The mechanical instability of the BPMs at HLS

Beam position monitor (BPM) used to measure the position of the electron beam in storage ring. The instability of the BPM itself will definitely influence the beam position readout. At Hefei Light Source (HLS), the mechanical instability of the BPM was studied with raster gauges. By tracing the displacement of the BPM chamber, it was observed that the BPM chamber moved from time to time. The movement corresponded to its surface temperature variation, which was caused by the synchrotron light. This paper recorded this phenomenon and gave the proper explanation.