Metal-as-Insulation HTS coils
Abstract In this article, we summarize what we have learned about Metal-as-Insulation (MI) winding behavior and technical challenges. Bailey et al. first proposed the use of Metallic Insulation (MI) for superconducting magnet in 1988 through a U.S. patent. High Temperature Superconductor (HTS) materials are highly thermally stable. This feature compared to classical Low Temperature Superconductor (LTS) enables the use of MI technology to improve the protection against quenches. Gupta was the first to propose the use of a metallic tape in an HTS winding to avoid too much radial currents in No Insulation (NI) in 2011. Hahn et al. presented preliminary results on a pancake sample the same year. We are proposing here to come back on the work done for about 10 years by research groups worldwide and will focus on the turn-to-turn contact resistivity Rct parameter. We will also give details of our LNCMI-CEA-Néel Institute MI HTS insert built in 2018 in the framework of the French National Research Agency (ANR) funding through the NOUGAT project. We tested this magnet many times between 2018 and 2021 and learnt a lot on this technology. This magnet is the first REBCO solenoid of this size using this technology and tested intensively at such high magnetic field (up to 32.5 T) so far. In this magnet, we firstly include a magnetic shielding technology consisting of REBCO NI turns inside the overbanding of each pancake. We give some details and effect of such technology inside an HTS MI insert in case of a fast discharge, a quench or an outsert failure. Finally, we discuss about the self-protection feature of MI coils and we propose a passive protection way for high Rct values.