The motion of a theta pinch hydrogen plasma in the presence of a local perturbing magnetic field is described. A small coil outside the plasma was excited by capacitors to produce a dipole field. Experiments were performed on a 3.5m long, collisional, high-beta plasma with a temperature of 60 eV, a radius of 1 cm and a density of 10¹⁶ particles cm^-3. Sinusoidal currents of 20kA and frequency 10⁶ rad s^-1 in a 3-turn perturbing field coil produced oscillating plasma displacements of 1 cm, which propagated along the plasma with a velocity of 20 cm mus^-1. The displacement was proportional to the coil current, inversely proportional to the axial magnetic field outside the plasma, and was spatiplly damped with an e-folding length of 20 cm. Using a step current waveform the plasma could be moved to an equilibrium position which was displaced from the initial unperturbed equilibrium. Experiments on a 2 m long low-beta plasma with a temperature of 10 eV showed that propagating plasma displacements of 0.5 cm could be obtained, which were damped in 5 cm. Predictions of the plasma displacement and velocity are made by considering the motion as long wavelength, m = 1 perturbations about an equilibrium position. Damping mechanisms are introduced by making an analogy between the excited waves and Alfven waves in a dissipative medium. Feedback stabilization of long wavelength gross modes in theta-pinch plasmas is shown to be possible, using a system of coils similar to those used to excite plasma motion. Experiments were performed to determine the effect of the initial conditions on the 3.5m theta-pinch parameters, and the results compared with computations. A computed heating rate was obtained; including partial ionization of the initial gas revealed a value of the initial rate of change of axial magnetic field below which no heating occurred. This cut-off was overcome experimentally by using bias fields.

The azimuthal magnetic field and current density structure of a Plasma Focus device, have been investigated by the observation of the Faraday rotation of a ruby laser probe beam. The magnetic field and current distributions play important roles in plasma compression, heating, confinement, stability, and particle acceleration. However, in over two decades of research, no satisfactory experimental data on the field or the current have been produced. The results reported are of the first measurements of magnetic field and current distribution in the Plasma Focus, using a non perturbing diagnostic technique. A full description of the experimental apparatus and method, the physics of the differential polarimetry (with refraction effects included), and theoretical reviews of Faraday rotation, birefringence, dichroism, and refraction, are given. It was found that during the collapse phase of the discharge, times t = -10 ns to t = 0 ns (peak compression), the current and field are confined to the plasma skin. The penetration depth is 0.56 mm, and the resistivity is classical. During the dense pinch phase, between t = 0 ns and t = +10 ns, the plasma develops a turbulent core, of radius 2 mm, in which the resistivity is highly anomalous (by a factor 6000). This results in a rapid diffusion (lasting approximately 10 ns) of field and current (typically 20 of the total) into the core. Particle acceleration is suppressed at this stage because the ion Hall term is less than unity. Outside this core, the resistivity is classical, and the current is carried in the plasma skin. At times t = +10 ns to t = +15 ns, axial current filamentation was observed. These filaments last less than 2.5 ns, and carry in excess of 12% of the current. Future studies of the filamentation should lead to a better understanding of the intense neutron production observed in Plasma Focus devices.

The frequency spectrum of density fluctuations in a Plasma Focus device has been investigated by Thomson scattering of laser light. A ruby laser scattering diagnostic has been developed to enable the co-operatively scattered ion feature, Si(k,w) to be measured simultaneously along different density fluctuation k vectors. Data derived from this apparatus is interpreted in terms of the time and space variations of local plasma parameters such as electron and ion temperatures, drift velocity, bulk velocity and level of turbulence. The total scattered intensity, Si(k) is shown to depend on the direction in whichthe fluctuations are sampled by the scattering geometry, with scattered intensities along a Je vector parallel to the current flow enhanced considerably above thermal levels. The data is found to be most satisfactorily interpreted in terms of a double radial shell model of the plasma, each shell with its own characteristic particle temperatures, density, drift and bulk velocity. At peak compression of the pinch onto the z-axis of symmetry for example, the core plasma has typically electron and ion temperatures Te = 200 eV, Ti = 250 eV respectively, while the outer sheath region exhibits a shot to shot variation in the temperature structure, with Te=2.3 keV, Ti=850 eV and Te = L.65 keV, Ti = 1.4 keV respectively. An examination of the relative enhancementand broadening of Si(k,w) along the various k-vectors has been interpretedin terms of current driven turbulence. The various mechanisms for inducing turbulence have been considered and lower-hybrid drift, electron-cyclotron drift and ion acoustic instabilities are thought to play a role in the various phases of the plasma discharge. The development of the multiple Je scattering diagnostic has overcome the ambiguity in interpreting scattering results from an irreproduceable inhomogeneous plasma. This problem has restricted the interpretation of the results in previous scattering experiments on the Plasma Focus using single Je vector scattering arrangements.

