Critical current densities and microstructures in rod-in-tube and tube type Nb3Sn strands—present status and prospects for improvement

In this work, the transport and magnetization properties of distributed barrier rod-in-tube (RIT) strands and tube type strands are studied. While tube type strands had smaller magnetizations and thus better stabilities in the low-field region, their 12 T non-Cu J c values were somewhat smaller than those of the RIT strands. Microstructures were investigated in order to find out the reasons for the difference in non-Cu J c values. Their grain size and stoichiometry were found to be comparable, leading to similar layer J c values. Accordingly it was determined that the lower A15 area fraction rather than the quality of the A15 layer was the cause of the discrepancy in non-Cu J c . Subsequently, the area utilizations of subelements were investigated. While for a RIT strand the fine grain (FG) A15 area occupies ∼60% of a subelement, for a tube type strand it is no more than 40%. Further analysis indicates that the low FG area fraction in a tube type strand is attributed to its much larger unreacted Nb area fraction. Finally, a simple change in strand architecture is proposed to reduce the unreacted Nb area fraction.