Magnetohydrodynamic stability analyses of tokamak edge plasmas

Edge Locilised Modes (ELMs) are edge phenomena in fusion plasmas that cause small bursts of energy and particles out of the plasma. In a fusion devices such as a tokamak, ELMs affect the plasma confinement and can cause divertor plate erosion. Therefore, for the operation of a tokamak fusion plasma it is important to understand the physical mechanisms behind the ELM phenomenon and to be able to minimise the detrimental effects of the ELMs. In this thesis, the ELMs are modelled using magnetohydrodynamic stability analysis. First an accurate equilibrium of the experimental plasma is created and then the stability of the equilibrium is analysed. The stability analyses show that the Type I or 'giant' ELMs in ASDEX Upgrade and JET plasmas are triggered by peeling-ballooning modes with low to intermediate toroidal mode number (n). The radial struture of these modes is relatively wide and is localised near the edge of the plasma. The ASDEX Upgrade plasmas with smaller Type II or 'grassy' ELMs are found to have narrower mode structure of the triggering instability. The stability against low-n modes is improved as well causing the triggering instability to shift to higher n. Increased plasma pressure in the core region is found to improve the stability of the edge against low-n instabilities. This can explain the easier access to Type II ELMs observed in such plasmas. The Type III ELMs in JET plasmas are found to be deep in the stable region against the low- to intermediate-n peeling-ballooning modes and are likelyto triggered by a different mechanism than other ELMs. Of various ELM-control methods, pellet triggering is studied and it is found that pellet-triggered ELMs are destabilised by the same mechanism as intrinsic Type I ELMs. In quiescent H-mode where no ELMs are observed, the plasma edge stability is significantly better than in similar ELMy plasmas. This can explain the absence of ELMs.reviewe

Recent progress in the quantitative validation of JOREK simulations of ELMs in JET

Future devices like JT-60SA, ITER and DEMO require quantitative predictions of pedestal density and temperature levels, as well as inter-ELM and ELM divertor heat fluxes, in order to improve global confinement capabilities while preventing divertor erosion/melting in the planning of future experiments. Such predictions can be obtained from dedicated pedestal models like EPED, and from non-linear MHD codes like JOREK, for which systematic validation against current experiments is necessary. In this paper, we show progress in the quantitative validation of the JOREK code using JET simulations. Results analyse the impact of diamagnetic terms on the dynamics and size of the ELMs, and evidence is provided that the onset of type-I ELMs is not governed by linear MHD stability alone, but that a nonlinear threshold could be responsible for large MHD events at the plasma edge.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 under grant agreement No 633053, and from the RCUK Energy Programme (grant number EP/I501045). To obtain further information on the data and models underlying this paper please contact PublicationsManagerccfe.ac.uk. This work used the HELIOS supercomputer (IFERC-CSC), Japan, under the Broader Approach collaboration, implemented by Fusion for Energy and JAEA. The views and opinions expressed herein do not necessarily reflect those of the European Commission or the ITER Organization. The HEC ARCHER computer (UK), as part of the Plasma HEC Consortium EPSRC grant EP/L000237/1, and the MARCONI computer at CINECA in Italy, were also used.Peer ReviewedPostprint (author's final draft

Untersuchung zur Lernkultur in Online-Kursen

Ausgehend von einer veränderten, durch Lern- und Kompetenzorientierung geprägten Lernkultur analysieren die Autorinnen zwölf mehrwöchige Online-Kurse mit insgesamt 130 Teilnehmer/innen. Die Autorinnen nehmen ein Klima der hohen Wertschätzung unter den Lernenden wahr sowie gegenseitiges Feedback in den Reflexions- und Diskussionsprozessen, welches das Lernen verstärkt. Die Hypothese, dass in rein virtuellen, mehrwöchigen Weiterbildungskursen eine veränderte Lernkultur gefördert und gelebt wird, wird mittels halbstrukturierter Interviews sowie qualitativer Inhaltsanalyse der Beiträge in den Diskussionsforen untersucht. (DIPF/ Orig.

PLASMA SHAPE AND FUELING DEPENDENCE ON THE SMALL ELMS REGIME IN TCV AND AUG

A series of experiments has been conducted at AUG and TCV to disentangle the role of fueling, plasma triangularity and closeness to a double null (DN) configuration for the onset of the small ELM regime. At AUG, the role of the SOL density has been revisited. Indeed, it turns out that a large density SOL is not a sufficient condition to achieve the type-II (small) ELM regime. This has been demonstrated with a constant gas fueled plasma close to DN which has been progressively shifted down, relaxing therefore the closeness to DN at constant. As the plasma is moved down, Type-I ELMs are progressively restored, finally being the unique ELM regime. It is observed that not only the pedestal top profiles are unchanged, but also the SOL profiles remained unaffected by transition from Type-II to Type-I ELMs. We conclude that the separatrix density is not the unique key parameter and it is hypothesized that the local magnetic shear, modified by the closeness to DN, could play an important role. A small ELM regime with good confinement has been achieved at TCV, a full carbon machine featuring an open divertor. A systematic scan in the fueling rate has been done for both medium and high triangularity shapes. For the latter case, a configuration close to a DN configuration, the stored energy and the pedestal top pressure increase by 5% and 30% respectively compared to the medium triangularity case. For both shapes, as the D2 fueling is increased, the Type-I ELM frequency decreases and small ELMs are observed in between large ones. Finally for the high triangularity, at the maximum fueling rate, the large ELMs are fully suppressed and only the small ELMs remain. As observed in JET and AUG, the pedestal pressure degrades with increasing fueling, up to 40% for the high triangularity scenario, although the stored energy remains almost unchanged. It is also observed that, for both shapes, the density at the separatrix increases with the fueling rate, reaching ne,sep/nG ~0.3 at ne,av/nG~0.75. The small ELM regime at TCV is associated with a coherent mode at about 30 kHz seen by the magnetic probes located at the outboard midplane. The outer target heat loads from IR tomography are reduced by more than a factor of 5 when transiting towards the small ELM regime