Thesis (M.S. in Physics)--Naval Postgraduate School, June 2004. / Thesis advisor(s): Ryan Umstattd. Includes bibliographical references (p. 27-28). Also available online.

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Com hipóteses de simetria plana e que a distribuição inicial de cargas, ocupa totalmente a amostra (tocando os dois eletródios), estudou-se o problema do movimento de carga espacial livre (M.C.E.L.) monopolar, em sólidos isolantes, submetidos à uma d.d.p. conhecida. Utilizando o método das características, reduziu-se o problema à um sistema de duas equações diferenciais ordinárias,de ia. ordem. O método foi aplicado para as distribuições linear, exponencial e p ~ x -1/2 , sob condições de curto circuito. são apresentados gráficos relativos à evolução temporal do perfil da densidade de cargas e da corrente externa. Estes resultados são comparados com outros obtidos por um cálculo aproximado, baseado em princípios variacionais . / Assuming planar symmetry and an arbitrary charge distribution that spreads through the sample, the one-carrier free - space-charge motion is studied in insulators solids. Using the method of characteristics we can reduce the problem t o the resolution of a system of two ordinary first order differential equations. Results are applied for linear, exponential and S.C.L. current charge distributions , under short-circuit conditions, The charge distribution for several times and the discharge currents are presented. The results are compared with those coming from an approximated method, based on variational principles.

This study uses an magnetohydrodynamics (MHD) model to simulate unsteady one-dimensional transport variables in argon plasma, under low pressure and weak ionization between two planar electrodes suddenly biased by a negative voltage or electric field. Plasma has been widely used in etching, ion implantation, light source, and nuclear fusion, etc. Studying transport processes of plasmas therefore is important. Ignoring magnetic field, collisions between ions and electrons, the computed results in this work shows density, velocity, voltage, electric field, energy and temperature transport phenomenon in different cases. The results give insight by theory and simulation the surfaces behavior in plasma. Keywords: magnetohydrodynamics (MHD), transport variables in sheath,space charge

High intensity proton synchrotrons provide beams for several types of facility around the world, including spallation neutron sources and high energy physics experiments. The defining feature of these particle accelerators, that of intense beams, is tightly coupled to what limits the intensity, which is the controlled loss of beam particles. Many different factors contribute to beam loss. Beam will be lost on injection to a synchrotron and may be lost on extraction or in transfer lines. Non-linearities in the accelerator lattice can introduce driving terms for resonant beam behaviour. Collective effects between the beam particles and with the beam environment modify the single particle behaviour considerably. High intensity loss that occurs in the transverse plane, due to space charge and image fields, was investigated. The rapid cycling synchrotron at the ISIS Spallation Neutron Source in the UK was the focus for all of the work. The ISIS Synchrotron has many particular features which were described. One such feature is the conformal rectangular vacuum vessel, which takes the shape of the design beam envelope with certain modifications. This vacuum vessel has a complex effect on beam image fields. Numerical tools to study the space charge and image fields at ISIS were created and reported. The tools included two Poisson solvers to study space charge and images which were benchmarked against commercially available algorithms. A two dimensional particle-in-cell tracking code was created using the space charge solvers in combination with either a smooth focusing lattice model or one which generated Twiss matrices. A variety of diagnostic tools were available. A survey of existing analyses for pencil beams in parallel plate and rectangular geometry was made. Results from the analysis were then compared with two dimensional simulations with round uniform beams in rectangular geometry. Differences and extensions to the analysis were summarised. Coefficients for higher order image terms were defined and tabulated. The two dimensional nature of the image field was discussed and values for the coefficients for certain higher order terms identified in the plane orthogonal to the beam offset. Solutions for closed orbits produced with single and harmonic kicks at low and high intensity were discussed and simulated. A model was proposed which included the higher order image coefficients produced by the closed orbits. A single particle model was then explored which obtained resonance conditions from the closed orbits and image coefficients. The effect of self-consistent coherent motion on the results was discussed. Particle-in-cell beam tracking simulations were used to explore the results of this analysis numerically. Image resonances were found and described for a variety of simulation parameters starting with a smooth focusing lattice and uniform density beam, then progressing to more realistic cases including waterbag beams, alternating gradient lattices and conformal vacuum vessels. Image resonances described by the models were reported as were others that needed further explanation. Their possible impact for ISIS was discussed. New experiments with coasting beams at ISIS were carried out to explore the relationship between tune and beam loss at low intensity. Such experiments are a vital first step to understanding high intensity behaviour. It was shown that ISIS has existing lattice nonlinearities (some known, some unknown) which will need to be taken into account for high intensity experiments and simulations. Finally this work was put into context by examing specific transverse space charge effects for a proposed ISIS upgrade and including ideas developed throughout the thesis. Estimates were made of the strength of space charge effects and emittance scaling using conventional methods. The particle tracking tools developed for the thesis were then used to study beam behaviour with lattice gradient errors, the effects of closed orbits and changes to the working point. The transverse calculations and simulations suggested that the upgrade was feasible.

