Optical alignment and tuning system for the HUST THz-FEL

A compact FEL oscillator with a radiation wavelength of 30-100μm is proposed by HUST and NSRL. The optical cavity is very sensitive to misalignment errors of the mirror, due to its near-concentric and symmetric structure. The magnetic axis of the undulator, the optical axis of the resonator, and the electron beam propagation axis must all be aligned with high precision for achieving saturated lasing. This paper introduces a high-precision, multi-degree-of-freedom controlled optical alignment system, which has the ability to align in the transverse and longitudinal directions. The alignment tolerances are given by theoretical analysis and numerical simulations with three-dimensional FEL code GENESIS and optical propagation code (OPC). To accomplish optical alignment, two auxiliary HeNe laser systems were introduced. By adjusting the HeNe laser beam spot on the wedge, the optical axis can be aligned to the magnetic axis, and the estimated errors meet the tolerances. Finally, the electron beam will be guided through the hole in the central wedge to complete the transverse alignment. The longitudinal alignment and tuning methods are also described.