Laboratory-scale wide-band FIR user facility based on a compact FEL

We have developed a laboratory-scale user facility with a compact far infrared (FIR) FEL. The FEL driven by a magnetron-based microtron operates in the wavelength range of 100–300 μm. The microtron accelerator has been upgraded to drive stable operation of the FEL by stabilizing current and bunch repetition rate of the electron beam. The FEL can be operated for several hours without further adjustment and the fluctuation in pulse energy during the period is less than 10% in rms value. The FIR radiation is divided by an FIR beam splitter (crystal quartz) and is detected by calibrated FIR detectors. The scheme facilitates high accuracy measurement of pulse energy with a fluctuation of ∼1%. When we used liquid-helium cooled detectors, the signal-to-noise ratio was more than 107, even in the case of a single pulse measurement. We could measure the transmission FIR spectrum of the vapour and liquid water as preliminary data of the molecular spectroscopy and imaging for living species. We are also preparing FIR imaging systems by 2-D scanning for high resolution and by the electro-optic method for single-pulse capturing. The area occupied by the compact FIR FEL is only 4 m2.