PARAMETER OPTIMIZATION FOR LASER POLISHING OF NIOBIUM FOR SRF APPLICATIONS

Surface smoothness is critical to the performance of SRF cavities. As laser technology has been widely applied to metal machining and surface treatment, we are encouraged to use it on niobium as an alternative to the traditional wet polishing process where aggressive chemicals are involved. In this study, we describe progress toward smoothing by optimizing laser parameters on BCP treated niobium surfaces. Results show that microsmoothing of the surface without ablation is achievable. BACKGROUND Surface topography plays a role in SRF cavity performance [1]. Different surface treatment methods are adopted to obtain clean and smooth inner surfaces of niobium cavities. The commonly used buffered chemical polishing (BCP) removes material rapidly but leaves sharp features, which may cause performance limitations. A proposed field enhancement model [2] explains how such local sharp features affect performance of the whole cavity. Electropolishing (EP) is widely used to smooth out sharpness, but EP is relatively slow. Both of these chemistry methods involve using hazardous acids and have potential hydrogen absorption problems due to direct contact between the metal and an aqueous liquid. Laser polishing avoids wet chemistry and can be much faster due to its controllable high density of energy. In this study we show that laser polishing is experimentally achievable by applying the proper energy density (or fluence). Laser polishing of metal surfaces began to be reported in the 1970’s and several processing strategies have evolved since. A recent review presents the history, the fundamentals and the process applications [3]. The broad theme in laser polishing is that the laser energy melts some or all of the surface; levelling proceeds by melt flow under the influence of surface tension until solidification intervenes. The events may be viewed in terms of lower and higher energy density regimes, shallow surface melting (SSM) and surface over-melting (SOM), respectively [4]. In SSM, melting occurs at prominences and capillary forces cause the melt to diffuse into depressions. In SOM, a liquid layer covers the surface. While surface tension driven levelling proceeds, other mechanisms can give rise to oscillations manifested as regular ridge structures. As noted earlier, elimination of sharp features is the most important aspect of SRF surface topography modification. EXPERIMENT