Simulations on the Fast-Ion Instability in the International Linear Collider Damping Rings

Abstract

Ions trapped by electron beams in the rings oscillate with a certain frequency in the beam potential. The oscillation, which is incoherent at the early stage, becomes coherent and shakes the beam with a frequency. This is the mechanism of the ion instability. We investigate the fast-ion instability in the two proposed International Linear Collider damping rings of OCS6 and FODO4 with a high intensity and a low-emittance in which the growth rate may be very rapid. We have performed a simulation based on the weak-strong model and analytical estimate to investigate characteristic phenomena of the fast-ion instability that is a critical issue in the damping rings. We investigated the effects of the ion frequencies, bunch filling patterns, vacuum pressures, and a feedback system on the fast-ion instabilities in the damping rings. The interactions between ions and beams in our simulations were performed using the magnetic components of the damping ring. The simulation results show that the bunch-by-bunch feedback of about 50 turns is sufficient to suppress the fast-ion instabilities in the damping ring. It is estimated that the tune shifts in the beam due to the ions are small.