Longitudinal and transverse dynamics of ions from residual gas in an electron accelerator

Abstract

The ion cloud produced from residual gas in an electron accelerator can degrade machine performances and produce instabilities. The ion dynamics in an accelerator is governed by the beam-ion interaction, magnetic fields and eventual mitigation strategies. Due to the fact that the beam has a nonuniform transverse size along its orbit, the ions move longitudinally and accumulate naturally at some points in the accelerator. In order to design effective mitigation strategies it is necessary to understand the ion dynamics not only in the transverse plane but also in the longitudinal direction. After introducing the physics behind the beam-ion interaction, we show how to get accumulation points for a realistic electron storage ring lattice. Simulations of the ion cloud dynamics, including the effect of magnetic fields on the ions, clearing electrodes and clearing gaps are shown. Longitudinal ion trapping due to the magnetic mirror effect in the dipole fringe fields is also detailed. Finally, the effectiveness of clearing electrode using longitudinal clearing fields is discussed and compared to clearing electrodes producing transverse field only.