Simulation of R.F window Brazing process: Thermal-Structural analysis of ceramic to metal vacuum brazing joints

Abstract

The creation of the cyclotron absolutely sited radio frequency at the centre of particle accelerators. In the cyclotron, RF system is a device that transforms electrical energy taken from the grid into energy transferred to a beam of particles inside the accelerating cavity. The power coupler, which transport and inject power to the cavity, needs at the same time to couple strongly the transmission line to the field distribution in the cavity. It also has to provide a way to adjust the matching between the line and the cavity, and finally to separate the vacuum of the cavity from the air of the line. The consequence of these multiple requirements is that the power coupler is usually a very critical device that requires a careful design and prototyping. R.F window is one of the main parts of the power coupling capacitor.
The present research work dealt with the simulation of R.F window brazing process & development of ceramic to metal (copper) outer joint of R.F window. The outer brazing joint is very critical as higher thermal expansion material, e.g. copper is placed outside of the lower linear thermal expansion coefficient material (ceramic). It results a large brazing gap – not suitable for capillary action. A detailed analysis was performed from simple model to FEM model (using theoretical approach & FEM code, Ansys.) to understand the behavior during brazing. Molybdenum & Graphite both had been considered separately as tooling fixture material for theoretical and FEM analysis. Literature review reported that maximum work in the field of ceramic to metal brazing have been adopted using molybdenum as tooling fixture. In this research work Graphite was selected as tooling fixture material to perform the vacuum brazing process of ceramic to copper outer joint test model. After design and dimensions, model drawings were developed. Fabrication of the test model assembly, brazed inside vacuum brazing furnace by using Cusil filler. This joint has qualified Helium leak test in order of 3.6 X 10-9 mbar l/s.