Performance evaluation of advanced cooling configurations for gyrotron machines

Two of the most significant issues of our time reside in the continuous increase in the energy demand and, on the other hand, the urgent need to reduce the carbon footprint of traditional energy sources. Nuclear fusion energy is expected to provide a solution for both instances: it should be a clean, safe and practically unlimited source of energy, at least in human time frameworks. This new technology relies on fusing light particles in an extremely hot and ionized gas, the plasma. For the external heating of the latter, different techniques are currently used and envisaged, and among them the gyrotron, a high-power (∼MW) linear-beam vacuum tube which employs the cyclotron resonance of electrons to generate high-frequency (∼ 100 GHz) EM waves to irradiate the plasma throughout its operation. During the functioning of the gyrotron, large heat loads are discharged on its inner walls, causing deformations. Those deformations then result in a frequency shift of the generated RF waves. To reduce the repercussion of heating and deformation, it is extremely important to effectively cooling the cavity. So far, European gyrotrons work at the very edge of their possibilities, mainly limited by thermal constraints rather than EM ones. Thus, an improvement of the cooling efficiency is strictly demanded in order to comply the requirements for DEMO perspective. Up to now, two different routes have been pursued to enhance the heat transfer between the coolant and the solid part in European machines: increasing the heat exchange area or promoting turbulence, augmenting the heat transfer coefficient. First option relies on the development of a porous structure of Raschig rings while the second one exploits mini-channels within the cavity cooling region. The aim of this thesis is the assessment and optimization of the cooling device performances for the gyrotron from both hydraulic, thermal and mechanical points of view, to minimize pressure drops, maximize heat removal capability and reduce deformations and stresses. First, the iterative procedure used for modelling the cavity is described: the MUCCA tool (Multi-physiCs tool for the integrated simulation of the Cavity), coupling thermal-hydraulic and thermo-mechanic simulations to electro-dynamic code EURIDICE, developed by the Department of Physics, National and Kapodistrian University of Athens. EURIDICE takes as input the deformed resonator cavity profile and returns as output the updated Ohmic heat load, which in turn is the input for the TH and TM part in a new step of an iterative process, which is continued up to convergence. Then, two specific cavity configurations have been analyzed: TH1507U, produced for W7-X, and TH1509U, envisaged for ITER. This work has been carried out using the software STAR-CCM+ for the thermal, fluid-mechanical calculations while for the thermo-mechanical part both STAR-CCM+ and Ansys Mechanical have been exploited. The study of the W7-X gyrotron has been implemented using the MUCCA tool, since a new cooling system equipped with mini-channels was designed by Thales and needed evaluation of performances. Concerning the TH1509U equipped with RR, the uniformization of the flow entering the gyrotron cooling device has been investigated based on previous works, where an optimization analysis was performed to design a multi-inlets device instead of a single one for the gyrotron TH1507U equipped with RR.