Steps towards x-ray sources based on linac-driven free-electron lasers

X-rays play a crucial role in the study of structural and electronic properties of matter on an atomic scale. With high-brilliance x-ray sources high resolution imaging and the observation of very fast chemical processes become possible. A high-brilliance x-ray free-electron laser (FEL) based on linear accelerator technology using the principle of self-amplified spontaneous emission (SASE) appears to be the most promising approach. However, the high electron beam quality required by the SASE FEL process presents challenges to the linear accelerator community. Recent results obtained in several SASE FEL test facilities in the visible and ultra-violet ranges have demonstrated the viability of injector systems to deliver high brightness electron bunches and longitudinal compression schemes to obtain a few kA peak current while preserving beam brightness over a small slice of the bunch length. Challenges to new facilities reside in understanding the underlying physics to extend the technology to the x-ray region (about 0.1 nm) and in developing high-resolution phase space diagnostics. After a brief introduction to the SASE principle, we review the encouraging results of the recent experiments and present technical concepts to meet the requirements in the FEL facilities presently under construction.