relevant studies, it is first necessary to develop a routine method of Langmuir probe data analysis in magnetized plasma that would be applicable in quite broad range of plasma conditions characteristic for Aline. The presence of magnetic field leads to the fact that conventional methods of data analysis are not applicable, because of completely different particle transport in magnetized plasma compared to the non-magnetized one, especially when using small cylindrical Langmuir probes parallel to the magnetic field or at a small angle to it. This case is realized in Aline and such probes are the most suitable for measurements in narrow sheaths.
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