Vapor-Deposited Nb3Sn Ribbons

Abstract

The electromagnetic performance of high-field type-II superconductors is significantly affected by structural factors such as crystal size, crystal orientation, or crystal strain. Because the vapor-deposition process for depositing single-phase, polycrystalline, stoichiometric Nb3Sn on a heated moving substrate permits control of these structural parameters, this technique is used for the deposition of continuously monitored layers of Nb3Sn on long lengths of ribbon substrate. The thickness of teh Nb3Sn layer is controlled and can be readily varied to meet specific magnet requirements. For small-bore, high-field, layer-wound magnets, a family of commerical 0.090-in.-wide ribbons has been developed. For larger-bore magnets, in which ``pancake''-type construction is desirable, a commercial line of ½-in.-wide ribbons is currently being offered. Custom-size ribbons have been made as narrow as 0.025 in. and niobium stannide has been deposited on wires as thin as 0.005 in. in diameter. This paper describes these different ribbons and discusses their electromagnetic performance. Normalized curves have been developed for calculation of short-sample critical currents as a function of volume of niobium stannide. For instance. from 20 to 95 kG, normalized critical current In (i.e., the ratio of critical current at any desired field to the critical current at 100 kG) can be expressed as follows: In=Ic/I100 kG=exp(−0.0152H+1.597). From 95 to 140 kG, the following approximate expression can be used: In=Ic/I100 kG=exp(−0.02581H+2.592),where Ic is the critical current at the desired magnetic field, I100 kG is the critical current at 100 kG, and H is the magnetic field in kG. The current density of the Nb3Sn layer in the various types of vapor-deposited ribbon remains essentially constant. When a constant current density at 100 kG is used for the various Nb3Sn layer thicknesses, the Nb3Sn thickness and the ribbon width can be incorporated into the expression for critical current from 20 kG to 95 kG as follows: Ic=(W/0.090)37.4×104(t−7.5×10−5)exp(−0.0152H+1.597). From 95 kG to 140 kG, the following expression applies: Ic=(W/0.090)37.4×104(t−7.5×10−5)exp(−0.02581H+2.592),where W is the ribbon width in inches and t is the Nb3Sn layer thickness in inches. These data are discussed in detail. The critical currents obtained in actual magnets are often lower than the short-sample performance. In many magnet applications, it is economically desirable to obtain the highest possible current density. To achieve these goals, it may be necessary to compromise the amount of normalized metal parallel to the superconductor. Specific examples of optimization of magnet current density as a function of silver plating and cooper cladding of the superconductor ribbon are described.