Optical preparation and measurement of atomic coherence at gigahertz bandwidth

We detail a method for the preparation of atomic coherence in a high density atomic medium, utilising a coherent preparation scheme of gigahertz bandwidth pulses. A numerical simulation of the preparation scheme is developed, and its efficiency in preparing coherent states is found to be close to unity at the entrance to the medium. The coherence is then measured non-invasively with a probe field.Comment: 7 pages, 5 figure

Large eddy simulation of subsurface phytoplankton dynamics: an optimum condition for chlorophyll patchiness induced by Langmuir circulations

Phytoplankton patchiness occurs on a plethora of spatial and temporal scales which can be extremely patchy in both horizontal and vertical directions. This patchiness directly affects the dynamics of the overall bloom, so understanding the mechanisms for patchiness to occur on each scale is therefore integral to the understanding of plankton bloom dynamics as a whole. This modelling study aims to introduce a mechanism for patch formations which has had very little exposure, but is ubiquitous to the oceanic mixed layer – patchiness induced by the interaction between the nutrient upwelling and Langmuir circulations. By combining a Large-Eddy Simulation which resolves Langmuir circulations, with a Nutrient-Phytoplankton-Zooplankton biological model, one can examine the horizontal and vertical patchiness which results from a flux of nutrients into the bottom of the mixed layer. Here, phytoplankton form significant horizontal patchiness in a depth interval where vertical currents from Langmuir cells are apparent and turbulent mixing is not; this comprises the lower region of the surface mixed layer. Aggregations have frequently been observed in lower regions of the surface mixed layer and have been attributed to the high nutrient flux associated with the pycnocline. This modelling study also shows patches occurring in this region and it is hypothesised that Langmuir cells are a catalyst for patchiness. The results clearly demonstrate that for certain levels of wind forcing, which are strong enough to introduce turbulent mixing only to the upper part of the mixed layer whilst inducing deeper Langmuir circulation, patchiness is greatly enhanced

An experimental approach to analysing rain droplet impingement on wind turbine blade materials

Leading edge erosion of wind turbine blades is an important issue within the industry and has been found to have a substantial impact on the annual energy output of generators. This forces operators to make blade repair a necessity, adding to the operation and maintenance costs of a project. A wind turbine’s tip speed can in some cases have an upper limit based on the erosion exhibited on the leading edge. This paper explores the variables of rainfall rate and impact velocity of the impinging droplets in an attempt to explore the recovery time of the tri-axial composite material used. It is shown that an increase in impact velocity results in a higher mass loss than an increase in the rainflow rate. Analysis using a scanning electron microscope reveals that pin holes in the laminate surface are exploited by the droplets, acting as initiation point for erosion of the composite. Overall the results suggest that the tip speed of the wind turbine blade is of greater importance when compared to the relevant rainfall conditions as to where the wind turbine is situated

How accurate are forecasts of costs of energy? A methodological contribution

Forecasts of the cost of energy are typically presented as point estimates; however forecasts are seldom accurate, which makes it important to understand the uncertainty around these point estimates. The scale of the differences between forecasts and outturns (i.e. contemporary estimates) of costs may have important implications for government decisions on the appropriate form (and level) of support, modelling energy scenarios or industry investment appraisal. This paper proposes a methodology to assess the accuracy of cost forecasts. We apply this to levelised costs of energy for different generation technologies due to the availability of comparable forecasts and contemporary estimates, however the same methodology could be applied to the components of levelised costs, such as capital costs. The estimated “forecast errors” capture the accuracy of previous forecasts and can provide objective bounds to the range around current forecasts for such costs. The results from applying this method are illustrated using publicly available data for on- and off-shore wind, Nuclear and CCGT technologies, revealing the possible scale of “forecast errors” for these technologies

Comparison of Muscle Activation During an Overhead Pres: Kettlebell v. Dumbbell

Please refer to the pdf version of the abstract located adjacent to the title

Magneto-x-ray effects in transition-metal alloys

We present a theory that combines the relativistic spin-polarized version of the Koringa-Kohn-Rostoker coherent-potential approximation theory and the macroscopic theory of magneto-optical effects enabling us to calculate magneto-x-ray effects from first principles. The theory is illustrated by calculation of Faraday and Kerr rotations and ellipticities for transition-metal alloys

Enhanced frequency up-conversion in Rb vapor

We demonstrate highly efficient generation of coherent 420nm light via up-conversion of near-infrared lasers in a hot rubidium vapor cell. By optimizing pump polarizations and frequencies we achieve a single-pass conversion efficiency of 260% per Watt, significantly higher than in previous experiments. A full exploration of the coherent light generation and fluorescence as a function of both pump frequencies reveals that coherent blue light is generated close to 85Rb two-photon resonances, as predicted by theory, but at high vapor pressure is suppressed in spectral regions that do not support phase matching or exhibit single-photon Kerr refraction. Favorable scaling of our current 1mW blue beam power with additional pump power is predicted.Comment: 6 pages, 4 figures. Modified to include referees' improvement

Wavelength Tunability of Ion-bombardment Induced Ripples on Sapphire

A study of ripple formation on sapphire surfaces by 300-2000 eV Ar+ ion bombardment is presented. Surface characterization by in-situ synchrotron grazing incidence small angle x-ray scattering and ex-situ atomic force microscopy is performed in order to study the wavelength of ripples formed on sapphire (0001) surfaces. We find that the wavelength can be varied over a remarkably wide range-nearly two orders of magnitude-by changing the ion incidence angle. Within the linear theory regime, the ion induced viscous flow smoothing mechanism explains the general trends of the ripple wavelength at low temperature and incidence angles larger than 30. In this model, relaxation is confined to a few-nm thick damaged surface layer. The behavior at high temperature suggests relaxation by surface diffusion. However, strong smoothing is inferred from the observed ripple wavelength near normal incidence, which is not consistent with either surface diffusion or viscous flow relaxation.Comment: Revtex4, 19 pages, 10 figures with JPEG forma