Jakob Rhyner

Magnetic properties and AC-losses of superconductors with power law current—voltage characteristics

Abstract In many high-Tc superconductors the critical current density jc is an ill-defined quantity due to the smooth current—voltage characteristic. Since jc is the basic parameter entering the critical state model, its application to such materials becomes problematic. In this paper, a theory of magnetic properties and AC-losses in superconductors with smooth current—voltage characteristics is proposed. It is applied to superconductors with a power law characteristic, E ≈ jα. The AC-losses are calculated analytically; simple scaling rules are obtained for their dependence on the frequency and the field amplitude. Moreover, it is shown that the normal ohmic conductor and the “perfect” type-II superconductor (critical state) emerge as limiting cases, α = 1 and α = ∞, from the theory.

Equivalence of the generalised grid and projection methods for the construction of quasiperiodic tilings

The two main techniques for the generation of quasiperiodic tilings, de Bruijns grid method (1981), and the projection formalism, are generalised. A very broad class of quasiperiodic tilings is obtained in this way. The two generalised methods are shown to be equivalent. The standard calculation of Fourier spectra is extended to the whole general class of tilings. Various applications are discussed.

Dealing with the White Death: Avalanche Risk Management for Traffic Routes

This article discusses mitigation strategies to protect traffic routes from snow avalanches. Up to now, mitigation of snow avalanches on many roads and railways in the Alps has relied on avalanche sheds, which require large initial investments resulting in high opportunity costs. Therefore, avalanche risk managers have increasingly adopted organizational mitigation measures such as warning systems and closure policies instead. The effectiveness of these measures is, however, greatly dependent on human decisions. In this article, we present a method for optimizing avalanche mitigation for traffic routes in terms of both their risk reduction impact and their net benefit to society. First, we introduce a generic framework for assessing avalanche risk and for quantifying the impact of mitigation. This allows for sound cost-benefit comparisons between alternative mitigation strategies. Second, we illustrate the framework with a case study from Switzerland. Our findings suggest that site-specific characteristics of avalanche paths, as well as the economic importance of a traffic route, are decisive for the choice of optimal mitigation strategies. On routes endangered by few avalanche paths with frequent avalanche occurrences, structural measures are most efficient, whereas reliance on organizational mitigation is often the most appropriate strategy on routes endangered by many paths with infrequent or fuzzy avalanche risk. Finally, keeping a traffic route open may be very important for tourism or the transport industry. Hence, local economic value may promote the use of a hybrid strategy that combines organizational and structural measures to optimize the resource allocation of avalanche risk mitigation.

Nonlinear dielectric composites

Polymer composites with high dielectric constant are widely used as shielding and field grading materials. An improvement of the refractive grading can be expected for a dielectric constant being a nonlinear function of the electric field or the temperature, as known for nonlinear resistive field grading. An increase of the dielectric constant in those areas where the highest electrical field occur will result in a homogenization of the field distribution. Such composite materials can be considered as having the smart functions of sensing and actuating. The influence of nonlinear properties of filler material on the resulting dielectric properties has been studied both theoretically and experimentally. Calculations using effective medium theory show how much of the nonlinearity of the filler is transferred to the composite. They are compared with experiments on composites containing ferroelectric, semiconducting and varistor-type filler material in a thermoset or thermoplastic matrix. Depending on the filler type, the dielectric constant increases by a factor of up to three, for example, on raising the temperature from 30 degrees C to 150 degrees C. Such an enhancement can be sufficient to rearrange the field distribution in stressed insulating parts.

