Recent Experimental Results on the Fast Ion Instability at the 2.5 GeV PLS

We report on the recent experimental results on the fast-ion instability at the 2.5 GeV Pohang Light Source (PLS). In order to enhance the fast-ion instability in the PLS, we inserted helium gas to the storage ring in order to increase the vacuum pressure. With the gaps in the bunch train which is large enough to avoid multiturn ion trapping, we observed increase of a factor of 3-4 in the vertical beam sizes for the increased vacuum pressure up to 40 nTorr. The instability growth rate is obtained as a function of vacuum pressure and is also compared with the simulation results.