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Open Access Publications from the University of California

Non-diffusive cross field transport in scrape-off-layer in Tokamak

Abstract

In recent years, coherent structures in edge plasmas are believed to be one of the major factors in cross-field transport. The present dissertation is dedicated to using theoretical and numerical methods to study the dynamics of individual coherent structure, or so-called blob, moving from bulk plasma to the chamber wall. Blob is a filament structure extended along magnetic filed lines. We focus on two different types of blob. One is ending up at target plates, the other is not, i.e. the SOL and HB blobs. Two dimensional blob models are derived. Characteristic spatial scale , time scale , and velocity for blob are obtained and calculated for different tokamaks. They are in agreement with experimental measurements. Scaling analysis shows blob dynamics sensitive to spatial scale length. Stability analysis shows that blobs with spatial scales less than may move as coherent structures to large distances. In the SOL model simulation we have found the most structurally stable blob has scale length around . Blobs smaller than evolve into mushroom-like structures. Blobs larger than are subject to the fingering instability. Blobs with spatial scales close to can coherently move to long distances. Simulation results show that high density background effectively narrows down blob size and the inertia term in vorticity equation drives blob to mushroom shape and builds up vortex dipole within the structure. We also compare results with and without the Boussinesq approximation. In the HB model simulation it is shown that blobs have wider stable range. Their steep nose and long relaxation tail can explain experimentally detected asymmetric profile. HB blob moves with a constant velocity. In the biasing potential model a critical magnitude of the potential barrier is derived. A strong deformation of the blob as coherent structure will be observed while blob pass through a barrier higher than the critical value. Simulation results confirm the theoretical prediction. In the rotational blob model simulation results show the suppression of radial velocity and the generation of poloidal velocity. Fingering and mushroom effects are inhibited, but blobs evolve into rotational instability at later stage

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