Superconducting screening on different length scales in high-quality bulk MgB2 superconductor

High quality bulk MgB2 exhibit a structure of voids and agglomeration of crystals on different length-scales. Because of this, the superconducting currents percolate between the voids in the ensuing structure. Magnetic measurements reveal that the superconducting currents circulate on at least three different length-scales, of ~1 micrometre, ~10 micrometre and whole of the sample (~millimetre). Each of these screenings contributes to the measured irreversible magnetic moment (Dm). The analysis of the field dependence of Dm for samples of subsequently decreasing size showed that the critical current obtained using the simple critical state model is erroneous. This leads to the artefact of the sample size-dependent critical current and irreversibility field. Our data analysis enables the separation of the contribution of each of the screening currents to Dm. The field dependence of each of the currents follows a stretched exponential form. The currents flowing around whole of the sample give a dominant contribution to Dm in the intermediate fields (1T < H < 4T at 20K) and they can be used to obtain the value of Jc from critical state model, which corresponds to the transport Jc

Interaction between superconductor and ferromagnetic domains in iron sheath: peak effect in MgB2/Fe wires

Interaction between the superconductor and ferromagnet in MgB2/Fe wires results in either a plateau or a peak effect in the field dependence of transport critical current, Ic(H). This is in addition to magnetic shielding of external field. Current theoretical models cannot account for the observed peak effect in Ic(H). This paper shows that the theoretical explanation of the peak effect should be sought in terms of interaction between superconductor and magnetic domain structure, obtained after re-magnetization of the iron sheath by the self-field of the current. There is a minimum value of critical current, below which the re-magnetization of the iron sheath and peak effect in Ic(H) are not observed

Improvement of critical current in MgB2/Fe wires by a ferromagnetic sheath

Transport critical current (Ic) was measured for Fe-sheathed MgB2 round wires. A critical current density of 5.3 x 10^4 A/cm^2 was obtained at 32K. Strong magnetic shielding by the iron sheath was observed, resulting in a decrease in Ic by only 15% in a field of 0.6T at 32K. In addition to shielding, interaction between the iron sheath and the superconductor resulted in a constant Ic between 0.2 and 0.6T. This was well beyond the maximum field for effective shielding of 0.2T. This effect can be used to substantially improve the field performance of MgB2/Fe wires at fields at least 3 times higher than the range allowed by mere magnetic shielding by the iron sheath. The dependence of Ic on the angle between field and current showed that the transport current does not flow straight across the wire, but meanders between the grains

Decision Making under Uncertainty and Competition for Sustainable Energy Technologies