Com hipóteses de simetria plana e que a distribuição inicial de cargas, ocupa totalmente a amostra (tocando os dois eletródios), estudou-se o problema do movimento de carga espacial livre (M.C.E.L.) monopolar, em sólidos isolantes, submetidos à uma d.d.p. conhecida. Utilizando o método das características, reduziu-se o problema à um sistema de duas equações diferenciais ordinárias,de ia. ordem. O método foi aplicado para as distribuições linear, exponencial e p ~ x -1/2 , sob condições de curto circuito. são apresentados gráficos relativos à evolução temporal do perfil da densidade de cargas e da corrente externa. Estes resultados são comparados com outros obtidos por um cálculo aproximado, baseado em princípios variacionais . / Assuming planar symmetry and an arbitrary charge distribution that spreads through the sample, the one-carrier free - space-charge motion is studied in insulators solids. Using the method of characteristics we can reduce the problem t o the resolution of a system of two ordinary first order differential equations. Results are applied for linear, exponential and S.C.L. current charge distributions , under short-circuit conditions, The charge distribution for several times and the discharge currents are presented. The results are compared with those coming from an approximated method, based on variational principles.

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Several results were reviewed on the charge injection into thin conjugated polymer films. The space charge limited tunneling current (SCTC) and space-charge limited current (SCLC) models were used to explain the charge injection and transport in conjugated polymer films such as those used in organic light emitting diodes. On the basis of regional approximation, the effect of the space charge on the current density of electrons tunneling from metal electrodes to the lowest unoccupied molecular orbital of a polymer film is calculated. The space charge is considered to decrease with increasing distance of the injecting electrode. If the space charge occupies only a limited region between the tunneling (short) distance and the collecting electrode, the current (I) is found to be independent on a power law of the applied voltage (V). However, if the space charge occupies all the region between the tunneling distance and the collecting electrode, I is found to vary exponentially on V for lower V values, and that follow approximately the law of Child at high V in region I, the law of Trap filled limited current at middle voltage in region II and the region III show Ohm¡¦s law at lower voltage. The space charge limited tunneling current (SCTC) theory gives the same results of the space charge limited conduction theory when the energy barrier for charge carrier injection is small or when the polymer layer is thick. Under various bias voltages, the charge transport mechanism was analyzed to investigate the effects on the thin conjugated polymer films induced by the parameters such as background concentration, trap-state density and space charge etc... The results show the transport current decrease with increasing the trap-state density and also increase the characteristic temperature parameters. In this study, the regional approximation is applied to calculate the charge injection and current transport mechanism in the organic thin film. The SCTC and SCLC models are compared to the experimental data. It shows that there is a good agreement between theory and experiment, concerning both current magnitude and current versus voltage dependence. It is very important that our model gave a new way to simplify the calculation and to discuss the charge transport of organic light emitting diode.