The circumference of the high energy photon source (HEPES) storage ring is about 1360 m, with 288 girders (96 three-magnet girders, 96 five-magnet girders, and 96 eight-magnet girders) and 1776 magnets. In order to improve the installation efficiency of the storage ring, each girder is usually first pre-aligned in the laboratory, and then transported to the storage ring to participate in the tunnel alignment. Based on simulation and physical design of the accelerator, the standard deviation for the pre-alignment adjustment of magnets on one girder in transverse and vertical directions must be below 10 μm.

To efficiently complete the pre-alignment work of HEPES and simultaneously ensure the pre-alignment accuracy, a four-station laser tracker multilateration measurement system is used for measuring and real-time monitoring the magnet position deviation. The ranging accuracy of laser trackers is much better than that of angle measurement. The multilateration measurement system uses four laser trackers to measure the distance to the target point on every magnet, and a rank-defect free-network adjustment model is used to solve the coordinates of the station and target points. The multilateration measurement system can avoid the errors of angle measurement of laser trackers, thus obtain a high-precision result. In order to improve the work efficiency for pre-alignment, a standardized operating process is established, including girder fiducialization and calculation of the theoretical values of pre-alignment, sextupole magnet and mover alignment, and conventional magnet alignment. In order to reduce the residual error of beam correction and improve the dynamic aperture, a sextupole magnet mover is developed to perform beam-based alignment online. The mover can realize transverse and vertical motions. Therefore, it is necessary to align the movement direction of the mover until it is parallel to the corresponding direction of the coordinate axis, and then carry out the alignment of the sextupole magnet. Since there is no elevation adjustment mechanism between mover and sextupole magnet, it is determined that the mover can be fixed on the girder directly and the shims are inserted in the interface of magnet and mover using a special lifting fixture to adjust the flatness errors.

Three work stations are built in the laboratory for pre-alignment. All work stations are designed to accomplish alignment of all three girders. Through the continuous optimization of the pre-alignment process, the pre-alignment efficiency for the three-magnet girder and the five-magnet girder are 0.5 d per girder and 1 d per girder, respectively. For the eight-magnet girder, three sextupole magnets and the mover need pre-alignment after the five conventional magnets are completed. The motion direction of the mover is made parallel to the theoretical direction by the lateral adjustment mechanism of the mover, and then the error in the elevation direction of the sextupole magnet is adjusted to within 10 μm by insertion of shims (Fig. 9). This pre-alignment scheme for the sextupole magnet and the mover has good results in practice, and the time taken to pre-alignment each eight-magnet girder is reduced from 5 d per girder at the beginning to 1 d per girder. The multilateration measurement scheme adopted for pre-alignment can be adjusted according to the size of the target to be measured and the size of the environment, so as to meet the needs for high-precision measurement and adjustment of different equipments.

In the actual pre-alignment process through continuous exploration, one needs to sum up standardized operating procedures of the storage ring unit pre-collimation, prepare the pre-collimation process control table, and strictly control the pre-collimation process and the results of each unit. In the end, 288 standard girders are pre-aligned on time with an accuracy of better than 10 μm. It shows that the pre-alignment process is reasonable and the accuracy of the measurement scheme meets the requirements. The alignment accuracy of the multilateration system can still be improved by increasing the height difference of four stations. Furthermore, this laser tracker-based multilateration method can be applied to other high precision fields, for example, the field of industrial measurement.