Design Study on a 100-kA/20-K HTS Cable for Fusion Magnets

Abstract

Due to the > 100 times higher thermal stability of high temperature superconductor (HTS) than that of low temperature superconductor (LTS) and the capability of HTS to be operated at a liquid-helium-free temperature, an HTS cable is being considered a possible alternative to LTS for fusion magnets such as Toroidal Field (TF) coils in Tokamaks, and Helical coils in Stellarators. This paper presents a first-cut design of a 100-kA/20-K HTS cable, which could be an option for fusion magnets. Thermal behaviors of the cable were analyzed and compared quantitatively with those of a 100-kA/4.2-K Nb3Sn cable using the CryoSoft code THEA. In the paper, we demonstrated that the conventional concepts of the "current sharing temperature (Tcs)" and the "minimum quench energy (MQE)" may not be suitable for design and analysis of the HTS cable. Instead, "thermal runaway temperature (TR)" and "minimum runaway energy (MRE)" were proved to be more effective. Also, the post-quench temperature rise of the HTS cable, simulated by the THEA, was compared with that by the conventional analyticZ-function approach. The results demonstrate that the Z-function approach, proven to be effective for an LTS cable, may significantly overestimate the post-quench temperature rise of an HTS cable.