Improve optical cavity quality of a waveguide FEL with hole-coupling by using a grating

We propose a method to improve the quality of a hole-coupled resonator for a free-electron laser oscillator. A mesh or grating with a reflectivity of 50 - 70%, which is quite easy to be produced, can be used to cover the hole. Both the extraction and diffraction losses can be reduced effectively and the optical cavity Q value can be improved to obtain higher net gain which allows the FEL oscillator to be operated in worse conditions. These advantages and properties are confirmed by numerical simulations using a three-dimension code with the parameters of a compact far-infrared FEL oscillator built in China Academy of Engineering Physics (CAEP). Furthermore, it can be used as an electron transparent mirror to make the FEL device more compact and less expensive.