• M.D. Roper
  STFC/DL, Daresbury, Warrington, Cheshire

The initial three FELs for the proposed UK NLS facility will cover the soft x-ray range 50 to 1000 eV in the fundamental and up to 5000 eV in the 3rd to 5th harmonics. The FELs will be seeded to produce pulse lengths of ~20 fs FWHM, equivalent to a transform limited bandwidth of ~0.1 eV. Whilst spectral filtering of the FEL pulses will not always be necessary, reasons for passing the FEL beam through a monochromator include: removing unwanted spectral content produced by SASE or spontaneous emission but without lengthening the pulse, improving the spectral resolving power from the inherent pulse bandwidth, and removing the fundamental radiation when using the harmonics. In this paper, a grating based monochromator is described that fulfills all three roles. It is based on the SX700 type of monochromator that uses a variable-included-angle plane-grating operating in collimated light. This flexible design allows different modes of operation optimised for minimising temporal stretch of the pulse or for achieving high spectral resolving power. The performance of a beamline operating over the fundamental range of NLS FEL2 (250 to 850 eV) and the harmonics to 2000 eV is calculated.