X-ray Free Electron Lasers based on high brilliance linacs

Due to the progress of accelerator technology in recent years it is now possible to consider the construction of a Free Electron Laser (FEL) that provides coherent radiation at wavelengths very far below the visible. In this paper, various projects are mentioned which are under way to establish the Self-Amplified Spontaneous Emission (SASE) principle at very short photon wavelengths as well as multiple harmonic generation. The basic principles are briefly explained and the expected performance is discussed. With respect to linac technology, the key prerequisite for such single-pass, high-gain FELs is a high intensity, diffraction limited, electron beam to be generated and accelerated without degradation. Key components are RF guns with photocathodes, bunch compressors, and related diagnostics. Once proven in the micrometer to nanometer regime, the SASE FEL scheme is considered applicable down to Angstrom wavelengths. It is pointed out that this latter option is of particular interest in context with the construction of a linear collider, which requires very similar beam parameters.