DSpace Collection:
https://dspace.lboro.ac.uk/2134/8269
2015-04-11T05:49:24ZStochastic neural network dynamics: synchronisation and control
https://dspace.lboro.ac.uk/2134/16508
Title: Stochastic neural network dynamics: synchronisation and control
Authors: Dickson, Scott M.
Abstract: Biological brains exhibit many interesting and complex behaviours. Understanding of the mechanisms behind brain behaviours is critical for continuing advancement in fields of research such as artificial intelligence and medicine. In particular, synchronisation of neuronal firing is associated with both improvements to and degeneration of the brain's performance; increased synchronisation can lead to enhanced information-processing or neurological disorders such as epilepsy and Parkinson's disease. As a result, it is desirable to research under which conditions synchronisation arises in neural networks and the possibility of controlling its prevalence.
Stochastic ensembles of FitzHugh-Nagumo elements are used to model neural networks for numerical simulations and bifurcation analysis. The FitzHugh-Nagumo model is employed because of its realistic representation of the flow of sodium and potassium ions in addition to its advantageous property of allowing phase plane dynamics to be observed. Network characteristics such as connectivity, configuration and size are explored to determine their influences on global synchronisation generation in their respective systems. Oscillations in the mean-field are used to detect the presence of synchronisation over a range of coupling strength values. To ensure simulation efficiency, coupling strengths between neurons that are identical and fixed with time are investigated initially. Such networks where the interaction strengths are fixed are referred to as homogeneously coupled. The capacity of controlling and altering behaviours produced by homogeneously coupled networks is assessed through the application of weak and strong delayed feedback independently with various time delays. To imitate learning, the coupling strengths later deviate from one another and evolve with time in networks that are referred to as heterogeneously coupled. The intensity of coupling strength fluctuations and the rate at which coupling strengths converge to a desired mean value are studied to determine their impact upon synchronisation performance.
The stochastic delay differential equations governing the numerically simulated networks are then converted into a finite set of deterministic cumulant equations by virtue of the Gaussian approximation method. Cumulant equations for maximal and sub-maximal connectivity are used to generate two-parameter bifurcation diagrams on the noise intensity and coupling strength plane, which provides qualitative agreement with numerical simulations. Analysis of artificial brain networks, in respect to biological brain networks, are discussed in light of recent research in sleep theory.
Description: This thesis is restricted until 18th December 2015. A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.2014-01-01T00:00:00ZComputational models of ion erosion
https://dspace.lboro.ac.uk/2134/12216
Title: Computational models of ion erosion
Authors: Tagg, Malcolm A.
Abstract: The work in this thesis examines computational models of ion
erosion. It computes solutions to a non-linear wave equation which
governs the surface motion. by the method of characteristics. Some
experimental work on the rocking and rotation of samples of quartz which
are Jon-milled for use as surface acoustic wave resonators Is compared with
theoretical predictions. The computations pay particular attention to the
motion of edges and the development of edges on the surface as the
erosion proceeds. The thesis discussed secondary effects and examines
the problems generated by erosion In microelectronic device manufacture.
A line segment algorithm is also devised and programmed and
applied to the modelling of ion reflection.
Finally. a binary collision algorithm Is presented written recursively In
Algol 68 in order to describe the sputtering process In terms of Individual
events. The computed sputtering yields are then used in the simulations of
surface erosion described previously.
Description: A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University. This thesis is closed access for copyright reasons as it comprises of published articles.1987-01-01T00:00:00ZSimulating radiation damage in plutonium
https://dspace.lboro.ac.uk/2134/6254
Title: Simulating radiation damage in plutonium
Authors: Robinson, Marc
Abstract: Modelling radiation effects has become an important area of research due to the continual advances in nuclear technology. Plutonium, as a radioactive source, undergoes continual atomic level radiation events which over time change the mechanical properties and structural integrity of the material. Studying how damage accumulates and evolves is key to gaining an understanding of the ageing processes and the radiation tolerance of plutonium. This thesis presents ﬁndings based upon computer simulation using classical molecular dynamics (MD). This, combined with long time scale dynamics (LTSD) techniques enable low level investigations of collision cascades and studies of point defect diﬀusion. Using the Modiﬁed Embedded Atom Method (MEAM) to govern atomic interactions, pure δ-Pu, Ga stabilised δ-Pu and Pu-Ga-He systems were studied. The ballistic stages of the radiation event were found to be similar for both pure Pu and Ga
stabilised Pu, with low impact energies producing a cascade structure comparable to other fcc metals. Isolated defects tended to be of type Pu, with displaced Ga atoms returning to lattice sites. A clear observation at high impact energies was the ease at which both Pu and Ga stabilised Pu amorphise, resulting in an increased recovery time. Over long time scales, calculations suggest dynamics will be dominated by interstitial diﬀusion, with vacancies more likely to migrate as clusters. Defect diﬀusion mechanisms were observed to avoid Ga-rich regions, with Ga atoms relatively immobile in the Pu matrix. Results suggest the inclusion of Ga does not effect defect production but strongly inﬂuences defect diffusion. Investigating the presence of He in Ga stabilised δ-Pu suggested He would tend to reside in a substitutional site and is then rendered immobile. Local vacancies induced He migration to a certain degree yet the mechanism for He diﬀusion which may lead to bubble growth is unclear. Calculations regarding a range of He bubble conﬁgurations indicated a 1:1 He:vacancy ratio concentration was favourable relative to isolated substitutional He atoms. Results also suggested there exists an additional stable bubble conﬁguration of around 2:1 He:vacancy ratio with a diameter of more than 1.2 nm. Studying the interaction of radiation damage with a local He bubble indicated that cascade structure can be greatly eﬀected, resulting in a signiﬁcant increase in the number of residual defects.
Description: This thesis is confidential. A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.2010-01-01T00:00:00Z