Design of the brazing cycle for the IFMIF/EVEDA-RFQ (Radio Frequency Quadrupole) modules using coupled thermal-structural finite element analyses

In the framework of the IFMIF-EVEDA project [1] the Radio-Frequency Quadrupole (RFQ) is a linear accelerating structure designed to both increase the energy of a 140mA Deuteron beam from 100keV to 5MeV and to focus the beam itself, by means of proper electric RF fields at the frequency of 175MHz [2, 3]. Such structure consists of 18 modules for a total length of 9.8m, made of OFE copper and AISI316LN Stainless Steel [4]. Due to the large amount of beam power (637kW) and RF power (about 600kW) as well as the necessity for the RFQ to operate in ultra-high vacuum (10-7 mbar), the component dimensions and materials adopted require the coupling of parts by means of vacuum furnace brazing performed typically in a range of temperatures between 850°C until 780°C. The deterioration of the mechanical and morphological properties of the CUC2 [5] as well as the need to limit the overall deformations of the modules (the soldering process and intermediate milling steps) induced to develop a design for a single step brazing. The main reason for this was the respect of the very tight geometrical tolerances with the corresponding need to limit beam losses under a few%. The brazing process was studied with a finite element thermal cycle considering the modules and the oven. The characterization of the component and oven geometry and materials and their thermal contacts will be described. Simplified thermal models are used for data determination, by adjusting the emissivity surface conditions and the reduction of surface area exposed. Validation is obtained by the comparison of the temperatures measured by the thermocouples on components, with the corresponding simulated ones. Thanks to a coupled thermal-structural model, which accounts for the thermal expansion curves of the adopted materials, a tailored brazing cycle could be designed. This made possible to reduce the geometric distortion of modules component with greater mass.