Generation of terawatt pulses by use of optical parametric chirped pulse amplification

Optical parametric chirped pulse amplifiers offer exciting prospects for generating new extremes in power, intensity, and pulse duration. An experiment is described that was used to investigate the operation of this scheme up to energies approaching a joule, as a step toward its implementation at the petawatt level. The results demonstrate an energy gain of 10(10) with an energy extraction efficiency of 20% and close to diffraction-limited performance. Some spectral narrowing during amplification was shown to be compatible with the time-varying profile of the pump beam and consistent with the measured recompressed pulse durations of 260 and 300 fs before and after amplification, respectively.     

Storage and behavior of As, Sb, Pb, and Cu in ombrotrophic peat bogs under contrasting water table conditions

Concentration depth profiles and inventories of solid-phase As, Sb, Pb, and Cu were determined in 21?Pb-dated cores from an ombrotrophic peat bog in northwest England. Cores were collected from the peat dome and adjacent to an eroding gully. Down-core distributions of As, Sb, Pb, and Cu in the dome core are almost identical. The water table is close to the dome surface with only short-term draw-down. Under these conditions, As, Sb, Pb, and Cu are immobile, allowing the reconstruction of trends in historical contaminant deposition. The peak in atmospheric deposition of As, Sb, Pb, and Cu (4.59, 2.78, 147, and 26.7 mg m?2 y?1, respectively) occurred during the late 19th century. Stable Pb isotope ratios reveal that Pb deposition during this period was from indigenous and foreign sources. The mean water table is much lower at the gully edge, and there are pronounced interannual fluctuations. These conditions have not affected the integrity of the Pb and Cu records but have caused postdepositional mobilization and redistribution of As and Sb. Cumulative inventories show significant loss of As and Sb at the gully edge site. Long-term water table draw-down in ombrotrophic peat bogs has the potential to alter the geochemistry and fate of previously deposited As and Sb.     

Shape adaptive activity controlled multistage gain shape vector quantisation of images

The performance of a shape adaptive activity controlled multistage gain shape vector quantiser for digital image compression is compared with that of more established full search multistage vector quantisation methods. The advantages of the former approach to the vector quantisation coding of images are clearly demonstrated via computer simulation of the respective coding algorithms.     

A plasma light source for the VUV spectral region

The 5-50-eV spectral range appears to be absent from the planned third generation synchrotron radiation sources (SRS) and the science requiring this radiation can be jeopardized once the second generation SRS light sources are switched off worldwide. Molecular photochemistry, photobiology, atomic physics, solid-state physics are but a few sciences requiring powerful VUV sources. We demonstrate that a compact, laser heated plasma light source can generate efficiently radiation in this spectral region, and that this radiation can be coupled very efficiently from the plasma to the exposure cell, by three simple VUV beam-lines, each providing a different spectral bandwidth     

Impacts of climate change on phosphorus loading from a grassland catchment: Implications for future management

Dynamic modelling was used to quantify the impact of projected climate change, and potential changes in population and land use, on phosphorus (P) export from a sub-catchment in SW Ireland using the Generalised Watershed Loading Functions (GWLF) model. Overall the results indicated that the increase in annual total phosphorus loads attributable to climate change was greater than that from either population or land use change, and therefore that future climate variability will pose an increasingly significant threat to the successful long-term implementation of catchment management initiatives. The seasonal pattern in projected P export mirrored changes in streamflow, with higher rates between January and April and lower rates in summer. The potential reduction in export in summer was, however, negated when increases in population were included in simulations. A change in the slurry spreading period from that stipulated in national regulations to the months between April and September could potentially mitigate against future increases in dissolved P export in spring. The results indicate that projected changes in climate should be included when undertaking modelling exercises in support of decision making for catchment management plans.     

A data optimization test technique for characterizing embedded ADCs

A novel data optimization test technique is presented which utilizes a BIST structure, an ADC model and histogram data to characterize embedded ADCs. A practical 8 bit ADC is modeled and then characterized using 20% less data points then conventional analysis with a 78% reduction in the amount of data required to be shifted off-chip. Comparisons between theoretical, modeled and practical results are also made in the paper.     

Arsenic retention and release in ombrotrophic peatlands

Organic matter can play an important role in the mobility and fate of As in the environment, but there is a lack of data on As biogeochemistry in ombrotrophic peatlands. The aim of this study was to investigate As retention and release in atmospherically contaminated ombrotrophic peat soils in the Peak District National Park (UK). Solid phase As concentrations in the peat soils exceed 25 mg kg^− 1. Solid phase As and Fe concentrations are closely correlated at sites where the peat is subjected to drying and oxic conditions. In a wetter zone of the bog, solid phase As and Fe distributions are decoupled, suggesting that As retention in these systems is not solely controlled by the presence of Fe oxides. Comparison of solid phase As and Pb distributions reveals that As has been subjected to post-depositional mobility in areas of water table fluctuation. Conversely, at permanently waterlogged locations As is immobile. Detailed stream water sampling reveals that As is released from the organic-rich uplands soils into the fluvial system. Dissolved As concentrations are highly variable, with values ranging from 0.20 to 7.28 μg l^− 1. Stream water As concentrations are elevated during late summer stormflow periods when there has been re-wetting of the peat after significant water table draw-down. Dissolved As is strongly correlated to dissolved organic carbon under stormflow and baseflow. The results of this study suggest that organic matter plays an important role in As dynamics in ombrotrophic peatlands, but further work is needed to identify the exact As binding and release mechanisms. Drying and re-wetting of ombrotrophic peat soils and associated changes in redox status has the potential to lead to increased As mobility. Further work is needed to provide information on how predicted climate change will influence As cycling at sites containing a legacy of atmospheric contamination.     

A spatial and seasonal assessment of river water chemistry across North West England

This paper presents information on the spatial and seasonal patterns of river water chemistry at approximately 800 sites in North West England based on data from the Environment Agency regional monitoring programme. Within a GIS framework, the linkages between average water chemistry (pH, sulphate, base cations, nutrients and metals) catchment characteristics (topography, land cover, soil hydrology, base flow index and geology), rainfall, deposition chemistry and geo-spatial information on discharge consents (point sources) are examined. Water quality maps reveal that there is a clear distinction between the uplands and lowlands. Upland waters are acidic and have low concentrations of base cations, explained by background geological sources and land cover. Localised high concentrations of metals occur in areas of the Cumbrian Fells which are subjected to mining effluent inputs. Nutrient concentrations are low in the uplands with the exception sites receiving effluent inputs from rural point sources. In the lowlands, both past and present human activities have a major impact on river water chemistry, especially in the urban and industrial heartlands of Greater Manchester, south Lancashire and Merseyside. Over 40% of the sites have average orthophosphate concentrations > 0.1 mg-P l^− 1. Results suggest that the dominant control on orthophosphate concentrations is point source contributions from sewage effluent inputs. Diffuse agricultural sources are also important, although this influence is masked by the impact of point sources. Average nitrate concentrations are linked to the coverage of arable land, although sewage effluent inputs have a significant effect on nitrate concentrations. Metal concentrations in the lowlands are linked to diffuse and point sources. The study demonstrates that point sources, as well as diffuse sources, need to be considered when targeting measures for the effective reduction in river nutrient concentrations. This issue is clearly important with regards to the European Union Water Framework Directive, eutrophication and river water quality.