Feasibility study of a local active correction system of magnetic field errors in RFX - mod

The active control system of MHD modes is a key feature of RFX - mod device and it allowed performing a variety of control experiments operating the machine both as a high current RFP and a low current, low q Tokamak. Expertise in the active control of plasma modes along with the highest accuracy in the equilibrium magnetic field configuration are stringent requirements in ITER operation scenarios. In RFX - mod, studies are under way to review the existing load assembly with the a im of further increasing the quality of the magnetic configuration. In spite of its overlapping edges, the poloidal cut of the copper shell still remains one of the major sources of field error. On the basis of the experience acquired in the active control , a feasibility study of a local correction system made up of a set of dedicated coils has begun. It includes the development of a cylindrical FE model of copper shell, stainless steel support structure, local coils, existing MHD saddle probes and new loca l ones. The frequency characterization of the system response provides basic information for checking the system effectiveness in producing the desired magnetic field at both the probes and the plasma boundary. Moreover, it provides data for the identifica tion of a dynamic model whose inputs are the local coil currents and whose outputs are the saddle probe magnetic field measurements. In particular, a map of the magnetic field components at the saddle probes radius is necessary to evaluate the spatial harm onic content of the local correction system and subsequently to analyse its interaction with the 192 saddle coils covering the whole support structure. Conversely, the transfer functions between neighbouring saddle coils and local system probes are studied . Different decoupling strategies of the two control systems are considered both in terms of harmonic components and raw signals.