K. Saeki et al., OPTICAL CAVITIES FOR IR-FELS AT THE FELI, Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment, 375(1-3), 1996, pp. 10-12
The g-parameters of two optical cavities whose length is 6.718 m are d
esigned to be stable. The FEL beam is outcoupled from the upstream mir
ror with an aperture by hole output coupling. The cavity mirrors are s
pherical and gold-coated on copper substrate. The outcoupling mirror h
as a hole of 0.5 mm diameter in the center. Our alignment system conce
pt is that the optical axis of two mirrors is adjusted to coincide wit
h the electron beam position. The electron beam position in the beamdu
ct of the undulator is detected using three OTR plates and images of O
TR at the plates are observed by CCD cameras.