Stabilization of Neoclassical Tearing Modes in tokamak fusion plasmas via extremum seeking

Abstract

The neoclassical tearing mode (NTM) instability produces magnetic islands in Tokamak plasmas that can degrade confinement and lead to plasma disruptions. NTMs are driven by a lack of bootstrap current inside the magnetic island where the pressure profile is flattened. Suppression of these islands is necessary for sustained energy confinement and efficient operation in Tokamak magnetic-fusion reactors. Compensating for the lack of bootstrap current by an electron cyclotron current drive (ECCD) has been proved experimentally as an effective method to stabilize NTMs. The effectiveness of this method is limited in practice by the uncertainties in the width of the island, the relative position between the island and the EC beam, and the EC power threshold for NTM stabilization. Heuristic search and suppress algorithms have been proposed and shown effective to improve the alignment of the EC beam with the island by just using an estimate of the island width. Making use of this estimate, a real-time, non-model-based, extremum-seeking optimization algorithm is proposed in this work for EC beam steering and modulation in order to minimize the island-beam misalignment and the time (control energy) required for NTM stabilization. The efficiency of the proposed method is compared with traditional search and suppress algorithms.