3D magnetic fields and plasma rotation in RFX-mod tokamak plasmas

Plasma rotation has been one of the topics of major interest in the fusion community over the last few years, given in particular its role on magnetohydrodynamic stability and turbulence suppression. Rotation can be affected by three-dimensional (3D) magnetic fields, e.g. due to intrinsic magnetic field errors or active coils. The reversed field pinch RFX-mod device (Sonato et al 2003 Fusion Eng. Des. 66 161) has been recently operated as an ohmic tokamak with q(a) < 2 thanks to the suppression of the m = 2, n = 1 mode through magnetic feedback control. In such discharges, plasma rotation is sensitive to the presence of 3D magnetic fields, applied through magnetic feedback or due to the growth of the m = 2, n = 1 mode. In particular, it has been observed that above a threshold value of m = 2, n = 1 radial magnetic field amplitude, plasma rotation reverses from counter- to co-I-p direction. The physical mechanisms that can play a role on momentum transport in these plasmas have been investigated, such as the neoclassical toroidal viscosity, the ambipolar electric field due to magnetic stochasticity, and the friction force due to neutrals coming from the wall. All these phenomena are expected to be important in determining the plasma rotation behaviour also in larger tokamaks. Since the RFX-mod experiment has a simple geometry (circular cross-section) and a wide choice of 3D magnetic fields can be applied with the 192 active coils available, it provides a unique test bed to study the basic physics mechanisms responsible for momentum transport.