Structural Design And Process Research Of The Waveguide For The Optical Cavity In A THz-FEL Facility

The Free Electron Laser(FEL) terahertz source can generate continuously tunable far infrared radiation with frequency range of 0.1THz-10 THz. The radiation power and efficiency are very high, so it is better than other terahertz sources. Optical cavity is one of core system of THz-FEL, whose main role is to provide a place for energy coupling of reflected light pulses and subsequent electron beams. So it is very significant for the development of high gain and low-loss optical cavity. In order to reach saturation energy quickly, the waveguide which can limit the light, reduce the losses and increase the gains is added.The waveguide of this paper will be installed in the 16 mm magnet gap of the undulator for the FEL facility. To meet the requirements of low-loss and high gain, it is necessary to do research to meet the high requirement of stiffness, strength, processing straightness, flatness and surface. Firstly, through comparative analysis, the waveguide can limit the light, significantly improve the gains and reduce the losses. So the waveguide is very important for the FEL facility. Secondly, considering the limit of strength, stiffness and installation space of the undulator gap, optimization design of overall waveguide structure is researched. But the length of waveguide is 1130 mm and there is a 40mm×10mm cavity whose surface requires coating and high-quality surface roughness 1.6?m. This special kind of structure can not be processed directly. So it is necessary to design a feasible mechanical structure. The improved structure is only 2mm. Its strength and stiffness are very poor, so it is too thin to be deformed easily. In order to obtain good processing quality, process research is studied. Through several tests of processing trial, the viable process route of waveguide is made. At the same time, considering the need of the welding for the waveguide and the requirement of optical cavity, it is necessary to do coating on the inner surface of the waveguide. Through several coating tests, the final waveguide surface treatment and coating materials are chosen.After the completion of the relevant process research, the formal waveguide is machined and welded. The result of test meets the requirement of accuracy and vacuum. The last waveguide's straightness is 0.07 mm, flatness is 0.10 mm, upper and lower surfaces parallel is 0.08 mm, and the accuracy is the best which we can get currently. The waveguide will be a good foundation for the laser of the THz-FEL facility.