Effective collecting area of a cylindrical Langmuir probe in magnetized plasma

International audienceLangmuir probe diagnostic on magnetic plasma devices often encounters more challenges in data processing than in non-magnetized plasmas, the latest itself being far from simple. In this paper, a theory of particle collection by a probe at the plasma potential in collisionless weakly ionized plasmas is constructed, accounting for velocities distributed according to the Maxwell equation and different mechanisms of particle collection depending on their speed. Experimental validation of the presented theory has been done with 2 cylindrical probes (rpr = 75 mum and Lpr = 1 cm and rpr = 0.5 mm and Lpr = 1 cm) parallel to B --> on a linear plasma device Aline, with magnetic fields of 0.0024-0.1 T and plasma densities of 1015-1017 m-3 in helium. Cylindrical probe measurements are compared to data from a planar probe perpendicular to the magnetic field, and the results for electron density, temperature, and plasma potential are presented. The introduced theory is initially constructed for a cylindrical probe but is applicable to various probe sizes, shapes, and orientations. Alongside the main subject, a number of associated issues are addressed with different details: a probe design issue relative to the magnetized environment, the "intersection" method of plasma potential evaluation, and the robustness of the conventional "1st derivative" method, a current bump near the plasma potential, lower limit for electron temperature estimation, and self-consistent calculation of electron temperature and density

Experimentelle und numerische Untersuchung der Turbulenz in Fusionsplasmen mittels reflektometrischer Synthesediagnostik

Anomalous energy and particle transport is closely related to micro-turbulence. Therefore plasma turbulence studies are essential for successful operation of magnetic confinement fusion devices. This thesis deals with the development of interpretative models for Ultra-Fast Swept Reflectometry (USFR), a diagnostic used for the measurement of turbulence radial wave-number spectra in fusion devices. While the interpretation of reflectometry data is quite straightforward for small levels of turbulence, it becomes much trickier for larger levels as the reflectometer answer is no longer linear with the turbulence level. It has been shown for instance that resonances due to probing field trapping can appear in turbulent plasma and produce jumps of the signal phase. In the plasma edge region the turbulence level is usually high and can lead to a non-linear regime of the reflectometer response. The loss of probing beam coherency and beam widening when the probing beam crosses the edge turbulence layer can affect USFR core measurements. Edge turbulence with a long correlation length leads to small beam widening and strong distortion of the probing wave phase. However backscattering effects from turbulence with short correlation lengths are also able to cause reflectometer signal change. To study turbulence wave-number spectra as well as reflectometer signal phase variations, signal amplitude variations can be analized. Unlike signal phase variation, amplitude does not suffer from resonant jumps, and can give more clear qualitative evaluation of turbulence structure. In the case when the turbulence amplitude peaked in the edge region, it can be detected as spectral peak near local Bragg resonance wave-number. USFR with a set of receiving antennas arranged poloidally was proposed to obtain more information on the edge turbulence properties. A displacement of the spectral peak appears when the receiving antenna is misaligned with the emitting one. Peak displacement measurements could provide additional information on probing beam shaping and turbulence properties and help in coherent mode observation as well. A 2D full wave code was applied as a synthetic diagnostic to Gysela gyro-kinetic code data to study Tore-Supra tokamak core turbulence. Radial correlation lengths computed from the amplitude of multi-channel fixed frequency reflectometry signals 5have shown good agreement with the turbulence correlation length directly computed from the simulation. The synthetic diagnostic was then applied to analyse the correlation length and wave-number spectra obtained by USFR in the ASDEX-Upgrade tokamak. A comparison between 1D and 2D results have shown different behaviour. However correlation lengths measured with UFSR signals are in the same order with turbulence ones

Étude expérimentale et numérique de la turbulence dans les plasmas de fusion à l'aide de diagnostics synthétiques pour la réflectométrie

