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The interaction of ultra-intense laser pulses >1018W/cm2 with thin foils drives the acceleration of protons to multi-MeV energies. Proton beam has unique properties such as low emittance, short pulse duration and high particle flux. However, the beams usually • exhibit large envelope divergence and quasi-Maxwellian energy spectra, which might be undesirable for a range of foreseen applications. Hot electrons generated during the interaction of intense lasers with foil targets escape the foil, charging it to potential of the order of the hot electron temperature. It is observed that the charge flows towards the ground with a velocity close to the speed of light in a localised pulse of a Gaussian profile with 6ps rise and 15ps decay, which retains its temporal profile over a few centimetres. Based on these findings, novel target geometry is envisioned to create an electrostatic lens which simultaneously focuses/collimates the proton beam and allows energy selection. This electrostatic lens demonstrates a reduction in beam diameter by 75% and an enhancement of the proton flux by an order of magnitude for 6.5 MeV protons in comparison to typical divergent proton beam. Particle tracing simulations corroborate the dynamics nature of the lens. Laser driven proton beam is employed as proton radiography technique to investigate the expansion of ablated plasma created by the interaction of intense laser >1015W/cm2 with solid target, into low density background plasma. High temporal and spatial resolution of the technique allows detection of the precursory stages that lead to formation of an electrostatic collision less shock at the boundary of blast shell of the expanding laser ablated plasma. The evolution of the electrostatic potential associated with the shock unveils the transition from a double layer into a symmetric shock structure, stabilized by ion reflection at the shock front. A PlC simulation supports the existence of super-critical electrostatic shocks.

The current momentum in plasmonics research was spurred on by the discovery of enhanced optical transmission. This phenomena results from the resonate coupling of electromagnetic surface waves on either side of a metal film. These waves are known as surface plasmon polaritons. By leveraging their ability to control light on the nanometric scale plasmonic structures are beginning to be integrated into a new generation of opto-electronics devices such as LEDS, photodetectors, photovoltaics and biosensors. This research has been made possible through the use new nanofabrication technologies. In particular, the Focused Ion Beam(FIB) has been key in the quick development and fabrication of the structures known as plasmonic crystals. These crystals are a periodic arrangements of apertures and gratings whose critical dimensions are of the same scale as the wavelengths of visible light. This is a requirement for efficient and controlled coupling between the incident electromagnetic wave and the resulting surface wave. By being able to selectively and precisely mill complicated geometries, the FIB allows for a mask less fabrication process of a variety of plasmonic devices. The structures fabricated in this manner offer the fast prototyping of systems examining fundamental optical interactions. The plasmonic crystals studied in this thesis look at systems where the signature feature is some form of asymmetry. In the first set of structures, study coaxial apertures in gold film on the high-index of refraction substrate, GaP. The GaP- gold substrate does not support SPP Bloch modes, resulting in an asymmetry in the transmission mechanisms. This asymmetry arises out of the interaction of the air-metal surface mode with the cavity mode associated with the sub-wavelength apertures. When the same coaxial structures are fabricated on glass substrates, the presence of a second set of surface modes creates a more complex system where the surface plasmon modes on each metal-dielectric interface interact via the resonate cavity mode. For both of the systems examined it is shown that this aperture geometry offers a significant improvement in transmission when compared to simple cylindrical apertures. The studies in the optical behaviors of GaP based plasmonic crystals showed that FIB patterning damages the optical transmission properties of the substrate. To improve the optical transmission, a FIB patterning strategy is developed which leads to over sevenfold increase in optical transmission for GaP based plasmonic devices. This is achieved through the use of low kV FIB patterning routines. The devices presented demonstrate the first example of fine structuring using a low kV FIB to produce functional devices. These devices demonstrate a 7x optical transmission enhancement when compared to standard fabrication approaches. Leveraging advances in FIB patterning capabilities to create complex structures allows for the exploration into the effects asymmetry on the transmission mechanisms possible in subwavelength apertures. By deforming the basic geometry of the coaxial aperture the boundary conditions of the optical system are altered. These new conditions open the possibility to excite different orders of surface modes on each side of the metal film. The structures fabricated demonstrate complex optical behaviors that provide for challenging experimental and numerical studies. The plasmonic saturates fabricated in this work, only begin to touch on the optical phenomena that can be explored through the selective application of material of structural asymmetry. Further, by the continual refinement of nanofabrication techniques, the ability to control the system studied is enhanced. Both of these processes work off each other to create the next generation of technologies.

