Femtosecond resolution timing jitter correction on a TW scale Ti:sapphire laser system for FEL pump-probe experiments

Intense ultrashort pulse lasers are used for fs resolution pumpprobe experiments more and more at large scale facilities, such as free electron lasers (FEL). Measurement of the arrival time of the laser pulses and stabilization to the machine or other sub-systems on the target, is crucial for high time-resolution measurements. In this work we report on a single shot, spectrally resolved, non-collinear cross-correlator with sub-fs resolution. With a feedback applied we keep the output of the TW class Ti:sapphire amplifier chain in time with the seed oscillator to ~3 fs RMS level for several hours. This is well below the typical pulse duration used at FELs and supports fs resolution pump-probe experiments. Short term jitter and long term timing drift measurements are presented. Applicability to other wavelengths and integration into the timing infrastructure of the FEL are also covered to show the full potential of the device

Centrifuge modelling to determine the influence of pile stiffness on pile capacity

The stiffness of piles relates to their ability to resist deformation in response to an applied force. The modulus of elasticity of an uncracked concrete piles typically varies between 30-40kN/m2. Under axial loading these high stiffness piles transfer the load through the pile to the base. This results in low mobilisation of shaft friction as the stiff pile displaces uniformly and therefore the magnitude of skin friction along the entire length of the pile is small. In addition, base resistance of deep piles is mobilised at very high loads which may exceed the working load of the pile shaft. The adhesion factor, α, for bored piles in London Clay can range between 0.45 and 0.6 suggesting that a significant proportion of the soil strength cannot be mobilised. This low mobilisation of shaft resistance means that the ultimate bearing capacity is much reduced. The research investigated the behaviour of a low stiffness pile under axial load and compared this with a conventional high stiffness pile. The results demonstrated that the low stiffness pile exhibited marginally greater capacity at working load and a noticeably improved capacity at ultimate load

A method for creating larger clay samples with permeability anisotropy for geotechnical centrifuge modelling

Long-term ground movements associated with geotechnical constructions are predominantly caused by the dissipation of excess pore-water pressures and are governed by the permeabilities of both the soil and the geotechnical structure. Natural soil has inherent anisotropy due to the layering and structure as a result of the natural deposition process. A significant factor that influences the rate of consolidation and seepage in natural soils is that the horizontal permeability can be orders of magnitude larger than the vertical permeability. This is often considered in numerical modelling during geotechnical design however, due to the lack of reliable field measurements available, validating these numerical models can be difficult. Geotechnical centrifuge techniques have successfully been used to investigate responses to complex construction events but are, generally, models created from reconstituted soil. This results in models with well-defined but homogeneous properties. There is a fundamental difference between centrifuge models and natural soil deposits. As a result, centrifuge models are better suited to simulating the short-term response of the soil to a construction event. The work presented outlines a procedure for creating large clay models suitable for geotechnical centrifuge testing with a sedimented structure. These models have anisotropy of the horizontal and vertical permeability allowing for more representative soil behaviour (in terms of dissipation of pore-water pressures) which can be used to investigate the long-term movements resulting from geotechnical construction events

Plans, Preferences or Going with the Flow: an Online Exploration of Women’s Views and Experiences of Birth Plans

Objective To explore women’s views of birth plans, and experiences of their completion and use. Design A qualitative, exploratory study, using Internet-mediated research methods. Setting The discussion boards of two well-known, UK-based, online parenting forums, where a series of questions relating to birth plans were posted. Participants Members of the selected parenting forums who had written and used, or who had chosen not to write or use, a birth plan. Findings Women responded with a range of views and experiences relating to the completion and use of birth plans. The benefits of birth plans were described in terms of communication with healthcare professionals, potentially enhancing awareness of available options, and maintaining a sense of control during labour and birth. However, many respondents believed the idea of ‘planning’ birth was problematic, and described a reluctance to write a formal plan. The support of healthcare professionals, particularly midwives, was considered essential to the success of both writing and using birth plans. Key conclusions Our findings show a continued debate among women on the benefits and challenges involved in writing and using birth plans, suggesting problems for a ‘one size fits all’ approach often seen in the use of birth plan templates. In the context of maternity policy supporting women’s choice and personalised care, and as a way of acknowledging perceived problems of ‘planning’ for birth, a flexible approach to birth plans is required, including the consideration of employing alternative nomenclature. Implications for practice Birth plans remain a point of contention in care contexts around the world. Midwives and other healthcare providers play a central role in supporting women to discuss available options, whether or not they decide to complete a formal birth plan

