Simulation of fast beam ion instability (FBII) in Indus-2 and its experimental observation

Indus-2 is a 2.5 GeVelectron storage ring and a national facility for synchrotron radiation users in India providing the photon beam in round the clock mode. Recently, the electron beam emittance of Indus-2 was reduced to one third of the normal operating optics (130 nm rad) with improved beam optics. Due to this reduced emittance, likelihood of fast beam ion instability (FBII) enhances, and that was explored using simulation study. FBII is generated due to the cascading interaction of electron beam with ions that comes across in the path of the beam. This interaction induces oscillation in center of mass of bunches of electron beam and that process increases the transverse beam size, which spoils the effort made in achieving the reduction in beam emittance. For correct understanding of this instability, simulations have been performed with different ring average pressure and considering different per bunch current values. Further, studies have been carried out for two sets of beam optics used for normal emittance and reduced emittance of the electron beam. The simulation method and results are presented in this paper. To observe the signature of this instability, an experiment has been conducted in Indus-2 and its limitations due to operational gas pressure are discussed.