Like other third generation light sources, main contributions to the be am lifetime of the Pohang Light Source come from the beam-gas scattering and the Touschek effect. The beam-gas scattering lifetime is measured with extremely low stored current in the long train of bunches. By investigating the vertical gap dependence of the beam-gas scattering lifetime, it is possible to decompose the beam-gas scattering lifetime to the elastic and inelastic parts. On the other hand, the Touschek lifetime is analyzed using the graph of the measured beam lifetime as a function of time. It is used that the rate of change of the pure Touschek lifetime is exactly 1. The deviation from 1 is caused by the beam -gas scattering effect and the radiative electron beam polarization. By analyzing the graph, it is possible to estimate the beam-gas scattering lifetime and the polarization contribution to the beam lifetime.
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