Flux trapping and compression in superconducting niobium - tin tubes

Abstract

Niobium-tin compacts have been produced by pressing and sintering the powdered elements. Tube magnetization experiments on the material at 4.2°K in fields of up to 54 kOe have yielded results that are consistent with the relation Jc = αc/(H + B0), where Jc is the critical current density in a local magnetic field H and αc and B0 are constants. The highest critical current density to date has been achieved by pressing at 30 ton in-2 and sintering at 975°C for 3 hours; the values of αc and B0 for niobium-tin prepared in this way are 4 × 106 kOe A cm-2 and 4 kOe respectively. The material is notable in that it seldom exhibits flux jumps; it is thought that this feature of its behaviour is due to the presence of liquid helium in the interconnected pores. The absence of flux jumps indicates that the material can be used in the construction of mechanical flux compressors and the design of a flux compressor, which can yield 50 kOe in a working space of 1 cm diameter, is discussed. Preliminary experiments on flux compression up to a field of 23 kOe have given results in accordance with the theoretical predictions.