REVIEW OF EXISTING SOFT AND HARD X-RAY FEL PROJECTS

The existing soft and hard X-Ray FEL Projects are discussed, along with the underlying design and technological strategies. We consider two main categories: large facilities, which will involve the joint efforts of big laboratories and will take benefit from the heritage of high energy Physics facilities and smaller devices, which are aimed at exploiting high quality accelerators with modest e-beam energy to reach shorter wavelengths with alternative schemes. We will discuss advantages and drawbacks of the different conceptions and give an outlook to the future developments, with particular attention to combinations of different solutions like exotic undulators, seeding and so on, aimed not only at improving X-ray beam qualities but also at reducing device complexity and cost.