Free electron laser

Abstract

A free electron laser FEL comprises an undulatorgenerating coherent EUV radiation receiving an upstream electron beam EBand emitting a downstream electron beam EBand at least an electron sourceoperable to produce an upstream electron beam EB, EBcomprising 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, a bunch compressor, and said undulator. The downstream electron beam EB, EBthat leaves the undulatorrecirculates through the second linear acceleratorin parallel with the upstream electron beam with a phase such that the downstream beam is decelerated by the second linear acceleratorand then recirculates through the first linear acceleratorin parallel with the upstream electron beam with a phase such that the downstream beam is decelerated by the first linear accelerator; and to direct the downstream beam to a beam dump. At least a first energy spreaderimparts a reversible change to the energy distribution of bunches of electrons and is located at a position in the beam path before the bunch compressorand so that it is only passed through by the upstream electron beam EB. A second energy spreaderreverses the change to the energy distribution of bunches of electrons imparted by the at least one first energy spreader, the second energy spreaderbeing located at a position in the beam path before the undulatorand so that it is only passed through by the upstream electron beam EB