Three-Dimensional Physics Studies in RFX-Mod

Full text: The discovery of Single Helical Axis States (SHAx) in the reversed field pinch (RFP) gives a unique opportunity to investigate the physics of three-dimensional fields in magnetized fusion plasmas. In RFX-mod high current experiments the plasma frequently reaches the Single Helical Axis (SHAx) state, in which the core electron temperature assumes a helical shape and significant gradients appear. The plasma magnetic topology is strongly helical in the core while it is almost axisymmetric at the edge, thanks to the active control of the edge radial field. The weak helical field produced by the plasma profoundly affects the shape of the equilibrium going from 2D to 3D, therefore three-dimensional tools are being developed for the description of these states. On one hand the SHEq code, based on a perturbative approach, is used to determine the magnetic field topology by computing the helical flux: the iota profile of the helical flux surfaces significantly differs from the axisymmetric one especially in the core. On the other hand, the VMEC code has been modified for the helical RFP, in order to describe the SHAx states by using tools developed by the Stellarator community. The VMEC code allows deepening the investigations on the use of helical fields to allow long pulse operations. Moreover, an investigation on the stability of the helical states is being performed by means of the TERPSICHORE code: preliminary runs suggest that the core magnetic core shear is an important parameter for ideal MHD stability. The effect of three-dimensional fields on transport is also being actively investigated. The magnetic shear is found to be correlated with regions with reduced transport: an investigation on the transport mechanisms underlying such barriers is being performed by adopting several approaches. Particle transport coefficients across helical surfaces have been estimated numerically by a mono-energetic test particle approach: the main contribution to transport is found to be due to neoclassical effects, even though a residual level of magnetic chaos may play a role on passing particles. Further investigations at low collisionality show that the estimated particle transport coefficients do decrease in these regimes. A different approach for transport characterization based on Stellarator tools, using VMEC or SHEq equilibria, is planned. (author)