Conductor manufacturers worldwide are now making ReBCO coated conductors with optimized $J_c$ performance for applications at low temperatures and at ultra-high fields > 25T. There are several approaches to improve the whole conductor $J_e$ of ReBCO conductors: artificial pinning centers (APC), increased ReBCO thickness, and thinner substrates are three such thrusts being actively studied. Recently NHMFL started a project to design and then construct a 40T superconducting magnet for which we have compared transport $I_c$ for 4mm wide short samples of commercially available conductors produced by SuperPower Inc., SuperOx, Samri, Shanghai, Fujikura, and Theva. Measurements were made in very high fields up to 31T at 4.2K and in fields up to 15T at various temperatures and compared with $I_c$(77K,SF). Conductors with a higher density of APC have a wider ab-peak in $I_c$($\theta$), which makes them useful in those parts of the coil with considerable radial field. Central regions of solenoid coils tend to have unnecessarily high $I_c$ due to the strong angular dependence $Ic$($\theta$), making problems for quench protection and encouraging conductor $I_c$ grading for bigger magnet systems. We observed that $I_c$(15T;4.2K) is proportional to ReBCO thickness and $I_c$(77K, SF) for SuperPower Inc. tapes with graded $I_c$. To study $J_c$ properties at the highest fields, ultra-narrow bridges were prepared from ReBCO tapes produced by SuperPower Inc. using photolithography, wet etching, and trimming by FIB. Transport $I_c$(B,T,$\theta$) dependencies were measured at temperature range 4.2K-30K in the NHMFL Hybrid DC magnet up to 45T. At 10K and 20K and fields above 20T in the BIItape orientation, we found that $J_c$ is field independent. In B⊥tape orientation the $J_c$(B,4.2K) dependencies follows the power function up to 45T. At 12K we observed that the $f_p$ maximum is at 25T for 7.5%Zr tapes.
The National High Magnetic Field Laboratory is supported by the National Science Foundation through NSF/DMR-1644779 and DMR-1839796, and the State of Florida.