Microstructure and -Value of RHQT Processed Nb Al Superconductors for NMR Uses

We have examined the critical current and -values of RHQT processed Nb Al conductors at 4.2 K in a wide range of fields from 14.5 to 25 T in relation to the microstructures obtained. For a point and a tail of Cu-clad rectangular conductor used for a winding, the -values were determined by fitting the voltage over a taps spacing of 110 or 220 mm to a relationship of , in a range from to V/m. Such -values were almost the same with each other, indicating the uniformity of a 370 m length of RHQT Nb Al conductor as the consistency does. The -value increased monotonously with decreasing a field from 25 T to 18 T, but got saturated to be around 35 at 4.2 K in fields less than 18 T. The -value at 21 T and 4.2 K, corresponding to 23.5 T and 1.8 K, was about 25, and confirmed to be large enough for NMR uses.