Experimental investigation of plasma topological properties in the Reversed Field Pinch

A description of some topological properties of the Reversed Field Pinch (RFP) equilibrium during Quasi Single Helicity (QSH) states is presented. Since a Hamiltonian description applies to magnetic field lines, a direct application of ideas and tools of Hamiltonian dynamics to magnetized plasmas is possible. These plasmas, produced in toroidal laboratory devices, are confined by a magnetic field. Since in the MHD framework SXR iso-emissive surfaces are representative of magnetic flux surfaces, soft X-ray (SXR) tomography provides a powerful, non-invasive diagnostic to reconstruct experimentally magnetic field maps. In particular, information on the presence or absence of a magnetic separatrix is achieved. The possibility of obtaining information on non-linear phenomena relies on the ability of mapping magnetic flux surfaces with high spatial and time resolution. SXR tomograpy diagnostic turns out to be a powerful tool. Moreover, the RFP configuration is a good environment for non-linear physics studies. In the paper correlations between the topology of magnetic islands and the level of magnetic chaos within the plasma are discussed. A classification of the presence /absence of magnetic separatrix is done as a function of the mode amplitudes. The evidence of the separatrix absence during Pulsed Poloidal Current Drive (PPCD) experiments is consistent with the general result of Hamiltonian dynamics stating that the separatrix is a preferred location for the onset of chaos.