Thermal mechanical analyses of the mm-wave miter bend for the ITER electron cyclotron upper launcher first confinement system

Each of the 4 ITER Electron Cyclotron Heating Upper Launcher (ECHUL) features 8 transmission lines used to inject microwave power of up to 1.31 MW (at the source) per line, into the plasma at 170 GHz. The microwaves are guided from the gyrotrons via (ø50 mm) waveguide transmission lines, into the plasma via the First Confinement System (FCS), consisting of a 'Z'-shaped set of straight corrugated aluminum waveguides, totaling ∼13 m per line, connected by miter bends (MB). The orientation of the transmission line is achieved by reflecting the microwaves with mirrors. The MBs perform these reflections at angles of ∼90°; they also form part of the primary vacuum boundary and serve as a Tritium barrier, for which SIC-1 safety classification requirements apply. By reason of ohmic dissipation, mm-wave power is converted into heat, reaching a peak thermal flux of approximately 5.3 MW/m² at the MB mirror center. Due to this intense peaked power density in the middle of the MB mirror, a dedicated cooling system needs to be designed and tested. In addition to handling the ohmic losses, the MB of the FCS shall be capable of resisting the applied external loads and imposed displacements and also comply with material and space restrictions. This study describes the evolution of several cooling concepts (evaluated via computational fluid dynamic analysis), leading to the optimization of the final chosen cooling system, which integrates the innovative use of vapor chamber technology. In addition, consideration is given to the final design parameters (material choice and structural dimensioning), concluding with the presentation of the MB assembly concept deemed most suitable for the final FCS design.