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Design and implementation of control methods for active feedback stabilization of resistive wall modes in the EXTRAP T2R reversed field pinch
Sensor signals may be affected by stray fields due to small misalignments of the sensor coils, a problem which cannot be completely avoided, and thus have to be taken into consideration when designing the controller algorithm. A compensation for stray fields can be implemented by using information from other magnetic diagnostics available. There is other magnetic activity as well as electrical noise that also adversely affect the sensor signals. This noise problem can be dealt with using suitable digital filters, based on knowledge of the various noise sources.
Perturbations of the plasma equilibrium caused by ideal MHD instabilities are sometimes observed in toroidal fusion devices. The unstable eigenmodes occurring in a reversed field pinch device are essentially helical deformations characterized by different toroidal mode numbers. The perturbations may be passively suppressed by an electrically conducting wall that is close to the plasma boundary. However, in cases when the magnetic field penetration time of the wall is shorter than the plasma life time, the perturbation grows on the time scale of flux penetration through the wall, resulting in unstable, slowly growing resistive wall modes (RWM) that require active stabilization.
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