Experimental results on the fast beam-ion instability

The so-called fast beam-ion instability, if it really exists, is going to be a serious problem for future accelerators such as B factories. A series of experiments has been conducted at the Pohang Light Source to test the existence of this new instability. An adequate amount of He gas was injected into the ring to enhance the ion effects. The results of the experiment strongly support the existence of the instability. The measured ion frequencies agree well with the linear theory. They even appear in normal operation condition with enough bunch current and bunch train length. Above all, measurements of the single pass beam position monitor clearly show the coherent oscillations with the amplitude increasing along the length of the bunch train, as predicted by the theory of this new instability.