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Description
The FHI Free-Electron Laser (FHI FEL) provides intense, pulsed mid-infrared (MIR) radiation continuously tunable from <3 µm to >50 µm. Since user operation started in 2013, the FHI FEL has enabled user groups to run experiments in diverse fields ranging from spectroscopy of clusters, nanoparticles, and bio-molecules in the gas phase to nonlinear solid-state spectroscopy and surface science. This has resulted in some 75 peer-reviewed publications so far. A 2-color upgrade of the FHI FEL was funded by the Max Planck Society in 2018 and is now nearing completion with first light expected to occur in the 1st half of 2022.
The FHI FEL Upgrade essentially includes the addition of a second short-Rayleigh-range undulator FEL beamline [1]. The second FEL branch will permit lasing in the far-infrared (FIR) regime from <5 µm to >160 µm. In addition, a 500 MHz kicker cavity has been inserted downstream of the electron accelerator to permit simultaneous operation of both FEL beamlines by deflecting alternate 1 GHz electron bunches into each of the two undulator branches shown in Fig. 1. The kicker cavity is powered by a 65-kW, 500-MHz solid-state amplifier and operates in a dipole mode using the strong electric field between two vanes to deflect the electron bunches alternatingly left and right. It can deflect 50 MeV electrons by an angle of ±2°. Two additional small dipole magnets upfront and behind the kicker cavity permit deflection up to ±2°. For 2-color operation the beam is alternatively bent -1°, -2°, -1° => - 4° by the dipole-kicker-dipole combination into the FIR line and -1°, +2°, -1° => 0° for the MIR beamline. Conventional single-color operation of either the MIR or the FIR FEL is also possible when the 500 MHz field is off: with both dipoles off, every electron bunch is sent to the MIR branch; with both dipoles on deflecting -2°, -2° => -4° every electron bunch will be sent to the FIR branch.
References
1. W. Schöllkopf et al., “The FHI FEL Upgrade Design,” Proc. FEL 2019 Conf., Hamburg,
Germany, August (2019).
