Upgrades of the Liquid Helium-Free Superconducting Magnet for the 18-GHz Electron Cyclotron Resonance Ion Source at NFRI

Abstract

The superconducting magnet is successfully installed on the 18-GHz ECR ion source at the National Fusion Research Institute (NFRI). The ECR ion source will be used as a multi-application source of highly charged ions and their interaction with matter. The magnetic system compromises of four superconducting coils sets for the axial mirror field and a permanent hexapole magnet for the radial mirror field. The previous superconducting magnet with a single cryocooler (1.5 W at 4.2 K) had been designed to reach peak mirror fields of 2.2 T in the injection and 1.5 T in the extraction regions (You et al., Rev. Sci. Instrum. 85, 02A916, 2014). However, it has not been successful in reaching the designed value (125 A) of current excitations; magnet coils have been shown to repeatedly quenched around 50–60 A of current ratings. After a detailed analyses and inspection, we found several weak points to be the possible cause of quenching. In this presentation, we describe important upgrades and technical improvements that have been made, followed by the test results of cooling and coil excitations. The four superconducting coils were successfully cooled to less than 4 K and excited accordingly to the designed current densities of 125 A/mm².