Stabilization of Neoclassical Tearing Modes in Tokamak Fusion Plasmas via Extremum Seeking
W. Wehner and E. Schuster
IEEE Multi-conference on Systems and Control
Saint Petersburg, Russia, July 8-10, 2009
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Abstract
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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.
