Modeling and Optimization of the Chemical Etching Process in Niobium Cavities

Niobium cavity is an important component of the integrated NC/SC high-power linacs. Over the years researchers in several countries have tested various cavity shapes. They concluded that elliptically shaped cells and buffered chemical polishing produce good results. The objective of this paper is to study the effect of chemical etching on the surface quality and to optimize this process. Chemical etching of the inner surface of the cavity is achieved by circulating acid through it. As the acid interacts with the surface, it eliminates imperfections and improves surface quality. During etching, a pipe with baffles is inserted within the cavity to direct the flow along the surfaces. A finite element computational fluid dynamics model is developed for the etching process. The problem is modeled as a two-dimensional, axisymmetric, steady state fluid flow problem. This model is used to evaluate the current etching process. An alternative design with an expanding baffle is proposed. The new design is optimized to improve the chemical etching process.