The aim of this research is to better understand the behaviour of pulsed discharges and electron dynamics for the purpose of tailoring the plasma properties for neutral beam etching (NBE) applications. A capacitively coupled plasma formed in a research system was used for a study of pulsed tailoring in an electropositive plasma. A combination of high time resolved optical diagnostics, plasma imaging and optical emission spectroscopy, and hairpin probe measurements were used to study the electron density and the energy distribution function during the ignition phase of a repetitively pulsed plasma. Two different waveforms were used to modulate the envelope of the input RF-voltages in order to control the ignition phase, by changing the increase rate of the electron density and evolution of the electron energy distribution function (EEDF). The results of this study indicate that the increase rate of the electron density and the EEDF, during operation, can be influenced and even controlled to some extent by pulse tailoring. Electron densities of the order of 1016 m-3 were obtained, and EEDFs of a highly non-Maxwellian nature were characterised during the ignition phase. Also, the ignition timescales were controlled by applying pulse tailoring from a few microseconds (typically 2 μs) to a few tens of microseconds (80 μs) for the different input waveforms. An inductively coupled plasma in an industrial plasma etching tool was used to study pulse tailoring in electropositive and electronegative discharges. The same environment was used to create a source to from energetic negative ions which could then be extracted and neutralised. Similar diagnostic techniques, as those used in the research source, in addition to RF-probes were used to characterise the inductive source. Optical emission spectroscopy and electron density measurements showed that the plasmas, almost instantaneously, ignite in the H-mode. The EEDFs were characterised by a Maxwellian distribution with an electron temperature ranging between 1.2 up to 1.6 eV, and electron densities of the order 1018 m-3 were measured, depending on the operating conditions. This source was also used for preliminary NBE studies. Neutralisation efficiencies ranging between 70% and 95% were measured, and etch rates of 25 and 30 nm/min were found. Finally, a novel technique was developed to monitor in-situ and in real time the plasma-induced damage on thin films, in particular low-k dielectrics. This technique uses wafer tiles to simultaneously measure the plasma properties and plasma-induced damage on the thin films. An analytical model, based on the ion flux probe model, was used to to extract the plasma parameters and mimic the existence of thin on the probe surface, by a resistor and a capacitor connected in parallel. The model managed to extract exact values of a dummy load, while a reasonable fit could not be achieved for thins silicon dioxide films. Initial results showed that, even for a well characterised material of a known permittivity and thickness, the model could not extract absolute resistivity and capacitance. Plasmas surface interactions were deduced to be the main factor for this result, which were not included in the model used in this study. However, plasma-induced damage/changes on low-k thins films by different plasma chemistries, such as argon and hydrogen, were qualitatively measured in real-time. The wafer probe was used to monitor the etch rate of an SF6 plasma for low-k thin films in realtime and in-situ. This technique could also be used to measure the neutralisation efficiency and possibly the etch rates of our NBE source in real time.

