A Study on the Sextupole Design with Iron Yoke Inside Solenoids for 56 GHz ECR Ion Sources

The third generation of electron cyclotron resonance (ECR) ion sources is in operation or under development in the world. The fourth generation ECR ion sources are expected to operate at a heating radio frequency greater than 40 GHz, requiring a higher confinement field. Such requirements provide the motivation to design the structure of the fourth generation ECR ion sources. The Nb
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Sn wire, which can generate fields in the 10-20 T range, is considered to construct the magnet coil analyzed in this study. First, for both solenoid-in-sextupole structure and sextupole-in-solenoid structure, the critical current of materials and the local magnetic field are analyzed and compared. By the comparison, sextupole-in-solenoid structure is selected to construct sextupole magnet for 56 GHz ECR ion source. Considering the characteristics of the wire and the Lorentz force on sextupole ends, iron yoke is considered to be added around sextupole to provide a structure for the sextupole coils. Then, the confinement fields and the safety margin of the wire are analyzed and compared using models with and without the iron yoke. Finally, a design of the sextupole with a shorter iron yoke is presented in this paper. With the suggested sextupole design, the length of the sextupole and the total size of the ECR ion source can be reduced.