Design of the new electromagnetic measurement system for RFX-mod upgrade

A major modification of the RFX-mod toroidal load assembly has been decided in order to improve passive MHD control and to minimise the braking torque on the plasma, thus extending the operational space in both RFP and Tokamak configurations. With the removal of the vacuum vessel, the support structure will be modified in order to obtain a new vacuum-tight chamber and the first wall tiles will be directly in front of the passive stabilising shell inside of it, so increasing both the poloidal cross section (by 28 mm radially) and the plasma-shell proximity. This implies the design of a new vacuum fit electromagnetic measurement system. The spatial resolution is constrained by the number of graphite tiles, 28 (poloidal) x 72 (toroidal), which cover the inner surface of the copper shell. The new local probes (of size 42 x 37 x 7 mm) will be installed in vacuum onto the copper shell, behind the graphite tiles, and shall operate up to a maximum temperature of 180°C to allow for baking cycles for first wall conditioning. Because of the reduced room available, triaxial pickup probes have been designed, with the additional advantage of allowing the minimization of alignment errors. The paper describes the detailed design of the new probe set, in particular highlighting the technological issues complying with different functional requirements: the thermo-mechanical interface with the copper shell, their protection, the electrical connection for the transmission of the signal with a minimum level of noise and the choice of materials suitable to the vacuum environment at relatively high temperature. The extensive tests carried out on probe prototypes to characterise their electromagnetic, thermal and vacuum behaviour are also reported.