From 3rd-to 4th-generation light sources: Free-electron lasers in the X-ray range

Activities on free-electron laser (FEL) x-ray sources, based on linear accelerators, to produce spatially coherent, ultra-short (similar to 100 fs) pulses with very high peak brilliance (10(28)-10(32) photons/s/mm(2)/mrad(2)/0.1% BW) are summarized. The scientific case includes time-resolved studies of dynamics on sub-ps scales, structural studies by imaging of non-periodic systems, and investigation of high energy-density phenomena such as non-linear x-ray optics and the production of warm dense matter. Examples are presented, with emphasis on the operational facilities, FLASH at DESY, Hamburg and LCLS in Stanford, California, and on the European XFEL project in Hamburg.