SASE FELs: Interactions with Atoms and Ions

Attainment of tunable laser-like radiation in the VUV and X-ray spectral regions is becoming a reality with the construction of single-pass Free Electron Lasers (FELs) based on the principle of Self-Amplified Spontaneous Emission (SASE). The paper introduces the development of short wavelength sources from an atomic physics perspective, describes the SASE FEL process and discusses examples from first user experiments at FLASH (Free Electron Laser at DESY (Hamburg). The perspective for further progress to shorter wavelength devices is outlined