Mathieu Therasse

University of Geneva, Genève, Geneva, Switzerland

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Publications (11)10.48 Total impact

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    ABSTRACT: We developed a hybrid model of the quench propagation in coated conductors in the current limitation condition. This model combines the finite element method, to study the thermal behaviour of the coated conductors, and analytical calculation of the heat dissipation. We demonstrate that the evaluation of the heat dissipation can be conducted on a larger mesh than the FEM thermal problem. The results obtained with this model are in very good agreement with experiments, without the need of using free parameters for adjustment. Parametric studies are then conducted to evaluate the influence of both the substrate thickness and the layer interface thermal properties on the transition propagation behaviour. 3D simulations of a thin superconducting line placed on a wider substrate are also presented. Significant transverse heat propagation is observed in spite of the low thermal conductivity of the substrate, though this has little to no influence on the transition propagation along the line. These results are discussed in the context of FCL design.
    Superconductor Science and Technology 01/2012; 25(9). · 2.76 Impact Factor
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    ABSTRACT: The development of superconducting fault current limiters (SFCL) are mainly based on coated conductors (CC) in spite of the low electric fields the CC can sustain due to their poor thermal behavior and the very low propagation velocities of the normal zone. Nowadays the SFCL demonstrators are made of coil but another way of building them is to use plates with a mean- dered line. Its main advantage is the increase of the propagation velocities of the normal zone thanks to the longitudinal and lateral diffusion of the heat. Indeed in a meander the heat generated by a local dissipative zone diffuses also laterally and switches the adjacent lines, increasing the apparent propagation velocity of the normal zone. There is also a possibility to increase the lateral propagation velocity of the heat by adding onto the Hastelloy substrate a thermal link (an Ag film for instance) between the lines without an electrical short circuit. These test structures were developed on CC allowing the simultaneous measurement of the propagation of a heat front in the substrate and in the Hastelloy/Ag bilayer. They are made of a heater line and several parallel superconducting strips where the propagation of the temperature front is recorded. We will discuss the benefit of this approach as compared to the standard coil geometry.
    IEEE Transactions on Applied Superconductivity 01/2011; 21(3):1213-1216. · 1.20 Impact Factor
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    ABSTRACT: A fundamental understanding of the quench phenomenon is crucial in the future design and operation of high temperature superconductors based fault current limiters. The key parameter that quantifies the quenching process in superconductors is the normal zone propagation (NZP) velocity, which is defined as the speed at which the normal zone expands into the superconducting volume. In the present paper, we used numerical models developed in our group recently to investigate the quench propagation in coated conductors. With our models, we have shown that the NZP in these tapes depends strongly on the substrate properties.
    Physica C Superconductivity 10/2009; · 0.72 Impact Factor
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    ABSTRACT: A fundamental understanding of the quench phenomenon is crucial in the future design and operation of HTS based Fault Current Limiters (FCLs). The key parameter that quantifies the quenching process in superconductors is the normal zone propagation (NZP) velocity, which is defined as the speed with at which the normal zone expands into the superconducting volume. Recent experimental measurements in YBCO tapes have shown that the NZP velocity in these materials is extremely slow in comparison with theoretical predictions. In the present paper, we compared experimental results with FEM models developed in our group recently. With our models, we have shown that the NZP of YBCO tapes depends strongly on the substrate properties.
    IEEE Transactions on Applied Superconductivity 07/2009; · 1.20 Impact Factor
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    ABSTRACT: The major drawback for the commercialization of fault current limiter (FCL) made of YBCO on sapphire is their expensive price. In the recent years, coated conductors (CC) have been extensively developed and, due to their lower prices, have been recently tested for current limitation application. One weakness of these CC is the very low electric fields they can sustain, typically below 1 V/cm as compared to 20-40 V/cm observed in YBCO films grown on sapphire. The limitation of this electric field in CC comes certainly from the very low propagation velocities of the dissipative state, a property which might be correlated with the poor thermal behavior of the architecture of these materials. Both the thermal conductivities of the Hastelloy substrate and of the conducting bilayer (superconducting DyBCO and Ag conducting layer) influence the thermal behavior of the CC and therefore have to be optimized to get the best performance. We have then investigated the thermal and electrical behavior and the propagation velocities in CC during constant current pulses above J<sub>c</sub> . The comparison with the results obtained on YBCO films grown on sapphire shows several differences. In CC, the flux flow resistivities are 2-3 orders of magnitude higher than in film grown on sapphire and quench propagation velocities are 2-3 orders of magnitude lower (of the order of cm/s). The propagation velocities in CC and in films on sapphire are analysed with a simple adiabatic model.
    IEEE Transactions on Applied Superconductivity 07/2009; · 1.20 Impact Factor
