Soft quench of superconducting REBCO magnets at high fusion fields

High temperature superconducting (HTS) cuprate REBCO magnets for magnetically confined fusion have important differences from the REBCO smaller magnets used in other high field applications that can change their response to the off-normal events that might precipitate a SC quench. Toroidal field magnets self-generate their ambient magnetic field and their large size implies a high stored SC energy that must be dissipated in a quench without overheating the magnet. Fusion fields, on the order of 15--20 T, have strong magneto-resistive effects on the REBCO superconductor and on the copper stabilizer layers. It gradually increases the SC dissipation as the temperature rises from operating (15--20 K) towards liquid nitrogen levels (77 K).

The maximum SC field, the irreversibility field H*(T), can fall below 18--20 T at temperatures well below 70 K. In addition, at even lower temperatures the SC resistivity may rise to a fraction of that of the copper stabilizer, prompting sharing of the SC current with the stabilizer before the superconductivity breaks down. Both boundaries are sensitive and gradual, unlike the relatively sharp and ``hard'' boundaries of low temperature SCs. They have important implications for fusion magnets, some of which were seen in the SPARC TFMC coil tests.