The suppression of radial hydromagnetic oscillations of a theta pinch plasmoid produced in air has been investigated with magnetic probes and a framing camera. In the presence of essentially static and uniform magnetic bias fields (produced by Helmholtz coils) it was found that the oscillations are suppressed by a flip instability of the plasmoid and the direction of flip is controlled by the transverse magnetic field. Using the suppression of radial hydromagnetic oscillation as the criterion for flip, it is found experimentally that if [formula omitted] is greater than a critical value, then the plasma ring flips. [formula omitted] is the strength of the bias field perpendicular to the axis of the theta coil, 'a' the radius of the discharge vessel and 'p' is the gas pressure. The experimental results are interpreted in terms of a snowplough model for the discharge, in which the plasma current forms two concentric cylinders. The predictions of the theory agree with the experimental observations. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Theoretical expressions for the transport coefficients of a single component gas with a nonzero but small local angular momentum density are obtained from a modified Boltzmann equation which takes into account the presence of degenerate internal states (specifically, rotational states). As is to be expected, a number of Onsager reciprocal relations are found connecting the transport coefficients. A linearization of the Boltzmann equation is carried out by means of a perturbation expansion about a local equilibrium state which is characterized by a local temperature, stream velocity and angular momentum density. This perturbation is expressed as a linear combination of the macroscopic gradients of the system, whose coefficients, being tensors, are expanded in terms of irreducible Cartesian tensors made up of the angular momentum pseudovector operator J and the reduced velocity vector W. The transport coefficients are then given by combinations of certain scalar expansion coefficients. Expressions for these expansion coefficients in terms of square bracket integrals are obtained with the aid of an iterative variational procedure based on a scalar product which allows for the lack of time reversal symmetry of the Boltzmann collision operator. Finally, the square bracket integrals are reduced to relative and center-of-mass coordinates and expressed in terms of generalized collision cross sections. The techniques developed for the rotating gas with a nonzero local angular momentum density are utilized to obtain an expression for the change in the thermal conductivity of a gas when placed in a magnetic field. It is shown that at saturation the ratio of the changes in the thermal conductivity with the magnetic field (a) parallel to, and (b) perpendicular to, the temperature gradient is 2/3. This value agrees with the experimental result for paramagnetic gases. / Science, Faculty of / Chemistry, Department of / Graduate

The temperature of a Helium plasma produced in a shock tube has been determined from absolute intensity measurements. The plasma was considered to be homogeneous, transparent and in local thermodynamic equilibrium. The emissivity of the plasma has been measured by comparing the plasma radiation to the radiation from a carbon arc through a simple optical system. The influence of errors in measured parameters and the presence of impurities is discussed. Temperatures which depend on large exponential terms are relatively insensitive to both errors in measurement and the impurity content of the plasma. The accuracy of absolute intensity measurements is at best comparable to relative intensity measurements. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Reignition curves have been obtained for a high current (40ka) spark gap channel in air at atmospheric pressure at various radial distances down to delay times of 50 µs. after initiation of spark. The reignition curves in the ‘thermal breakdown' regime are obtained by a constant voltage generator having a variable output ranging from 100 volts to 2.5 kv, while those in the 'spark breakdown' regime are obtained by the use of a restriking voltage generator having a higher voltage output ranging from 1 kv to 15 kv. The experimental results show that the gas at large radial distances recovers faster than that at a smaller distance. A special feature of the recovery curves is the occurrence of a dip which is thought to be the effect of a thermal wave. Gas temperatures are derived by using the equation of state, Paschen curves and the reignition measurements. A temperature profile of the spark channel has been obtained which indicates that the spark channel has a fairly uniform temperature up to a radial distance of 2.5 cm. Beyond this distance the temperature drops rapidly to ambient temperature at a radial distance of 4 cm, Deionization and recovery mechanisms are predicted theoretically and are verified during the earlier recovery period in which 'thermal breakdown' occurs and during the later recovery period when 'spark breakdown' is observed. