Enhanced critical current densities in Nb3Sn superconductors for large magnets

The latest achievements in the development of high performance Nb3Sn strands are reported. In the field of high energy physics (HEP) critical current densities up to 3000 A mm(-2) (at 12 T, 4.2 K and 10 mu V m(-1)) can be achieved. Current actions are aimed at reducing the filament size to about 50 mu m and decreasing production costs by using less expensive base materials. In order to take advantage of the progress made by the HEP R&D program, the fusion community has initiated several strand procurement actions worldwide to produce advanced Nb3Sn strand types fulfilling the updated strand specification of the International Thermonuclear Experimental Reactor project. Recently developed internal-tin strands reach critical current densities of more than 1100 A mm(-2) and acceptable non-Cu hysteresis losses of about 700 kJ m(-3). An important milestone towards higher production rate and yield was reached: a billet of about 100 kg was successfully drawn to the final strand diameter without problems. In powder-in-tube conductors, the composition and number of filaments have been modified to meet the European target specification. Although the bronze technique itself sets certain limitations on J(c), strands with critical current densities around 800 A mm(-2) have been produced on an industrial scale.