This dissertation addresses the main challenges faced in the transition to a more sustainable energy sector by applying modelling tools that could design more effective managerial responses and provide policy insights. To mitigate the impact of climate change, the electric power industry needs to reduce markedly its emissions of greenhouse gases. As energy consumption is set to increase in the foreseeable future, this can be achieved only through costly investments in more efficient conventional generation or in renewable energy resources. While more energy-efficient technologies are commercially available, the deregulation of most electricity industries implies that investment decisions need to be taken by private investors with government involvement limited to setting policy measures or designing market rules. Thus, it is desirable to understand how investment and operational decisions are to be made by decentralised entities that face uncertainty and competition. One of the most efficient thermal power technologies is cogeneration, or combined heat and power (CHP), which can recover heat that otherwise would be discarded from conventional generation. Cogeneration is particularly efficient when the recovered heat can be used in the vicinity of the combustion engine. Although governments are supporting on-site CHP generation through feed-in tariffs and favourable grid access, the adoption of small-scale electricity generation has been hindered by uncertain electricity and gas prices. While deterministic and real options studies have revealed distributed generation to be both economical and effective at reducing CO2 emissions, these analyses have not addressed the aspect of risk management. In order to overcome the barriers of financial uncertainties to investment, it is imperative to address the decision-making problems of a risk-averse energy consumer. Towards that end, we develop a multi-stage, stochastic mean-risk optimisation model for the long-term and medium-term risk management problems of a large consumer. We first show that installing a CHP unit not only results in both lower CO2 emissions and expected running cost but also leads to lower risk exposure. In essence, by investing in a CHP unit, a large consumer obtains the option to use on-site generation whenever the electricity price peaks, thereby reducing significantly its financial risk over the investment period. To provide further insights into risk management strategies with on-site generation, we examine also the medium-term operational problem of a large consumer. In this model, we include all available contracts from electricity and gas futures markets, and analyse their interactions with on-site generation. We conclude that by swapping the volatile electricity spot price for the less volatile gas spot price, on-site generation with CHP can lead to lower risk exposure even in the medium term, and it alters a risk-averse consumer’s demand for futures contracts. While extensive subsidies have triggered investments in renewable generation, these installations need to be accompanied by transmission expansion. The reason for this is that solar and wind energy output is intermittent, and attractive solar and wind sites are often located far away from demand centres. Thus, to integrate renewable generation into the grid system and to maintain a reliable and secure electricity supply, a vastly improved transmission network is crucial. Finding the optimal transmission line investments for a given network is already a very complex task since these decisions need to take into account future demand and generation configurations, too, which now depend on private investors. To address these concerns, our third study models the problem of wind energy investment and transmission expansion jointly through a stochastic bi-level programming model under different market designs for transmission line investment. This enables the game-theoretic interaction between distinct decision makers, i.e., those investing in power plants and those constructing transmission lines, to be addressed directly. We find that under perfect competition only one of the wind power producers, the one with lower capital cost, makes investment and to a lower degree under a profit-maximising merchant investor (MI) than under a welfare-maximising transmission system operator (TSO), as the MI reduces the transmission capacity to increase congestion rent. In addition, we note that regardless of whether the grid expansion is carried out by the TSO or by the MI, a higher proportion of wind energy is installed when power producers exercise market power. In effect, strategic withholding of generation capacity by producers prompts more transmission investment since the TSO aims to increase welfare by subsidising wind and the MI creates more flow to maximise profit. Under perfect competition, a higher level of wind generation can be achieved only through mandating renewable portfolio standards (RPS), which in turn results also in increased transmission investment

On finite--temperature and --density radiative corrections to the neutrino effective potential in the early Universe

Finite-temperature and -density radiative corrections to the neutrino effective potential in the otherwise CP-symmetric early Universe are considered in the real-time approach of Thermal Field Theory. A consistent perturbation theory endowed with the hard thermal loop resummation techniques developed by Braaten and Pisarski is applied. Special attention is focused on the question whether such corrections can generate any nonzero contribution to the CP-symmetric part of the neutrino potential, if the contact approximation for the W-propagator is used.Comment: 11 pages, revtex styl

Uni-directional transport properties of a serpent billiard

We present a dynamical analysis of a classical billiard chain -- a channel with parallel semi-circular walls, which can serve as a model for a bended optical fiber. An interesting feature of this model is the fact that the phase space separates into two disjoint invariant components corresponding to the left and right uni-directional motions. Dynamics is decomposed into the jump map -- a Poincare map between the two ends of a basic cell, and the time function -- traveling time across a basic cell of a point on a surface of section. The jump map has a mixed phase space where the relative sizes of the regular and chaotic components depend on the width of the channel. For a suitable value of this parameter we can have almost fully chaotic phase space. We have studied numerically the Lyapunov exponents, time auto-correlation functions and diffusion of particles along the chain. As a result of a singularity of the time function we obtain marginally-normal diffusion after we subtract the average drift. The last result is also supported by some analytical arguments.Comment: 15 pages, 9 figure (19 .(e)ps files

Egorov property in perturbed cat map

We study the time evolution of the quantum-classical correspondence (QCC) for the well known model of quantised perturbed cat maps on the torus in the very specific regime of semi-classically small perturbations. The quality of the QCC is measured by the overlap of classical phase-space density and corresponding Wigner function of the quantum system called quantum-classical fidelity (QCF). In the analysed regime the QCF strongly deviates from the known general behaviour in particular it decays faster then exponential. Here we study and explain the observed behavior of the QCF and the apparent violation of the QCC principle.Comment: 12 pages, 7 figure