XUV/X-ray Radiation Damage to Amorphous-Carbon Optical Coatings

Amorphous carbon optical coatings are often applied to optics on the new generation of free-electron lasers working in the XUV and soft x-ray spectral regions. The response of these coatings to short-wavelength radiation applied at relatively low fluence for long periods was investigated with a focused beam of high-order harmonics. Material expansion of several nanometers was found on irradiated surfaces by atomic force microscopy (AFM). The results of these experiments were compared to those obtained for the same material with single shots from a soft x-ray Ne-like Zn laser. Introduction The next generation of short-wavelength free-electron lasers (FEL) are either in the commissioning phase (VUV FEL in Hamburg) or under construction (LCLS in Stanford, XFEL in Hamburg). Operation of these facilities could be seriously limited by radiation-induced damage to mirrors manipulating the output beam. Grazing-incidence mirrors coated with amorphous-carbon (a-C) films [Jacobi et al. 2002, Steeg et al. 2004, Stoermer et al. 2004] seem to be suitable not only for the operational conditions of a VUV FEL (6 nm < λ < 60 nm, τ ~ 100 fs, Ep ~ 50 μJ) but also for future facilities working at even shorter wavelengths. Initial experiments have already been carried out to investigate the response of amorphous carbon coatings to short-wavelength radiation. Previously presented [Bittner et al. 2004, Bittner et al. 2005] results, obtained with a neon-like zinc soft x-ray laser (λ=21.2 nm, τ =90 ps, Ep ~ 1 mJ; one shot per 25 minutes [Rus et al. 2002]), showed strong volume expansion (Fig. 1a) following material exposure to single pulses of relatively high fluence (~1 J/cm). Recent Raman measurements confirm that this is due to the massive graphitization of the amorphous carbon layer (Fig. 1b). WDS'05 Proceedings of Contributed Papers, Part II, 373–376, 2005. ISBN 80-86732-59-2 © MATFYZPRESS