Design Margin Comparison of a Sextupole-in-Solenoid and a Closed-Loop-Coil Type Nb-Ti Superconducting ECR Magnets

Third-generation electron cyclotron resonance ion sources (ECRISs) have relied on a combination of sextupole and solenoid fields from Nb-Ti superconductors to produce the magnetic fields required for 28 GHz operation. The VENUS ion source at Lawrence Berkeley National Laboratory (LBNL) was the first third-generation ion source optimized for 28 GHz operation, and since that time a similar design has been adopted at other heavy-ion facilities such as RIKEN, GANIL and FRIB. These sources utilize a sextupole-in-solenoid design and represent the high-field limit of this design using Nb-Ti superconductor. To get past this limit without moving to a more difficult-to-use superconductor material, LBNL is developing a fourth-generation, 45 GHz ECRIS, called MARS-D, with a closed-loop coil structure. Since both the sextupole and solenoid fields are efficiently produced by the closed-looped coil, the 45 GHz ECRIS can be realized using the Nb-Ti conductor. In the MARS-D design, the maximum field on the conductor is 8.3 T at 4.2 K, and the operation point is about 90% of the short sample limit. As this tight load line margin may leave the source susceptible to quench, we investigate and compare the design parameters with the most recent VENUS -type superconducting ECRIS which is already under operation at FRIB in this paper. Additionally, we investigate the advantages of the MARS-D -type structure by keeping the coil design constant and only replacing the conductor with the one used in FRIB and VENUS.