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    ABSTRACT: We report on the electrical and thermal characterisation of DyBa2Cu3O7 (DyBCO) coated conductors. Detailed I–V curves of these coated conductors are presented showing that the flux flow resistivity of these CC is 2–3 orders of magnitude higher than the one observed in HTS thin films grown onto sapphire substrates. This high flux flow resistivity, which might reflect the presence of grain boundaries, influences the switching behaviour of the superconducting line. The thermal characteristics of these coated conductors have been investigated on two aspects; the propagation velocity of the normal zone and the heating of the line. The propagation velocity is very low of the order of cm/s and a very fast increasing of the temperature of DyBCO switched line is observed. From all these measurements, the electrical and thermal behaviour of the coated conductors will be discussed.
    Physica C Superconductivity 01/2008; 468(21):2191-2196. · 0.72 Impact Factor
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    ABSTRACT: The adjunction of constrictions along the meander of a superconducting fault current limiter (FCL) greatly improves its behavior thanks to a better distribution of the dissipative zones at the occurrence of a short circuit. This design works perfectly for symmetrical short circuit (i.e. short circuit at the maximum voltage). However for asymmetrical short circuits (at voltages close to 0), we are facing a problem due to the small number of the initially switched constrictions. To solve this problem, we test the possibility to speed up the transition into the normal state of the whole meander by heating it locally. This thermally assisted transition is realized by growing a gold layer on the backside of the substrate and by patterning it into a meander with its dissipative parts lying just underneath the constrictions of the FCL. This gold meander can be either connected in parallel with the superconducting meander or a capacitor bank can supply the current. In order to confirm the benefit of the thermally assisted transition we have carefully measured the behavior of the FCL during constant current and low voltage pulses as a function of the power injected into the gold line. We present results showing that the response of the FCL to the generated heat is very fast; typically less than 100 mus. Furthermore the distribution of the dissipated power across the wafer, during asymmetrical AC short circuit, is clearly improved.
    IEEE Transactions on Applied Superconductivity 07/2007; · 1.20 Impact Factor
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    ABSTRACT: Tl-1223 and Tl-2223 superconducting films have been prepared using spray pyrolysis and MOCVD for the deposition of the precursor layer and a solid source of thallium for the subsequent thallination step. For the former phase, critical temperatures and critical current densities up to 113 K and 2.5 MA/cm2 (77 K, 0 T) have been obtained after 3 to 6 hours at 860-870°C. Pure Tl-2223 films are by far more difficult to prepare. This can be achieved by operating at 890-900°C or with a fluorine containing source. The resulting films have then poor superconducting properties due to the presence of holes. Working at 875°C yields multiphase films (Tl-2223 + Tl-2212 or Tl-2223 + Tl-1223) with however better properties
    Journal of Physics Conference Series 07/2006; 43(1):281.
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    ABSTRACT: To facilitate the up scaling of thin film based fault current limiters (FCL), a complete control of the localization of the dissipated power is needed. This control can be achieved by a new design where constrictions, which are regularly located along the YBCO/Au meander, have the aim of localizing and homogenizing this dissipated power at the beginning of a short circuit. To minimize the initial power peak, the resistivity of the constrictions has to be as small as possible whereas the resistivity of the connecting paths should be increased in order to lower the long-term power. We have tested these expected improvements on 5 kW (340 V, 16 A) FCL, on 2'' wafers, in AC conditions. The observed behavior of the FCL during short circuits, initiated at fault angles, confirm that the dissipated power is minimized and distributed, in a controlled way, along the meander. Finally we present measurements of the self-restoring time of the FCL and test on a 10 kW FCL made of two wafers connected in parallel.
    IEEE Transactions on Applied Superconductivity 07/2005; · 1.20 Impact Factor
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    ABSTRACT: We present a new design of an YBCO/Au meander which allows to control the distribution of the dissipated power on a fault current limiter (FCL) at the beginning of a fault. To achieve that, we create N constrictions regularly located along the meander. These constrictions increase the effective thermal propagation velocity of the normal zone by a factor 2N. At the beginning of a short circuit, we show that the meander is initially in a current source mode and therefore it has to sustain a power density proportional to its resistivity rho . At longer time, it operates in a voltage mode regime and the power density decreases with rho . Therefore it seems impossible to minimise both the initial and the long term dissipated power. However, this problem can be solved with our novel design by choosing the resistivity of the different parts of the meander in accordance with their working conditions (current or voltage regime). The resistivity of the constrictions is decreased by increasing the Au layer thickness, whereas the resistivity of the rest of the meander can be increased as much as possible. We present results on a 5 kW FCL which show the benefits of both the constrictions and their adapted resistivity.
    01/2004;
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    ABSTRACT: The elaboration of Tl-2223 films by a two step process has been investigated. The films contain both Tl-2212 and Tl-2223 phases. We find that the kinetics for the Tl-2212 --> Tl-2223 transformation is very slow due to the texture of the Tl-2212 phase. Critical current densities up to 1.3 MA/cm2 (self-field, 77 K) are however measured. Pure Tl-2223 is obtained when the films are thallinated with a fluorinated source of thallium.
    Journal de Physique IV (Proceedings) 01/2004; 122:117-122. · 0.29 Impact Factor