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

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
$\hbox{Nb}_{3}\hbox{Sn}$
cable using the CryoSoft code THEA. In the paper, we demonstrated that the conventional concepts of the “current sharing temperature
$(T_{cs})$
” and the “minimum quench energy (MQE)” may not be suitable for design and analysis of the HTS cable. Instead, “thermal runaway temperature
$(T_{R})$
” 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 analytic
$Z$
-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.