Plasma shape effect on the n=0 stability of RFX-mod-shaped tokamak plasmas

The achievement of new operational conditions in the RFX-mod-shaped tokamak allowed for the investigation of n = 0 stability and its relationship with a plasma shape cross-section. For this purpose, plasma response models have been produced from CREATE-L code equilibrium reconstructions based on experimental data, and their properties related to n = 0 stability have been studied in detail. In particular, we find that a pure proportional controller is unable to stabilize the n = 0 mode in all models related to experimental shots with the new operational conditions due to the peculiar property that the plasma response models are non-minimum phase systems. This phenomenon has been correlated with increased poloidal beta and enhanced plasma shape conditions. A physical interpretation is given as the rise of a combination of n = 0 vertical instability and n = 0 horizontal instability leading to actuators being ineffective at achieving complete stabilization. This conjecture has been confirmed by exploring the relationship between n = 0 stability of plasma equilibrium and the shape of a plasma cross-section with both vacuum equilibrium field harmonic analysis and field decay index investigation.