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Posterior lumbar spinal dynamic stabilisation devices are intended to relieve the pain of spinal segments while prolonging the lifespan of adjacent intervertebral discs. This study focuses on the design of such a device, one that has the correct stiffness to stabilise the spinal segment by the correct amount. An initial literature survey covers contemporary topics related to the lumbar spine. Included topics are lumbar anatomy and kinematics, pathology of degenerative disc disease and treatment thereof, other spinal disorders such as spondylolisthesis and spinal stenosis, as well as the complications associated with lumbar dynamic stabilisation. The influence of factors such as fatigue and wear, as well as the properties of appropriate biomaterials are considered when determining the basis of the device design and development. Stabilising the spinal segment begins with correct material selection and design. Various designs and biomaterials are evaluated for their stiffness values and other user requirements. The simplest design, a U-shaped spring composed of carbon fibre-reinforced poly-ether-ether-ketone (CFR-PEEK) and anchored by polyaxial titanium pedicle screws, satisfies the most critical user requirements. Acceptable stiffness is achieved, fatigue life of the material is excellent and the device is very imaging-friendly. Due to financial constraints, however, a simpler concept that is cheaper and easier to rapid prototype was chosen. This concept involves a construct primarily manufactured from the titanium alloy Ti6Al4V extra-low interstitial (ELI) and cobalt-chrome-molybdenum (CCM) alloys. The first rapid prototype was manufactured using an additive manufacturing process (3D-printing). The development of the device was performed in three main stages: design, verification and validation. The main goal of the design was to achieve an acceptable stiffness to limit the spinal segmental range of motion (ROM) by a determined amount. The device stiffness was verified through simple calculations. The first prototype’s stiffness was validated in force-displacement tests. Further validation, beyond the scope of this study, will include fatigue tests to validate the fatigue life of the production-ready device. / MIng (Mechanical Engineering), North-West University, Potchefstroom Campus, 2014

Posterior lumbar spinal dynamic stabilisation devices are intended to relieve the pain of spinal segments while prolonging the lifespan of adjacent intervertebral discs. This study focuses on the design of such a device, one that has the correct stiffness to stabilise the spinal segment by the correct amount. An initial literature survey covers contemporary topics related to the lumbar spine. Included topics are lumbar anatomy and kinematics, pathology of degenerative disc disease and treatment thereof, other spinal disorders such as spondylolisthesis and spinal stenosis, as well as the complications associated with lumbar dynamic stabilisation. The influence of factors such as fatigue and wear, as well as the properties of appropriate biomaterials are considered when determining the basis of the device design and development. Stabilising the spinal segment begins with correct material selection and design. Various designs and biomaterials are evaluated for their stiffness values and other user requirements. The simplest design, a U-shaped spring composed of carbon fibre-reinforced poly-ether-ether-ketone (CFR-PEEK) and anchored by polyaxial titanium pedicle screws, satisfies the most critical user requirements. Acceptable stiffness is achieved, fatigue life of the material is excellent and the device is very imaging-friendly. Due to financial constraints, however, a simpler concept that is cheaper and easier to rapid prototype was chosen. This concept involves a construct primarily manufactured from the titanium alloy Ti6Al4V extra-low interstitial (ELI) and cobalt-chrome-molybdenum (CCM) alloys. The first rapid prototype was manufactured using an additive manufacturing process (3D-printing). The development of the device was performed in three main stages: design, verification and validation. The main goal of the design was to achieve an acceptable stiffness to limit the spinal segmental range of motion (ROM) by a determined amount. The device stiffness was verified through simple calculations. The first prototype’s stiffness was validated in force-displacement tests. Further validation, beyond the scope of this study, will include fatigue tests to validate the fatigue life of the production-ready device. / MIng (Mechanical Engineering), North-West University, Potchefstroom Campus, 2014

This thesis is concerned with two main questions. Do systemic banking crises substantially affect the income distribution in a country? Is income inequality a destabilising factor for the macro-economy? In order to answer the first question, this thesis examines a panel of 26 countries since 1900 and assembles a new database of crises, finding that the impact of major banking crises on the national income shares detained by the income groups within the richest decile is mostly small in magnitude. Indeed, the estimated impact is never bigger than a standard deviation of the specific top shares under investigation. Results are also confirmed in a separate analysis for the United States and are robust to a series of checks. These findings lend indirect support to the structuralist hypothesis that only substantial changes in government policies and institutional frameworks can bring about radical changes in income distribution. The analysis also highlights interesting heterogeneity across different income groups, country groups and time periods. The second question is addressed by making use of a newly assembled database on different dimensions of economic inequality. The new data helps to reject the statistical validity of the hypotheses that either growing inequality or a high level of inequality may systematically precede the onset of major banking crises. In addition, simulations based on the UK Family Expenditure Survey data find that even a full equalisation of income would increase the aggregate consumption by 3 percentage points at most. These findings, taken together, point out that an increase in income inequality may not concur to reduce the pressure on aggregate demand or be adduced as a structural factor of financial instability. Nonetheless, the evidence is not yet clear cut as the work further documents that periods of increasing income inequality in the UK were also associated with a reduction of the saving rates across the whole income distribution since 1968. The analysis contends that such evidence of under-saving behaviour may be consistent with the relative income hypothesis and some of its recent formulations such as the ’expenditure cascades’ theory.

