7 T Niobium-Titanium-Based Persistent-Mode Superconducting Magnet for an Electron Beam Ion Source

A high field magnet is a key element of cryogenic electron beam ion sources (EBISs), which are known for generating highly charged ions through the magnetic compression of an electron beam. Herein, we report the design, fabrication, and evaluation of a 7 T niobium-titanium superconducting magnet capable of persistent-mode operation. The magnet was designed using finite element analysis by considering its magnetic, thermal, and mechanical properties. The designed magnet was then fabricated, assembled, and evaluated for various design parameters in a recondensing-type liquid helium cryostat. After several quench trainings, the magnet reached a target magnetic field of 7 T with an operating current of 200 A, a magnetic field uniformity of 0.24%, and an electron beam focusing length of 1.3 m inside the bore. The magnet was successfully operated in the persistent-mode for 9.5 days (228 hours) and achieved a field-decay rate of 0.42 ppm
$\cdot \text{h}^{-1}$
. The magnet evaluation results confirm that our superconducting magnet system can be applied to an EBIS to carry out stable and effective electron beam compression.