Upgrade of the magnetic fault detection system of RFX-mod2

The magnetic fault detection system of RFX (named RGM, from the italian acronym Rilevamento Guasti sistemi Magnetici), originally developed in early 1990s and refurbished in 2000s, has been designed and manufactured with the aim of detecting, within one millisecond, electromagnetic faults of the windings and their circuits occurring during the operation of RFX. RGM is designed to guarantee a high degree of reliability to ensure the integrity of the machine. The magnetic configuration of RFX is characterized by a high level of symmetry: different windings are connected in order to carry the same current (i.e., the four sectors of magnetizing winding are connected in series, the field shaping windings has up-down symmetry of the currents). RGM exploits these specific features to detect imbalances between symmetric circuits, ensuring a fast and reliable intervention in case of fault. Other types of protection are foreseen to detect abnormal currents or voltages in critical parts of the system. However, the RGM system needs to be updated for several reasons. Firstly, the modifications planned for the upgraded RFX-mod2 are incompatible with the layout of some of the original sensors. Moreover, RGM does not allow operations with non-symmetric plasma cross-section (i.e., upper/lower single null configurations), as these configurations would require breaking the symmetry of the windings, giving an erroneous fault identification (false positive). This paper describes the upgrade of the RGM system, which will be equipped with several additional magnetic sensors to overcome the aforementioned limitations, and refurbished with re-designed electronic boards based on state of the art technologies and components. A model-based approach has been followed: the modelling activity involved simulations with both lumped-parameter models of the plasma and passive structures, and more detailed FEM models describing the non-linear evolution of plasma in presence of the passive structures. These simulations, which extend and update the effort done to design the original RGM, have the aim of analyzing several fault conditions to properly characterize the evolution of current/ voltages during these abnormal events, as well as the electrodynamic forces acting on the copper shell, and tune the protections to guarantee a suitable fast detection of the faults.