Doppler-Kohärenz-Abbildung von Ionendynamiken in den Plasmaexperimenten VINETA.II und ASDEX Upgrade

In magnetically confining plasma experiments, measurement of ion dynamics is of great importance to study the plasma behaviour in magnetic fields such as the exhaust particle flows in the divertor areas. The plasma exhaust heat flux in a future nuclear power plant is estimated to be too high for the proposed wall materials. This is due to the relatively small areas, where the exhaust plasma interacts with the wall, that result in high local heat loads in these small interaction areas. A solution for the exhaust problem for future high power plasma experiments is one of the major challenges for magnetic confinement fusion as an energy source. High quality experimental measurements are necessary to improve understanding of the scrape-off-layer (SOL) and divertor physics as well as to validate simulation results of edge codes such as e.g. EMC3-Eirene or SOLPS. Understanding the plasma exhaust and edge behaviour is crucial to make correct assumptions about a future, large-sized power plant. This thesis is concerned with the development of a diagnostic measuring impurity ion flows in the SOL and divertor as well as basic physical understanding of these measured flows. Doppler coherence imaging spectroscopy (CIS) is a relatively new technique for the observation of plasma bulk ion dynamics in magnetically confined plasma experiments. It is a passive optical diagnostic that measures 2D images of the line-integrated ion flow and temperature, thus having the potential to vastly increase our knowledge about the SOL physics. Since its invention, the Doppler CIS has been further developed and tested in several plasma experiments such as DIII-D, H-1NF, MAGPIE and MAST. The Doppler CIS has the advantage of a relatively simple hardware set-up with high entendue, providing high flow sensitivity and an order of magnitude more data at higher signal-to-noise than traditional systems. However, absolute flow calibration has proven to be difficult for many impurity ion species present in the divertor of larger plasma experiments. This is due to lack of nearby calibration lines, ambient temperature changes of the diagnostic hardware as well as the difficulty to create a calibration light source equivalent to the plasma. The diagnostic was used successfully for first Doppler CIS ion flow measurements in the small, low-temperature linear plasma experiment VINETA.II and the medium-sized tokamak ASDEX Upgrade (AUG). The main physics objective of these studies is the research of ion dynamics in the two experiments. In VINETA.II, drifts due to electric fields were found to dominate ion dynamics. In the SOL and divertor of AUG, though there is a complex interplay of several drive mechanisms influencing the impurity ion dynamics, flows were found to be rather stable in the set of discharges where the Doppler CIS was employed. The physical background of SOL and divertor flows is reviewed in detail. This work focuses on the general characteristics of impurity ion flows in the poloidal field divertor and on bulk plasma ion flow in VINETA.II. Doppler CIS measurements from both experiments are presented: C-III, He-II and D-I in AUG as well as Ar II flows in VINETA.II. A flexible diagnostic set-up was designed to directly calibrate each flow measurement immediately before and after an investigated plasma discharge, making absolutely calibrated flow measurements possible without the use of active or passive temperature control. In AUG, they show a change of flow sign between the two divertor legs that is expected due to magnetic topology. The measured flow speeds are in the range of several kilometers per second and are mainly parallel to the magnetic field lines.In Plasmaexperimenten mit magnetischem Einschluss ist die Messung von Ionen-Dynamiken essentiell um das Plasmaverhalten in Magnetfeldern zu verstehen. Insbesondere ist das ausströmende Plasma im (magnetisch offenen) Rand- und Divertorbereich vom großen Interesse, da in einem