Estimations of beam tails and beam lifetimes in PLS-II

Recent light sources are characterized by beams with a small emittance, high beam current and small vertical gap in insertion devices. We investigated beam tail distributions and beam lifetimes for a light source at PLS-II by using a fast simulation method. The simulations were performed to obtain transverse and longitudinal tail distributions due to beam-residual gas bremsstrahlung, beam-residual gas scattering and intra-beam scattering. The simulation results on the tail distributions were compared with a solvable model and showed good agreements. The simulated beam lifetime was estimated as a function of vacuum pressure and vertical aperture in the ring. The simulated beam lifetime at PLS-II was estimated to be around 7 h for a beam current of 400 mA in a half vertical gap of 3 mm and a vacuum pressure of 1 nTorr. It was shown that the beam lifetimes, due to the beam-residual gas bremsstrahlung and beam-residual gas scattering, were greatly affected by the vacuum pressures and vertical aperture, and their contributions to the beam lifetime were larger than the intra-beam scattering. Our results show that a control of the vacuum pressure is important in the recent light sources with a small emittance, high beam current and small vertical gap.