The longitudinal "trapping" phenomenon in the simultaneous acceleration of intense H+ and H- in an RFQ

The phenomenon of longitudinal "trapping" in the simultaneous acceleration of intense H+ and H- beams in an RFQ accelerator is described in this paper. A new code named PARMTEQVC is compiled based on an old version of t-code PARMTEQ for the computer simulation. In this code, the 2D PIC method is applied to calculate the space charge forces. Multi-bunch effects are also taken into account. In our simulation the RFQ structure parameters which were designed only for single beam (H+ or H-) acceleration are adopted. The simulation result shows that more ions near the ends of buckets are "trapped" into the bunches with opposite charge state. These ions leave their own buckets gradually and become lost in either longitudinal or transverse direction, leading to a descent of the total transmission rate. Further studies reveal that the depth of longitudinal potential well decreases in the case of the simultaneous acceleration, which is attributed to the weaker bunching forces because of the longitudinal space charge interaction between the positive and negative ion bunches. It seems that a new RFQ structure should be specially designed for the simultaneous acceleration in order to neutralize the transverse space charge effect and increase the beam transmission rate at the same time. The design ideas are then presented. (C) 2004 Elsevier B.V. All rights reserved.