Use of the buffer layers as a current flow diverter in 2G HTS coated conductors

Abstract

This paper presents a simple and effective approach to increase the normal zone propagation velocity (NZPV) in (RE)BaCuO thin films grown on a flexible metallic substrate. The concept is an extension of the current-flow-diverter concept already known in the literature. The key idea behind the novel approach is to use a specific geometry of the silver thermal stabilizer that surrounds the superconducting tape. More specifically, a very thin layer of silver stabilizer is deposited on top of the superconductor layer, typically less than 100nm, while the remaining stabilizer (still silver) is deposited on the substrate side. Normal zone propagation velocities up to 170cm s −1 at 77K have been measured experimentally for the first time with this novel architecture, corresponding to a stabilizer thickness of 20nm on top of the superconductor layer. This is at least one order of magnitude faster than the NZPV speeds measured on commercial 2G HTS tapes. Our results clearly demonstrate that a very thin stabilizer on top of the superconductor layer leads to high normal zone propagation velocities. The experimental values are in good agreement with predictions realized by finite element simulations. Furthermore, the propagation of the normal zone during the quench was recorded in situ and in real time using a high-speed camera. Due to high Joule losses generated on both edges of the tape sample, a 'Ushaped' profile could be observed at the boundaries between the superconducting and the normal zones, which matches very closely the profiles predicted by the simulations. The most direct application of this new HTS tape architecture is in high field magnets, since faster quench propagation allows easier detection and protection, which is likely to accelerate substantially the development of HTS magnets used as high-field MRI/NMR systems, etc.

Normal zone propagation in various REBCO tape architectures

“…The buffer layers-CFD (or bCFD) architecture was proposed in [26] and experimentally demonstrated in [18,27]. In this work, tapes with a bCFD architecture were fabricated using a sulfidation process.…”

Concepts of static vs. dynamic current transfer length in 2G HTS coated conductors with a current flow diverter architecture

“…The FE1 model consists of a 3D FE electro-thermal model that simulates the dynamics of quench propagation in HTS tapes, as described in detail in [23,36]. The model uses the Joule heating module in COMSOL 4.3b, which solves simultaneously the heat equation and the current continuity equation, which are strongly coupled.…”

Concepts of Static vs. Dynamic Current Transfer Length in 2G HTS coated conductors with a Current Flow Diverter Architecture

“…As a final observation, we confirmed that the normal zone propagation velocity (NZPV) in HTS tapes is proportional to λ d , not to λ s , which strongly suggests that λ d is a more general definition of the CTL than λ s . Therefore, in order to perform a fair comparison between any type of HTS tape architecture, in particular CFD-like architectures [22,38], the dynamic CTL λ d should always be used.…”

“…The FE1 model consists of a 3-D finite element electrothermal model that simulates the dynamics of quench propagation in HTS tapes, as described in details in [23,36]. The model uses the Joule heating module in COM-SOL 4.3b, which solves simultaneously the heat equation and the current continuity equation, which are strongly coupled.…”

“…The transition from the superconducting state to the normal state was modeled by putting the conductivity of the superconducting and the normal state in parallel, such as two resistance in parallel in an electrical circuit. Once again, this is presented in details in [23,36].…”