Quench Detection and Protection of the HTS Insert Coils

As a result of extremely high upper critical fields B_c2, high temperature superconductors (HTS) have the potential to be used as high field insert coils in magnet systems where the background field is provided by low temperature superconductors (LTS) with the aim of application of such systems for high energy physics, nuclear magnetic resonance and energy storage. Among the superconductors discovered in late 1980s, one that is widely studied is YBCO coated conductor (CC). Despite the advanced 2G conductor technology in recent years allowing for manufacturing of long lengths of YBCO CC, up to 1 km, the crucial issue in practical application of the hybrid systems with YBCO insert coil remain the quench detection and protection of the insert. Unlike in LTS magnets, the quench propagation in HTS is significantly slower, which makes quench detection and consecutively protection, very challenging. Following a new approach for quench protection a non-insulated HTS insert coil was manufactured at the CRPP. In case of quench, the current is expected to by-pass the normal zone and prevent the coil from a permanent damage. The idea of non-insulated free-standing coils has been already studied and brought promising results in terms of self-protection of such coils. However, in a hybrid system of LTS-HTS, the quench behaviour needs careful evaluation that includes magneto-thermo-electrical study. The objective of this work is to study and discuss the applicability of HTS insert coils (insulated and non-insulated) by addressing the issue of their quench detection and protection schemes. The starting point for the analysis is studying the existing design according to CRPP specifications taking into account various operating modes together with detection and protection schemes. Finally, general guidelines for the design of a successful LTS-HTS magnet system will be discussed.

3D Modeling of Coupling Between Superconducting Filaments via Resistive Matrix in AC Magnetic Field

Bertrand Dutoit, Francesco Grilli, Yong Yang

The ac loss of superconducting composite depends strongly on coupling between superconducting filaments via the resistive matrix. The established technique for loss reduction using twisted filaments relies on the decoupling of the filaments below a critical coupling field Bc, which increases with the reduction of the twist pitch and the matrix conductivity. Although the concept of Bc may be clearly demonstrated using two infinite slabs of finite length, further details on its correlation with the filament/conductor geometry are not yet available. The main obstacle is due to the fact that any accurate analysis of such a problem must be carried out in 3d. In this paper, we describe the initial results from 3d modeling using Cedrat‘s Flux3D, for which a superconductor module for handling power-law E-J characteristics was developed. Using a simple model of two rectangular superconductors connected through a normal metal, we demonstrate the feasibility for quantitative modeling of their coupling behavior over a wide range of field sweep rates for different conductor geometries. Typical examples were given for cases not addressed by the existing approximate theory, as well as for the evolution of field profile for varying field sweep rate. ample of a very low resistivity in the matrix, it is also suggested that onset of coupling is strongly dependent on the shape of the conductor and individual filaments. Qualitatively speaking, conductors with a large aspect ratio have a greater tendency to couple due to a larger demagnetising effect, as confirmed by experimental studies. While the mechanism that promotes the coupling of the filaments through the resistive matrix in ac magnetic fields is well understood conceptually, quantitative calculation of the critical coupling field is only limited to the simplified scenario of infinite slabs. The main difficulty is that we are dealing with a 3d problem with conductors of finite length and induced currents flowing both along and perpendicular to the long axis. Fortunately 3d calculations of superconductors in magnetic fields are becoming a reality thank to the rapid enhancement in computing power to desktop computers and progress in the implementation of numerical techniques for modeling superconductors.

2003

Finite element method analysis of the coupling effect between superconducting filaments of different aspect ratio

Bertrand Dutoit, Francesco Grilli, Yong Yang

Using 3D finite element method (FEM) modelling, the present work investigates the coupling effect between two finite superconductors through a resistive matrix. This effect, related to the finite length of the conductors, is typically three-dimensional and cannot be analyzed by the widely utilized 2D models. Superconductors are modelled with the non linear power law E=Ec(J/Jc)^n, which has been implemented in the FEM software. The main focus is to demonstrate the feasibility of such calculations and to establish the correlation between the coupling effect and the aspect ratio of the conductor cross-section, in order to extend the existing theory, which is only precise for superconductors of infinite slabs, fully penetrated by the magnetic field. The effect of other parameters such as the conductor length and the gap between the superconducting filaments is also considered. The latter is a parameter which does not feature in the approximate theories.

2003

Modelling the E-J relation of high-Tc superconductors in an arbitrary current range

Joseph Duron, Bertrand Dutoit, Francesco Grilli

For describing the E-J relation of high-Tc superconductors (HTS) in power applications, where the applied current I is generally limited by Ic, the critical state model, a piecewise linear generalization, or a simple power-law of the type E=Ec(J/Jc)^n are most often used. The power-law cannot be used for modelling the E-J relation with I>>I_c due to the unbound exponential increase of the electric field for currents above Ic, while in reality the non-linear HTS resistivity is limited by its normal state value. This paper presents a modified E-J model for describing the V-I characteristic of HTS tapes with applied currents largely exceeding Ic. This model is based on the power-law in combination with a parallel metallic branch and has a limited resistivity - the HTS one in the normal state. It can be used for black-box modelling of superconductors in a unlimited current range, as well as for numerical modelling of superconducting devices, which can be operated at currents far exceeding Ic; for example fault-current limiters or cables with over-critical current excursions. The model has been tested in a simple numerical implementation and the modified power-law has been implemented in finite element method simulations. It is shown that for bulk material with currents above 1.3-2Ic$ (depending on the n-value), the usual power-law results in excessive AC loss estimation.

2004

3D Modeling of Coupling Between Superconducting Filaments via Resistive Matrix in AC Magnetic Field

Bertrand Dutoit, Francesco Grilli, Yong Yang

2003