A common points selection method based on the uniformity of FLTMMS for pre-alignment of particle accelerators

High-precision pre-alignment of a magnet is a crucial step in the construction of a particle accelerator such as the Hefei Advanced Light Facility (HALF) or High Energy Photon Source (HEPS). To improve the accuracy and efficiency of pre-alignment, there are plans to build a four-station laser tracker multilateration measurement system (FLTMMS) at HALF. In the coordinate conversion and self-calibration processes of a FLTMMS, an unreasonable distribution of common points can lead to a loss of accuracy. A method for selecting common points based on uniformity in the FLTMMS is therefore proposed. In this method, the coordinate transformation space is divided according to the uniformity in different directions. The feasibility of our method is compared and analysed through coordinate conversion and self-calibration experiments. Simulations show that with the same number of common points, this method leads to higher accuracy in coordinate conversion and self-calibration. The proposed method compensates for the loss of accuracy due to aggregation of common points. Comparative analyses show a clear positive relationship between the accuracy improvement effect and the difference in uniformity. The distribution of common points will change the accuracy distribution of FLTMMS. Our approach of providing essential references for pre-aligning HALF magnets has been applied to HEPS with extensive engineering experience, to verify the practicality of this method for engineering applications. In the actual measurements, self-calibration is performed using an edge measurement network, and coordinate conversion is conducted using angle measurements. Based on actual measurements, the self-calibration accuracy is 1.7 mu m and a conversion accuracy is 14.8 mu m. The results show that under the premise of satisfying precision, the method can reduce the number of common points, thus decreasing the cost of common points construction. Our study provides instructions for pre-alignment at HALF in the future.