Optimization Method for Extracting Stabilizer Geometry and Properties of REBCO Tapes

A good knowledge of material properties is a critical aspect for modeling high-temperature superconductor (HTS) devices. However, the electrical resistivity of coated conductors above the critical current is limited. The major challenge in characterizing this regime lies in the fact that for I > Ic, heating effects and thermal instabilities can quickly destroy the conductor if nothing is done to protect it. In our previous works we proposed the overcritical current model, obtained by combining fast pulsed current measurements with finite element analysis (Uniform Current (UC) model). In this work, we assessed the impact of the uncertainties of the input parameters on the quantities calculated with the UC model (temperature and current in each layer of the tape). Firstly, sensitivity and uncertainty analyses were performed and it was found that the input parameters that mostly affect the UC model are the electrical resistivity and the thickness of the silver layer. Afterwards, an optimization method to correctly estimate the geometry and the resistivity of the silver layer was developed. This method combined experimental measurements of resistance R(T) of the tape and pulsed current measurement. The development of this strategy allowed us to determine the parameters that significantly impact the UC model and helps to minimize their uncertainties. This can enable a precise estimation of the overcritical current resistivity. 10%

Quench behavior of high-temperature superconductor tapes for power applications: a strategy toward resilience

Nicolò Riva

High-Temperature Superconductors (HTS) can be superconducting in liquid nitrogen 77 K, holding immense promises for our future. They can enable disruptive technologies such as nuclear fusion, lossless power transmission, cancer treatment devices, and technologies for future transportation. In the past years, the numerical models to describe the electrical resistivity of REBCO commercial tapes for devices working near and above the critical current, have been shown to be not accurate or very empirical. The resistivity in this regime, in fact, is not very well known. The lack of this knowledge is a significant issue in developing quality simulation tools. The major challenge in retrieving such properties lies in the fact that when I>Ic, heating effects, and thermal instabilities can quickly destroy the conductor if nothing is done to protect it. Moreover, due to the current sharing between the layers, it is difficult to know the amount of current carried by the superconducting layer and its resistivity. The present work aims to understand better the overcritical current regime combining ultra-fast pulsed current measurements performed on HTS REBCO based coated conductors with Finite Element Modeling. The experimental activities were carried out mostly at EPFL and in part at PM and KIT. The modeling activities were carried out between EPFL and KIT. The major result is a resistivity relationship describing the overcritical current regime to be used in numerical simulations of REBCO tapes. The first part of the thesis illustrates a post-processing method based on the so-called Uniform Current (UC) model to estimate the REBCO material's resistivity in the overcritical from experimental measurements. Pulsed current measurements as short as 15 us and with current magnitude up to 5 Ic were performed in liquid nitrogen bath 77 K on samples from various manufacturers, without damaging the tapes. The second part of the thesis discusses a post-processing method based on regularization of data to treat the experimental measurements extracted in the overcritical current regime. The output of this technique is a look-up table that can be shared with interested partners and used in numerical modeling afterward. The third part of the thesis presents the overcritical current model (rho-\eta\beta): a mathematical relationship of the overcritical current regime based on measurements performed between 77 K and 90 K and in self-field conditions. We compare such models with the power-law model, and we provide a short discussion of the fitting parameters and their typical values. The last part of the thesis discusses the overcritical current model, based on experimental measurements obtained as outlined above. The model was validated experimentally and used to show that for the case of a superconducting fault current limiter when the power-law model is used to model its electro-thermal response, the device quenches faster than with the overcritical model. In conclusion, this work can help optimize the use of superconductors and, consequently, the stabilizer. More interestingly, it opens the study of the overcritical current regime, a new exciting aspect of REBCO commercial tapes. This project has received funding from the Swiss Federal Office of Energy SFOE under grant agreement SI/500193-02.

EPFL2021

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

Quench behavior of high-temperature superconductor tapes for power applications: a strategy toward resilience

Nicolò Riva

EPFL2021