Variable-Depth Complementary Spiral cooling channel design for the steerable ECRH mirrors of DTT

The steerable launching mirrors, essential for directing microwave beams into the plasma, play a pivotal role in the Electron Cyclotron Resonance Heating (ECRH) system for the Divertor Tokamak Test (DTT) facility, currently under construction in Frascati, Italy. Due to the substantial heat and electromagnetic induced loads acting on the mirrors, implementing internal channels for active water cooling, together with a proper choice of the materials, is necessary to keep temperature and deformation under control. Three different channel configurations are studied. First, the single-channel spiral cooling path with a constant cross-section, defined in a previous design stage, has been examined. Then, a constant-depth complementary spiral geometry that increases heat exchange area has been defined and analyzed. Finally, a variable-depth complementary spiral channel is proposed and optimized to increase heat exchange efficiency. In all cases, single-channel geometries are considered to enhance safety and malfunctioning detectability. The study is based on Computational Fluid Dynamics simulations. In order to reduce electromagnetic loads on the mirrors in case of plasma disruption to a tolerable extent, a reduced electrical conductivity of the mirror bulk material with respect to pure copper is necessary: this requires the use of material different than copper alloys, which have in turn a lower thermal conductivity. In this case, high cooling efficiency is mandatory. With this goal in mind, first, the performances of the different configurations in terms of mirror temperature and pressure drop are compared considering a reference material with 100 W/(m⋅K) thermal conductivity. Then, the variable-depth configuration is tested for different and more realistic mirror materials. Finally, a comparison between the developed geometry and previous solutions is provided.