High speed optical measurements are important for laser fusion studies. At present, streak cameras provide the most versatile direct optical recording techniques with picosecond time resolution, but they possess limited dynamic range. Their poor performance is believed to be partly due to space charge effects at the electron optical crossover point. This was experimentally investigated in this thesis, but found to be relatively unimportant. Effects inside the photocathode are suggested as an alternative explanation. The dynamic ranges, sweep speeds and linearity of two commercial streak camera systems were measured using a modelocked dye laser and an etalon. The spatial resolution was measured with a test chart. These two streak cameras were then used to study the temporal evolution of the backscattered fundamental and harmonic emission spectra from micro balloon targets when they were irradiated by a neodymium laser. The spectra at the fundamental frequency displayed the characteristic red shift of Brillouin scattering. The spectral features of the 2wq harmonic are explained in terms of ion acoustic turbulence driven by the electron return current. The harmonic spectra were observed to consist of two wings, symmetrically shifted to the blue and red, with a separation depending on the electron temperature. Both the 2w and 3w/2 harmonic emission were found to consist of picosecond pulsations which are attributed to density perturbations driven by the unstable ablation flow across a steepened density profile. Finally, proposals are made for the development of diagnostic techniques in laser produced plasmas.

A study of harmonic generation in Laser produced plasmas is presented. Experiments were performed on Royal Holloway College's carbon-dioxide laser system and on the Rutherford Appleton Laboratory's neodymium-glass laser facility. Various targets were irradiated. The backscattered radiation was spectrally analysed in the vicinity of the incident(w0) and twice incident frequency(2w0). Optical and x-ray diagnostics were also undertaken. Theoretical models for harmonic generation in laser produced plasmas by Cairns, Erokhin, Silin and others are reviewed and compared to the experimental results. It is shown that theories due to Cairns and Silin give reasonable estimates of plasma temperature, from the experimental shift of the second harmonic (2w), though the former requires the plasma density scale-length to be known to a greater accuracy. However, Cairns explains satisfactorily the observed structure of the second harmonic spectra. Other theories account well for the observed dependence of the intensity of the second harmonic to that incident. Finally, an attempt is made to explain features of the 2w spectrum in terms of plasma motion, resonance absorption, density profile modification and ponderomotive forces.

An experimental investigation of the impurity behaviour of the Reversed Field Pinch (RFP) HBTX1A/B is presented. The investigation was initiated by determining the impurity composition of the plasma through spectral surveys utilising both photographic and multi-channel techniques. The principal low Z impurities were found to be carbon and oxygen while the principal high Z impurity was iron with smaller amounts of chromium and nickel. The investigation was extended using quantitative techniques in the Extreme Vacuum Ultraviolet (EVUV), Vacuum Ultraviolet (VUV) and Ultraviolet(UV)/visible regions of the spectrum. A novel technique involving a scintillator imaged on to an Optical Multichannel Analyser (OMA) was used for quantitative work in the VUV region and a unique scanning toroidal mirror system was also developed for this range. It was found that the impurity concentration was a small fraction of the electron density (5% 0, -1% C, -0.2% Fe) and, in conjunction with the bolometric results from other workers, it was found that the impurity radiation was a small fraction (-4%) of the global input power (for I/N >7 x 10-14 A.m) and did not lead to radiative cooling at any position in the discharge. The value of the resistivity calculated from the impurity composition was significantly less than the value obtained from helicity balance calculations, assuming a perfect boundary. The impurity diffusion coefficient for carbon was determined using a unique technique based on spatial measurements of CV emission. The measured value of 100-150 m2.s-1 was anomalously high compared to neoclassical predictions. It was not possible to determine the diffusion mechanisms of either the bulk plasma or the impurities, hence this measurement represents the first step in a more detailed investigation of RFP impurity dynamics.

<p> The conditions needed for the amplication of radiation by an ensemble of magnetized, relativistic electrons that are collisionally slowing down are investigated. The current study is aimed at extending the work of other researchers in developing solid-state sources of Terahertz radiation. The source type considered here is based on gyrotron-like dynamics of graphene electrons, or it can alternately be viewed as a solid state laser source that uses Landau levels as its band structure and is thus similar to a quantum cascade laser. Such sources are appealing because they offer the potential for a compact, tunable source of Terahertz radiation that could have commercial applications in scanning, communication, or energy transfer. An exploration is undertaken, using linear and nonlinear theories, of the conditions under which such sources might be viable, assuming realistic parameters. Classical physics is used, and the model involves electrons in monolayer graphene assumed to be pumped by a laser, follow classical laws of motion with the dissipation represented by a damping force term, and lose energy to the electromagnetic field as well. The graphene is assumed to be in a homogeneous magnetic field, and is sandwiched between two partially-transmissive mirrors so that the device acts as an oscillator. </p><p> This thesis incorporates the results of two approaches to the study of the problem. In the first approach, a linear model is derived semi-analytically, which is relevant to the conditions under which there is gain in the device and thus stable operation is possible, versus the regime in which there is no net gain. In the second approach, a numerical simulation is employed to explore the nonlinear regime and saturation behavior of the oscillator. The simulation and the linear model both assume the same original equations of motion for the field and particles that interact self-consistently. The model used here is very simplied, but the aim here is to elucidate the basic principles and scaling behavior of such devices, not necessarily to calculate what the exact dynamics, outputs, and parameters of a fully commercially realized device will be.</p><p>

