Robust control of resistive wall modes using pseudospectra

Details Robust control of resistive wall modes using pseudospectra Robust control of resistive wall modes using pseudospectra

May 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009njph...11e3015s&link_type=abstract

New Journal of Physics, Volume 11, Issue 5, pp. 053015 (2009).

Physics

1

Scientific paper

A novel methodology to design robust feedback controllers for the stabilization of resistive wall modes (RWMs) in tokamaks is presented. A linear state space model is used that describes the system composed of the plasma, the resistive wall, the active coils and the magnetic field sensors. The full three-dimensional (3D) geometry of the wall and the coils is taken into account. The control system is represented by a parametrized matrix whose robust stability properties are optimized under variations of the parameters. To make the optimization process feasible, a reduced state space model is constructed by means of a novel technique involving an orthogonal projection. The orthogonality is essential to the robust stability concept used. The latter is based upon the idea of matrix pseudospectra and therefore accounts for the sensitivity of eigenvalues. Furthermore, the transient growth of perturbations is investigated. Namely, in some theoretically stable linear systems, initial perturbations can grow by large factors before they eventually decay, rendering them practically unstable.
A detailed analysis of a simple, ITER-like test case provides the following general, conceptual insights into the RWM stabilization problem: (i) it is important to consider not only the eigenvalues themselves, but also their sensitivity, (ii) transient amplification might be an issue requiring consideration and (iii) the transient peak can be substantially reduced by a careful choice of the optimization objective and the sensor configuration.

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