Introduction

摘要

Free-electron laser (FEL) user facilities for the short-wavelength regime from vacuum-ultraviolet to hard x-rays are operational for more than a decade now. In Europe FLASH (Hamburg) and FERMI (Trieste), world-wide LCLS (Menlo Park, U.S.A.), SACLA (Harima, Japan) and PAL-XFEL (Pohang, South Corea; from 2017) are the facilities to be listed here. In Europe, two new facilities with presently nine end-stations for experiments turn on in the current year: SwissFEL (Villigen) and European XFEL (Hamburg). X-ray FEL radiation provides exquisite beam properties in terms of pulse duration, coherence, and pulse energy. High repetition rate facilities like FLASH and European XFEL also provide high average flux. Short-wavelength FELs combine features of conventional x-ray sources and of ultrashort and highly intense optical laser sources. At the same time the FEL sources are highly complementary to these x-ray sources, like e.g. provided by synchrotron radiation from storage rings, and optical laser sources.