Este trabalho procura caracterizar uma cavidade eletromagnetica ressonante toroidal teorica e experimentalmente. Do ponto de vista teorico, foi obtida a relacao de dispersao para as autofrequencias da cavidade, resolvendo as equacoes de maxwell usando teoria de perturbacao. Duas solucoes em ordem zero foram obtidas: uma em termos de funcoes hipergeometricas e outra em termos de uma serie de frobenius. Os resultados foram comparados com a literatura. Baseados nestes autovalores calculados, foi projetado e construido um toroide em cobre, com seccao transversal circular e razao de aspecto 1,25 (\'R IND.0\'=0.125m e \'R IND.0\'=0.100m). As medidas experimentais realizadas incluem as autofrequencias e seus respectivos indices de merito. Pelo que se sabe, estas medidas foram as primeiras feitas num toroide. Os dados experimentais tambem foram comparados com os resultados teoricos, permitindo estabelecer qual dos tratamentos e mais compativel com a realidade e fornecendo elementos para um aprimoramento da teoria / Electromagnetic resonance in toroidal cavities - theory and experience

<p>This thesis demonstrates a measurement of a plasma fluctuation velocity-space cross-correlation matrix using laser induced fluorescence. The plasma fluctuation eigenmode structure on the ion velocity distribution function can be empirically determined through singular value decomposition from this measurement. This decomposition also gives the relative strengths of the modes as a function of frequency. Symmetry properties of the matrix quantify systematic error. The relation between the eigenmodes and plasma kinetic fluctuation modes is explored. A generalized wave admittance is calculated for these eigenmodes. Since the measurement is a localized technique, it may be applied to plasmas in which a single point measurement is possible, multipoint measurements may be difficult, and a velocity sensitive measurement technique is available.

<p> The absence of a protecting magnetic field, such as the dipole magnetic field around Earth, makes the interaction of solar wind with unmagnetized objects particularly interesting. Long-term evolution of the object&rsquo;s surface and atmosphere is closely tied to its interaction with the outer space environment. The ionospheric plasma layer around unmagnetized objects acts as an electrically conducting transition layer between lower atmospheric layers and outer space. This study considers two distinct types of unmagnetized objects: Titan and comet 67P/Churyumov-Gerasimenko (67P/CG). For many years, Titan has been a key target of the National Aeronautics and Space Administration (NASA) Cassini mission investigations; and the European Space Agency (ESA) Rosetta spacecraft explored comet 67P/CG for more than two years. </p><p> Ionospheric composition and primary ion production rate profiles for Titan are modeled for various solar activity conditions. Photoionization is the main source of ion production on the dayside; on the nightside, electron-impact ionization is the main ionization source. This dissertation uses model results and in-situ measurements by the Ion and Neutral Mass Spectrometer (INMS) and the Langmuir Probe (LP) onboard the Cassini spacecraft to show that while the solar activity cycle impacts the primary ion species significantly, there is little effect on heavy ion species. Solar cycle modulates the Titan&rsquo;s ionospheric chemistry. The solar cycle effects of on each ion species are quantified n this work. In some cases the solar zenith angle significantly overshadows the solar cycle effects. How each individual ion reacts to changes in solar activity and solar zenith angle is discussed in details. A method to disentangle these effects in ion densities is introduced. </p><p> At comet 67P/CG, the fast-moving solar wind impacts the neutral coma. Two populations of electrons are recognizable in the cometary plasma. These are the hot suprathermal electrons, created by photoionization or electron-impact ionization, and the cold/thermal electrons. Even though photoionization is the dominant source of ion production, electron-impact ionization can be as high as the photoionization for certain solar events. At 3 AU, electron energy spectra from in-situ measurements of the Ion and Electron Sensor (IES) instrument exhibit enhancement of electron fluxes at particular energies. Model-data comparisons show that the flux of electrons is higher than the typical solar wind and pure photoionization fluxes. The probable cause of this enhancement is the ambipolar electric field and/or plasma compression. </p><p> This research also discusses formation of a new boundary layer around the comet near perihelion, similar to the diamagnetic cavity at comet 1P/Halley. At each crossing event to the diamagnetic cavity region, flux of suprathermal electrons with energies between 40 to 250 eV drops. The lower flux of solar wind suprathermal electrons in that energy range can cause this flux drop. </p><p>