Vector potential theory of ac losses in superconductors

Abstract A novel scheme for calculation of ac losses in superconductors is described. It is based on a formulation of the loses in terms of the vector potential. The method is a generalisation of ideas put forward by W.T. Norris in the 70s. In contrast to the widely used conformal mapping methods, the vector potential approach is not restricted to (infinitely) thin sheets, but allows for the analytical calculation of ac losses in conductors with arbitrary cross-section. It is restricted to situations where a so-called kernel or electric centre exists, i.e. a region or line inside the superconductor along which the electric field vanishes throughout the ac cycle. Fortunately many theoretically and practically important problems are of this type. Analytical expressions are derived for a series of problems involving rectangular superconductors, single conductors as well as assemblies of several conductors. Explicit solutions are obtained for the back - conduction for the reduction of ac losses and for gap losses occurring in transformer windings and cables. The formalism can be easily applied to other than rectangular, e.g. circular or more complicated cross-sections.

Quasiperiodic tilings: a generalized grid-projection method

In 1988 we discovered generalized grid--projection method. Since then the method proved to be useful for description of symmetries of quasicrystals also for analysis of interacting spins.

Making SDGs work for climate change hotspots

The impacts of climate change on peoples livelihoods have been widely documented. It is expected that climate and environmental change will hamper poverty reduction, or even exacerbate poverty in some or all of its dimensions. Changes in the biophysical environment, such as droughts, flooding, water quantity and quality, and degrading ecosystems, are expected to affect opportunities for people to generate income. These changes, combined with a deficiency in coping strategies and innovation to adapt to particular climate change threats, are in turn likely to lead to increased economic and social vulnerability of households and communities, especially amongst the poorest.

Analysis of the low field critical state in ceramic high-Tc superconductors

Abstract Results of magnetization measurements on polycrystalline YBa2Cu3O7−δ samples at very low fields (below 10 Oe) are presented. Using the critical state model we are able to deduce the “true” zero field critical current density which in transport measurements is masked by self-field effects. Our method is useful to test the material quality obtained by different sintering methods. Our analysis does not rely on a particular form of the critical state model; we rather show that the various versions used in the literature are in close quantitative agreement in the low field limit.

Development and validation of risk profiles of West African rural communities facing multiple natural hazards

West Africa has been described as a hotspot of climate change. The reliance on rain-fed agriculture by over 65% of the population means that vulnerability to climatic hazards such as droughts, rainstorms and floods will continue. Yet, the vulnerability and risk levels faced by different rural social-ecological systems (SES) affected by multiple hazards are poorly understood. To fill this gap, this study quantifies risk and vulnerability of rural communities to drought and floods. Risk is assessed using an indicator-based approach. A stepwise methodology is followed that combines participatory approaches with statistical, remote sensing and Geographic Information System techniques to develop community level vulnerability indices in three watersheds (Dano, Burkina Faso; Dassari, Benin; Vea, Ghana). The results show varying levels of risk profiles across the three watersheds. Statistically significant high levels of mean risk in the Dano area of Burkina Faso are found whilst communities in the Dassari area of Benin show low mean risk. The high risk in the Dano area results from, among other factors, underlying high exposure to droughts and rainstorms, longer dry season duration, low caloric intake per capita, and poor local institutions. The study introduces the concept of community impact score (CIS) to validate the indicator-based risk and vulnerability modelling. The CIS measures the cumulative impact of the occurrence of multiple hazards over five years. 65.3% of the variance in observed impact of hazards/CIS was explained by the risk models and communities with high simulated disaster risk generally follow areas with high observed disaster impacts. Results from this study will help disaster managers to better understand disaster risk and develop appropriate, inclusive and well integrated mitigation and adaptation plans at the local level. It fulfills the increasing need to balance global/regional assessments with community level assessments where major decisions against risk are actually taken and implemented.

Limiting interface model for strongly coupled oxide ceramics

Abstract The transport properties and the nature of the breakdown of superconductivity in oxide ceramics with “strong” intergrain coupling are discussed. As an alternative to the XY -model which is not applicable to these materials, the “limiting interface” model is proposed. An extension of the limiting interface model allows for the calculation of the current-voltage characteristic which is expected to contain interesting information on the spatial structure of the superconducting-to-normal transition.