Reducing loss of a hole-coupled resonator for a free-electron laser with partially reflecting meshes

We propose a method to reduce the loss of a hole-coupled resonator for a free-electron laser (FEL) oscillator. A mesh 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 to obtain higher net gain, which allows the FEL oscillator to be operated under worse conditions. These advantages and properties are confirmed by numerical simulations using a three-dimensional code with the parameters of a compact far-infrared FEL oscillator built at the China Academy of Engineering Physics. Furthermore, it can be used as a electron transparent mirror to make the FEL device more compact and less expensive.