Total reflection mirrors for VUV Free Electron Lasers

Abstract

Publisher Summary This chapter, through an experiment, discusses the use of total reflection mirrors for vacuum ultraviolet VUV free electron lasers (FEL). In the experiment, single-layer amorphous carbon coatings were produced on planar, well-polished silicon substrates by magnetron sputtering at GKSS. In order to optimize the deposition process, the coatings were routinely characterized with unpolarized Cu-κα radiation using a conventional x-ray reflectometer. The optical characterization of the coatings in the VUV and soft x-ray spectral region was done at H ASYLAB /DESY using the soft x-ray reflectometer at beamline Gl. For the comparison, carbon coatings produced by other deposition methods, that is, plasma-enhanced chemical vapour deposition (PE-CVD) and pulsed laser deposition (PLD) were also investigated. Annealing experiments have been performed to test thermal stability. The radiation stability of the carbon mirrors was investigated in the free electron laser interaction with solids (FELIS) experiment at the TESLA Test Facility (TTF)-FEL. The ablation of carbon was analyzed by a time-of-flight (TOF) spectrometer as a function of the radiation power density. All measurements were performed at room temperature under UHV conditions. The experiment showed that the PE-CVD coating (dashed line) has the lowest reflectivity in the energy region of 50–200 eV. The low density of this coating can possibly be explained by the incorporation of hydrogen during the deposition process. The PLD film has high reflectivity and very high density but also high roughness. The sputtered amorphous carbon coating has the highest reflectivity (95–96%). The density is about 2.2 g/cm3, and the roughness is much lower than that of the PLD film. The amorphous carbon coating produced by magnetron sputtering shows the best combination of properties.