1. See for example lightsources.org.
2. W.A. Barletta, et al., Nuclear Instruments and Methods in Physics Research Section A 618 (2010) 69.
3. E. d׳Humieres, P. Balcou, in: Sandor Varro (Ed.), Compact X-FEL Schemes, Free Electron Lasers, March 14, 2012, under CC BY (ISBN 978-953-51-0279-3).
4. G.B. Schroeder, et al., in: Proceedings of the FEL 2012 Conference, Nara, Japan, 2012.
5. Nearly copropagating sheared laser pulse FEL undulator for soft x-rays