Conventional free electron laser (FEL) oscillators minimize the optical mode volume around the electron beam in the undulator by making the resonator Rayleigh length about one third to one half of the undulator length. This maximizes gain and beam-mode coupling. In compact configurations of high-power infrared FELs or moderate power UV FELs. the resulting optical intensity can damage ... https://www.nacrack.com/flash-find-Orthofeet-Edgewater-Men-s-Athletic-Shoe-hot-choice/