Lasing down to the deep UV in the NIJI-IV FEL

Lasing has been obtained at the wavelengths down to the deep UV on the NIJI-IV free electron laser (FEL) system. The shortest wavelength obtained was 228 nm and the laser peak power was estimated to be ∼200 and ∼40 mW at 300 and 240 nm, respectively. Suppression of the fast head–tail instability in the electron beam led to a drastic enhancement in the laser gain which enabled us to achieve such short-wavelength lasing even in a compact storage ring such as NIJI-IV. Low-loss cavity mirrors having Al2O3/SiO2 dielectric layers are being prepared to obtain FEL operation at much shorter wavelengths.