MEASUREMENT OF LOSSES IN THE CAVITY OF A SUBMILLIMETER FREE-ELECTRON LASER

A procedure is described for measuring the mode spectrum and power losses in the quasi-optical cavity of a free-electron laser, using the radiation from an H2O-laser operating at a wavelength of 118 mum. The measurements show that the losses in the cavity may be significant even if a principal-mode field in the open resonator is only slightly diaphragmed by the walls of the electron guide. Note that most of the losses are due to a mode transformation by geometric irregularities of the electron-guide walls inside the cavity.