Predicting the energetic and spectral characteristics of Z-pinch sources is a very delicate task. Z-pinch plasmas conditions are spread across a wide range of parameter space and often far from Local Thermodynamic Equilibrium (LTE). In addition, the increasing optical depth of the plasma at stagnation can have a strong influence on its own dynamics, suggesting that simultaneous solution of both the magnetohydrodynamic (MHD) and radiative response is required. Unfortunately, the estimation of the frequency and time dependent radiative properties of Z-pinches is computationally challenging and the recent improvements made to the parallel architecture of the 3D resistive Eulerian MHD code GORGON have only reinforced the strong emphasis already placed on optimizing the physics solvers used in Z-pinch simulations. To address these issues, we have developed a simple and fast pseudo NLTE code based on a Screened Hydrogenic Model (SHM) that can be run in-line with GORGON or as a post processing tool with synthetic spectra capabilities. Making use of a computationally inexpensive modification of the Saha equation, this highly optimized code has demonstrated a good ability to represent Non-LTE plasma conditions. In order to handle the amount of data generated by the spectral treatment of the billions of numerical cells constituting the simulation grids, an original data structure derived from a self-balancing binary search tree has been developed, enabling the use of Non-LTE DCA calculations in a large scale three dimensional environment for the first time. The implementation of this model is described in detail and comparisons with a commercial package are offered. Results from Z-pinch simulations performed with the new code are discussed and possible future improvements are presented.

Non-ideal plasmas are difficult to model numerically and achieve experimentally. Here I focus on reaching the warm dense matter (WDM) regime at one end of the energy scale and the high energy density (HED) regime at the other. Both regimes access common, yet hard to observe, features in planetary core physics and astrophysics respectively. We have implemented several experiments developing both ionic, isochoric heating of matter to HED states, and probing the release isentrope through the WDM regime. Using sub-picosecond, ~ 100 J laser pulses, ion beams were produced in a thin foil interaction and then used to rapidly and uniformly heat ~ 20 µm wires to ~15 eV temperatures before hydrodynamic expansion could occur. Thus the partially ionised, strongly coupled WDM regime was accessed and then explored using streaked XUV and UV imaging of the expansion into vacuum, together with measurement of the ion spectra deposited in the sample. The calibrated ion spectra indicates the energy deposition occurs over ~ 400 ps and as such further investigation of the stopping power of warm dense matter is required. The high absorption of these laser pulses in cluster gases (> 90%) was used to create HED blast waves. Under hydrodynamic scaling laws these blast waves can be used to model processes occurring within astrophysical phenomena. Schlieren, interferometry and ion probing methods have been used to spatially and temporally resolve the blast wave profiles. We describe high energy scaling experiments in the search for the thermal cooling instability, in which radiation from the blast wave modi es both its density profile and propagation dynamics. A unique perpendicular, time delayed colliding blast wave geometry has been used to identify up-stream characteristics of the front propagation and to pre-heat material leading to shock front acceleration. We show that ions of an energy greater than ~ 200 keV are required to probe the transient electromagnetic fields believed to occur at the shock front. The development and characterisation of low energy (0.1 - 10 MeV) proton beams was necessary for studies of both non-ideal plasma regimes, particularly for probing high density media and transient electromagnetic fields. The application of ions to recover stopping power is discussed for WDM and HED plasmas.

The single and dual frequency ICPs are studied in this thesis, mostly in oxygen; a simple electronegative gas. The diagnostics used here include; PROES, laser photodetachment and, Hairpin and Langmuir probes. Many issues of the fundamental understanding were investigated which include; the influence of the electronegativity on the electron density, E-H mode transition, collisionless heating, RF biased ICPs and the pulsed ICPs. The changes in the electronegativity are found here to be related to the heating mechanisms in the plasma. ICPs are well known to operate in E or H mode, depending on the applied power. The PROES measurements showed that both capacitive and inductive power couplings are simultaneously present in stable E and H modes, close to the transition. The individual contribution of capacitive and inductive power is also estimated here, using some new techniques. In pulsed ICPs, E and H like modes with simultaneous contribution of the capacitive and inductive couplings are also observed. The PROES measurements in the H-mode of ICPs, operated in the collisionless pressure regime, shows axially two maxima structure in the discharge, possibly because of the negative power absorption in the middle of the ICPs. The spatio-temporal excitation measurements in a RF biased ICPs (or dual frequency ICPs) showed that the coupling of the bias with tile capacitive coupling of the coil play a . significant role in the electron heating. The power and pressure regime where RF bias significantly influences the electron density and heating, is identified. The measurements also indicate the limitations of the independent control of the ion flux and energy. The phase between the ICP and bias could be used to decrease or enhance the influence of bias on electron heating. The electron density measurements showed that the RF bias can improve the radial uniformity and edge to centre electron density ratio.

