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Modelling of a system for active feedback stabilization of resistive wall modes installed at the EXTRAP T2R reversed field pinch device
For the digital controller, the effect of sampling and delay is important to model. In the case of the amplifier characteristics, the frequency response and the saturation effect should be included in the model. The components will then be assembled to a complete model of the active control system. Simulations with the model will be done for predicting the performance of the actual experimental system. In a next step, the plasma response to externally applied magnetic fields is incorporated into the system model using a cylindrical linear MHD description of the plasma (already available). Combining the MHD model description of the plasma with the modelling of the active system components, predictive simulations of plasma experiments with active control is a further goal of this project.
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 may grow 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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