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) superconductors are very attractive for compact fusion energy systems that can operate over a temperature range of 4 ľ 50 K. 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! While Ic of REBCO tapes at 77 K, 0 T is not reflective of their in-field Ic at low temperatures, we have discovered that Ic at 65 K, 4 ľ 8 T correlates very well (correlation coefficient > 0.95) with Ic in high magnetic fields at low temperatures. Reel-to-reel measurement of in-field Ic of REBCO tapes at 65 K, 4 T will be valuable to confirm the consistency and uniformity of Ic for use in fusion magnets. The strong correlation found between in-field Ic at 65 K and in-field Ic at lower temperatures motivated us to develop a reel-to- reel (R2R) scanning hall probe microscopy (SHPM) method for continuous in-field Ic measurement at 65 ľ 77 K over 100% tape length to determine the uniformity and consistency of in-field Ic at lower temperatures. In this project, we will utilize our in-field R2R SHPM tool to determine the Ic uniformity at 65 K, 4 T of long tapes made by various REBCO tape manufacturers that are currently delivering product to compact fusion companies. Next, the Ic of several segments of these long tapes will be measured at 20 K in magnetic fields up to 13 T to examine the predictive quality of the R2R measurement at 65 K, 4 T. Statistical analysis methods will be used to fill in the gaps in the low temperature in-field data. We will examine any universal trends in the correlation between in-field Ic data at high temperature and low temperature, in tapes manufactured by different companies. The reasons for any differences in the correlation coefficients of tapes of different companies will be examined using a palette of characterization techniques. In addition to serving as a metrology tool for tape and magnet manufacturers to determine uniformity in in-field Ic at low temperatures, 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. In this project, we will identify these tape locations exhibiting such drop-outs and contrary trends and evaluate their influence on magnet performance. In Phase II, we will augment our R2R in-field SHPM tool for measurements at temperatures below 65 K and higher speed.