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Bi-2212 round wire offers advantages in low screening current due to its filament structure compared to REBCO tapes among high-temperature superconductors (HTS). Additionally, established methods for creating superconducting joints are well known for Bi-2212. These characteristics have led to extensive research into its use in nuclear magnetic resonance (NMR) magnets, which require high temporal magnetic field homogeneity, and its potential application in the central solenoid of fusion reactors, where low heat dissipation due to AC loss is critical.
A recently identified application that demands high temporal magnetic field stability is the solid-state spin qubit experiment for quantum computers. This experiment requires precise magnetic field alignment to the nitrogen-vacancy (NV) axis of a diamond sample and exceptional temporal stability.
In this study, we present the fabrication and testing of a conduction-cooled Bi-2212 solenoid test coil designed for solid-state spin qubit experiments. The electromagnetic design of the coil was optimized to minimize conductor usage while maintaining magnetic field strength. The bobbin was designed and fabricated with conduction cooling in mind. We fabricated two types of current leads: 1) copper current leads subjected to heat treatment, and 2) replaceable current leads that are replaced after heat treatment. Furthermore, because the solid-state spin qubit experiment is conducted off-center within the magnet, we measured the temporal stability of the magnetic field both at the experiment spot and at the magnet's center.
This work was supported in part by National R&D Program through the National Research Foundation of Korea(NRF) funded by Ministry of Science and ICT(2022M3I9A1073924), in part by the Applied Superconductivity Center, Electric Power Research Institute of Seoul National University, and in part by Kiswire Advanced Technology Co., Ltd.
