Stability of superconducting coils made of NbTi wires internally doped with large heat capacity Gd₂O₃powder

Abstract

During the past decade we have been carrying out R&D aimed at increasing the stability of low temperature superconducting magnets. The main idea is to enhance the superconductor temperature margin by means of introduction into the winding of several volume per cent of specific substances with enormously large heat capacities (LHCS) at liquid helium temperatures. Two doping techniques have been developed at the Kurchatov and Bochvar institutes: the 'external' one-introduction of a dopant into the epoxy compound-and the 'internal' one-the introduction of doping filaments directly into NbTi and Nb 3 Sn wires. Up to now, our experiments with LHCS internal doping have been carried out on short samples only. In this paper we report the first experimental and numerical investigation of the stability of small-scale internally doped coils made of NbTi wires of a new type, with Gd 2 O 3 ceramic filaments. The coil wound from doped wire and an identical control coil without any doping were subjected to electromagnetic pulses with 1.4-7.4 ms duration. Minimum quench energies for the doped coil turned out to be about 80% larger than those for the undoped one. The gain is especially pronounced in the range of large transport currents (∼0.9I c ). Comparative effectiveness analyses of the two (external and internal) LHCS doping techniques are also presented.

Improvement of stability of Nb3Sn superconductors by introducing high specific heat substances

“…The specific heat of superconductors (such as NbTi and Nb 3 Sn) and matrix (Cu) can hardly be significantly increased; however, there is a class of materials with high specific heat at low temperatures, and the addition of such substances to a superconducting wire in a proper architecture can improve its overall C [5][6][7]. Experiments have shown that the high specific heat of these substances was not fully utilized when they were added to epoxy, which has low thermal diffusivity [8]; however, when they were directly added into superconducting wires (e.g., being placed between the Ta barrier and the outside Cu sheath in bronze-process Nb 3 Sn wires [9], or being filled into holes that were drilled in the Cu matrix in NbTi billets [10]), the minimum quench energies (MQE) of these conductors were significantly increased (e.g., by factors of five to seven as reported in [9]). Nevertheless, these schemes of adding high-C substances significantly increased billet fabrication difficulty and undermined wire drawability, making it difficult to obtain practical long-length wires.…”