Free electron laser

A free electron laser FEL comprises an undulator 24 generating coherent EUV radiation receiving an upstream electron beam EB2 and emitting a downstream electron beam EB4 and at least an electron source 21a, 21b operable to produce an upstream electron beam EB1, EB2 comprising bunches of electrons. A beam path is configured to direct the upstream electron beam through: a linear accelerator system (LINAC) comprising at least a first and a second linear accelerators 22a, 22b, a bunch compressor 28b, and said undulator 24. The downstream electron beam EB3, EB4 that leaves the undulator 24 recirculates through the second linear accelerator 22b in parallel with the upstream electron beam with a phase such that the downstream beam is decelerated by the second linear accelerator 22b and then recirculates through the first linear accelerator 22a in parallel with the upstream electron beam with a phase such that the downstream beam is decelerated by the first linear accelerator 22a; and to direct the downstream beam to