IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 18, NO. 2, JUNE 2008 989 Niobium Tin Conductors for High Energy Physics, Fusion, MRI and NMR Applications Made by Different Techniques E. Gregory, M. Tomsic, X. Peng, R. Dhaka, V. R. Nazareth, and M. D. Sumption Abstract—Niobium tin was one of the first superconductors to be made into relatively small high field magnets forty-six years ago. Conductors made from it are widely used for a range of different applications and development efforts are still being carried out to improve properties and reduce costs. The paper describes some of the work being carried out by the authors to make such improved conductors for fusion, high-energy physics and other applications. Three different types of niobium-tin conductors are described: 1. Low loss material with modest current density for fusion magnets. 2. High current density, higher loss material for high field high en- ergy physics magnets. 3. Tubular type, low cost material with inter- mediate current density and small subelement size that could have application in a wide range of different magnets. Some of the ad- vantages and drawbacks of each type of conductor are described together with illustrative properties. Index Terms—Copper, fusion, high energy physics, niobium, tin, tubular superconductors. I. INTRODUCTION I N 1961 wire was made and wound into a small 8.8 T coil [1]. For the next quarter of a century, was used in many applications of increasing size and no other supercon- ducting material could compete with it for fields in excess of 10 T or if temperatures slightly above 4.2 K were expected during use. For lower fields and 4.2 K operations however, Nb-Ti was usually the material of choice because of lower cost and less problems in coil manufacture. In 1986, the advent of high temperature superconductors caused considerable reduction in funding and development of both and Nb-Ti. Despite this for the next twenty years these two materials were the ones that were used for most of the practical superconductor applications. Most of the developmental work that has been done has been in improvement of critical current density and in cost re- duction. Magnetic Resonance Imaging (MRI) has been the pri- mary commercial application. High Energy Physics (HEP) and Fusion have been the two main government funded applications. Manuscript received August 22, 2007. This work was supported by the U.S. Department of Energy under a Phase I SBIR Grant DE-FG02-05ER84380 and a Phase II SBIR Grant DE-FG02-05ER84191. E. Gregory is with Supergenics I LLC, Jefferson, MA 01522-1333 USA (e-mail: ericgregory@charter.net). M. Tomsic and X. Peng are with Hyper Tech Research Inc., Columbus, OH 43212-1155 USA (e-mail: tomsic@voyager.net; xpeng@hypertechre- search.com). R. Dhaka, V. R. Nazareth. and M. D. Sumption are with LASM, Ohio State University, Columbus, OH 43210 USA (e-mail: dhaka@matsceng.ohio-state. edu; vrnazareth@matsceng.ohio-state.edu; sumption@matsceng.ohio-state. edu). Digital Object Identifier 10.1109/TASC.2008.920569 Very little development work has been done on Nb-Ti in recent years but, in , efforts are still being made to meet the ever-increasing needs of the HEP and fusion communities and to reduce costs. Some of these efforts that Supergenics I LLC has carried out in cooperation with Hyper Tech Research Inc. and the Labo- ratory for Applied Superconductivity and Magnetism of Ohio State University (LASM OSU) will be discussed in this paper. II. FUSION APPLICATIONS The work on the development of fusion conductors has been carried out under a Supergenics I LLC SBIR contract, the aim of which was to make material meeting the ITER toroidal field coil specifications of and at 12 T and , an RRR of 100. In the Phase I work two small, 88.9 mm (3.5 ) diameter billets were assembled. One had 162 Nb filaments and the other had the same number of Nb7.5 wt.%Ta filaments. These were extruded and drawn to the restack sizes. Both 19 and 37 subelement restacks were then fabricated. The material with Nb7.5 wt.%Ta filaments was examined with cores of both Sn and Sn2 wt.%Ti. The material with Nb filaments was however only examined with Sn2 wt.%Ti cores. Almost no wire breakage was encountered but the best achieved at 12 T in the non-Cu was 830 with a of 506 . This was achieved in a 19 restack of material with Nb filaments having Sn2 wt.%Ti cores, after 220 h at 660 and a ramp rate of 6 . The “n” value at 12 T was 20 but the RRR values were well within specification although no chrome-plated material was tested. Distortion and “sausaging” of the filaments were believed to be the main causes of the low and poor “n” values. It was difficult in the small billets to drill the holes with the neces- sary close spacing without them merging into one another. In Phase II therefore a 203 mm (8 ) diameter billet was chosen as a starting design. Based on the Phase I results it was decided to change the amount of Nb by increasing the diameter of rods more than simply scaling up the size. This was done even though it meant decreasing the spacing between the filaments. No particular problems were experienced in extrusion and drawing of any of the wires down to 0.83 mm diameter. A cross section of the 19-subelement restack at this size is shown in Fig. 1 and a 37-subelement restack in Fig. 2. Samples of the 19-subelement restacks were put onto ITER barrels and heat treated at 660 for 150 h, 180 h and 220 h after ramping to this temperature at 6 . The were all above 1330 at 12 T but the losses were all above 2400 , (Table I). 1051-8223/$25.00 © 2008 IEEE