Advanced cooling of the coaxial insert in high-power coaxial gyrotrons for fusion machines

In high-power Electron Cyclotron Heating systems for fusion applications, coaxial-cavity gyrotrons enable improved mode control and higher output power compared to conventional hollow-cavity designs, but impose severe thermal loads on the coaxial insert. Advanced cooling solutions are therefore required to sustain higher ohmic heat fluxes without modifying the external geometry of the component. This work investigates innovative internal cooling layouts for the coaxial insert of a 2 MW, 170 GHz gyrotron cavity operating under single-phase coolant conditions. An integrated multiphysics framework is adopted, coupling electrodynamic simulations for the evaluation of ohmic losses with thermo-hydraulic and thermo-mechanical analyses to assess thermal performance, pressure drop, coolant phase stability, and structural integrity. Starting from a fin-based reference configuration, the cooling layout is progressively upgraded by introducing longitudinal fins and advanced architected geometries based on Triply Periodic Minimal Surfaces (TPMS), namely Gyroid and Split-P. The different configurations are systematically compared in terms of maximum sustainable heat flux and associated hydraulic and mechanical constraints. The results show that TPMS-based layouts significantly outperform conventional finned solutions. Compared to a fin-based configuration with enlarged return channel, the Gyroid and Split-P designs increase the maximum sustainable heat flux by more than 45% and 70%, respectively, while maintaining acceptable margins to boiling and thermo-mechanical deformation. Among the investigated options, the Gyroid-based layout provides the most favorable balance between heat transfer enhancement and pressure drop. Overall, the study demonstrates that advanced internal cooling layouts can substantially extend the heat flux handling capability of coaxial gyrotron inserts, supporting the development of next-generation high-power gyrotrons for fusion applications.