Plasma turbulence studies are essential for successful operation of magnetic confinement fusion devices. Ultra-Fast Swept Reflectometry (USFR), a diagnostic widely used for the measurement of turbulence radial wave-number spectra. While the interpretation of reflectometry data is quite straightforward for small levels of turbulence, it becomes much trickier for larger levels as the reflectometer answer is no longer linear with the turbulence level. It has been shown for instance that resonances due to probing field trapping can appear in turbulent plasma and produce jumps of the signal phase. In the plasma edge region the turbulence level is usually high and can lead to a non-linear regime of the reflecetometer response. The loss of probing beam coherency and beam widening when the probing beam crosses the edge turbulence layer can affect USFR core measurements. Edge turbulence with a long correlation length leads to small beam widening and strong distortion of the probing wave phase. However backscattering effects from turbulence with short correlation lengths are also able to cause reflectometer signal change. To study turbulence wave-number spectra together with reflectometer signal phase variations, signal amplitude variations can be analyzed. Unlike signal phase variation, amplitude does not suffer from resonant jumps, and can give more clear qualitative evaluation of turbulence structure. In the case when the turbulence amplitude peaked in the edge region, it can be detected as spectral peak near local Bragg resonance wave-number. USFR with a set of receiving antennas arranged poloidally was proposed to obtain more information on the edge turbulence properties. A displacement of the spectral peak appears when the receiving antenna is misaligned with the emitting one. In perspective peak displacement measurements can provide additional information on probing beam shaping and turbulence properties and help in coherent mode observation. A 2D full wave code was applied as a synthetic diagnostic to Gysela gyro-kinetic for study of Tore-Supra tokamak core turbulence. Radial correlation lengths computed from the amplitude of multi-channel fixed frequency reflectometry signals have shown good agreement with the turbulence correlation length. The synthetic diagnostic was then applied to analyze the correlation length and wave-number spectra obtained by USFR in the ASDEX-Upgrade tokamak. A comparison between 1D and 2D results have shown different behavior. However correlation lengths measured with UFSR signals are in the same order with turbulence onesLa thèse porte sur l'interprétation des données de réflectométrie pour extraire les caractéristiques de la turbulence construites à partir de simulations numériques bidimensionnelles. Il a été démontré que la résonance due au piégeage de l'onde peut apparaître dans le plasma fluctuant et produire des sauts de phase. Cette interprétation à faible niveau de turbulence est directe. Cependant, le niveau de turbulence du bord du plasma est généralement élevé menant ainsi le réflectomètre à un comportement non linéaire. En conséquence, il y a une perte de cohérence de l'onde de sondage et un élargissement du faisceau-sonde après la traversée de la couche de turbulence. Cette étude a montré qu'une petite longueur de corrélation de la turbulence conduit à un faible élargissement et à de fortes variations de la phase du faisceau-sonde. Pour étudier comment une forte turbulence de bord affecte le signal de réflectométrie à balayage ultra rapide en fréquence (USFR) obtenu lors d'un sondage. Pour étudier cela des séries de simulations 2D ont été réalisées. Simultanément les spectres de variations de phase et de variations d'amplitude du réflectomètre ont été analysés. Il a été constaté que des pics spectraux correspondant à une diffusion accrue dans la région de turbulence de bord peuvent être observés dans les spectres de variations d'amplitude du signal. Un USFR utilisant une configuration de réflectométrie poloïdale a été proposé pour accéder à plus d'informations sur la turbulence de bord où le déplacement poloïdal des antennes réceptrices entraîne un glissement du pic de diffusion. En perspective, ces mesures peuvent fournir des informations supplémentaires sur, la déformation du faisceau-sonde, les propriétés de la turbulence et faciliter la mesure du signal cohérent porteur de l'information sur la turbulence de cœur du plasma. En plus, le code 2D "full-wave" a été appliqué en tant que diagnostic synthétique aux données de simulation gyro-cinétique du code de turbulence Gysela pour une décharge du tokamak de Tore-Supra. Les signaux de réflectométrie à fréquence fixe ont montré un bon accord entre la longueur de corrélation d'amplitude du signal avec celle de la turbulence utilisée comme donnée d'entrée. Il en a été de même pour analyser la longueur de corrélation et les spectres de nombre d'onde obtenus par un USFR pour le tokamak ASDEX-Upgard

Electromagnetic wave behaviour in turbulent plasmas as a possible support for diagnosing fusion plasma properties

International audience- Why to study the turbulence in Fusion Plasmas and to do what with this?- What are the scales to consider, using probing electromagnetic waves?- What are the possible ways to describe wave-turbulence interaction?- What are the diagnostics to probe turbulence in Fusion Plasmas?- How do they operate, under which plasma conditions?- What is the status of the code development in 3D?- Conclusion

Preliminary design of the COMPASS upgrade tokamak

COMPASS Upgrade is a new medium size, high magnetic field tokamak (R = 0.9 m, Bt = 5 T, Ip = 2 MA) currently under design in the Czech Republic. It will provide unique capabilities for addressing some of the key challenges in plasma exhaust physics, advanced confinement modes and advanced plasma configurations as well as testing new plasma facing materials and liquid metal divertor concepts. This paper contains an overview of the preliminary engineering design of the main systems of the COMPASS Upgrade tokamak (vacuum vessel, central solenoid and poloidal field coils, toroidal field coils, support structure, cryostat, cryogenic system, power supply system and machine monitoring and protection system). The description of foreseen auxiliary plasma heating systems and plasma diagnostics is also provided as well as a summary of expected plasma performance and available plasma configurations