Performance optimization for the insert of gyrotron coaxial cavities for nuclear fusion applications

In nuclear fusion machines, plasma heating is provided by the Electron Cyclotron Heating System where gyrotrons are typically employed to produce radiofrequency waves to be injected into the plasma chamber. The gyrotrons for the next generation of tokamaks and stellarators target a delivered power of 2 MW in continuous mode operation. While the hollow-cavity gyrotron concept requires significant R&D improvements in the thermal management of the cavity to minimize thermal deformation and frequency shift, the coaxial-cavity gyrotron concept facilitates improved mode competition control, eventually increasing the output power per tube while keeping the heat load on the cavity wall at an acceptable level. However, the coaxial insert placed in the center of the cavity , in turn, requires a dedicated active double-pass cooling system to keep its own temperature and deformation under control. Since the mode competition control by the coaxial insert can be improved when the thermal loading of the insert can be increased, it is very beneficial to have an insert cooling system which is as effective as possible. The aim of this thesis is to investigate different options for the cooling of the coaxial insert of a 2 MW 170 GHz gyrotron cavity, keeping the insert’s outer diameter unchanged. The investigation encompasses electrodynamic simulations to compute the spatial profile of the Ohmic heat load, as well as thermo-hydraulic and thermo-mechanic simulations, using the commercial software Star CCM+. The identified maximum heat load fulfils the constraints of keeping the fluid in single phase. Moreover, the maximum stress reached in the cavity coaxial insert remains within the allowable limits of the material, and the resulting radial and axial thermal deformations do not impair the functionality of the component. The adoption of simple longitudinal fins connecting the outer and inner shells inside the insert is sufficient to increase the peak heat load that the insert can withstand by ~50%. As a final case study, the investigation included internal geometries based on Triply Periodic Minimal Surfaces (TPMS), specifically the gyroid structure. This configuration demonstrated promising results in terms of thermal performance and structural integrity, confirming the potential of TPMS-based solutions for advanced cooling applications. These findings demonstrate the feasibility of an effective thermal management strategy for the coaxial insert without altering its external geometry, paving the way for enhanced performance and reliability of high-power coaxial-cavity gyrotrons in next-generation nuclear fusion devices.