Design and Development of a 28 GHz Nb$_{{\text{3}}}$Sn ECR Ion Source Superconducting Magnet

Worldwide several electron cyclotron resonance (ECR) ion sources have been developed and in operation for heavy ion accelerators using Nb-Ti superconducting magnets. The Versatile ECR ion source for NUclear Science (VENUS) at the Lawrence Berkeley National Lab (LBNL) and the newly commissioned 28 GHz superconducting ECR ion source at the Facility for Rare Isotope Beams (FRIB) were developed by LBNL. Both sources adopt a scheme with a sextupole magnet inside a mirror-type solenoid to confine the ions and electrons. Nb-Ti coils limit all the existing ECR ion sources to operate below $\sim$ 9 T at 4.2 K. Nb $_{{\text{3}}}$ Sn potentially enables next generation ECR ion sources with a higher field limit ( $\sim$ 22 T at 4.2 K). As an example, a 45 GHz ECR ion source Nb $_{{\text{3}}}$ Sn magnet is currently being developed by the Institute of Modern Physics (IMP) in China. Clearly conductor characteristics of Nb $_{{\text{3}}}$ Sn are very much different and new development are needed to meet challenges such as coil fabrication. FRIB and LBNL team up again to develop ECR ion sources based on Nb $_{{\text{3}}}$ Sn. Here as the first step, this paper describes the design of a second 28 GHz superconducting ECR ion source using Nb $_{{\text{3}}}$ Sn coils at FRIB. We present conductor selection and characteristics, magnetic design, mechanical design and cold mass assembly, coil fabrication challenges and potential solution, quench protection, and the development and prototyping efforts so far.