Reel-to-reel Metrology Tool for In-field Critical Current Characterization of Long REBCO Tapes for Fusion Magnets

The ultra-high critical magnetic fields of RE-Ba-Cu-O (REBCO RE=rare earth) High Temperature Superconductors (HTS) are very attractive for compact fusion energy systems that can operate over a temperature range of 4 – 50 K in magnetic fields beyond the realm of Nb3Sn wires. Several designs of high-current cables are being developed for these systems using stacks of REBCO tapes. The consistency and uniformity in critical current (Ic) of REBCO tapes need to be verified in a magnetic field over long tape lengths. At this time, REBCO tapes are qualified 100% over their length only at 77 K, 0 T which does not reveal inhomogeneity in Ic in high magnetic fields at lower temperatures. In fact, the Ic in high magnetic fields at low temperature can vary by a factor of 3 for tapes with the same Ic at 77 K, 0 T! In the Phase I project, we utilized our in-field reel-to-reel (R2R) scanning hall probe microscopy (SHPM) tool to determine the in-field Ic uniformity at 65 K, of 100-m-long tapes made by three REBCO tape manufacturers that are currently delivering product to compact fusion companies. The Ic of several segments of these long tapes were measured at 20 K in magnetic fields up to 13 T to examine the predictive quality of the R2R in-field Ic measurement at 65 K. A strong correlation was observed between the low-field Ic at 65 K and 77 K and the high-field (13 T) Ic at 20 K, in the tapes made by all three companies. Some differences were found in the correlation coefficients in the Ic of the three types of tapes. In the Phase II project, we will upgrade our in-field R2R SHPM tool for measurements at 20 K, different magnetic field orientations, and possibly higher magnetic fields. We will upgrade our Hall probe measurement sensor for a significantly higher measurement speed. Using the upgraded R2R in-field SHPM tool, we will evaluate the uniformity in in-field Ic at 20 K of tapes being delivered to compact fusion companies. In addition, the in-field R2R SHPM can also serve to identify drop-outs and trends that are missed in 77 K, 0 T R2R Ic measurements. We will specifically identify these tape locations exhibiting such drop-outs and contrary trends, examine the root-cause for these variations using a palette of characterization techniques, and evaluate their influence on magnet performance. The reel-to-reel in-field Ic measurement tool developed in this project will greatly benefit compact fusion companies to evaluate REBCO tapes used in their magnets, over 100% of the tape length. Such a capability will be a tremendous leap over the current practice of short coupon measurement of in-field Ic at the ends of long tapes, which is both very time consuming and misses Ic variations over the tape length. The availability of comprehensive Ic data over the tape length from a R2R in-field SHPM tool will enable compact fusion companies to confidently predict the performance of their superconducting magnets.