Control-oriented Modelling for Neoclassical Tearing Mode Stabilization via Minimum-seeking Techniques
W. Wehner and E. Schuster
Nuclear Fusion 52 (2012) 074003 (13pp)
Abstract
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Suppression of magnetic islands driven by the neoclassical tearing mode (NTM) is necessary for efficient and
sustained operation of tokamak fusion reactors. Compensating for the lack of bootstrap current, due to the pressure
profile flattening in the magnetic island, by a localized 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 beam, and the
ECCD power threshold for NTM stabilization. Heuristic search and suppress algorithms have been proposed and shown
effective in improving the alignment of the ECCD beam with the island, using only an estimate of the island width.
Making use of this estimate, real-time, non-model-based, extremum-seeking optimization algorithms have also been
proposed not only for beam steering but also for power modulation in order to minimize the island-beam misalignment
and the time required for NTM stabilization. A control-oriented dynamic model for the effect of ECCD on the magnetic
island is proposed in this work to enable both control design and performance analysis of these minimum-seeking type
of controllers. The model expands previous work by including the impact of beam modulation parameters such as the
island-beam phase mismatch and the beam duty-cycle on the island width dynamics.