Feedback control for magnetic island suppression in tokamaks

A real-time feedback control system has been developed that finds, tracks, suppresses and/or stabilizes resistive magnetic instabilities in a nuclear fusion plasma. In a tokamak, magnetic fields confine a fusion plasma in a topology of toroidally nested magnetic surfaces. The power produced by the fusion reactions increases with the plasma pressure. At high pressure, however, the magnetic field topology can break-up and magnetic islands are formed. Magnetic islands grow nonlinearly, reduce the plasma energy confinement and deteriorate the steady-state operation of tokamaks. Control of magnetic islands allows for high performance plasmas and pulse length extension. For effective magnetic island suppression, localized, high-power millimeter waves must be aligned precisely with the island centre at centimeter accuracy. The injected millimeter waves induce a local driven current that suppresses the island. This is known as Electron Cyclotron Resonance Heating and Current Drive (ECRH/ECCD). Suppression experiments, reported earlier, use a fixed alignment and power settings or offer a coarse closed-loop optimization of the alignment of ECRH/ECCD with an island. In this work, an advance is made towards a fully autonomous closedloop system that detects magnetic islands, establishes and maintains an accurate alignment between the suppressing ECRH/ECCD and a magnetic island via a feedback controlled steerable mirror and manipulates the applied ECRH/ECCD power in real-time. This feedback control system allows dynamic island tracking and guarantees stability robustness against uncertainties, disturbances and perturbations. In addition, it will be shown that tailoring of the applied ECRH/ECCD power level via feedback manipulation allows to apply exactly the right amount of power to enforce complete suppression of an island or can be used for the stabilization of a magnetic island at a specific width. Besides the actuators for manipulation of magnetic islands, the feedback control system requires sensors for real-time detection and monitoring of island control variables such as the island location in the magnetic topology, the island’s size and its rotation frequency. A prototype line-of-sight Electron Cyclotron Emission (ECE) system was developed earlier at the TEXTOR tokamak, Forschungszentrum Jülich, Germany and is used here as the dedicated sensing scheme for feedback control of magnetic islands. The line-of-sight ECE system incorporates magnetic island monitoring and actuation in a single transmission path, thereby minimizing latencies and avoiding the need for geometrical transformations between sensor and actuator. The magnetic island control system is setup following a systematic design approach. The control problem is specified in terms of input/output manipulation and analyzed using common control engineering techniques and system theory. Particularly, the following steps are made: 1) formulation of the magnetic island control problem and related sub-problems, 2) development of simulation models relevant for the design of magnetic island controllers based on existing plasma physics and including models for the actuators, sensors and other hardware, 3) comparison of relevant control methods and design of linear and nonlinear control strategies, 4) testing and performance assessment of the designed controllers in simulations, where the parameter sets of the TEXTOR tokamak and the future ITER reactor serve as case studies, 5) setup of a prototype real-time magnetic island control system for the TEXTOR tokamak, 6) design and implementation of algorithms to derive magnetic island control variables from diagnostic data, 7) implementation of control algorithms in a real-time data-acquisition and control system, 8) demonstration and analysis of the successful application of a prototype real-time magnetic island control system in experiments on TEXTOR, which in particular show successful real-time detection and monitoring of magnetic islands, controlled alignment of the stabilizing ECRH/ECCD power deposition at the right location and with the right timing for magnetic island suppression. An experimental demonstration of the tracking capabilities of the feedback system in the presence of an alignment perturbation is also given. Further development of the magnetic island control system is discussed and suggestions for improvement are given. Many of the techniques used for magnetic island control are also applicable to other tokamak problems. An outlook on future advances for the control of tokamak plasmas is therefore included.