Study of magnetic effects on DTT ECRH front-steering mirror

In the final configuration of the Divertor Tokamak Test (DTT) facility, 32 front-steering launchers of the Electron Cyclotron Resonance Heating (ECRH) system will be distributed in 4 sectors of the machine. Two different antennas are hosted in the equatorial and in the upper port of each ECRH sector, with 6 and 2 single launchers respectively. This setup will deliver up to 35.2 MW installed power to the plasma, making such ECRH system the most powerful ever implemented at the time of its completion. To comply with compactness and performance requirements, a fully in-vessel driving system has been proposed for the steering mirrors. The system relies on UHV-compliant piezoelectric walking drives to provide biaxial steering capability. The main drawback of piezoelectric drives is their low driving force, in an environment where magnetic torques can be very high because of variable magnetic fields, as those induced by the non-axisymmetric in-vessel coils for plasma control, and especially in case of disruption events. Therefore, the materials of the water-cooled steering mirrors must be chosen in such a way as to minimize magnetic torques while guaranteeing adequate heat conduction. Numerical and analytical models of the magnetic torques acting on the steering mirror have been developed. The models have been applied to the steering mirror to quantify magnetic torques due to various sources and dynamic regimes. The currently proposed Copper-Chromium-Zirconium mirror was proven to be critical. Therefore, compliance requirements for the mirror have been computed, and different solutions have been numerically evaluated for magnetic torque mitigation and heat conduction capability, including combination of different materials and implementation of a tungsten-coated dielectric mirror.