We report levitation force cycles resulting from measurements carried out on a YBCO and a MgB2 disk cooled down in the field of a permanent magnet. In both cases the amplitude of the levitation force tends toward maximum values as the temperature decreases. Otherwise, the cycles are almost closed at low temperature and strongly hysteretic in the high temperature range. The hysteresis of the force cycles is attributed to the distribution of the currents induced in the sample by the field of the magnet. The saturation of the levitation forces at low temperature is related to that of the magnetic moment of the disks. We show that this type of measurement allows for the determination of the critical current density of superconductors in a restricted domain of temperatures.

Applications of bulk superconductors concern superconducting motors and generators, the levitation of vehicles, the generation of high magnetic fields with small size cryo-magnets, the shielding of magnetic fields and other applications. For all of them, it is essential to determine the critical current density, and to understand the effect of the shape and size of the bulks on the properties of interest. In this contribution, we show how the combination of levitation force and trapped field measurements allow one to determine the characteristics and the potential performances of superconducting disks using analytical modeling. As examples of applications we detail the effects of the magnetizing field and of the bulk sheet critical current density on the levitation force. An important result of the reported measurements is that in field-cooled samples, the shielding currents possibly do not flow along the whole thickness of the disks.

Cette étude emploie la théorie de la cointégration d’Engels et Granger pour analyser les relations entre développement touristique, croissance économique et réduction de la pauvreté en Haïti. Elle permet de tirer des enseignements et déduire des implications sur les liens de causalité entre ces trois variables. Les résultats montrent que ces dernières sont cointégrées et qu’au seuil de 5%, les relations, d’une part entre croissance économique et développement touristique sont unidirectionnelles, c'est-à-dire que la croissance économique engendre du développement touristique en Haïti et non l’inverse.  D’autre part, il existe également une relation unidirectionnelle entre réduction de la pauvreté et développement touristique. Autrement dit, la réduction de la pauvreté qui, souvent mais pas toujours dépend  de la croissance économique se traduirait ici par du développement touristique et non l’inverse.
This study, using cointegration and causality tests, investigates the relationship among tourism development, economic expansion, and poverty reduction in Haïti. Empirical studies have shown that the three variables are cointegrated with a long-run stable relationship among the three. At 5% level, the causality tests suggest one-way Granger causal relation between economic growth and tourism development, and between poverty reduction and tourism development. On the other hand, the paper presents arguments in support of the proposition that poverty reduction as a source of tourism development, offers a convincing case for the use of policy instruments focused to drive a tourism-based economy

The Bean model considers that in field cooled superconducting cylinders with diameter R, the currents flow over all the thickness of the superconductor along circular paths, the minimum radius of which depends on the magnetizing field and the critical current density. A combination of trapped field and levitation force measurements reported recently has shown, however, that in YBCO and MgB 2 disks the current flows in fact in a restricted region with thickness t of the superconductor. In this contribution, from measurements carried out on two YBCO and two MgB 2 disks, we report the dependence on temperature of t and J p , the current density in this region, as well as that of the field trapped by the samples. The results confirm that t decreases as the temperature decreases. This behaviour is ascribed to the conservation of the magnetic energy stored in the superconductor, which depends on the magnetizing source and not on the measurement temperature. As a consequence, t behaves as -J , p 2 3 while the field trapped along the axis of the cylinder behaves as J . p 1 3 These claims are substantiated by the experimental results. The possibility that J p is equal to the depairing current is investigated.

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We present preliminary results suggesting that MgB 2 is a superconductor that could be used in Maglev vehicles. For this purpose, we compare the levitation capability of MgB 2 samples to those of YBCO bulks.

International audienceThe goal of this text is to explore the role played by the « searching-subject » in the productionof knowledge. In geography, such a reflexion brings up two intermingled questions: one is thenature fieldwork, as a source of information and of experience thus – of practices thereof ; theother is reflexivity, its nature and how it enlights the fieldwork and the researcher’spositionning. Auto-ethnography is one sure way to deal with both. I focus here on what it is as amethod and, more importantly, its narration. The fieldwork and geographic experience I amrefering to is mine, that is North America, where I am from and through which I still tend to seethe world.Ce texte se penche sur le rôle du chercheur dans la démarche de production du savoir. Engéographie, la réflexion sur ce sujet passe d’abord par un retour sur le terrain, comme sourced’information et d’expérience, et donc de pratiques, puis par la prise en compte de la réflexivité,ce qu’elle est, mais surtout ce qu’elle éclaire du terrain et du positionnement du chercheur.L’auto-ethnographie permet justement de conjuguer le deux. Ce texte focalise sur la nature et lesmodalités du récit auto-ethnographique. L’expérience du terrain dont il est question et le récitauto-ethnographique que j’en fais, renvoient ici à l’Amérique du Nord, mon espace d’origine etl’espace privilégié de mon regard de géographe