zukünftigen Kernfusionsreaktor die Wärmeflüsse auf die Wand als sehr groß eingeschätzt werden. Dies liegt daran, dass die Regionen, in denen das ausströmende Plasma auf die Wand trifft, räumlich klein sind und die lokale Wärmebelastung pro Fläche dadurch sehr groß wird. Die Lösung dieses Problems ist eine Voraussetzung, um die Kernfusion mit magnetisch eingeschlossenen Hochtemperaturplasmen nutzen zu können. Genaue Messungen werden benötigt, um das Verständnis des Plasmas im magnetisch offenen Randbereich und am Divertor zu verbessern, sowie um entsprechende Computersimulationen von Codes wie EMC3-Eirene oder SOLPS zu verifizieren. Das Verstehen und korrekte Beschreiben des Randschichtplasmas ist entscheidend, um die richtige Auslegung für das ausströmende Plasma bei zukünftigen, großen Hochtemperatur-Plasmaanlagen treffen zu können. Diese Arbeit beschäftigt sich mit der Entwicklung einer spektralen Diagnostik, die die Verunreinigungsflüsse im offenen Randbereich messen kann. Der Fluss von Wand-Verunreinigungen spielt eine wichtige Rolle für den Betrieb eines Plasmaexperimentes und lässt auch Rückschlüsse auf die ausströmenden Ionenflüsse des Hauptplasmas zu. Das Doppler-Kohärenz-Abbildungsverfahren ist eine relativ neue Methode zur Messung von Ionendynamiken in Plasmaexperimenten. Optisch passiv nimmt sie Bilder aus dem sichtbaren Bereich vom Plasma auf und kann den linienintegrierten Ionenfluss und die Ionentemperatur in ihnen sichtbar machen. Beide Parameter können bisher vor allem nur lokal in der Randschicht gemessen werden. Die Doppler-Kohärenz-Abbildungs-Diagnostik (KAD) hat deshalb das Potential, unser Verständnis dieser beiden Parameter für die Randschichtphysik deutlich zu vergrößern. Seit ihrer Erfindung wurde die Doppler KAD weiterentwickelt und in mehreren Plasmaexperimenten (DIII-D, WEGA, H-1NF, MAGPIE, MAST...) eingesetzt. Sie zeichnet sich durch einen relativ simplen Aufbau mit großem Entendue aus und liefert ca. eine Größenordnung mehr Daten mit höherem Signal-zu-Rausch Verhältnis als traditionelle spektroskopische Systeme. Allerdings ist die Kalibration der Flussmessung für viele spektrale Plasmalinien sehr schwierig. Dies liegt am teilweisen Fehlen von geeigneten Kalibrationslinien, Temperaturveränderungen der Diagnostik-Kristalle und an dem Umstand, dass die Diagnostik nicht in der Objektebene innerhalb des Torus kalibriert werden kann. Erfolgreiche Messungen mit der Diagnostik wurden am linearen Niedertemperatur-Plasmaexperiment VINETA.II und dem Tokamak ASDEX Upgrade (AUG) durchgeführt. Das physikalische Ziel dieser Messungen war die Untersuchung der Ionendynamiken in beiden Experimenten. In VINETA.II wurden azimutale Plasma Driften aufgrund von einem radialen elektrischen Feld gemessen. Im offenen Randschichtbereich und am Divertor von AUG gibt es ein komplexeres Zusammenwirken von verschiedenen Flussursachen. Trotzdem wurden in den mit der Doppler KAD untersuchten Entladungen für verschiedene Verunreinigungsspezies relativ konstante Flüsse beobachtet. Die verschiedenen physikalischen Ursachen von Ionenflüssen werden im Detail für poloidale Feld-Divertoren (eingesetzt in AUG) sowie für das Hauptplasma in VINETA.II beschrieben. Messungen aus beiden Experimenten werden präsentiert: C III, He II und D-α in AUG sowie für Ar II Flüsse in VINETA.II. Ein flexibler Diagnostik-Aufbau wurde realisiert, um direkt vor und nach jeder Plasmaentladung kalibrieren zu können. Dies ermöglichte absolut kalibrierte Flussmessungen ohne die Nutzung von aktiver oder passiver Temperaturkontrolle. In AUG wurde ein Wechsel der Flussrichtung (vom Beobachter weg bzw. zum Beobachter hin) zwischen den beiden Divertorbeinen beobachtet. Dies wird aufgrund der Magnetfeldtopologie erwartet. Die gemessenen Flussgeschwindigkeiten liegen im Bereich von mehreren Kilometern pro Sekunde und sind hauptsächlich parallel zu den Magnetfeldlinien.EC/H2020/633053/Implementation of activities described in the Roadmap to Fusion during Horizon 2020 through a Joint programme of the members of the EUROfusion consortiu

