X-ray free-electron lasers: from dreams to reality

The brightness of x-ray sources has been increased one to ten billion times by x-ray free-electron lasers (XFELs) that generate high intensity coherent photon pulses at wavelengths from nanometers to less than one angstrom and a duration of a few to 100 femtoseconds. For the first time XFELs allow for experimental exploration of the structure and dynamics of atomic and molecular systems at the angstrom-femtosecond space and time scale, creating new opportunities for scientific research in physics, chemistry, biology, material science and high energy density physics. This paper reviews the history of this development, concentrating on the Linac Coherent Light Source (LCLS), the world's first hard x-ray XFEL. It also presents the physical principles on which XFELs are based, their present status and future developments, together with some recent experimental results in physics, chemistry and biology. LCLS success has spurred the worldwide construction of more XFELs; SACLA in Japan, XFEL and FLASH in Germany, Swiss FEL, Korean XFEL, Fermi in Italy. The characteristics of these other sources are also discussed.