Design and high-power test of a short prototype of high gradient S-band accelerating structure for the FERMI free electron laser linac upgrade

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.