L. Kochannek
H. Ehlers
S. F. Mikhailov
J. Yan
V. Popov
P. Wallace
G. Swift
M. Ahmed
Y. Wu
L. Jensen
D. Ristau

Enabling Storage Ring FEL for Lasing below 170 nm and production of 120 MeV circular polarized γ-rays by VUV mirrors

23. Juni
Type: Konferenzbeitrag
The operation of a storage ring free-electron laser (FEL) is constrained by the shortest wavelength that its cavity can support. To address this limitation, significant efforts have been made in recent decades to develop high-reflective FEL mirrors supporting progressively shorter wavelengths. However, the harsh environment of FEL, e.g., vacuum condition, synchrotron radiation of bending magnets, intensive lasing wavelength and higher harmonics of undulators are limiting the utilizable materials for resistant mirrors. Oxides have been demonstrated to be suitable materials, for optical coating systems operating at wavelength down to 189.7 nm [1]. Due to the intrinsic band gap absorption of oxides, mirrors for higher energetic lasing wavelengths are only possible to fabricate with fluoride materials such as LaF3 and MgF2. Unfortunately, these materials were found to be highly unstable under FEL conditions. Stefan Günster et. al. developed a hybrid design for FEL mirrors combining a protective oxide layer and a fluoride multilayer coating. While these mirrors enabled FEL lasing at wavelengths as short as 176.4 nm, they already showed changes in reflection after a dose of 28.6 mA*h [2]. These long-standing limitations on fluoride-based FEL, have been overcome due to a recent collaboration work between LZH and Duke. By combining the resources to investigate the process of fluoride decomposition, we were able to create a set of robust and high-reflective fluoride-based FEL mirrors of >95 \% reflection. Single layer and multilayer coatings were fabricated at LZH and irradiated at Duke under realistic FEL undulator conditions. LZH analysed the properties of the coatings before and after irradiation using a VUV spectrometer and SIMS analysis. This knowledge was used to further optimize the coatings. The resulting mirrors were demonstrated to enable FEL lasing between 168.6 and 179.7 nm, setting a new record for the shortest wavelength achieved by all storage ring FELs (s. Fig 1a). The Duke FEL is the laser driver for the High Intensity Gamma-ray Source (HIGS). These new VUV FEL mirrors also enabled the HIGS to produce high-energy, polarized Compton gamma-ray beams up to 120 MeV (s. Fig. 1b), a high-energy record for the HIGS, with a good lifetime of more than 70 hours and an overall dose of 5985 mA*h [3].