Centrifuge Modelling of Long-term Tunnelling Ground Movements

The increasing demand for public transport provision in cities has resulted in a requirement for enlarged public transport infrastructure. Where underground railways represent an important component of these systems, tunnel construction will inevitably lead to some degree of ground movement that can cause damage to surface structures and buried structures and services. It is important, therefore, that in the design of tunnels in urban environments these ground movements are predicted reliably. Predicting short-term ground movements resulting from tunnelling is standard when assessing the potential for damage to adjacent infrastructure. However, long-term tunnelling-induced ground movements and how these develop are understood less well and a research programme, based on geotechnical centrifuge modelling, is being conducted to improve our fundamental scientific understanding of this. The first stage of the programme has been to develop an apparatus that simulates the construction of a tunnel with a lining of known stiffness and permeability and allows construction ground loss to be replicated correctly. This paper describes the initial development of the apparatus along with results and analysis that demonstrates the suitability of the technique for the proposed study. The results obtained were observed to represent the short-term settlements that might be expected above a tunnel excavated in clay. The results also prove the modelling technique suitable for application in a full parametric study in which the geometry and boundary conditions of the model will be varied together with the permeability of the tunnel liner

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

Atomic excitation during recollision-free ultrafast multi-electron tunnel ionization

Modern intense ultrafast pulsed lasers generate an electric field of sufficient strength to permit tunnel ionization of the valence electrons in atoms. This process is usually treated as a rapid succession of isolated events, in which the states of the remaining electrons are neglected. Such electronic interactions are predicted to be weak, the exception being recollision excitation and ionization caused by linearly-polarized radiation. In contrast, it has recently been suggested that intense field ionization may be accompanied by a two-stage `shake-up' reaction. Here we report a unique combination of experimental techniques that enables us to accurately measure the tunnel ionization probability for argon exposed to 50 femtosecond laser pulses. Most significantly for the current study, this measurement is independent of the optical focal geometry, equivalent to a homogenous electric field. Furthermore, circularly-polarized radiation negates recollision. The present measurements indicate that tunnel ionization results in simultaneous excitation of one or more remaining electrons through shake-up. From an atomic physics standpoint, it may be possible to induce ionization from specific states, and will influence the development of coherent attosecond XUV radiation sources. Such pulses have vital scientific and economic potential in areas such as high-resolution imaging of in-vivo cells and nanoscale XUV lithography.Comment: 17 pages, 4 figures, original format as accepted by Nature Physic

First Results from Commissioning of the Phin Photo Injector for CTF3

Installation of the new photo-injector for the CTF3 drive beam (PHIN) has been completed on a stand-alone test bench. The photo-injector operates with a 2.5 cell RF gun at 3 GHz, using a Cs2Te photocathode illuminated by a UV laser beam. The test bench is equipped with transverse beam diagnostic as well as a 90-degree spectrometer. A grid of 100 micrometer wide slits can be inserted for emittance measurements. The laser used to trigger the photo-emission process is a Nd:YLF system consisting of an oscillator and a preamplifier operating at 1.5 GHz and two powerful amplifier stages. The infrared radiation produced is frequency quadrupled in two stages to obtain the UV. A Pockels cell allows adjusting the length of the pulse train between 50 nanoseconds and 50 microseconds. The nominal train length for CTF3 is 1.272 microseconds (1908 bunches). The first electron beam in PHIN was produced in November 2008. In this paper, results concerning the operation of the laser system and measurements performed to characterize the electron beam are presented

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

Centrifuge modelling of screw piles for offshore wind energy foundations

Screw piles (helical piles) can provide a viable, cost-effective and low-noise installation alternative to increasing the size of existing foundation solutions (e.g. monopiles) to meet the demand for the advancement of offshore wind energy into deeper water. Significant upscaling of widely used onshore screw pile geometries will be required to meet the loading conditions of a jacket supported offshore wind turbine. This increase in size will lead to greater installation force and torque. This paper presents preliminary results from centrifuge tests investigating the requirements to install screw piles designed for an offshore wind energy application using specially developed equipment. Results indicate that the equipment is suitable to investigate these screw pile requirements and that significant force is required for such upscaled screw piles, with 19 MN vertical force and 7 MNm torque for the standard design. Optimisation of the screw pile geometry, reduced these forces by 29 and 11% for the vertical and rotational forces respectively

A low-cost miniature immersible pore water pressure transducer

The ability to measure pore water pressure accurately in geotechnical models is vital for allowing researchers to quantify effective stress and, frequently, its temporal variation. The earliest use of electrical devices to measure pore water pressure in specific locations within a model employed a standard laboratory pressure transducer located outside the boundaries of the model connected to tubing inserted into the model. However, this technique was superseded by the development (some 40 years ago) of submersible miniature pore water pressure measuring devices that could be located within a model to measure pore pressure directly at specified locations. In particular, miniaturisation allowed these transducers to be used in small scale centrifuge models. This facilitated, across the full spectrum of geotechnical engineering modelling, both enhanced understanding of mechanisms, by permitting quantitative analysis, and validation of numerical techniques. The earliest miniature transducers, the PDCR81 device manufactured by Druck, rapidly became universally adopted by the geotechnical modelling community. The production of this device was halted about 10 years ago, but other manufacturers have developed similar miniature pore water pressure transducers. Whilst the newer devices have been demonstrated to be effective their unit cost is relatively expensive; owing to the low volume of manufacture. This can result in forced limitations in the number that may be in a model, or a reluctance to use them in zones of models where they might be damaged. The requirement for a cheaper but equally reliable device prompted the work described in this paper. This is now achievable because of the development for the consumer market of a mass produced robust, immersible pressure device that has a very low unit cost. The paper will describe the development of a miniature transducer, employing this device, that can be located within models in a similar way to the PDCR81

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