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The glow phase of high pressure spark discharges initiated by the Townsend mechanism of breakdown in hydrogen, nitrogen and carbon dioxide is studied using 16 ohm coaxial cable discharge techniques. Before the establishment of the spark channel in these low over-voltaged spark discharges, there exist two distinct transition stages, namely, a diffuse glow phase and a filamentary glow-to-channel transition phase. The optical appearance of the diffuse glow is similar to that of a normal d.c. glow discharge. It consists of a negative glow, a Faraday dark space, and a positive column. From our experimental results it is concluded that for the glow phase the cathode fall voltage and the ratio of the axial potential gradient to the gas pressure in the positive column are fixed for each gas. These values are independent of the impedance of the external circuit and the gas pressure. Comparisons with low pressure d.c. glow discharges indicate further that the cathode fall voltages are in agreement in both types of glow for the gases' investigated. In addition, a qualitative discussion about the transition nature of this diffuse glow phase is given on the basis of results obtained from spectroscopic investigations. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Two investigations are reported on the application of resonantly enhanced multiphoton ionization (RMPI) to gaseous mixtures of an atomic vapor and a few Torr of argon. Photon fluxes large enough to ionize atoms by RMPI were produced by focussing down the light from a tunable pulsed dye laser. The irradiances generated were of the order of 500MWcm⁻². Ionization was detected by a voltage biased wire electrode that simply collected the photoelectrons either directly or after some gas multiplication. One investigation was the measurement of the dependence of RMPI processes in mercury on the polarization of the incident light for comparisons with theoretical calculations. The processes were four-photon resonant absorptions to either a ¹S₀ or ¹’³D₂ level followed by single photon ionization. Complete photoionization of all atoms excited to the resonant levels is established and the measured polarization dependences are found to agree with the calculated polarization dependence for the resonant excitation step of the RMPI process. Unexplained observations of the distortions in the polarization dependence of the ion yield and the absorption linewidth of the 6d ¹D₂ resonance are discussed. The second investigation was a study of the density dependent electric dipole forbidden two-photon resonant transition ²S → ²P in three-photon RMPI spectra of the Rydberg states of potassium. Stark interactions are shown to be unlikely and too weak. From the characteristics of the spectra, the excitation process is identified as a laser-assisted collision interaction. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Improvements to the laser excited interferometer previously constructed in this laboratory are described. The use of a rotating retro-reflector in the interferometer cavity, and electronic circuitry, permit direct recording of electron densities. The sensitivity of the interferometer is 5 x 1015 cm-2, and the temporal resolution is 0.1 usee. Pyrex tubes, terminated by windows, enclosing the laser beam, have been used to improve spatial resolution. The effects of the tubes, and of all other sources of error associated with interferometric measurements of electron densities, have been carefully assessed. A Z-pinch discharge in 4 torr helium has been studied with this instrument, and with spectroscopic determinations of electron temperature. Under the conditions chosen, the pinching plasma does not reach the axis of the discharge tube, but stops at a radius of 2.7 cm. The shock front caused by the pinching action has been shown to be weak. Longitudinal structure, due to heat flux, characteristic cathode behaviour, and variations in timing of the pinching action, has been found. The discharge has been assessed as a spectroscopic source and as a medium for laser scattering experiments. The characteristics of two suitable regions are presented. These are the axial region and the hollow cylinder of plasma formed by the arrest of the pinching plasma. Both are free of instabilities. At the appropriate time the latter is also current free, and shows longitudinal electron density variations of less than 2%. The length of time that the current spends near the wall of the vessel is determined by the balance between kinetic and magnetic pressures. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Orientador: Francisco Carlos de Prince / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-07-15T15:34:23Z (GMT). No. of bitstreams: 1 Machado_AldionsoMarques_M.pdf: 1211023 bytes, checksum: 9c6ebec2a32623cf24b633586f83a665 (MD5) Previous issue date: 1984 / Resumo: O trabalho aqui apresentado começou a ser desenvolvido em setembro de 1981 quando iniciávamos o projeto de um sistema de LPE com o objetivo de ser usado no crescimento de camadas epitaxiais para obtermos as características elétricas e óticas necessárias à fabricação de Transistores de Efeito de Campo (FET). Este Transistor exige camadas com características bem definidas. A camada que separa o substrato da região ativa do dispositivo é a de obtenção mais difícil, pois deve possuir alta pureza e alta resistividade. O seu objetivo é evitar a difusão de impurezas provenientes do substrato para a região ativa do dispositivo, evitando assim que haja alterações nas características elétricas desta região. No decorrer deste trabalho estudamos