Confinement degradation and plasma loss induced by strong sawtooth crashes at W7-X

Sawtooth-like crashes were observed during electron cyclotron current drive experiments for strikeline controls at the optimised superconducting stellarator Wendelstein 7-X (W7-X). The majority of the crashes did not have a relevant impact on plasma performance. However, a limited number of events, characterised by a large plasma volume affected by the instability, have been related to a strong deterioration performance and even to the premature termination of the plasma. The hot plasma core expelled during these sawtooth crashes can reach the plasma edge, where plasma surface interaction can occur and impurities can be released. The x-ray tomography shows a strong radiation increase starting from the edge and moving towards the inner plasma regions. This results in the cooling down and shrinking of the plasma, which eventually leads to a poor coupling of the ECRH to the electrons, that can in turn result in a plasma loss. A relation between the size and amplitude of the sawtooth crashes and the impurity increase is reported.EC/H2020/633053/EU/Implementation of activities described in the Roadmap to Fusion during Horizon 2020 through a Joint programme of the members of the EUROfusion consortium/Eurato

Stereoscopic Imaging of Dusty Plasmas under Microgravity Conditions

In this thesis, a stereoscopic camera system is presented that is designed for the use on parabolic flights for the investigation of dusty plasmas under microgravity conditions. This camera system consists of three synchronously triggered high-speed cameras observing a common volume of approximately (15 × 15 × 15) mm³ size. In this volume, the three-dimensional trajectories of a large number of particles surrounded by a dense dust cloud were reconstructed. For this task an intricate set of reconstruction algorithms has been developed, including a four-frame linking algorithm and a complex combined 2D/3D tracking algorithm for a reliable tracking of 3D particles. Furthermore, these algorithms effectively suppress so-called ghost particles in the evaluation process which are reconstructed from falsely identified 2D particle correspondences. Dusty plasmas under microgravity conditions are of special interest due to their complex structure and the variety of observable dynamic phenomena. Under typical discharge conditions, a central dust-free void is formed, surrounded by a dense particle cloud. Since the void is inherently dust-free, particles shot into the void can be uniquely identified and used to probe plasma properties inside this region. In the dust cloud itself, processes like self-excited dust-density waves can be observed under suitable experimental conditions. Using the presented camera setup and reconstruction algorithms, two parts of a dusty plasma under microgravity on parabolic flights are investigated. Initially, the force field creating and sustaining the central void is deduced and characterized. The combination of ion drag and electric field force is measured and compared to current models of the ion drag, showing a good agreement with these models. While previous investigations on the forces were limited to two-dimensional slices through the void, our measurements represent the first three-dimensional quantitative analysis of a large fraction of the void region. From this analysis the structure of the force field is determined and separated into a radial and a non-radial (or orthogonal) contribution. It is shown that the radial contribution dominates in the central void, while non-radial forces increase in magnitude close to the void edge. The radial domination is also observed in the velocity distribution of the probe particles which is significantly shifted to radially outward directed velocities for particles leaving the void. Assuming a strictly radial force profile in the horizontal mid-plane of the void, the friction coefficient determining the interaction of the probe particles with the neutral gas background is experimentally determined and shown to match the theoretical expectation. Subsequently, particles at the outer surface of the dust cloud are reconstructed. There, the particles are found to oscillate due to dust-density waves propagating through the high-density dust cloud. For the investigation of the correlation between waves and oscillating particles, the instantaneous wave and oscillation properties are determined and the instantaneous phase difference is obtained. Modeling the probe particles as driven, damped harmonic oscillators, these phase differences between waves and particles are interpreted with respect to the resonance frequency of the oscillating particles. Spatial variations of the phase difference are observed that may be attributed to different frequencies of the dust-density waves, or to changes of the resonance frequency induced by changing local plasma parameters. From a few measurements of particles oscillating at their resonance frequency, information about the surrounding plasma or properties of the particles themselves can be deduced. However, a larger number of reconstructed trajectories is necessary in order to interpret the phase differences on a reliable data