Numerical simulation of simultaneous acceleration of positive and negative ions in an RFQ

By means of a numerical method, beam dynamics was analyzed for an RFQ, where mixtures of positive and negative ions were injected into the quadrupole channel. In order to simulate simultaneous bunching of ions with opposite charges, the motion of particles injected into the cavity during two RF periods was traced under consideration of 3D Coulomb forces between particles. Effects of neighbor bunches were also taken into account. In the input region of the structure, the transverse beam divergence due to space charge forces was suppressed by the charge neutralization. However, it was found that the bunch formation in the cavity was disturbed by the attractive forces between positive and negative ions, leading to longitudinal beam loss. Dependence of the beam transmission efficiency on the input beam intensity as well as on the blend ratio of two ion species is reported. These results are compared with those obtained when injecting ions of single species.