várias formas de obtenção de camadas de alta pureza. Nos concentramos naquelas que utilizavam um sistema de LPE do mesmo tipo que dispomos, isto é, um sistema que utiliza tubos de quartzo, bote de grafite e atmosfera, de hidrogênio. Um procedimento que se destacou, devido ao seu grande uso, foi o cozimento prolongado da solução de crescimento. Alguns trabalhos analisados mostraram que este cozimento prolongado apresentava melhores resultados quando realizado em uma temperatura bem definida, que para este tipo de sistema é 775ºC. Resolvemos adotar o procedimento de trabalho sugerido por J.K. Abrokwah [16], onde além de cozermos a solução de crescimento a 775ºC, cozemos também o substrato, com sua superfície exposta ao fluxo de hidrogênio. Com este procedimento esperávamos conseguir camadas epitaxiais com densidades de portadores livres em torno de 1014 cm-3. No decorrer do trabalho nos deparamos com algumas dificuldades que atribuímos principalmente à qualidade do substrato por nós utilizado. Nesta tese expomos estas dificuldades tentamos explicá-las baseados nos resultados obtidos, sugerimos maneira de evitá-las. Mostramos que podemos obter camadas epitaxiais com densidade de portadores livres em torno de 1015 cm-3 e que densidades menores podem ser obtidas, mas as amostras crescidas sob as mesmas condições não fornecem os mesmos resultados devido à qualidade do material utilizado / Abstract: Not informed. / Mestrado / Física / Mestre em Física

Orientador: Paulo Hiroshi Sakanaka / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-07-17T03:26:30Z (GMT). No. of bitstreams: 1 Constantino_Cesar_M.pdf: 1523594 bytes, checksum: a208edff7c9d5981659fd3c377559f60 (MD5) Previous issue date: 1979 / Resumo: Estuda-se numericamente o efeito do termo dissipati vo da equação de Korteweg-de Vries-Burgers sobre a evolução de um pulso quadrado e de um sóliton. Para o pulso quadrado, o efeito de amortecimento é mais forte que o previsto por uma análise assintótica. Para tempo pequeno, todas as soluções são oscilatórias. O sóliton com amortecimento se propaga com velocidade constante e igual à do sóliton ideal de mesma amplitude inicial. Observa-se distorsão não-linear e diminuição da amplitude durante a propagação. O quadrado da meia-largura do sóliton com amortecimento varia linearmente com o inverso de sua amplitude, como ocorre com o sóliton ideal, mas o sóliton com amortecimento é mais largo que o sóliton ideal de mesma amplitude / Abstract: The effect of the dissipative term of the Korteweg de Vries Equation on the evolution of a square pulse and of a soliton is studied numerically. For the square pulse, it is found that the dissipative effect is stronger than predicted by an asymptotic analysis. For small time, however, all the solutions are oscillatory. The soliton with dissipation propagates with a constant velocity equal to that of the ideal soliton of the same initial amplitude. Nonlinear distorsion and damping of amplitude are observed during the propagation. The squared half-width of the soliton varies linearly with the inverse of its amplitude, as in the non-dissipative case, but the width is greater than that of the ideal soliton of the same amplitude / Mestrado / Física / Mestre em Física

This dissertation describes two experimental studies of a vibrofluidized granular medium and a preliminary study of two-phase fluid flow in a porous medium using Nuclear Magnetic Resonance (NMR). The first study of granular medium is to test a scaling law of the rise in center of mass in a three-dimensional vibrofluidized granular system. Our granular system consisted of mustard seeds vibrated vertically at 40 Hz from 0g to 14g. We used Magnetic Resonance Imaging (MRI) to measure density profile in vibrated direction. We observed that the rise in center of mass scaled as ν 0α/Nlβ with α = 1.0 ± 0.2 and β = 0.5 ± 0.1, where ν 0 is the vibration velocity and Nl is the number of layers of grains in the container. A simple theory was proposed to explain the scaling exponents. In the second study we measured both density and velocity information in the same setup of the first study. Pulsed Field Gradient (PFG)-NMR combined with MRI was used to do this measurement. The granular system was fully fluidized at 14.85g 50 Hz with Nl ≤ 4. The velocity distributions at horizontal and vertical direction at different height were measured. The distributions were nearly-Gaussian far from sample bottom and non-Gaussian near sample bottom. Granular temperature profiles were calculated from the velocity distributions. The density and temperature profile were fit to a hydrodynamic theory. The theory agreed with experiments very well. A temperature inversion near top was also observed and explained by additional transport coefficient from granular hydrodynamics. The third study was the preliminary density measurement of invading phase profile in a two-phase flow in porous media. The purpose of this study was to test an invasion percolation with gradient (IPG) theory in two-phase flow of porous media. Two phases are dodecane and water doped with CuSO4. The porous medium was packed glass beads. The front tail width σ and front width of invading phase were extracted from fitting of the invading front profile. The front tail scaled as σ∞Ca −α, where Ca is capillary number and α is 0.4 ± 0.08. The result is very close to IPG predication 0.25.