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Cylindrical Model of RWM in RFP Plasmas and Application on RFX-mod

The RWM instabilities have been demonstrated to be successfully controlled by the feedback equipment implemented in the experiments of RFX-mod [1] . The observations give an evidence that the system of the active control has a sufficient efficiency and a large flexibility, therefore can provide an excellent environment to apply and to test RWM physical modeling and various control scenarios. Apart from some similar behaviour in both RFP and Tokamak configurations, the RWMs in RFP plasmas are current driven instabilities; while in Tokamak the RWMs are normally driven by the plasma pressure. Furthermore, in RFPs the external kink instabilities, having their rational surfaces outside the plasma, exist as so called externally non-resonant modes (ENRM),which have their rational surfaces located at q q(0) > 0. This fact leads to a more severe condition on the stabilization by plasma rotation and dissipation. In addition, the current driven RWM instability in a RFP, where the strong poloidal magnetic field leads to a “bad curvature” dominant along the entire poloidal angle, has much weaker ballooning structure than in a tokamak. In the present work, we study RWM instabilities in cylindrical RFP plasmas by MHD theory, in which the effects of the plasma pressure, compressibility, plasma inertia, longitudinal rotation and parallel viscosity (tensor) have been taken into account. The resistive wall is modeled with a finite thickness, which allows to treat a large equilibrium flow in the plasma and ωτb>>1 (ω is the mode frequency and τb is the wall penetration time scale length). In the RFP configuration the poloidal asymmetry in the equilibrium magnetic field is much weaker than in a tokamak, the existing studies in toroidal geometry of RFPs have shown a weak influence of the toroidal coupling effects on the growth rate of the RWM modes [2] . As a first step of the study, in present work we adopt a periodic cylindrical model while the effects of the toroidal geometry will be investigated in a future work.