Optimization of the new designed FEL beam transport line

A mid-infrared free electron laser (MIR-FEL) (target wavelength: 5 ~ 20 μm) facility named KU-FEL (Kyoto University Free Electron Laser) was constructed to aid various energy science researches at the Institute of Advanced Energy, Kyoto University. In December 2011, KU-FEL was upgraded by replacing its undulator and optical cavity mirrors. By this upgrade, the tunable range of KU-FEL was improved to 5.5 ~ 15 μm. According to replacing the cavity mirrors, size and divergence of FEL beam at the emitting point was changed. Therefore, we designed and constructed a new MIR-FEL transport line. By using calculation code Zemax (http://www.zemax.com), the condition for keeping the FEL beam radius less than 25 mm during the transportation length of 24 m was determined. In addition, the FEL intensity profile was measured after passing through the constructed transport line. The beam waist at the out-coupling …