Performance of tungsten plasma facing components in the stellarator experiment W7-X: Recent results from the first OP2 campaign

The transition to reactor-relevant materials for the plasma facing components (PFCs) is an important and necessary step to provide a proof of principle that the stellarator concept can meet the requirements of a future fusion reactor by demonstrating high performance steady-state operation. As a first step to gain experience with tungsten as plasma-facing material in the Wendelstein 7-X (W7-X) stellarator, graphite tiles coated with an approximately 10 µm MedC tungsten layer (NILPRP Bucharest) were installed to complete the ECRH beam dump area in two of the five plasma vessel modules over an area of approximately one square meter each. In addition, tungsten baffle tiles are installed (40 tiles in total) with either bulk tungsten as part of NBI shine-through target or with a tungsten heavy alloy (W95-Ni3.5-Cu1.5) to replace the graphite tiles that were previously thermally overloaded. Based on an advanced diffusive field line tracing method and EMC3-Eirene simulations, the overloaded baffle tiles were redesigned by making the tiles thinner (i.e. moving the plasma-facing surface (PFS) away from the hot plasma region) and by reducing the local angle of incidence through toroidal displacement of the watershed. Significant erosion of the tungsten tiles can only be expected if sputtering by impurity ions such as carbon or oxygen ions contributes. However, the resulting central concentration of tungsten and the corresponding radiation losses are expected to be marginal. The expected deposition of carbon on the tungsten surfaces in the baffle regions mitigates further the possible tungsten enrichment in the core plasma. In OP2.1, no adverse effects of the installed tungsten PFCs on the plasma performance were observed during normal plasma operation. With the design changes made in the baffle area, the predicted heat load reductions could be experimentally confirmed

Confirmation of the topology of the Wendelstein 7-X magnetic field to better than 1:100,000

Fusion energy research has in the past 40 years focused primarily on the tokamak concept, but recent advances in plasma theory and computational power have led to renewed interest in stellarators. The largest and most sophisticated stellarator in the world, Wendelstein 7-X (W7-X), has just started operation, with the aim to show that the earlier weaknesses of this concept have been addressed successfully, and that the intrinsic advantages of the concept persist, also at plasma parameters approaching those of a future fusion power plant. Here we show the first physics results, obtained before plasma operation: that the carefully tailored topology of nested magnetic surfaces needed for good confinement is realized, and that the measured deviations are smaller than one part in 100,000. This is a significant step forward in stellarator research, since it shows that the complicated and delicate magnetic topology can be created and verified with the required accuracy

Major results from the first plasma campaign of the Wendelstein 7-X stellarator

After completing the main construction phase of Wendelstein 7-X (W7-X) and successfully commissioning the device, first plasma operation started at the end of 2015. Integral commissioning of plasma start-up and operation using electron cyclotron resonance heating (ECRH) and an extensive set of plasma diagnostics have been completed, allowing initial physics studies during the first operational campaign. Both in helium and hydrogen, plasma breakdown was easily achieved. Gaining experience with plasma vessel conditioning, discharge lengths could be extended gradually. Eventually, discharges lasted up to 6 s, reaching an injected energy of 4 MJ, which is twice the limit originally agreed for the limiter configuration employed during the first operational campaign. At power levels of 4 MW central electron densities reached 3 1019 m-3, central electron temperatures reached values of 7 keV and ion temperatures reached just above 2 keV. Important physics studies during this first operational phase include a first assessment of power balance and energy confinement, ECRH power deposition experiments, 2nd harmonic O-mode ECRH using multi-pass absorption, and current drive experiments using electron cyclotron current drive. As in many plasma discharges the electron temperature exceeds the ion temperature significantly, these plasmas are governed by core electron root confinement showing a strong positive electric field in the plasma centre.Peer reviewe

Major results from the first plasma campaign of the Wendelstein 7-X stellarator

\u3cp\u3eAfter completing the main construction phase of Wendelstein 7-X (W7-X) and successfully commissioning the device, first plasma operation started at the end of 2015. Integral commissioning of plasma start-up and operation using electron cyclotron resonance heating (ECRH) and an extensive set of plasma diagnostics have been completed, allowing initial physics studies during the first operational campaign. Both in helium and hydrogen, plasma breakdown was easily achieved. Gaining experience with plasma vessel conditioning, discharge lengths could be extended gradually. Eventually, discharges lasted up to 6 s, reaching an injected energy of 4 MJ, which is twice the limit originally agreed for the limiter configuration employed during the first operational campaign. At power levels of 4 MW central electron densities reached 3 10\u3csup\u3e19\u3c/sup\u3e m\u3csup\u3e-3\u3c/sup\u3e, central electron temperatures reached values of 7 keV and ion temperatures reached just above 2 keV. Important physics studies during this first operational phase include a first assessment of power balance and energy confinement, ECRH power deposition experiments, 2nd harmonic O-mode ECRH using multi-pass absorption, and current drive experiments using electron cyclotron current drive. As in many plasma discharges the electron temperature exceeds the ion temperature significantly, these plasmas are governed by core electron root confinement showing a strong positive electric field in the plasma centre.\u3c/p\u3