C -band disk-loaded-type accelerating structure for a high acceleration gradient and high-repetition-rate operation

A high-acceleration-gradient linear accelerator (LINAC) for an x-ray free electron laser (XFEL) offers the advantages of a short accelerator length and low construction costs. In addition, the high pulse repetition rate of the LINAC, which can drive multiple x-ray beam lines, provides additional user opportunities for experiments involving XFEL. A $C$-band disk-loaded-type accelerating structure was developed to achieve a high acceleration gradient of $g50\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ and an rf-pulse repetition rate of 120 pps, which is twice as high as that of the XFEL facility, SACLA (60 pps). The structure has a quasiconstant gradient and a traveling wave type with an accelerating mode of $\mathrm{TM}01\ensuremath{-}2\ensuremath{\pi}/3$. To reduce the surface electric fields, we employed a cross section with an ellipsoidal curvature around an iris aperture. The accelerating structure was manufactured for SACLA. High-power rf conditioning was conducted to investigate its performance. Owing to the conditioning, the acceleration gradient reached a value of more than $50.1\text{ }\text{ }\mathrm{MV}/\mathrm{m}$. The structure was operated without any serious issues at a repetition rate of 120 pps. The accelerating structures were installed in the dedicated accelerator for EUV-FEL at SACLA beam line-1. Finally, we obtained accelerated electron beams with the structures operated at an acceleration gradient of $41.4\text{ }\text{ }\mathrm{MV}/\mathrm{m}$.