Simulation of transient energy distributions in sub-ns streamer formation

Breakdown and streamer formation is simulated in atmospheric pressure nitrogen for a 2D planar electrode system. A PIC code with multigrid potential solver is used to simulate the evolution of the non-equilibrium ionization front on sub-nanosecond timescales. The ion and electron energy distributions are computed, accounting for the inclusion of inelastic scattering of electrons, and collisionally excited metastable production and ionization. Of particular interest is the increased production of metastable and low-energy ions and electrons when the applied field is reversed during the progress of the ionization front, giving insight into the improved species yields in nanosecond pulsed systems

Development of 9 wt.% Cr steels for next generation power plant

There is a need to increase the thermal efficiency of coal fired power plants. High chromium ferritic steels have historically been used to manufacture steam pipes, tube and headers. Over the last forty years there has been a continuous development of the 9 – 12 wt. % chromium martensitic alloys which has allowed the service temperature to be increased from 510°C – 550°C1. There is now a high demand for Ultra Super Critical coal fired power plants which operate at 650°C and hence have an increased thermal efficiency. The aim of this research project was to develop a MarBN steel (Martensitic Steel Strengthened with Boron and Nitrides) with an optimised composition in order to allow MarBN to be used at 650°C. In order for MarBN to be used within high temperature applications it is vital to understand how microstructural changes can have an impact on their creep strength. This is a key area that the present research has focused upon. The long term creep strength of 9 wt. % chromium ferritic steels is derived from the stability of the microstructure over its service life, which has a direct relationship to the stability of the precipitates and the lath structure within the microstructure

An empirical study of corporate bond pricing with unobserved capital structure dynamics.

This work empirically examines six structural models of the term structure of credit risk spreads: Merton (1974), Longstaff & Schwartz (1995) (with and without stochastic interest rates), Leland & Toft (1996), Collin-Dufresne & Goldstein (2001), and a constant elasticity of variance model. The conventional approach to testing structural models has involved the use of observable data to proxy the latent capital structure process, which may introduce additional specification error. This study extends Jones, Mason & Rosenfeld (1983) and Eom, Helwege & Huang (2004) by using implicit estimation of key model parameters resulting in an improved level of model fit. Unlike prior studies, the models are fitted from the observed dynamic term structure of firm-specific credit spreads, thereby providing a pure test of model specification. The models are implemented by adapting the method of Duffee (1999) to structural credit models, thereby treating the capital structure process is truly latent, and simultaneously enforcing cross- sectional and time-series model constraints. Quasi-maximum likelihood parameter estimates of the capital structure process are obtained via the extended Kalman filter applied to actual market trade prices on 32 firms and 200 bonds for the period 1994 to 2000. We find that including an allowance for time-variation in the market liquidity premium improves model specification. A simple extension of the Merton (1974) model is found to have the greatest prediction accuracy, although all models performed with similar prediction errors. At between 28.8 to 34.4 percent, the root mean squared error of the credit spread prediction is comparable with reduced-form models. Unlike Eom, Helwege & Huang (2004) we do not find a wide dispersion in model prediction errors, as evidenced by an across model average mean absolute percentage error of 22 percent. However, in support of prior studies we find an overall tendency for slight underprediction, with the mean percentage prediction error of between -6.2 and -8.7 percent. Underprediction is greatest with short remaining bond tenor and low rating. Credit spread prediction errors across all models are non-normal, and fatter tailed than expected, with autocorrelation evident in their time series. More complex models did not outperform the extended Merton (1974) model; in particular stochastic interest-rate and early default accompanied by an exogenous write- down rate appear to add little to model accuracy. However, the inclusion of solvency ratio mean-reversion in the Collin-Dufresne & Goldstein (2001) model results in the most realistic latent solvency dynamics as measured by its implied levels of asset volatility, default boundary level, and mean-reversion rate. The extended Merton (1974) is found to imply asset volatility levels that are too high on average when compared to observed firm equity volatility. We find that the extended Merton (1974) and the Collin-Dufresne & Goldstein (2001) models account for approximately 43 percent of the credit spread on average. For BB rated trades, the explained proportion rises to 55 to 60 percent. For investment grade trades, our results suggest that the amount of the credit spread that is default related is approximately double the previous estimate of Huang & Huang (2003). Finally, we find evidence that the prediction errors are related to market-wide factors exogenous to the models.Credit risk, bond pricing, Kalman Filter, Merton model, credit spreads, structural credit models

X - Ray Flares and Their Connection With Prompt Emission in GRBs

We use a wavelet technique to investigate the time variations in the light curves from a sample of GRBs detected by Fermi and Swift. We focus primarily on the behavior of the flaring region of Swift-XRT light curves in order to explore connections between variability time scales and pulse parameters (such as rise and decay times, widths, strengths, and separation distributions) and spectral lags. Tight correlations between some of these temporal features suggest a common origin for the production of X-ray flares and the prompt emission.Comment: 7th Huntsville Gamma-Ray Burst Symposium, GRB 2013: paper 15 in eConf Proceedings C130414

The Hurst Exponent of Fermi GRBs

Using a wavelet decomposition technique, we have extracted the Hurst exponent for a sample of 46 long and 22 short Gamma-ray bursts (GRBs) detected by the Gamma-ray Burst Monitor (GBM) aboard the Fermi satellite. This exponent is a scaling parameter that provides a measure of long-range behavior in a time series. The mean Hurst exponent for the short GRBs is significantly smaller than that for the long GRBs. The separation may serve as an unbiased criterion for distinguishing short and long GRBs.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Societ

Cylindrical, periodic surface lattice — theory, dispersion analysis, and experiment

A two-dimensional surface lattice of cylindrical topology obtained via perturbing the inner surface of a cylinder is considered. Periodic perturbations of the surface lead to observation of high-impedance, dielectric-like media and resonant coupling of surface and non-propagating volume fields. This allows synthesis of tailored-for-purpose "coating" material with dispersion suitable, for instance, to mediate a Cherenkov type interaction. An analytical model of the lattice is discussed and coupled-wave equations are derived. Variations of the lattice dispersive properties with variation of parameters are shown, illustrating the tailoring of the structure's electromagnetic properties. Experimental results are presented showing agreement with the theoretical model