New resonator for the Israeli FEL

The Israeli FEL resonator (W-band 75-110 GHz) was re-designed in order to reduce the overall round-trip losses and to control the radiation out-coupling. In its new configuration, the resonator consists of an overmoded corrugated rectangular waveguide and two radiation mode splitters, separating the high-energy e-beam from the mm-wave radiation. The electron input splitter is based on Talbot effect in an overmoded rectangular waveguide. The radiation out-coupling takes place in the output splitter. The splitter is based on a novel design. It combines Talbot effect between two parallel plates with free space propagation and with focusing by two curved cylindrical mirrors in a confocal imaging scheme. The waveguide and the splitters were tested, showing improved performance in comparison with the former resonator. The measured unloaded Q-factor of the new resonator is increased by a factor of ∼3, up to Q=25,000. Accordingly, the round-trip losses are ∼23%. Rotating grids control the radiation out-coupling, allowing optimization of the radiation power and the extraction efficiency. The design layout and the testing results are presented.