Assessment of the performance of different cooling configurations for the launcher mirrors of the ECRH system of the DTT facility

In the framework of the Divertor Tokamak Test (DTT) facility, under construction at Frascati, Italy, the Electron Cyclotron Resonance Heating (ECRH) system is under design to provide auxiliary heating power to the plasma by means of (up to) 32 gyrotrons, operating at 170 GHz and supplying up to 1.2 MW for 100 s. The microwave power will be launched into the plasma by a pair of steerable mirrors (M1, elliptical and curved, and M2, elliptical and flat) located at the end of the transmission line. The mirrors are subject to nuclear and radiative loads from the plasma, and to heating due to the microwave beam they reflect. An active cooling by subcooled pressurized water in forced flow, located within the CuCrZr block on the back side of both mirrors, is needed to keep their deformation under control. In the paper, the targets for an optimal cooling of M1 and M2 are identified, discussed and ranked. Different cooling concepts, ranging from a single spiral pipe to a split flow in parallel radial manifolds, connected by azimuthal mini-channels, are then investigated, assuming addictive manufacturing could be used to realize the CuCrZr block embedding the cooling channels. The performance of the difference alternatives is first carefully evaluated by means of a 3D conjugate heat transfer analysis performed using a CFD commercial tool. The hot spot temperature, as well as the temperature uniformity on the mirror surface are compared, while the pressure drops determined by the different cooling configurations are evaluated. The thermal deformation is also assessed for the different alternatives for M1 and M2, as well as the possible benefits to reduce the effects of magnetic braking due to the proximity to the machine. Eventually, the pros and cons of the cooling alternatives are discussed in the light of the different targets.