Laser Beam Profiling and Further Improvements to the FHI FEL

The free-electron laser facility at the Fritz-HaberInstitut (FHI FEL) startet regular operation in November 2013. Currently, the FEL provides continuously tunable, powerful pulsed laser radiation at any wavelength between 3.6 and 50 μm covering the mid-infrared (MIR) wavelength range. The first 11⁄2 years of operation have opened up new possibilities for research in gas phase spectroscopy of molecules and clusters, non-linear optics, and surface chemistry at the FHI. The EPICS software framework was chosen to build the control system of this facility. Actions have been taken for laser beam profiling and characterization as well as improvements to the infrastructure like the archiver system. This paper presents the next upgrade step and lessons learned during the first time of operation. DESIGN OF THE FHI FEL The FHI FEL includes two oscillator FELs; a MIR branch for wavelengths up to about 50 μm and a farinfrared (FIR) branch for wavelengths from, possibly, about 40 to 400 μm. The MIR FEL has been commissioned and is fully operational whereas the FIR FEL has been projected as a future upgrade. A normalconducting linear accelerator provides electrons of up to 50 MeV energy with a beam transport system feeding either of the two FEL branches or the electron diagnostics beamline as illustrated in Fig. 1 [1]. Electron Accelerator The accelerator system was designed and build by Advanced Energy Systems, Inc. It combines a thermionic electron gun, a sub-harmonic buncher cavity, and two Sband (2.99 GHz) standing-wave copper linacs. The first of the two linacs accelerates the electron bunches to a constant energy of 20 MeV. The second linac accelerates or decelerates the electrons to a final energy between 15 and 50 MeV. The accelerated electron bunches from bunch trains (macro-bunches) containing thousands of micro-bunches at a repetition rate of 1 GHz. The microbunch length can be compressed down to a minimum of 1 ps rms by a chicane between the linacs. The macrobunches are repeated at 5 or 10 Hz [2]. Undulator and Cavity In the MIR FEL a planar hybrid-magnet undulator is located within the IR cavity. The undulator is 2 m long containing 50 periods of 40 mm. It employs permanent magnets made out of NdFeB. At a minimum gap of nominally 16.5 mm, a maximum root-mean-square undulator parameter Krms of more than 1.6 is reached. This, in combination with the minimum electron energy of 15 MeV, corresponds to a theoretical maximum wavelength of more than 50 μm for the MIR system. The 5.4 m long MIR FEL cavity is formed by an end mirror and an out-coupling mirror. These are gold-plated copper mirrors of concave spherical shape. The waist of the cavity mode is located at the undulator center. A motorized in-vacuum mirror changer permits to select one out of 5 out-coupling mirrors with different outcouplinghole diameters of 0.75, 1.0, 1.5, 2.5, and 3.5 mm for optimized performance at different wavelength regions. In addition, the cavity end mirror is mounted on a precision translation stage allowing for fine adjustment of the cavity length with 1 μm repeatability [2]. Figure 1: Overview of the FHI FEL installation. LASER BEAM PROFILING The IR pulses coupled-out from the FEL cavity pass through the evacuated IR beamline and propagate a distance of 18 m from the FEL vault to the diagnostic station located in the lab building. There the spectrum of the IR radiation is measured by an in vacuum CzernyTurner grating spectrometer allowing for online monitoring of the FEL spectrum for each individual macro pulse. In addition, various commercial IR detectors are in use to determine the intensity of the FEL-pulses at different levels of sensitivity and temporal resolutions [2].