Simulation of incoherent ion effects in electron storage rings

Abstract

Ion trapping occurs when a negatively charged beam ionizes residual gas inside an accelerator vacuum chamber, and the resulting ions become trapped in the beam potential. In addition to the well-understood coherent instability, trapped ions can cause incoherent effects, such as emittance growth and tune spread. Typically, simulation of ion effects is done using a weak-strong model, in which the ions are modeled using macroparticles, but the beam is assumed to be a fixed Gaussian distribution, with only centroid motion allowed. This type of model necessarily neglects intra-bunch effects. Recently, a residual gas ion modeling capability has been incorporated into the particle tracking code elegant. Both the beam and ions are modeled using macroparticles, so incoherent effects can be studied. The code models ion generation, movement of ions between bunches, and beam-ion interactions. It has also been parallelized and can be used in combination with other elegant elements. The code has been used to study ion instability in the present APS storage ring. Once several important effects are included (multiple ionization, transverse impedance, and charge variation between bunches), the simulations show good agreement with the measured data.