Understanding and optimization of the coating process of the radio-frequency Nb3Sn thin film superconducting cavities using tin vapor diffusion method

Abstract

A study was conducted to understand and optimize the coating process of Nb3Sn thin film superconducting radio-frequency (SRF) cavities using the tin (Sn) vapor diffusion method. The characterization of nanometer-scale Sn droplets, millimeter-scale Sn spots, and locally extremely thin patchy areas was carried out. The causes of their occurrence were analyzed, and their influence on RF performance was distinguished and clarified. Furthermore, a method of achieving high-quality Nb3Sn films was explored by increasing the adsorption of residual Sn vapor to suppress the generation of nanometer-scale Sn droplets while under the premise of ensuring uniform film growth by increasing the Sn vapor flux. It was seen that the performance of the 1.3 GHz single cell Nb3Sn SRF cavity coated at IMP was significantly enhanced after the optimization of the coating process with a maximum accelerating gradient (Eacc,max) of over 18 MV/m and an unloaded quality factor (Q0) of more than 1 × 1010 at Eacc = 12 MV/m and 4.2 K. This study provides quantitative insights for understanding the coating process and provides an essential reference for coating high-performance Nb3Sn SRF cavities using the Sn vapor diffusion method.