Investigations Of A Few Problems On Error Field Locked Modes In Tokamaks

Many investigations indicate that error field will play a significant role in future fusion reactor operations. During the tokamak operation, the error field will still induce the mode locking phenomenon, and further lead to island growth and even major disruption. Meantime, as a remarkable physical phenomenon, error field locked mode receives extensive attention and investigation, and researches are carried out from the points of experiment, theory and simulation. Experimentally, the world’s notable tokamakdevices, such as JET, DIII-D, COMPASS, C-Mod etc also took series of investigation. Theoretically, error field locked mode physics also receives extensive studies in the nuclear fusion theoretical cycle. And the simulation was mainly developed in recent years. Nowadays, the simulation are made forward to the real configurations of tokamak plasmas, i.e. from simple study on physical regimes in the slab geometry to coupled three dimensional configuration, and from single fluid to two-fluid and further kinetics. With more than twenty years’ studies, error field locked mode physics have become more and more clear. However, revisiting these theories, we still find the necessity of further developments of the existing theories. In this thesis, a comprehensive introduction to error field locked mode is shown at first, and then follows deep analyses to the relevant theory and experiment. On the basis of these, proper answers to these problems are given. The context of this thesis is as follows:In introduction, the key physics problems in tokamaks in the previous special topic are summarized at first. After then the research history and progress on error field relevant investigation is reviewed. Besides, the significance of error field locked mode in tokamak physics problems is also pointed out.In chapter2, the newest significant theoretical progress on error field penetration, i.e. linear and nonlinear theories at the two-fluid frame and also linear kinetic theory are present. Some key details in the linear theory under the condition of two-fluid approximation are also shown.In chapter3, nonlinear error field penetration theories under the condition of magnetohydrodymics (MHD) approximation are reviewed and re-analysed. A discussion on nonlinear error field penetration theory under the condition of MHD approximation is given first. With detailed analysis, the previous transition regime shows little smooth transition characteristic is found. A new transition regime that shows well smooth transition for nearly all parameters is given. To better verify this analysis, a relevant simulation is given. The relation between critical error field amplitude and critical island width, and the detailed scaling results verify our opinion. Besides, we then present the electric current profiles in the three nonlinear regimes at the time of critical error field amplitude. These again verify our theoretical analysis. At the last part of chapter3, extra discussions on the crucial Rutherford regime are shown.In chapter4, the error field penetration theory is applied in tokamaks. First, a discussion on the two assumptions that are employed when the theoretical scalings transform into experiment parameters is present. On the basis of these, the better assumption is selected, and the more reasonable density scalings are then obtained. Based on these above, an analysis on the error field locked mode relevant experiment investigations is given. After then, follows a comprehensive classification. At the end of chapter4, the theoretical analysis on the very recent error field locked mode experiment results in J-TEXT tokamak is shown. And the present error field locked mode experiments cover all three nonlinear regimes in the frame of MHD. The theoretical result matches well with the experiment data.In chapter5, error field penetration in reversed magnetic shear configuration is investigated. To give the comprehensive discussion, three states, i.e. stable, marginal and unstable states are considered. The significance is that once error field induces mode locking at the outer rational surface when the double tearing mode is initially stable, the double tearing mode will grow up. This will induce plasma disruption. Hence significant attention should be paid.At last a summary and prospect ends this thesis.