KINETICS OF RECOMBINATION PROCESSES

In order to solve nonlinear kinetic equations for recombination reactions when the production of participating species is driven externally at a time-dependent rate, the authors establish a framework based on the correspondence between these equations and a Schrodinger-type equation in which the product of the recombination constant with the production rate plays the role of the potential. The required solutions of the kinetic equations can therefore be obtained by solving the equivalent Schrodinger-type equation directly, or by identifying an appropriately chosen time-dependent function as a particular integral of this equation and deducing the production rates with periodic as well as aperiodic time dependences. The approach is seen to reveal a variety of solutions which can be exploited to assist identification of the mechanisms operating and to aid extraction of rate constants in conjunction with the experiment

Finite-element simulations of hysteretic ac losses in a magnetically coated superconducting tubular wire subject to an oscillating transverse magnetic field

Numerical simulations of hysteretic ac losses in a tubular superconductor/paramagnet heterostructure subject to an oscillating transverse magnetic field are performed within the quasistatic approach, calling upon the COMSOL finite-element software package and exploiting magnetostatic-electrostatic analogues. It is shown that one-sided magnetic shielding of a thin, type-II superconducting tube by a coaxial paramagnetic support results in a slight increase of hysteretic ac losses as compared to those for a vacuum environment, when the support is placed inside; a spectacular shielding effect with a possible reduction of hysteretic ac losses by orders of magnitude, however, ensues, depending on the magnetic permeability and the amplitude of the applied magnetic field, when the support is placed outside.Comment: 7 pages, 4 figure

Magnetic-dipole induced appearance of vortices in a bilayered superconductor/soft-magnet heterostructure

The penetration of the magnetic field of an infinitesimal magnetic dipole into a bilayered type-II superconductor/soft-magnet heterostructure is studied on the basis of the classical London approach. The critical values of the dipole moment for the first appearance of a single magnetic vortex and, respectively, a magnetic vortex-antivortex pair in the superconductor constituent are obtained, when the magnetic dipole faces the superconductor or the soft-magnet constituent. This reveals that the soft-magnet constituent inhibits penetration of vortices into the superconductor constituent, when the dipole faces the soft-magnet constituent.Comment: 2 pages, 1 figure; accepted in Physica C for the special issue of Preceedings of the 8th Int. Conference on Materials and Mechanisms of Superconductivity and High Temperature Superconductors (M2S-HTSC), Dresden, Germany, July 9-14, 200

Superconductor strip in a closed magnetic environment: exact analytic representation of the critical state

An exact analytic representation of the critical state of a current-carrying type-II superconductor strip located inside a cylindrical magnetic cavity of high permeability is derived. The obtained results show that, when the cavity radius is small, penetration of magnetic flux fronts is strongly reduced as compared to the situation in an isolated strip. From our generic representation it is possible to establish current profiles in closed cavities of various other geometries too by means of conformal mapping of the basic configuration addressed.Comment: 2 pages, 1 figure; accepted in Physica C for the Proceedings of M2S-HTSC, Dresden, 200

Reading between the lines: Prediction of political violence using newspaper text

This article provides a new methodology to predict armed conflict by using newspaper text. Through machine learning, vast quantities of newspaper text are reduced to interpretable topics. These topics are then used in panel regressions to predict the onset of conflict. We propose the use of the within-country variation of these topics to predict the timing of conflict. This allows us to avoid the tendency of predicting conflict only in countries where it occurred before. We show that the within-country variation of topics is a good predictor of conflict and becomes particularly useful when risk in previously peaceful countries arises. Two aspects seem to be responsible for these features. Topics provide depth because they consist of changing, long lists of terms that make them able to capture the changing context of conflict. At the same time, topics provide width because they are summaries of the full text, including stabilizing factors.</jats:p

The Hard Problem of Prediction for Conflict Prevention

There is a growing interest in prevention in several policy areas and this provides a strong motivation for an improved integration of forecasting with machine learning into models of decision making. In this article we propose a framework to tackle conflict prevention. A key problem of conflict forecasting for prevention is that predicting the start of conflict in previously peaceful countries needs to overcome a low baseline risk. To make progress in this hard problem this project combines a newspaper-text corpus of more than 4 million articles with unsupervised and supervised machine learning. The output of the forecast model is then integrated into a simple static framework in which a decision maker decides on the optimal number of interventions to minimize the total cost of conflict and intervention. This exercise highlights the potential cost savings of prevention for which reliable forecasts are a prerequisite

Financial Valuation of PBGC Insurance with Market-Implied Default Probabilities

In this paper, we use financial valuation techniques to measure the unfunded liabilities associated with the Pension Benefit Guaranty Corporation (PBGC) single-employer pension insurance program. This is an alternative approach to the calculations of expected future PBGC payouts in the PBGC exposure reports. The PBGC insurance is akin to an exchange option, a financial instrument that allows a party to exchange one risky asset for another. Calculating the value of this option for each PBGC-covered plan provides a measure of the fair market price of the PBGC guarantee that is consistent with the finance principles of risk-neutral pricing. That is, the market valuation method reflects the fact that bad outcomes tend to coincide with times when losses are particularly painful. The valuation we perform also reflects the fact that PBGC insurance is triggered only in the case of bankruptcy by drawing on the default probabilities implied by the credit ratings of insured plans. Under the baseline parameters, the PBGC’s insurance of the unfunded liabilities has a financial value of $358 billion, net of the estimated present value of PBGC premiums