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Design study of an HHG-seeded harmonic cascade free-electron laser

Dunning, D J and Thompson, N R and McNeil, B W J (2011) Design study of an HHG-seeded harmonic cascade free-electron laser. Journal of Modern Optics, 58 (16). pp. 1362-1373. ISSN 0950-0340

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Abstract

The UK New Light Source (NLS) project identified a strong scientific case for a next-generation light source to deliver continuous coverage of the photon energy range 50–1000 eV with variable polarisation, 20 fs pulse widths, a high degree of temporal coherence and 1 kHz repetition rate. Three separate seeded free-electron lasers (FELs) were proposed which in combination satisfy these requirements. It was proposed to use a high harmonic generation (HHG) seed source tuneable from 50–100 eV to give direct seeding at the fundamental FEL photon energy up to 100 eV, with one or two stages of harmonic up-conversion within the FEL to reach the higher photon energies. This paper focuses on the optimisation of a two-stage harmonic cascade FEL design operating at the highest (fundamental) photon energy of 1000 eV. Topics investigated include the configuration of the modulator undulators, the required seed power and the effects of the temporal structure of the unfiltered HHG seed on the FEL output. FEL simulations using realistic electron beam distributions tracked from the gun to the FEL are presented, illustrating the predicted coherence properties of the FEL output.