What are the main macroeconomic factors that help understand economic growth in Brazil since 1870? Are institutions (and changes in institutions) a deep cause of economic growth in Brazil? Are these effects fundamentally and systematically different? Does the intensity and the direction (the sign) of these effects vary over time, in general and, in particular, do they vary with respect to short- versus long-run considerations? This thesis tries to answer these questions focusing on within country over long periods of time. It uses the power-ARCH (PARCH) econometric framework with annual time series from 1870 to 2003. The results suggest that financial development (domestic and international) exhibit the most robust first-order effects on growth and its volatility. Political instability, trade openness and public deficit play important yet secondary roles since the effects of the first two do not extent to the long-run (that is, they are restricted to the short-run) and those off the latter are sensitive to the measures of the variables used in our analysis.

The problem of heating the solar corona requires the conversion of magnetic energy into thermal energy. Presently, there are two promising mechanisms for heating the solar corona: wave heating and nanoflare heating. In this thesis, we consider nanoflare heating only. Previous modelling has shown that the kink instability can trigger energy release and heating in large scale loops, as the field rapidly relaxes to a lower energy state under the Taylor relaxation theory. Two distinct experiments were developed to understand the coronal heating problem: the avalanche effect within a multiple loop system, and the importance of thermal conduction and optically thin radiation during the evolution of the kinked-unstable coronal magnetic field. The first experiment showed that a kink-unstable thread can also destabilise nearby threads under some conditions. The second experiment showed that the inclusion of thermal conduction and optically thin radiation causes significant change to the internal energy of the coronal loop. After the initial instability occurs, there is continual heating throughout the relaxation process. Our simulation results show that the data is consistent with observation values, and the relaxation process can take over 200 seconds to reach the final relaxed state. The inclusion of both effects perhaps provides a more realistic and rapid heating experiment compared to previous investigations.

In this project we investigate the feasibility of detecting the signatures of Pop III stars in metal poor second generation stars and in gas clouds at high redshifts. First, the nucleosynthetic yields of Pair Instability Supernova and how they are manifested in gas clouds are presented. Next, some basic quantities of radio astronomy are explained and the requirements of SKA are shown. Then, the minimum detectable hydrogen column density of SKA for gas clouds at high redhsift is calculated and after that the basic principles of spectroscopy and the requirements of the HiReS instrument of E-ELT are demonstrated. Finally, suggestions about where the observations with HiReS should focus are made.

Anterior cruciate ligament injury leads to increased sagittal tibial translation, and perceptions of instability and low confidence in the knee joint are common. Many patients have remaining problems despite treatment and are forced to lower their activity level and prematurely end their career in sports. The effect of ACL reconstruction and/or rehabilitation on dynamic knee stability is not completely understood. The overall aim of this thesis was to study the dynamic knee stability during and after rehabilitation in individuals with ACL injury. More specific aims were 1) to elaborate an evaluation method for muscle strength, 2) to evaluate the effect of exercises in closed and open kinetic chain, and 3) to evaluate dynamic knee stability in patients with ACL deficiency or ACL reconstruction. Sagittal tibial translation and knee flexion angle were measured using the CA‐4000 computerised goniometer linkage. Muscle activation was registered with electromyography. The intra‐ and inter‐rater reliability of 1 repetition maximum (RM) of seated knee extension was clinically acceptable. The inter‐rater reliability of 1RM of squat was also acceptable, but the intra‐rater reliability was lower. The systematic procedure for the establishment of 1RM that was developed can be recommended for use in the clinic. One specific exercise session including cycling and a maximum number of knee extensions and heel raises did not influence static or dynamic sagittal tibial translation in uninjured individuals. A comprehensive rehabilitation program with isolated quadriceps training in OKC led to significantly greater isokinetic quadriceps strength compared to CKC rehabilitation in patients with ACL deficiency. Hamstring strength, static and dynamic translation, and functional outcome were similar between groups. Five weeks after ACL reconstruction, seated knee extension produced more anterior tibial translation compared to the straight leg raise and standing on one leg. All exercises produced less or equal amount of anterior tibial translation as the 90N Lachman test. Five weeks after the ACL reconstruction the static and dynamic tibial translation in the ACL reconstructed knee did not differ from the tibial translation on the uninjured leg. Patients in the early phase after ACL injury or ACL reconstruction used a joint stiffening strategy including a reduced peak knee extension angle during gait and increased hamstring activation during activity, which reduces the dynamic tibial translation. Patients with ACL deficiency that completed a four months rehabilitation program used a movement pattern that was more close to normal.

