Upgraded electromagnetic measure system for RFX-mod

In order to extend the operational space of RFX-mod in both RFP and Tokamak configurations, a major refurbishment of the load assembly is under study. It includes the removal of the vacuum vessel to increase the plasma-shell proximity and modifications of the support structure to obtain a new vacuum-tight chamber. This entails the design of a new electromagnetic measure system, taking into account the equilibrium and MHD control system requirements in both RFP and shaped Tokamak configurations. The required bandwidth is 0-500 Hz for the control system and up to 50 kHz for the diagnostics of the high m-n tearing mode rotation. The presence of the copper shell forces to mount most of the sensors onto the inner surface of the shell itself, in vacuum, protected by the first wall graphite tiles. Triaxial pick-up sensors are being considered for both the reduced available room and their convenience for the compensation of alignment errors. The spatial resolution is constrained by the number of tiles: 28 (poloidal) x72 (toroidal) fully covering the toroidal inner surface. A basic set of 7x72 sensors is foreseen to assure the calculation of the MHD mode spectrum up to (m=0,. . . ,3; n=0,. . . , ± 35). Moreover, at least 6 poloidal arrays will include 14 sensors to compute the harmonic content needed by the plasma boundary reconstruction algorithm (m=0,. . . ,6). Being available both the magnetic field radial and poloidal component, the poloidal flux loops could be reduced to 2. On the contrary, 12 equally spaced measurements of the toroidal flux, at the middle of each toroidal winding sector, would be of interest for RFP experiments, complemented by the corresponding arrays of triaxial measurements to detect the tokamak diamagnetic component. The installation of an additional set of 4x48 saddle probes is also envisaged to assure a redundant full MHD active control capability.