Stability of DC transport in HTS conductor with local critical current reduction

Abstract

A common feature of commercially available conductors based on high-temperature superconducting compounds is the fluctuation of critical current along the length. Fortunately, the practice adopted by manufacturers nowadays is to supply the detailed I c(x) data with the conductor. Compared to knowing just the average of critical current, this should also allow a much better prediction of the conductor performance. Statistical methods are suitable for this purpose in the case when the fluctuations are regular at the low end of critical current distribution. However, a different approach is necessary at the presence of ‘weak spots’ that drop out of any statistics. Because of the strong nonlinearity of the current–voltage curve, such a location could transform into a ‘hot spot’ at transporting direct current (DC), with an abrupt increase of temperature endangering the conductor operation. We present a set of analytical formulas including the prediction of the maximum DC that could be carried sustainably before the thermal runaway appears. It is necessary to know the cooling conditions as well as the properties of the conductor constituents and their architecture. A formula for the voltage appearing on a weak spot, and its dependence on the DC, is also proposed. For this purpose the result of previous theoretical work has been slightly modified after comparing it with numerical iterative computations and finite element modeling. We demonstrate that the derived model allows a powerful analysis of experimental data comprising an estimation of the weak spot parameters i.e. its critical current and the length of the defect zone.

Experimental and Numerical Analysis of High-Temperature Superconducting Tapes Modified by Composite Thermal Stabilization Subjected to Thermomechanical Loading

“…Figure 12 shows this situation at lower temperatures: at 75 °C, which can be defined as a safe state and at 120 °C, which is not critical for the composite coating but poses a danger for the superconductor. We suppose that places with high strain concentration would lightly deform the (RE)BCO crystal lattice and by this way they will contribute to the reduction of current transport capability with possible effect on hot spot formation in the HTS layer [46]. An investigation of such hillock-like defects in cross-section by SEM showed structural defects in the superconductor, sometimes with a crack inside (Figure 11c).…”

“…Their formation is not yet quite clear, but since they are observed indeed only in specific places of the tape (bellow ends of composite coating), they could by related to the maximal strain developed in the HTS layer, as indicates the calculated model of strain distribution in particular layers in Figure 11d at 150 • C. Figure 12 shows this situation at lower temperatures: at 75 • C, which can be defined as a safe state and at 120 • C, which is not critical for the composite coating but poses a danger for the superconductor. We suppose that places with high strain concentration would lightly deform the (RE)BCO crystal lattice and by this way they will contribute to the reduction of current transport capability with possible effect on hot spot formation in the HTS layer [46]. The best performing composite, based on the above mentioned evaluation of the maximal strain and strain in path distribution, seems to be the composite SB11-SiC20.…”

Thermal Runaway of Conduction-Cooled Monofilament and Multifilament Coated Conductors

“…Numerical and experimental studies have been conducted on the initiation of thermal runaway in monofilament coated conductors [13][14][15][16][17][18][19][20][21]. However, the initiation of thermal runaway in multifilament coated conductors has not been experimentally studied.…”

Design, Fabrication and Soundness Test of A Bi2223 Magnet Designed for Cooling by Liquid Hydrogen

“…UENCH protection is crucial for HTS magnets because of slow propagation of a normal zone. Height of resistive voltage to initiate thermal runaway should be an important index for quench detection of HTS magnets to prevent the failure [1], [2]. The critical heat flux in liquid hydrogen is ten times higher than that in liquid helium and is approximately half of that in liquid nitrogen.…”