The FELiChEM is an infrared free electron laser (FEL) facility, currently under construction, and it consists of two oscillators that generate middle- and far-infrared lasers covering the spectral range of 2.5-200.0 μm. Each oscillator includes an undulator, a pair of gold-plated spherical mirrors with adjustable poses, a resonant cavity composed of a vacuum chamber, and three POP-IN detectors. The magnetic axis of the undulator, optical axis of the resonator, and electron beam propagation axis must be aligned with high precision to achieve saturated lasing. To monitor the beam current, the reflected light from the spherical mirrors must pass through three 1-mm holes on the POP-IN detectors controlled by stepper motors via a collimating telescope. According to physical calculations, the transverse inclination of the resonator mirror and the transverse off-axis deviation should be less than 50 μrad and 0.1 mm, respectively, ensuring that the three POP-IN operating points are coaxial with the electron beam centerline and the coaxial accuracy is 0.15 mm.
Through the analysis of relevant engineering experience and actual measurement conditions, we analyze and install the two oscillators based on a laser tracker, photoelectric autocollimator, and collimating telescope. Initially, a laser tracker is used to install major equipment, such as magnets, undulators, vacuum chambers, and mirror supports, based on the installation control network. The installation accuracy is better than 0.15 mm and 0.1 mm in the beam and transverse directions, respectively. In accelerator physics, the transverse direction refers to two directions perpendicular to the beam direction. After this equipment is installed, a laser tracker and photoelectric autocollimator are used to install and adjust a gold-plated spherical mirror. First, based on the laser tracker, the photoelectric autocollimator is positioned in the space coordinate system with high precision to ensure that the axis of the photoelectric autocollimator is coaxial with the optical axis of the FEL, and the coaxial accuracy is better than 0.1 mm. Then, based on the photoelectric autocollimator, the postures of the upstream and downstream mirrors are adjusted to be within 10″. Based on the same principle, a laser tracker and collimating telescope are used to collimate and locate the three POP-IN detectors. First, two reference target points are located on the optical axis of the FEL based on the laser tracker and installation control network. The two target points must be adjusted with high precision, and the internal coincidence accuracy with the optical axis of the FEL must reach 0.03 mm. Taking the above two target points as a reference, two high-precision reticle target balls are used to adjust the height of the optical axis center of the collimating telescope to the coaxial laser optical axis, and the coaxial accuracy is better than 0.1 mm. The three POP-IN detectors are pushed into the vacuum chamber successively to reach the working point, and a collimating telescope is used to adjust them. The coaxial measurement accuracy of a single POP-IN is better than 0.05 mm. The total coaxial accuracy is better than 0.15 mm. During the operation of the infrared FEL, the radiation is relatively large. To ensure real-time monitoring and adjustment of the resonator mirror, a set of laser online alignment systems is added. The adjustable aperture is shaped and then sampled by two flat mirrors, and the light spot emitted by the flat mirror is analyzed to determine the coincidence of the incident light and outgoing light. The device has a detection accuracy of ±30 μm for the spot center position.
The requirements for the alignment and positioning of electron guns, accelerator tubes, magnets, and other equipment during device installation are not stringent. The alignment is completed using a laser tracker and 1.5-inch reflective target balls, with a transverse positioning precision and beam direction positioning accuracy better than 0.1 mm and 0.15 mm, respectively. A combination of a laser tracker and photoelectric autocollimator completes the installation and adjustment of the middle- and far-infrared oscillators (Fig. 3). A combination of a laser tracker and collimating telescope completes the installation and adjustment of the POP-IN detectors (Fig. 5). Real-time monitoring of the oscillator mirror is possible during the FEL alignment process. A set of laser online alignment systems is added (Figs. 4 and 6), which has been proved to be stable and reliable through offline and online debugging.
By referring to the engineering experience and technical solutions of relevant scientific research institutions, a technological solution combining a laser tracker, photoelectric autocollimator, and a collimating telescope is selected based on the reliability of the specific project implementation and the needs of the infrared FEL project. This scheme includes offline calibration experiments and on-site installation. The smooth output of an infrared FEL device demonstrates that the scheme is feasible and reliable.
