SESRI 300 MeV同步加速器注入线的传输效率与接受效率

Transmission efficiency and beam reception of the SESRI 300 MeV synchrotron injection line

为了完成SESRI 300 MeV同步加速器的束流调试,使用Tracewin软件和加速腔电磁场分布文件建立了该加速器注入线从离子源出口到注入点的全尺寸模型,在多粒子模式下计算了两种典型粒子束(质子、^(209)Bi^(32+))在不同状态下的加速传输,得到了注入线上束流相空间的变化过程和注入线的传输效率与接受效率.研究结果表明,300 MeV同步加速器注入线在加速不同荷质比的离子束流时,电磁场参数设置基本与荷质比成反比.束流的接受效率主要由射频四极加速腔出口发射度决定.质子束和重离子束的接受效率差别较大.在射频四极加速腔出口发射度为0.1πmm·mrad时,^(209)Bi^(32+)束接收效率达到92.13%,质子束的接收效率为68.18%.分析束流传输过程表明,当横向发射度过大时,束流会因为纵向能散展宽和相位展宽而无法被最终接受.在现有配置下,质子束存在横向聚焦力不够、接受效率较低的问题.通过额外增加2个四极铁能够将质子束的接受效率由68.18%提升至83.61%.

SESRI 300 MeV synchrotron in Harbin Institute of Technology is now under construction and the whole equipment has been installed and tested.Before commissioning beam,the beam transport through the injection line is simulated by using a full-scall model through the Tracewin code.The field distribution of RFQ cavity is calculated with CST,and the results are substituted into the Tracewin code to generate the accurate results.The envelop mode and multi-particles mode are used in the beam simulation with two typical beams(H_(2)^(+)and ^(209)Bi^(32+),the lightest beam and the heaviest beam).Both beams are accelerated from 4 keV/u to 2 MeV/u by an RFQ cavity and two IH-DTL cavities.Then the H_(2)^(+)beam is stripped into a proton beam by a carbon foil and accelerated to 5.6 MeV with the third IH-DTL cavity.Simulation results show that the strength of the magnetic field and the acceleration field are proportional to the mass charge ratio.The beam transmission efficiency and the injection line reception are inversely proportional to the beam transverse emittance.The ^(209)Bi^(32+)beam transmission efficiency and beam reception(momentum spread less than±0.2%)are 72.16%and 46.72%with transverse emittanceε=0.12πmm·mrad(ECR source output)andε=0.4πmm·mrad(RFQ output).H_(2)^(+)beam transmission ratio and beam reception are 24.19%and 17.89%withε=0.2πmm·mrad(ECR source output)andε=0.5πmm·mrad(RFQ output).In order to obtain high transmission efficiency and beam reception,the transverse emittance should be limited to 0.1πmm·mrad after the RFQ.With this limitation,the ^(209)Bi^(32+)beam transmission efficiency and the reception are increased to 96.68%and 92.63%,respectively,and the H_(2)^(+)beam transmission efficiency and the rception rise to 74.40%and 68.18%,respectively.If two additional quadrupole magnets are added,the H_(2)^(+)beam transmission efficiency and beam reception can be increased to 90.73%and 83.61%,respectively,which will fulfill the requirement for long-time operation.The phase space change process shows that loss of ^(209)Bi^(32+)beam is caused mainly by longitudinal defocusing(energy spread and phase width spread),the loss of proton beam is caused both by the longitudinal defocusing and by the transverse defocusing(beam envelop spreading),that is why two additional focusing magnets should be added in proton beam acceleration.Results also show that by using field distribution calculation in the simulation process the greater influence of the cavity design details can be confirmed such as beam off-axis caused by dipole field in the IH-DTL cavity and beam loss caused by unperfect field in the RFQ.Tracking with field distribution is shown to be a useful method to link the cavity design process,beam line design process,and beam commission process.