Thermo-hydraulic analysis and thermal fatigue life estimation of TH1509U-A gyrotron’s electron beam collector

Gyrotron devices use the phenomenon of electron cyclotron resonance (ECR) for the heating of plasma in fusion reactors. A substantial amount of power, carried by the electron beam, is not actively employed and must be exhausted by the gyrotron’s beam collector. In this thesis, the commercial softwares STAR-CCM+ and Ansys are used to carry out a thermo-hydraulic and thermo-mechanical analysis on the collector of the TH1509U-A gyrotron, manufactured by THALES for the Divertor Tokamak Test (DTT) facility in Frascati. The component has a concentric cylindrical shells structure, and it is cooled by subcooled pressurized water flowing into the annular section between the shells. The innermost shell receives the spent electron beam and is equipped with a series of deep cavities (fins with aspect ratio=2.85) with azimuthal symmetry. The analysis is performed considering a permanent regime: the transient thermal load (reaching peaks of 30MW/m2) is substituted with a time-averaged one (with peaks of 3.5MW/m2) . The thermo-hydraulic problem is solved both for a 3D and 2D section, exploiting the axial symmetry of the domain. The aim is assessing to what extent a 2D simulation can be used to accurately capture the thermal-hydraulic behavior of the collector, when compared to the 3D solution. The results highlights how the main features of the flow field, and namely the number of vortices in the cavities, is significantly dependent on the model: two main vortices are predicted inside the cavity in the 3D solution, while three are predicted in the 2D one. In both cases, the temperature field in the fluid shows that boiling is likely to occur for the given operating conditions. The 2D model, being the most conservative, is used as the reference one for the thermo-mechanical analysis. The temperature field in the solid is used to evaluate thermal stresses on the component and provide an estimation of its thermal fatigue life, which results to be in the range of 10^4 cycles.