Coherent structures in the boundary plasma of EAST Tokamak

Ning Yan

Research output: Book/ReportPh.D. thesis

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In recent years, with the application of fast camera in fusion plasma, as well as
other diagnostic of spatial-temporal resolution such as Langmuir probe, it has become generally clear that the turbulence transport is mostly dominant by cross-field propagation of coherent structures, namely blobs or filaments in low-confinement mode (L-mode). Analogously, the fine structures associated with the edge-localized modes (ELMs), i.e., ELM filaments, have been shown to be the main carriers of the transport in the high-confinement mode (H-mode). The filaments carry particles and heat, impinging upon the plasma-facing material, leading to intensive transient heat load and particle load on the local areas of both the divertor target plates and the first wall, which damages the material and causes enhanced recycling and impurity generation, then further pollutes the core plasma. In this project, we carried out experiment in the boundary plasma using multi-pin Langmuir probe in L-mode discharge. It was found that the coherent structures (Blobs and Holes) are created in the edge shear layer of poloidal flows where the plasma shows steep pressure gradient. Simulations have been performed using the ESEL code, which is a 2D fluid
turbulence-simulation code based on the interchange instability as the main drive for the turbulence and structure motion in the scrape-off layer (SOL) plasma, with the input parameters from the EAST experiments. The simulations successfully reproduce the statistical characteristics of the SOL turbulence as well as the profiles of the plasma main parameters measured by the Langmuir probes on EAST. Our observations suggest that the coherent blobs/filaments are generated from the interchange driven instability. On the other hand, an improved understanding on the particle and heat transport by ELM filaments has been obtained though intensive research in different machines. Nevertheless, it is still rather unclear about the ability of current transport in ELM filaments in the international fusion community. A Langmuir-magnetic probe array has been used to study the electromagnetic features of the ELM filaments in the SOL or slightly inside the separatrix. It is observed that the topological configuration of density and potential in the ELM filaments deviate from each other. Furthermore, isolated electromagnetic filaments have been clearly identified during the type-I-like ELMs. They propagate radially outwards and poloidally in the electron diamagnetic drift direction. Based on the reconstruction of the topological structure from the associated magnetic perturbations, we demonstrate
that the ELM filaments carry mono-polar current, which provides important evidence for current transport by filamentary structures at the plasma edge. These studies further improve our understanding on the characteristics of filaments and the transport mechanism in the fusion plasma boundary.
Original languageEnglish
PublisherDepartment of Physics, Technical University of Denmark
Number of pages133
Publication statusPublished - 2014

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