basis. The presented camera setup in combination with the evaluation algorithms is a flexible system for the investigation of three-dimensional dusty plasmas. Its robust construction allows the operation of the system in challenging environments such as on parabolic flights, where spatial limitations and vibrations produced by the aircraft make special demands on such a diagnostic tool. This versatility makes our stereoscopic camera setup and the reconstruction process a suitable standard diagnostic for the application with dusty plasmas; this system will therefore be used in future research amongst other things for the investigation of boundary layers in extended three-dimensional dust clouds under microgravity.In dieser Arbeit wird ein stereoskopisches Kamerasystem vorgestellt, das für die Untersuchung staubiger Plasmen unter Schwerelosigkeit auf Parabelflügen entwickelt wurde. Dieses Kamerasystem besteht aus drei synchronisierten Hochgeschwindigkeitskameras, die ein gemeinsames Volumen von etwa (15 × 15 × 15) mm³ Größe beobachten. In diesem Volumen wurden die dreidimensionalen Trajektorien einer großen Anzahl von Partikeln rekonstruiert, die von einer Staubwolke hoher Partikeldichte umgeben sind. Hierfür wurde ein komplexer Satz an Algorithmen entwickelt, der einen Vier-Frame-Algorithmus für das Verfolgen einzelner Partikel sowie einen kombinierten 2D/3D-Tracking-Algorithmus für eine zuverlässige Bestimmung der 3D Partikeltrajektorien enthält. Darüber hinaus unterdrücken diese Algorithmen sogenannte Ghost-Partikel im Auswerteprozess, die aus falsch identifizierten Korrespondenzen von Partikelprojektionen entstehen. Staubige Plasmen unter Schwerelosigkeit sind besonders interessant aufgrund ihrer komplexen Struktur und der Vielfalt an beobachtbaren dynamischen Prozessen. In typischen Entladungsbedingungen bildet sich ein zentrales, staubfreies Void aus, das von einer Partikelwolke hoher Dichte umgeben ist. Da dieses Void naturgemäß vollständig staubfrei ist, können in das Void eingebrachte Testpartikel eindeutig identifiziert und damit als Sonden für lokale Plasmaparameter verwendet werden. In der Staubwolke selbst können, unter geeigneten Entladungsbedingungen, Prozesse wie selbsterregte Staubdichtewellen beobachtet werden. Mit dem hier vorgestellten stereoskopischen Kamerasystem und den Rekonstruktionsalgorithmen wurden nun zwei verschiedene Ausschnitte von Staubwolken unter Schwerelosigkeit auf Parabelflügen dreidimensional untersucht. Zunächst wird das Kraftfeld charakterisiert, das der Voidbildung und -erhal\-tung zugrunde liegt. Hierfür wird die Überlagerung aus Ionenwindkraft und elektrischer Feldkraft gemessen und mit gängigen Modellen für den Ionenwind verglichen, wobei sich eine sehr gute Übereinstimmung zwischen Messung und Modell zeigt. Während frühere Untersuchungen des Kraftfeldes im Void auf zweidimensionale Schnitte durch das Void beschränkt waren, stellt unsere Messung die erste quantitative dreidimensionale Analyse des Kraftfeldes in einem großen Teil des Voids dar. Aus dieser Analyse wird die Struktur des Kraftfeldes bestimmt und dessen radiale und nicht-radiale (orthogonale) Komponenten ermittelt. Es zeigt sich, dass in der Voidmitte radiale Kräfte überwiegen, während in der Nähe des Voidrandes der Beitrag nicht-radialer Kräfte zur Gesamtkraft zunimmt. Der grundsätzlich radiale Charakter des Kraftfeldes zeigt sich auch in den Geschwindigkeitsverteilungen der Testpartikel. Die Geschwindigkeitsverteilung derjenigen Partikel die das Void verlassen zeigt ein deutlich radial nach außen gerichtetes Profil, das durch das radiale Kraftfeld hervorgerufen ist. Unter der Annahme eines vollständig radialen Kraftfeldes in der Projektion auf eine Ebene parallel zu den Elektroden kann der Reibungskoeffizient, der die Wechselwirkung zwischen Partikeln und neutralem Gashintergrund bestimmt, experimentell ermittelt werden. Ein Vergleich mit dem theoretisch erwarteten Wert zeigt auch hier eine sehr gute Übereinstimmung. Als zweiter Teil dieser Arbeit wurden Partikel am äußeren Rand der Staubwolke rekonstruiert. Dort werden die Partikel durch Staubdichtewellen in der Staubwolke zu Oszillationen angeregt. Für eine Untersuchung der Beziehung zwischen Wellen und Testpartikeln werden die instantanen Eigenschaften von Welle und Teilchen bestimmt und daraus die instantane Phasendifferenz berechnet. Diese Phasendifferenzen zwischen Wellen und schwingenden Testpartikeln werden im Bild des gedämpften, harmonischen Oszillators mit Blick auf die Resonanzfrequenz der Partikel interpretiert. Hier wird eine räumliche Verteilung der Phasendifferenzen beobachtet, der zwei Mechanismen zugrunde liegen können: entweder ändert sich die Schwingungsfrequenz von Welle und Testpartikel relativ zur Resonanzfrequenz, oder aber die Parameter des umgebenden Plasmas verändern die Resonanzfrequenz selbst. Aus einer zeitweisen Übereinstimmung von Treiberfrequenz und Resonanzfrequenz einiger Partikel können im Idealfall Erkenntnisse über das umgebende Plasma oder die Partikel selbst gewonnen werden. Die aktuelle Datengrundlage ist jedoch derzeit noch nicht ausreichend, um verlässliche Aussagen über eine räumliche Verteilung der Phasendifferenzen oder den zugrunde liegenden Mechanismus treffen zu können

