Grid-enabling the Astra Gemini Laser Data

Astra Gemini is a dual beam Ti:Sapphire laser capable of delivering up to 0.5 PW in each of its two beams*. The system can fire once every 20 seconds (producing over 1000 shots per day) and has over 200 diagnostic channels, including spectra, pulse length, traces, near and far-field images. This combination of multiple diagnostics and high shot rate leads to an unprecedented amount of performance and diagnostic data to save and analyse. To cope with this demand a system has been developed to automatically capture and analysis laser data on every shot, store it in an Oracle database and retrieve it on demand. A graphical user interface has been written to extract, sort and display the data in a tabular form. Powerful functions have been implemented to allow any parameters to be selected and plotted against one another to analyse performance trends and fluctuations. Metadata about each diagnostic can also be input to build a holistic picture of the laser system and help with future analysis. To increase the value it is planned to incorporate the target area experimental diagnostics into the system and make the data available to participating experimenters anywhere around the world

Twin-tunnelling-induced changes to clay stiffnesses

Tunnels used for transportation in urban environments are often constructed in pairs. Projects in which tunnels are constructed sequentially and within close proximity are referred to as ‘twin tunnelling’. Case studies and recent research indicate that the prediction of settlements for such schemes cannot be determined using existing simple methods derived from consideration of a single tunnel. To establish the reasons for the observed variation in settlements, a series of centrifuge tests was undertaken on various twin-tunnel arrangements in overconsolidated clay. The tests consisted of preformed cavities from which a specific quantity of supporting fluid could be drained, with precision, creating a predetermined magnitude of tunnelling volume loss. Data were obtained for surface and subsurface displacements, changes in pore-water pressure near the tunnels and the support pressure within the tunnels. The systematic use of cavity contraction models was found to be an informative method of explaining the observations. Use of an elastic–perfectly plastic cavity contraction model coupled with observations from the experiments enabled the shear stiffness of the clay around the tunnel to be described. Further analysis demonstrated a reduction in shear stiffness of the soil prior to and during the second tunnel excavation, explaining the increase in volume loss observed in that event

Explanation for twin tunnelling-induced surface settlements by changes in soil stiffness on account of stress history

In this article, a group of representative centrifuge tests were selected for numerical modelling to explain the surface settlements induced by sequential twin tunnelling. Both Modified Cam Clay model (MCC) and Three-Surface Kinematic Hardening model (3-SKH) were adopted in the simulation, which indicated the use of 3-SKH model conduced to mimicking more closely centrifuge model response. Via performing more contrastive numerical analyses with 3-SKH model, the influence of the first tunnel event on the stiffness of the soil around the second tunnel was quantitatively investigated, whereby the mechanism behind the observed surface settlements was finally made clear

Predictions of changes in pore-water pressure around tunnels in clay

Any underground construction causes changes to the stress state in the ground and this change generally causes the generation of excess pore-water pressures in saturated fine grained soils. Subsequent dissipation of these pressures can lead to settlements and potential damage and hence there is a need to understand and predict these changes in pore-water pressure. Simple plasticity and non-linear elastic solutions have been used to calculate pore-water pressure changes as a tunnel is constructed in clay. These are compared with previous centrifuge tests involving the simulation of tunnel excavation as well as new tests specifically designed to investigate the generation and subsequent dissipation behaviour of excess pore-water pressures. The paper reports on the new tests, presents the findings within the simple plasticity and non-linear elastic analysis framework

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

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

Numerical analysis of deep ground movements induced by circular shaft construction

: The prediction of the excavation-induced displacements by shaft construction in urban areas is an important design issue. A series of experiments were conducted by Le et al. (2019) using a geotechnical centrifuge to evaluate the distribution with depth of soil movements induced during a shaft construction. Those results have been confirmed and expanded in this work by means of numerical analyses. Following this validation, numerical modelling has been used to analyse the influence of OCR and of the shaft geometry, namely its depth H and diameter D!"#$%. In the abovementioned experimental study (Le et al., 2019) the presence of existing underground structures in the proximity of the shaft had been neglected. Therefore, in this study the effect of an existing shallow tunnel in the vicinity of the shaft excavation was taken into account and numerically analysed. The results show that the induced displacement field is affected by the tunnel, depending on its distance from the shaft. Nevertheless, when the distance between the shaft and the existing tunnel is larger than the tunnel axis depth, the tunnel presence may be neglected in the prediction of shaft-indued displacements. Based on the numerical results, further centrifuge tests will be carried out to model the interaction between the shaft excavation and the tunnel

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

The PHIN photoinjector for the CTF3 Drive beam

A new photoinjector for the CTF3 drive beam has been designed and is now being constructed by a collaboration among LAL, CCLRC and CERN within PHIN, the second Joint Research Activity of CARE. The photoinjector will provide a train of 2332 pulses at 1.5 GHz with a complex timing structure (sub-trains of 212 pulses spaced from one another by 333 ps or 999 ps) to allow the frequency multiplication scheme, which is one of the features of CLIC, to be tested in CTF3. Each pulse of 2.33 nC will be emitted by a Cs2Te photocathode deposited by a co-evaporation process to allow high quantum efficiency in operation (>3% for a minimum of 40 h). The 3 GHz, 2 1/2 cell RF gun has a 2 port coupler to minimize emittance growth due to asymmetric fields, racetrack profile of the irises and two solenoids to keep the emittance at the output below 20 p.mm.mrad. The laser has to survive very high average powers both within the pulse train (15 kW) and overall (200 W before pulse slicing). Challenging targets are also for amplitude stability (<0.25% rms) and time jitter from pulse to pulse (<1ps rms). An offline test in a dedicated line is foreseen at CERN in 2007