Tests of varied sample preparation methods for centrifuge modelling

Centrifuge modelling is an established technique capable of investigating the ground’s response to complex geotechnical events. Centrifuge models are often created from reconstituted soil, with well-defined boundary conditions and known soil parameters. Clay soil models may be prepared by mixing clay powder with distilled water to form a slurry. This slurry is placed within a soil container and subjected to a vertical stress (usually in a consolidation press or consolidated inflight). This creates an isotropic model but there is a fundamental difference between this soil model and naturally occurring soil deposits. The structure and fabric present within a naturally occurring clay is not reproduced by this preparation process. It is well-established that structure and fabric in naturally deposited soils are as significant in their effect on soil behaviour as, for instance, the stress history. Inherent structure and fabric within clay soils creates anisotropy which can vary with depth, this is particularly apparent when considering the permeability. Creating a soil model for centrifuge modelling with representative permeability anisotropy would allow for a better representation of consolidation driven events and the ability to observe long-term behaviour of complex geotechnical events. Currently, there are limited methods of doing so, leading to a considerable gap in knowledge associated with the behaviour of layered ground. This paper describes the development of the equipment and experimental procedure for quantifying the structure developed by different sample preparation techniques for centrifuge modelling

Changes in household wealth in communities living in proximity to a large-scale copper mine in Zambia

Large-scale mining can alter the living conditions of surrounding communities in positive and negative ways. A health impact assessment conducted in the context of a newly developed large-scale copper mine in rural Zambia gave us the opportunity to measure changes in health determinants over time. We conducted periodic household surveys at baseline in 2011, during the construction phase in 2015 and during the operational phase in 2019. Data collected included economic indicators that were based on the standardized list of household assets used in the Zambia Demographic and Health Survey, which we subsequently converted into a wealth score using principal component analysis. We compared mean wealth scores in six communities directly impacted by the mine with comparison communities, as well as the rest of the North-Western province of Zambia. A difference-indifferences linear regression model was used to compare changes over time. Mean wealth of the communities near the mine was significantly lower at baseline than that of the North-Western province (? 0.54 points; pvalue< 0.001) in 2011, but surpassed the regional average in 2019 (+1.07 points; p-value <0.001). Mean wealth increased more rapidly in communities directly impacted by mine than in the comparison communities (+0.30 points, p-value <0.001). These results suggest a positive impact on living conditions in communities living near this copper mine. Our findings underscore the potential of the mining sector to contribute to economic development in Zambia

Enhanced He-alpha emission from "smoked" Ti targets irradiated with 400nm, 45 fs laser pulses

We present a study of He-like 1s(2)-1s2p line emission from solid and low-density Ti targets under similar or equal to 45 fs laser pulse irradiation with a frequency doubled Ti: Sapphire laser. By varying the beam spot, the intensity on target was varied from 10(15) W/cm(2) to 10(19) W/cm(2). At best focus, low density "smoked" Ti targets yield similar to 20 times more He-alpha than the foil targets when irradiated at an angle of 45 degrees with s-polarized pulses. The duration of He-alpha emission from smoked targets, measured with a fast streak camera, was similar to that from Ti foils

Changes in socioeconomic determinants of health in a copper mine development area, northwestern Zambia

In 2011, an industrial copper mine was developed in northwestern Zambia. A health impact assessment was conducted to anticipate and address potential health impacts. To monitor these impacts, three community-based surveys were conducted in the area (2011, 2015 and 2019). We analysed these data to determine how household socioeconomic indicators - considered determinants of health - have changed in the area over time. In mine-impacted communities, between 2011 (pre-construction) and 2019, significant changes were observed for: (i) average household size (-0.6 members); (ii) proportion of mothers that have not completed primary school (+20.4%); (iii) ownership of economic assets (e.g. phones +29.3%; televisions +15.6%); (iv) access to safe drinking water (+27.4%); and (v) improved housing structures (e.g. finished roof +58.6%). When comparing changes between 2015 and 2019 in impacted communities to nearby comparison communities, there was (i) an increased proportion of mothers that had not completed primary school in comparison communities vs. no change in impacted communities; and (ii) increased ownership of economic assets in impacted vs. comparison communities in 2019. This study found generally positive changes in the socioeconomic development of impacted compared to comparison communities, with the most pronounced improvements in the early phases of mine development

Relativistic plasma surfaces as an efficient second harmonic generator

We report on the characterization of the specular reflection of 50 fs laser pulses in the intensity range 10(17)-10(21)Wcm(-2) obliquely incident with p-polarization onto solid density plasmas. These measurements show that the absorbed energy fraction remains approximately constant and that second harmonic generation (SHG) achieves efficiencies of 22 +/- 8% for intensities approaching 10(21)Wcm(-2). A simple model based on the relativistic oscillating mirror concept reproduces the observed intensity scaling, indicating that this is the dominant process involved for these conditions. This method may prove to be superior to SHG by sum frequency mixing in crystals as it is free from dispersion and retains high spatial coherence at high intensity