Spatially resolved three-dimensional particle dynamics in the void of dusty plasmas under microgravity using stereoscopy

Three-dimensional (3D) dynamical properties of fast particles being injected into the void region of a dusty plasma under microgravity conditions have been measured. For that purpose, a stereoscopic camera setup of three cameras has been developed that is able to track and reconstruct the 3D trajectories of individual dust particles. From more than 500 particle trajectories, the force field inside the void region and its influence on particle movement are derived and analyzed in 3D. It is shown that the force field is dominated by forces pointing radially out of the void and that this radial character is reflected in the velocity distributions of particles leaving the void. Furthermore, the structure of the force field is used for measuring the neutral gas friction for the particles inside the void

Radiative edge cooling experiments in Wendelstein 7-X start-up limiter campaign

Impurity seeding experiments during the start-up limiter campaign of Wendelstein 7-X provide first evidence for a localization effect of the 3D magnetic edge structure on the seeded impurities and their radiation distribution and cooling effect. Moreover, species dependencies have been seen. Nitrogen was observed to cool the entire edge plasma, with a stronger electron temperature reduction measured at the downstream position at the limiter. The radiation was limited at the periphery of the confined region. Both the temperature reduction and the radiation enhancement were directly correlated to the injection of the coolant gas. Mitigation of the limiter heat loads was also measured. Neon was observed to affect also the confined plasma with a long-lasting radiation and a cooling of the entire plasma.This work was supported in part by the U.S. Department of Energy (DoE) under grant DE-SC0014210 and by discretional funding of the Department of Engineering Physics and the College of Nuclear Engineering at the University of Wisconsin-Madison, USA. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 and 2019–2020 under grant agreement No 633053

Effect of toroidal plasma currents on the Wendelstein 7-X Scrape-Off Layer

The role of toroidal plasma currents for the island divertor scrape-off layer in the stellarator Wendelstein 7-X is investigated using reciprocating electric probes. Experiments show that small amounts (of a few kA) of plasma current are sufficient to significantly affect the scrape-off layer plasma conditions, whereas higher plasma currents above 10kA result in more drastic changes. This behavior is linked to the effect of the plasma current on the rotational transform profile, which can result in significant shifts of the edge magnetic islands. These shifts affect the interaction of the islands with the divertor and can eventually result in a transition from a diverted to a limited plasma configuration. The probe observations are complemented by further edge diagnostics including plasma flow measurements, divertor Langmuir probes, divertor thermography and impurity spectroscopy.This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement number 633053

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

Development of the self-modulation instability of a relativistic proton bunch in plasma

Self-modulation is a beam–plasma instability that is useful to drive large-amplitude wakefields with bunches much longer than the plasma skin depth. We present experimental results showing that, when increasing the ratio between the initial transverse size of the bunch and the plasma skin depth, the instability occurs later along the bunch, or not at all, over a fixed plasma length because the amplitude of the initial wakefields decreases. We show cases for which self-modulation does not develop, and we introduce a simple model discussing the conditions for which it would not occur after any plasma length. Changing bunch size and plasma electron density also changes the growth rate of the instability. We discuss the impact of these results on the design of a particle accelerator based on the self-modulation instability seeded by a relativistic ionization front, such as the future upgrade of the Advanced WAKefield Experiment

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