Lasing Towards the VUV in the NIJI-IV FEL

The lasing wavelength in the NIJI-IV free electron laser (FEL) reached 212 nm in the deep ultraviolet (UV) range. Adoption of Al2O3/SiO2 multilayer mirrors to the laser cavity, whose light absorption loss is sufficiently small even below 220 nm, compared with HfO2/SiO2 usually used in the UV, was essential to shorten the wavelength in the deep UV range. The laser gain was estimated to be ∼2% around 214 nm with a peak beam current of ∼4A (average beam current of ∼20 mA) from the measured loss of the cavity degraded after the lasing experiments which agreed well with an analytic gain calculation. Lasing in the vacuum ultraviolet (VUV) range is also expected even on the compact storage ring NIJI-IV with possible enhancement of the laser gain.