Journal of the Korean Physical Society, Vol. 74, No. 1, January 2019, pp. 24∼29 Performance of an Indium-sealed S-band RF Photoelectron Gun for Time-resolved Electron Diffraction Experiments Hyun Woo Kim, Kyu-Ha Jang, In Hyung Baek, Kitae Lee and Young Uk Jeong ∗ University of Science and Technology, Daejeon 34113, Korea and Radiation Center for Ultrafast Science, Korea Atomic Energy Research Institute, Daejeon 34057, Korea Jinhee Nam University of Science and Technology, Daejeon 34113, Korea Moonsik Chae, Mi Hye Kim, Young Chan Kim and Key Young Oang Radiation Center for Ultrafast Science, Korea Atomic Energy Research Institute, Daejeon 34057, Korea Sunjeong Park Korea University, Sejong 30019, Korea Jang-Hui Han Pohang Accelerator Laboratory, POSTECH, Pohang 37673, Korea Nikolay A. Vinokurov Budker Institute of Nuclear Physics, Novosibirsk 630090, Russia (Received 7 December 2018, in final form 18 December 2018) We have developed a one-and-half -cell S-band radio-frequency (RF) photoelectron gun (pho- togun) fed by a coaxial coupler. The RF photogun is dedicated to ultrafast-electron-diffraction experiments by generating electron bunches of 3-MeV energy and a-few-pC charge, which is not strict condition compared to those for X-ray free-electron lasers. Brazing of RF cavities is well- developed process for making RF guns or RF accelerators. Sometimes, however, a failure occurs in the brazing process, causing the entire electron gun or accelerating cavity to spoil. Axial-symmetric design of the RF photogun permits indium sealing for cavity cells, a photocathode plate, and a coupling RF part. We firstly report that the indium-sealed RF photogun successfully meets the required performance and long-term stability for ultrafast electron diffraction experiments. We have stably operated the RF photogun for more than three years with the electron beam conditions of ∼ 3-MeV energy, up-to-10-pC charge, and a repetition rate of 50 Hz. The quantum efficiency of the copper photocathode had improved from 10 -6 to 10 -5 depending on vacuum condition from 10 -8 to 5 × 10 -10 Torr, respectively. Measured emittance and energy spread of the generated electron beam showed 0.3 mm-mrad and less than 0.25%, respectively, for a bunch charge of ∼ 2 pC, which agree well with those obtained by ASTRA simulation. PACS numbers: 41.75.Fr, 41.75.Ht, 42.25.Fx Keywords: Radio-frequency gun, Photoelectron gun, Time-resolved electron diffraction, Ultrafast electron diffraction, Ultrafast science DOI: 10.3938/jkps.74.24 I. INTRODUCTION Radio-frequency (RF) photoelectron gun (photogun) is the leading technology to generate high-brightness electron beam required in next-generation radiation ∗ E-mail: yujung@kaeri.re.kr sources. The RF photogun, which accelerates the elec- tron beam quickly by the strong electric field inside the gun, is suitable for generating electron beams with ex- cellent beam qualities and high brightness. Many of the X-ray free-electron lasers (X-FEL) currently in operation or under development are using the RF photogun tech- nology [1–4]. In relativistic ultrafast electron diffraction (UED) experiments, where the beam brightness is rela- pISSN:0374-4884/eISSN:1976-8524 -24- c 2019 The Korean Physical Society