Composite stacks for reliable > 17 T trapped fields in bulk superconductor magnets

Kai Yuan Huang,Jan Srpčič,John H Durrell,Martin Boll,Devendra K Namburi,Yunhua Shi,Jan Jaroszynski,Difan Zhou,Eric E Hellstrom,Anthony R Dennis,Mark D Ainslie,David A Cardwell,Mykhaylo Filipenko

doi:10.1088/1361-6668/ab5e12

Abstract

Trapped fields of over 20 T are, in principle, achievable in bulk, single-grain high temperature cuprate superconductors. The principle barriers to realizing such performance are, firstly, the large tensile stresses that develop during the magnetization of such trapped-field magnets as a result of the Lorentz force, which lead to brittle fracture of these ceramic-like materials at high fields and, secondly, catastrophic thermal instabilities as a result of flux movement during magnetization. Moreover, for a batch of samples nominally fabricated identically, the statistical nature of the failure mechanism means the best performance (i.e. trapped fields of over 17 T) cannot be attained reliably. The magnetization process, particularly to higher fields, also often damages the samples such that they cannot repeatedly trap high fields following subsequent magnetization. In this study, we report the sequential trapping of magnetic fields of ∼ 17 T, achieving 16.8 T at 26 K initially and 17.6 T at 22.5 K subsequently, in a stack of two Ag-doped GdBa2Cu3O7-δ bulk superconductor composites of diameter 24 mm reinforced with (1) stainless-steel laminations, and (2) shrink-fit stainless steel rings. A trapped field of 17.6 T is, in fact, comparable with the highest trapped fields reported to date for bulk superconducting magnets of any mechanical and chemical composition, and this was achieved using the first composite stack to be fabricated by this technique. These post-melt-processing treatments, which are relatively straightforward to implement, were used to improve both the mechanical properties and the thermal stability of the resultant composite structure, providing what we believe is a promising route to achieving reliably fields of over 20 T.

Highlights

Introduction(RE)BCO materials generally exhibit relatively low thermal conductivity [10, 11], and so, when a significant amount of heat, Q = E ∙ J, is generated due to flux movement during magnetization, catastrophic thermal instabilities and flux jumps can occur [4, 12]
The (RE)Ba2Cu3O7-δ family of bulk high temperature superconductors [or (RE)BCO; where RE = rare earth element or yttrium] can be used as trapped-field magnets [1–3] with superior performance compared with conventional hard ferromagnets
The composite stack was magnetized initially by field cooling in 18 T at 26 K with a constant ramp-down rate of 0.02 T/min, which resulted in a trapped field of 16.8 T immediately after the complete removal of the applied field

Summary

Introduction

(RE)BCO materials generally exhibit relatively low thermal conductivity [10, 11], and so, when a significant amount of heat, Q = E ∙ J, is generated due to flux movement during magnetization, catastrophic thermal instabilities and flux jumps can occur [4, 12] To address these issues and to achieve record trapped fields of over 17 T, bulk superconducting samples have been reinforced by various techniques, including resin and alloy impregnation, carbon fiber wrapping [4] and shrink-fit stainless steel rings [5]. To form the composite assembly, stainless steel discs were sandwiched between layers of bulk superconductor to define a strengthened laminated structure These results are significant since the trapped fields reported here were achieved using the first composite stack to be fabricated by this technique. The two composite bulk samples were combined into a stack using Stycast epoxy resin with their top surfaces (i.e. the position of the seed crystals during meltprocessing) both pointing towards the center of the stack

Trapped field measurements

Results and discussion

Conclusions