Biogeography-Based Optimization of the Resonator Cooling in a MW-Class Gyrotron for Fusion Applications

The performance of MW-class gyrotrons, candidate technology for the plasma external heating in magnetic-confined fusion machines, crucially depends on the heat sink capability and thermomechanical stability of the resonator cavity. Because of the high and nonlinear heat flux and consequent high temperature reached on the cavity inner surface, high thermomechanical stresses are typically observed, and large displacements jeopardize the operation. An efficient cooling, typically with subcooled water in forced flow, beside reducing the displacement and stress of the cavity, allows to select operative modes with high beam/wave interaction efficiency. An optimized solution for the cavity cooling is addressed here, which should minimize the frequency shift of the radio frequency wave by controlling the displacements on the inner wall of the cavity while respecting the yield strength limit. The optimization study is based on a biogeography-based optimization (BBO) algorithm, which targets the optimal profile of the heat transfer coefficient (HTC) to the coolant, capable to guarantee that the maximum stress in the resonator is lower than its yield strength while minimizing the inner wall displacements. The optimum analytical HTC is translated into an engineering design and its thermal-hydraulic as well as mechanical performance (hot spot temperature, pressure drop, and displacements/stresses) is compared to the current existing cooling solution, showing potential advantages.