Thermal stability and current transfer in twisted-pair helium gas-cooled MgB₂ DC cables

Abstract

CERN’s helium gas-cooled, twisted-pair superconducting cable design, typically consisting of intercalated superconducting and copper (stabilising) tapes, is a novel design which offers low cable inductance for high-current DC power transmission. The electrical and thermal contacts between the copper and superconducting tapes, which are given by wrapping the complete ‘sandwich stack’ of tapes together in Kapton, are important parameters for current sharing, cryogenic stability and quench propagation. Observation of both IC and superconductor-stabiliser contact inhomogeneities due to the mechanical twisting process and the gas-cooling regime has shed light on overcoming the unique challenges these create in cable operation.
Both an MgB2 twisted-pair cable assembly manufactured at CERN and a short-length single-sandwich strand made at the University of Southampton have been studied and a novel method for measuring contact resistance in situ has be designed and validated. Tests included measuring thermal runaway propagation velocity (vp) and minimum quench energy (MQE) plus long-period stability tests for the cable, along with characterisation of current transfer as well as contact resistance measurements between the superconductor and stabiliser layers in the single-sandwich. The results suggest that an optimised cable operation strategy may exist, being a trade-off between: high stabilisation with lower superconductor-stabiliser contact resistance; and fast propagation resulting in easier quench detection with higher superconductor-stabiliser contact resistance. The results of the investigations are relevant to both the CERN LHC High Luminosity upgrade and future superconducting power transmission projects.

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ORCID for Yifeng Yang: orcid.org/0000-0002-3874-6735

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