High Field Superconductors for Fusion Energy Applications

Abstract

The fabrication and processing by solid-state heat-treatment, mechanical ball milling and hot isostatic pressing of microcrystalline and nanocrystalline niobium carbonitride is reported. This material is subjected to a number of characterisation measurements including x-ray diffraction, resistivity, ac-susceptibility, dc-extraction and heat capacity. The resultant measurement data are used to assess the adequacy of the material’s processing and quality with respect to the fundamental superconducting characteristics, transition temperature, T_c, upper critical magnetic field, B_c2, and critical current density, J_c. It is shown that a substantial increase in B_c2 from ~ 11 T (in the microcrystalline material) to ~ 21 T (in the nanocrystalline material) has been produced. A fortyfold increase in J_c from 1.8 x 107 Am^(-2) (in microcrystalline material measured at 3 T and 6 K) to 7.4 x 108 Am^(-2) (in nanocrystalline material measured at 3 T and 5.9 K) has also been produced. These substantial increases have been made with only a 32 % reduction in T_c from ~17.6 K to ~ 11.9 K, well above the temperature of liquid helium.

The accurate large quantity metrology of 10,000 Nb3Sn samples for the International Thermonuclear Experimental Reactor toroidal field coils is also reported and an overview analysis of the data provided. In particular, all seven measurement types; critical current, hysteresis loss, residual resistivity ratio, diameter, chromium plating thickness, twist pitch and copper to non-copper volume ratio are discussed in relation to the accuracy with which they were performed. The methodology in performing the heat-treatments and measurements is discussed and the detail of the necessary equipment set up is given. The results from some additional experiments that deal with the effect of heat-treatment cleanliness and sample geometry on various measurement types is provided.