Adaptive Extremum Seeking Control of ECCD for NTM Stabilization
J. Woodby, L. Luo, E. Schuster, G. Bateman, A.H. Kritz, F.D. Halpern
Division of Plasma Physics (DPP) Annual Meeting of the American Physical Society (APS)
Philadelphia, Pennsylvania, October 30-November 3, 2006
Abstract
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Neoclassical Tearing Modes (NTMs) drive magnetic islands to grow to
their saturated widths, at which they can persist stably in the plasma.
The presence of magnetic islands leads to a local flattening of the
current density and pressure profiles, which degrade plasma confinement.
Since the bootstrap current density is proportional to the pressure
gradient, this current is nearly absent within each island. One common
method of stabilizing NTMs and therefore shrinking the island widths
involves replacing the lost current via Electron Cyclotron Current
Drive (ECCD). In order for ECCD to be successful at shrink- ing the
island widths, the current must be driven at the flux surfaces that
contain the islands. Moreover, in order to shrink each island with
minimal ECCD power, the current must be deposited as close to the
center of the island as possible. The difficulty lies in determining
the locations of both the island flux surface and the ECCD deposition
in real time. The Extremum Seeking feedback method is consid- ered in
this work for non- model based optimization of ECCD suppression of NTMs
in tokamaks. Both ECCD steering and plasma position change will be
considered as mechanisms to maximize in real-time the alignment between
the island flux surface and the current deposition location, and thus
to minimize the ECCD power required for NTM stabilization.