The cavity optics within high power free-electron lasers based on energy-recovering accelerators are subjected to
extreme conditions associated with illumination from a broad spectrum of radiation, often at high irradiances. This is
especially true for the output coupler, where absorption of radiation by both the mirror substrate and coating places
significant design restrictions to properly manage heat load and prevent mirror distortion. Besides the fundamental lasing
wavelength, the mirrors are irradiated with light at harmonics of the fundamental, THz radiation generated by the
bending magnets downstream of the wiggler, and x-rays produced when the electron beam strikes accelerator diagnostic
components (e.g., wire scanners and view screens) or from inadvertent beam loss. The optics must reside within high
vacuum at ~ 10-8 Torr and this requirement introduces its own set of complications. This talk discusses the performance
of numerous high reflector and output coupler optics assemblies and provides a detailed list of lessons learned gleaned
from years of experience operating the Upgrade IR FEL, a 10 kW-class, sub-ps laser with output wavelength from 1 to 6
microns.
The IR Upgrade FEL at the Thomas Jefferson National Accelerator Facility (JLab) was used to make measurements of the absorption in laser cavity mirrors, both high reflectors and outcouplers. Measurements were made at 10, 6, and 3 um, by determining the temperature rise of the cooling water of the FEL cavity mirrors while operating at high average power, and by using a laser vacuum calorimeter and interpreting the data using the ISO 11551 standard.
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