<p>Plasma-based particle acceleration holds the promise to make the applications that revolve around accelerators more affordable. The central unifying theme of this dissertation is the modeling of certain non-linear and relativistic phenomena in plasma dynamics to devise mechanisms that benefit plasma-accelerators. Plasma acceleration presented here has two distinct flavors depending upon the accelerated particle mass which dictates the acceleration structure velocity and potential. The first deals with ion acceleration, where acceleration structure velocities are a significant fraction of the speed of light, with major applications in medicine. The second focusses on the acceleration of electrons and positrons for light-sources and colliders where the acceleration structures are wakefields with phase-velocities near the speed of light.</p> <p>The increasing Lorentz factor of the laser-driven electron quiver momentum forms the basis of Relativistically Induced Transparency Acceleration (RITA) scheme of ion acceleration. Lighter ions are accelerated by reflecting off a propagating acceleration structure, referred to as a snowplow, formed by the compression of ponderomotively driven critical layer electrons excited in front of a high intensity laser pulse in a fixed-ion plasma. Its velocity is controlled by tailoring the laser pulse rise-time and rising density gradient scale-length. We analytically model its induced transparency driven propagation with a 1-D model based on the linearized dispersion relation. The model is shown to be in good agreement with the weakly non-linear simulations. As the density compression rises into the strongly non-linear regime, the scaling law predictions remain accurate but the model does not exactly predict the RITA velocity or the accelerated ion-energy. Multi-dimensional plasma effects modify the laser radial envelope by self-focussing in the rising density gradient which can be integrated into our model and filamentation which is mitigated by a matched laser focal spot-size. We show that the critical layer motion in RITA compares favorably to the bulk-plasma motion driven by radiation pressure or collision-less shocks.</p> <p>Non-linear mixing of the laser, incident on and reflected off the propagating critical layer modulates its envelope affecting the acceleration structure velocity and potential, in the process setting up a feedback loop. For long pulses the envelope distortion grows with time, disrupting the accelerated ion-beam spectral shape. We model the Chirp Induced Transparency Acceleration (ChITA) mechanism that over- comes this effect by introducing decoherence through a frequency chirp in the laser. </p> <p>In a rising density gradient, the non-linearity of electron trajectories leads to the phase-mixing self-injection of electrons into high phase-velocity plasma wakefields. The onset of trapping depends upon the wake amplitude and the density gradient scale-length. This self-injection mechanism is also applicable to controlling the spuriously accelerated electrons that affect the beam-quality. </p> <p>Non-linear ion dynamics behind a train of asymmetric electron-wake excites a cylindrical ion-soliton similar to the solution of the cylindrical Korteweg-de Vries (cKdV) equation. This non-linear ion-wake establishes an upper limit on the repetition rate of the future plasma colliders. The soliton is excited at the non-linear electron wake radius due to the time-asymmetry of its radial fields. In a non-equilibrium wake heated plasma the radial electron temperature gradient drives the soliton. Its radially outwards propagation leaves behind a partially-filled ion-wake channel. </p> <p>We show positron-beam driven wakefield acceleration in the ion-wake channel. Optimal positron-wakefield acceleration with linear focussing fields is shown to require a matched hollow-plasma channel of a radius that depends upon the beam properties. </p> / Dissertation

In simulation of closely- or separately-joined rock masses, stability of rock blocks is of primary concern. However, there seems to be no approach that can handle general modes of simultaneous sliding and truly large rotation under general forces, including non-conservative forces such as water forces. General causes of failure for rock blocks, such as limit points, bifurcation points, and dynamic instability (divergence and flutter), have never been addressed. This research implements a formulation, called BS3D(an incremental-iterative algorithm introduced by Tonon), for analyzing general failure modes of rock blocks under conservative and non-conservative forces. Among the constitutive models for rock fractures developed over the years, Barton's empirical model has been widely used because it is easy to apply and includes several important factors associated with fracture characteristics. Although Barton's failure criterion predicts peak shear strength of rock fractures with acceptable precision, it has some weaknesses in estimating the peak shear displacement, post-peak shear strength, dilation, and surface degradation in unloading and reloading. In this dissertation, modifications are made to Barton's original model in order to address these weaknesses. The modified Barton’s model is validated by a series of direct shear tests on rock fractures and implemented in BS3D to consider the dilatant behavior of fractures. The mechanical behavior of a rock block formed in the roof of a tunnel is governed by its geometry, the mechanical characteristics and the deformability of the fractures forming the block, the deformability of the block and that of the surrounding rock mass, and the stresses within the rock. BS3D, after verification and validation, is used to investigate the effect of dilatancy on stability of rock blocks formed in the roof of a circular tunnel. High-velocity plunging jets, issuing from hydraulic artificial or natural structures, can result in scouring of the rock riverbed or the dam toe foundation. Assessment of the extent of scour is necessary to ensure the safety of the dam and to guarantee the stability of its abutments. BS3D is used to investigate effect of high-velocity jet impact on stability of rock blocks in plunge pools. / text