Analysis of Vertical Stability and Resistive Wall Modes in RFX-mod Tokamak Discharges Including 3D Effects

The RFX-mod experiment (R/a = 2.0/0.46 m), originally designed to produce high current Reversed Field Pinch plasmas (plasma current up to 2 MA), is currently operated also as a low current tokamak (Bt ~ 0.55 T, Ip ~150 kA @ qa ?2). Circular, double-null [1] and single null configurations have been successfully already achieved. Such tokamak configurations may be prone to several MHD instabilities, usually classified resorting to the toroidal mode number n: current-driven n=1 Resistive Wall Modes [2], due to relatively low safety factor; n=0 RWM (vertical instability) [1], due to non-vanishing elongation of the plasma configuration. For the active control of these instabilities, RFX-mod is equipped with a state-of-the-art MHD control system made by 192 (4 poloidally x 48 toroidally) independently fed active coils, with more than 600 magnetic sensors acquired in real time; the feedback system is operated under the MARTe framework. The evolution and the control of these instabilities, being RWM, are evidently strongly dependent on the features and the geometry of the (active and passive) conducting structures surrounding the plasma. In order to provide the required accuracy in their description, several computational tools will be applied to the plasma configurations under analysis: linearized axisymmetric models (CREATE_L [3]), linearized 3D models (CarMa0 [4], CarMa [5]), nonlinear evolutionary equilibrium models including 3D volumetric structures (CarMa0NL [6]). The different assumptions and approximations of the various models will allow a clear identification of the key phenomena ruling the evolution of the aforementioned instabilities in RFX-mod tokamak discharges and hence will provide fundamental information in the planning and the execution of related experiments and in refining the control system design. [1] G. Marchiori et al., 41st EPS Conference on Plasma Physics, Berlin, June 2014, paper P5.040 [2] G. Marchiori et al., 38th EPS Conference on Plasma Physics, Strasbourg, June 2011, paper P2.110 [3] R. Albanese and F. Villone, Nucl. Fusion 38 (1998) 723 [4] F. Villone et al., Plasma Phys. Control. Fusion 54 (2012) 085003 [5] F. Villone et al., Phys. Rev. Lett. 100 (2008) 255005 [6] F. Villone et al., Plasma Phys. Control. Fusion 55 (2013) 09500