Description
RFXmod2 — starting operations in 2027 as an improved version of the former RFXmod — is the largest Reversed Field Pinch (RFP) fusion device in the world. It features a flexible power supply, along with advanced feedback control systems, that allows it to operate in other magnetic configurations, such as the tokamak. RFP plasmas typically exhibit almost cyclic relaxation phenomena (namely, dynamo events), associated with magnetic reconnection processes, with the destabilization of Alfvénic eigenmodes, and with the generation of toroidal magnetic flix. In particular, for high plasma current (≈1MA), these events lead to the transition from self-generated helical equilibria, produced by the action of a dominant kink-resistive mode, towards a more turbulent and multimodal state characterized by the overlap of magnetic islands. The latter circumstances produce strong chaoticity of the magnetic field map, which results in the degradation of the confinement properties.
It has been observed that active dynamical modification of the magnetic edge toroidal field and its associated poloidal current drive can induce the transition towards improved confinement regimes, with the generation of internal thermal barriers. The associated steep pressure gradients, however, act as free energy source for various electrostatic and magnetic plasma instabilities, like the micro-tearing modes. Considering that RFXmod2 will feature an improved magnetic boundary (with respect to RFXmod), it is expected that these kinds of instabilities will play a crucial role in the efficiency of the device.
In this work we present a characterization of magnetic fluctuations that are expected to be present in RFXmod2 based on the measurements provided by the high-frequency in-vessel sensors of RFXmod. Similarly, the effect of the plasma equilibrium on the measured magnetic spectral properties will be here discussed. Finally, the dynamic modification of the current profile through the Oscillating Poloidal Current Drive (OPCD) technique will be widely analyzed as a method to stimulate the Quasi-Single Helicity (QSH) state.