Manufacture of ITER feeder sample conductors

The ITER feeders are the components that connect the ITER magnet systems located inside the main cryostat to the cryogenics, power-supply and control system interfaces outside the cryostat. The feeder busbars rely on the Cable-In-Conduit Conductor (CICC) design concept as all the conductors for the ITER magnet systems. There are two types of busbars for the feeder systems. One is the Main Busbar (MB) for the TF, CS and PF feeders, and the other is the Corrector Busbar (CB) for the CC feeders. The busbar cable is wound from multiple stage sub-cables made with Cu and superconducting strands. The superconducting material is NbTi for the busbar strands of all feeder systems. All Feeder conductors are provided by China. The R&D programs are needed to acquire knowledge on the behavior of such conductors. Since the conductors are new, some full size copper dummy conductors have been produced for the testing of the cabling parameters, definition of automatic TIG welding of seamless jacket section, elaboration of cable insertion and compaction. Then, two short qualification conductor samples (MB and CB) are prepared in ASIPP, and NbTi advanced strands are produced by Western Superconductor Technology (WST). The details of manufacturing procedures for Feeder conductor samples will be described in this paper. (C) 2013 Elsevier B.V. All rights reserved.

Modeling and Characterization of Coated Conductors Applied to the Design of Superconducting Fault Current Limiters

François Roy

The Superconducting Fault Current Limiter (SFCL) appears to be a device of great interest to efficiently build the electrical grid of tomorrow. With the recent progress made by the superconducting wires manufacturers, there are needs coming from the industry to evaluate the potential of such devices. In the present thesis work, the behavior under external field and transport current of the last generation of wire is investigated. This study is conduct both experimentally and numerically in order to link the physics occurring at the wires level to the design of SFCLs as a whole. From the nature of the material, the resistance appears non-uniformly in high temperature superconductors. For the purpose of building SFCLs it is important to obtain a fast and uniform resistive transition (quench) when a fault occurs through those conductors. This in order to reduce the local heat generation that may damage the device. This fast quenching property is related to the Normal Zone Propagation Velocity (NZPV). In this work the NZPV is measured using a localized magnetic field to initiate quenches in commercial coated conductors. Those velocities have been measured to be larger than 14 cm/s for pulsed currents above the critical value. The NZPV experiments have demonstrated that the superconductor non-uniformity (generated by the localized field) helps to reduce the initial delay before the quench initiation for transport currents in the range of the critical value. However, for larger transport currents the effect of the non-uniformity on the delay is less important since, with increasing transport current amplitudes, the normal state transition has shown to occur more as a consequence of the heat generated in the stabilizer than as the unique consequence of the advancement of the normal zone in the superconductor. From the experimental measurements, it has been shown that a reduction of the liquid-nitrogen temperature (subcoooled) increases the NZPV. This effect has been observed taking into account of the increase of the critical current associated with the temperature reduction. Nevertheless, it is not clear if it is the heat transfer or the estimation of the critical current that is responsible for this effect. In order to validate the numerical models, time-resolved voltage traces obtained from the experiments have been compared to the outputs of the models. Those are based on the thermal- and electrical-diffusion equations. From the simulations, it has been demonstrated that the NZPV can be increased by three methods: by using a thick diffusive substrate, by inserting a resistive interface between the superconductor and the stabilizer as well as by increasing the heat generation in the stabilizer. In light of those results, it seems that the insertion of a resistive layer is the most promising approach to improve the NZPV in coated conductors. As a matter of fact, a resistive interface increases the normal-zone size and keeps an acceptable temperature level along the conductor during quenches. The present work allowed to simulate the flux-flow regime in coated conductors. Comparing those simulations to experimental data have shown that the power-law may be inappropriate to simulate this regime under weak external magnetic fields. In addition, it appears that the role of transient heat transfer with the surroundings needs to be studied in more details to determine the specifications of a prospected SFCL made of coated conductors.

EPFL2010

Evaluation of the Applicability of Phenomenological HTS Models for Numerical Analysis of Quenches in Coated Conductors: Simulations vs. Experiments

Bertrand Dutoit, François Roy, Frédéric Sirois

The electrical resistivity of coated conductors is strongly related to the inter-dependent set of parameters (J, H, T), respectively current density, magnetic field and temperature. On the one hand, it is difficult to isolate the contribution of each of these parameters on the resistivity measured experimentally. On the other hand, numerical methods, which may allow this separation, require a good knowledge of the fundamental laws governing the superconducting transition which are, up to now, derived from curve fitting with experimental data. In this paper, we investigate the influence of phenomenological formulas on the outputs of a recently developed finite element model. The outputs are compared against experimental voltage curves, which have been obtained under pulsed transport currents between 80 and 160 A and external magnetic fluxes of 0 to 350 mT. The comparisons indicate that the numerical models may reproduce well the measurements, using the right set of phenomenological laws parameters. Nevertheless, the solution may still be inaccurate at low field values and high current amplitudes, where the curvature of the simulated E-J curves is more pronounced, indicating that further refinement is required in order to obtain models valid over a wider range of parameters.

2011

Characterization of the electrical resistance of high temperature superconductor coated conductors at high currents using ultra-fast regulated current pulses

Bertrand Dutoit, François Roy, Frédéric Sirois

This paper focuses on the experimental determination of the electrical resistance (R) of commercial high temperature superconductor (HTS) coated conductors (CCs) at currents well above the critical current. The major novelty of this work rests on the unique experimental capability of applying constant current pulses in the sample (up to 1000 A) for durations as short as 15 mu s, which allows very precise control of the amount of energy dissipated in the sample (the Joule effect), as well as the resulting temperature rise. By varying the applied current and the duration of the pulses, we show that we can achieve a relatively accurate characterization of R(I, T) simply from the measured dynamical V-I characteristics of the CCs. The resistance model obtained in this way is very important, as R(I, T) is the most fundamental design parameter in many practical HTS applications, especially in fault current limiters.

2010

Modeling and Characterization of Coated Conductors Applied to the Design of Superconducting Fault Current Limiters

François Roy

EPFL2010