Enabling Storage Ring FEL for Lasing Below 170 nm and Production of 120 MeV Circularly Polarized $\gamma$-Ray by VUV Mirrors

The operation of a storage ring free-electron laser (FEL) is constrained by the shortest wavelength that its cavity can support. To address this limitation, significant efforts have been made in recent decades to develop high-reflective FEL mirrors supporting progressively shorter wavelengths.