Nb$_3$Sn is a promising advanced material under development for superconducting radiofrequency (SRF) cavities. Past efforts have been focused primarily on small development-scale cavities, but large, often multi-celled cavities, are needed for particle accelerator applications. In this work, we report on successful Nb$_3$Sn coatings on Nb in a 1 m-long 9-cell Nb sample-host cavity at Fermilab. The geometry of the first coating with only one Sn source made it possible to study the influence of Sn flux on the microstructure. Based on these results, we postulate a connection between recently observed anomalously large thin grains and uncovered niobium spots observed in the past by other authors [Trenikhina 2018]. A phenomenological model to explain how these anomalously large grains could form is proposed. This model is invoked to provide possible explanations for literature results from several groups and to guide key process parameters to achieve uniform vapor-diffusion coatings, when applied to complex structures as the multi-cell cavity under study.

中文翻译：

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通过样品-宿主 9 单元腔开发和理解用于射频应用的 Nb3Sn 薄膜

Nb$_3$Sn 是一种有前途的先进材料，正在开发用于超导射频 (SRF) 腔。过去的努力主要集中在小型开发规模的空腔上，但粒子加速器应用需要大的、通常是多单元的空腔。在这项工作中，我们报告了费米实验室 1 m 长的 9 单元 Nb 样品-宿主腔中 Nb 上成功的 Nb$_3$Sn 涂层。仅具有一个 Sn 源的第一个涂层的几何形状使得研究 Sn 通量对微观结构的影响成为可能。基于这些结果，我们假设最近观察到的异常大的细晶粒与其他作者过去观察到的未发现铌点之间存在联系 [Trenikhina 2018]。提出了一个现象学模型来解释这些异常大的颗粒是如何形成的。