First lasing of the Darmstadt cw free electron laser

Abstract

The Darmstadt cw FEL designed for wavelengths between 3 and 10 μm driven by the superconducting electron accelerator S-DALINAC first lased on December 1st, 1996 and has operated thereafter successfully in the wavelength region between 6.6 and 7.8 μm. The pulsed electron beam employed had a micro pulse length of about 2 ps, with a repetition rate of 10 MHz and a peak current of 2.7 A while its energy was varied between 29.6 and 31.5 MeV. A wedged pole hybrid undulator, with 80 periods each of 0.032 m length and a magnetic field strength of 0.15–0.4 T, was located in between a 15.01 m long optical cavity equipped with two high reflectivity (99.8%) mirrors of 0.05 m diameter. Due to the low beam current special care with respect to the electron and optical beam properties was necessary to meet the stringent conditions in order to reach a minute small signal gain of at least a few percent resulting in amplification. Saturation was obtained after about 2000 repetitions of the photon pulse inside the cavity. The Darmstadt FEL experiment is unique with respect to its variable electron beam macro pulse structure and it is thus the first linac-based FEL providing a cw photon beam. First experiments using the FEL were performed for characterizing laser light parameters as well as for tissue ablation. Macropulse lengths of 4–8 ms at an average outcoupled power of 100 mW corresponding to a peak power density of 100 MW/cm 2 at a beam spot of 150 μm were employed to investigate the thermal ablation of bovine cornea.