Studies on RFX-mod discharges with spontaneous rotation

The Reversed Field Pinch (RFP) has a magnetic configuration characterized by a wide spectrum of m=0, 1 tearing modes (TM), involved in the dynamo process. These modes have the natural tendency to lock in phase together and to the wall, such to create stationary bulges at the plasma surface. In RFX-mod a sophisticated system of 4 x 48 independently fed saddle coils feedback induces slow rotations in order to avoid a strict wall-locked condition [1]. Recent experiments demonstrated the possibility to recover the tearing modes fast rotation by lowering the plasma current under a certain threshold [2], and opened a new RFX-mod scenario characterized by the "natural" rotation of the dynamo modes. Cases of back transition to spontaneous rotation of m=2, n=-1 TM have been also observed in RFX-mod tokamak discharges. The results obtained in the exploitation of this scenario will be presented in this work. In particular the relationship between the magnetic data and the impurity flow measurements will be interpreted with the aid of the RFXlocking, a cylindrical zero-pressure spectral code solving the single-fluid momentum equation with realistic magnetic boundary conditions, which include passive conductive structures and feedback coils. [2]. The influence of the diamagnetic drift on the TM frequency will be investigated. The dependence of the locking-unlocking threshold on the main gas will be also discussed by comparing He and D discharges. [1] P. Zanca, L: Marrelli, G. Manduchi, G. Marchiori, Nuclear Fusion 47 (2007) 1425. [2] P. Innocente, P. Zanca, M. Zuin, T. Bolzonella and B. Zaniol, Nucl. Fusion 54, 122001.