Located on the site of the Elettra Sincrotrone in Trieste, Italy, the FERMI free-electron laser (FEL) is a user facility fed by a 1.5 GeV, 10 to 50 Hz, S-band radio-frequency linear accelerator (linac). The FEL is based on an external laser seeding scheme which allows lasing down to a fundamental wavelength of 4 nm. In order to achieve shorter wavelengths and improved beam quality, a high gradient upgrade to the facility has been proposed which will extend the current capabilities of the system allowing the generation of beams up to 1.8 GeV while keeping breakdown rates low enough for high machine up-time. This article presents the detailed RF analysis of a 3.0 m S-band, high gradient (HG) structure designed for the new FERMI linac upgrade. This HG structure is designed to provide a reliable accelerating gradient of 30 MV/m with a very low breakdown rate. To ensure the low breakdown rate, Electric-Coupled (EC) RF couplers, also known as waveguide couplers, were designed for the HG structure.

To demonstrate the reliability and feasibility of the upgrade plan, a short prototype was built in collaboration with Paul Scherrer Institute (PSI), Switzerland. Using a newly commissioned S-band cavity test facility, the short prototype was successfully conditioned to an accelerating gradient of 40 MV/m with a pulse length of 600 ns at a breakdown rate of $8\times 10^{-8}$ bpp. A comprehensive overview of the testing facility, its data processing tools and the conditioning of this short prototype will be illustrated. Concluding the paper is a visual inspection of the cells for signs of damage resulting from RF breakdown during the high power testing.

FERMI自由电子激光器（FEL）位于意大利的里雅斯特的Elettra Sincrotrone现场，是一个用户设备，由1.5 GeV，10至50 Hz的S波段射频线性加速器（linac）供电。FEL基于外部激光播种方案，该方案可发射到4 nm的基本波长。为了获得更短的波长并改善光束质量，已提出对该设施进行高梯度升级，这将扩展系统的当前功能，从而允许产生高达1.8 GeV的光束，同时将击穿率保持在足够低的水平，以实现较高的机器升级率。时间。本文介绍了针对新型FERMI直线加速器升级设计的3.0 m S波段高梯度（HG）结构的详细RF分析。这种HG结构旨在提供可靠的30 MV / m的加速梯度，并且击穿率非常低。为了确保低击穿率，为HG结构设计了电耦合（EC）RF耦合器，也称为波导耦合器。

为了证明升级计划的可靠性和可行性，与瑞士的Paul Scherrer研究所（PSI）合作制造了一个简短的原型。使用新近调试的S带腔测试设备，将短原型成功调整为40 MV / m的加速梯度和600 ns的脉冲长度，击穿速率为$8\times \mathrm{1个}{0}^{-8}$ bpp。将说明测试设备，其数据处理工具以及此简短原型的条件的全面概述。本文的结论是对电池进行目视检查，以检查在高功率测试过程中射频击穿导致的损坏迹象。