New developments on free electron lasers based on self-amplified spontaneous emission

After a brief introduction to the basic concept of free electron lasers operating in the self-amplified spontaneous emission (SASE) mode, the paper presents the rapid progress achieved during the past 3 years in successfully proving this FEL process at shorter and shorter wavelengths, meanwhile reaching 80 nm. The technical challenges are discussed to be overcome in building FEL facilities for wavelengths much below the visible, and these challenges are compared with the present state-of-the-art. Finally, approved and proposed SASE FEL projects are briefly summarized