Analytic model for the nonlinear interaction of tearing modes of different pitch in cylindrical geometry

An analytic model has been developed for describing the nonlinear interaction of tearing modes of different pitch in cylindrical geometry for equilibria characterized by flat safety factor profiles. The analysis shows that the m = 2/n = 1 tearing mode can destabilize odd m modes, particularly the 3/2 mode. The model compares well with our three-dimensional (3-D) code with respect to the time evolution of the 0/0, 1/1, 2/1, 3/2, and 5/3 modes. Scaling rules are obtained for the position and location in time of the maximum or peak in the 3/2 growth rate. The characteristic time of destabilization of the odd m modes predicted by the model correlates well with the observed time scale for the major disruption in tokamaks

Poloidal magnetic field fluctuations in tokamaks

Elementary nonlinear tearing mode theory in a two-dimensional cylindrical geometry is used to predict accurately the amplitude of the m = 2 poloidal magnetic field fluctuations (Mirnov oscillations) at the limiter of a tokamak. The input required is the electron temperature radial profile from which the safety factor profile can be inferred. The saturation amplitude of the m = 2 tearing mode is calculated from the safety factor profile using a nonlinear ..delta..' analysis. This gives an absolute result (no arbitrary factors) for the amplitude of the perturbation in the poloidal magnetic field everywhere, in particular, at the limiter. An analysis of ORMAK and T-4 safety factor profiles (inferred from electron temperature profiles) gives results that are in agreement with the experimental data. A study of a general profile shows that as a function of the safety factor at the limiter, a maximum occurs in the amplitude of the Mirnov oscillation. The magnitude of the maximum increases with a decrease in temperature near the limiter

Stabilization of Tearing Modes to Suppress Major Disruptions in Tokamaks

It is shown, for q-profiles which lead to a disruption, that the control of the amplitude of the 2/1 tearing mode avoids the disruption. Q-profiles measured in T-4 and PLT before a major disruption were studied. Two methods of controlling the 2/1 mode amplitude have been considered: (1) Feedback stabilization with the feedback signal locked in phase with the 2/1 mode. (2) Heating slightly outside the q = 2 surface. In both cases it is only necessary to decrease the 2/1 mode amplitude to suppress the disruption. It is not always necessary to stabilize the unstable modes fully