Predictive Integrated Modeling of Low Aspect Ratio Tokamaks and Large Scale Instabilities in Tokamaks

Download or read book Predictive Integrated Modeling of Low Aspect Ratio Tokamaks and Large Scale Instabilities in Tokamaks written by Canh Ngoc Nguyen and published by . This book was released on 2004 with total page 372 pages. Available in PDF, EPUB and Kindle. Book excerpt: Models for large scale instabilities in tokamaks are developed, and they are tested together with models for transport and other physical processes by comparing the results of BALDUR integrated predictive modeling simulations with experimental data from tokamaks. Simulations are carried out for the low aspect ratio tokamaks, Mega Ampere Spherical Tokamak and the National Spherical Torus Experiment, to test the applicability of models that were developed for conventional tokamaks. The results indicate that neoclassical transport dominates over anomalous transport in the inner third of the plasma. Sawtooth oscillations, which are the result of an instability that periodically redistributes the central part of the plasma profiles, play a significant role by radially spreading the neutral beam injection heating profiles across the broad sawtooth mixing region. Two models for sawtooth oscillations, the Porcelli and the Kadomtsev models, are combined and tested against experimental data from the Joint European Torus and the Tokamak Fusion Test Reactor. Most of the sawtooth crashes are triggered by the m = 1 resistive internal kink mode in the semi-collisional regime. The median sawtooth period increases with increasing magnetic reconnection fraction. The best overall agreement with experimental data is obtained with a magnetic reconnection fraction of approximately 55%. Finally, plasma pressure effects are included in a quasi-linear model for saturated tearing modes, which are instabilities that produce magnetic islands. The model is used in a stand-alone code, and in the BALDUR code, to calculate the widths of saturated magnetic islands in tokamak plasmas with arbitrary cross-section and plasma pressure. The widths of tearing mode islands increase with decreasing aspect ratio and with increasing elongation. Also, the island widths increase when the gradient of the current density increases at the edge of the islands or when the current density inside the islands is suppressed. The widths oscillate in time in response to periodic sawtooth crashes. Local enhancements in the transport produced by magnetic islands have a noticeable effect on global plasma confinement in simulations of low aspect ratio, high beta tokamaks, where saturated tearing mode islands can occur with widths greater than 15% of the plasma minor radius.