Advanced active feedback schemes for MHD studies in hot magnetized plasmas

Magnetized plasmas represent a very good example of macroscopic complex systems where long and small range dynamics typical of neutral fluids are further complicated by electro-magnetic interactions. Increasing our understanding of laboratory based magnetized plasmas within the magneto-hydrodynamic (MHD) framework is important also to advance our knowledge on some basic phenomena of astrophysical plasmas such as ideal MHD, resistive reconnection and magnetic dynamo. RFX-mod is a toroidal experimental facility operated by Consorzio RFX in Padova, capable of confining hot (Te>1keV) hydrogen, deuterium or helium plasmas by the application of magnetic fields up to 0.6 T. The plasma is confined in toroidal geometry with major radius R=2m and minor radius a=0.46m. Thanks to its flexibility, RFX-mod can explore different magnetic configurations including the so called tokamak, Reversed Field Pinch, low-q and ultra low-q, each of which exhibits specific MHD stability properties. A peculiar feature of RFX-mod is the additional presence of a set of 192 active coils entirely covering the toroidal outer surface of the plasma [1]. These coils can be independently fed and are controlled by a fast (duty cycle of few ms) real time control software where different external "reactions" to plasma MHD activity can be implemented. This allows a unique flexibility in imposing different boundary conditions to MHD dynamics and in studying how to optimize the magnetic field properties in view of improving the MHD stability margins of the configuration confining the plasma [2,3]. Purpose of the present work is to present recent experimental results obtained in RFX-mod that significantly contribute to the wider effort of efficiently confine a fusion relevant plasma. In particular we will show detailed experiments where the intrinsic 3D nature of the stability problem is highlighted by properly manipulating the structure of the magnetic field produced by the 192 external coils [4]. The definition of the so-called "mode rigidity" will be critically reinterpreted for the case of the ideal kink instability developing in the presence of resistive passive and active magnetic boundaries. In the final discussion we will explain how these investigations not only allow a deeper understanding of the physics mechanisms underlying MHD stability in RFX-mod plasmas, but also constitute a subject where Consorzio RFX is providing an important support to the design and realization of new international magnetic fusion facilities such as JT-60SA in Japan and ITER in France.