Advanced Experimental Investigations on Cooling Concepts of Cavities for Megawatt-Class CW Gyrotrons

The ongoing research and development of high-power fusion gyrotrons demands for an effective cavity cooling system for optimum gyrotron operation. Since the last decade, fundamental experimental research of advanced cooling techniques using mini-channels for high-power gyrotron cavities is carried out at KIT. In this work, the latest improvements in the test set-up for the thermal-hydraulic investigation of a cavity cooling is described. In order to increase the overall accuracy in the experiments and to eliminate potential uncertainties a set of improvements of the test set-up are considered: a new flowmeter with flow rates that range up to 12 l/min and with improved measurements accuracy; faster thermocouples, with reaction times $\lt 0.1\mathrm{s}$. Additionally, following modifications are introduced: a coating of the inner surface of the copper cavity with a Nickel layer of $100 \mu \mathrm{m}$ thickness in order to increase the heat load using induction heating; 3D printing Additive Manufacturing (AM) of the mock-up in order to directly compare the cooling performances and to validate the 3D printing AM technique for this application; (3) installation and test of a new mock-up with mini-channels and with Raschig rings for a direct comparison between the two cooling techniques. The obtained experimental results provide input to validate numerical models used for the cavity cooling optimization.