This thesis contains work of a computational and theoretical nature. The floating potential of dust grains immersed in plasma is investigated via particle-in-cell simulation for a range of parameters. In particular, work is focused on the charging of grains large with respect to the electron Debye length. Numerical fits are given for the floating potential of large grains in stationary and flowing plasma. A modified version of the well known orbit-motion-limited (OML) theory is developed for large dust grains. The modified OML theory is shown to be in good agreement with simulation. This modified theory is then adapted for use with flowing plasmas. In the case of flowing plasma, for low ion temperatures and flow speeds upwards of Mach 1, interesting and unexpected effects are seen in the potential and density distribution around dust grains, these are investigated and discussed. Finally, the application of this work is outlined with particular focus on dust grains in a tokamak plasma environment.

This thesis describes the numerical modelling of H-mode plasmas on the Joint European Torus (JET), both during and between edge localised modes (ELMs). The multi-fluid code EDGE2D-EIRENE has been benchmarked against experimental data from the inter-ELM phase of an ITER-relevant JET plasma. Despite matching all other reliable experimental measurements to within a factor ∼ 1.5, the simulated radiation was located primarily at the targets, whereas the experimental radiation was located primarily at the X-point. This discrepancy occurred because the majority of radiating carbon ions in the simulation were located near the targets. In addition, the inner target Dα measurement was underestimated in the simulation by a factor of 9. A wide parameter scan did not change these two discrepancies. The inclusion of a thermoelectric parallel current in the simulations was found to be consistent with experimental measurements and to give more realistic degrees of in/out power asymmetry. A fuelling scan was carried out from the benchmarked simulation and compared to experiment. For similar increases in upstream density, the simulated and experimental outer target profiles were seen to agree well. This lead to an experimentally verified scaling of [Equation appears here. To view, please open pdf attachment] for the maximum parallel energy flux density at the outer target as a function of the electron separatrix density at the outer mid-plane. Preliminary simulations with nitrogen seeding are also discussed. The ELM transport phase has also been modelled. By making a simple modification to the analytic ion free streaming equations (Fundamenski et al. 2006 PPCF, 48:109-156), the equations are shown to match the impulse responses for target particle and energy fluxes, as calculated by a 1d kinetic code, to a high degree of accuracy. The linearity of the force-free Vlasov equation means that a distributed ELM source in time and space can be modelled by convolving the impulse responses with the ELM source function. Although this introduced some error (due to the fact that, in reality, the potential evolves in time for a distributed ELM source), the agreement with even a complex 1d3v particle-in-cell code was remarkably good. A new code is presented that tracks portions of the ELM distribution function in three-dimensional space as they free stream to the vessel walls. The code is simple and fast and is shown to reproduce similar scalings for the maximum energy density as were found in experiment. It also reproduced an experimentally observed profile for the energy density at the outer target, and encouragingly the agreement was seen to improve when a more physically realistic distribution for the ELM influx was used in the poloidal direction. The simulated rise and fall times were too short compared to experiment when an impulse ELM influx was used and reasons for this discrepancy are discussed. Also demonstrated is the ability of the code to model the effect of stochasticity of filament size on the surface energy flux density at the target.

The terahertz regime has until recently been some what neglected due to the difficulty of generating and measuring terahertz radiation. Terahertz time domain spectroscopy has allowed for affordable and broadband probing of this frequency regime with phase sensitive measurements (chapter 3). This thesis aims to use this tool to add to the knowledge of the interactions between electromagnetic radiation and matter specifically in regard to plasmonics. This thesis covers several distinct phenomena related to plasmonics at terahertz frequencies. The generation of terahertz radiation from metal nanoparticles is first described in chapter 4. It is shown that the field strength of the plasmon appears to relate to the strength of the generated field. It is also shown that the power dependence of the generated terahertz radiation is not consistent with the optical rectification description of this phenomenon. An alternative explanation is developed which appears more consistent with the observations. A simple model for the power dependence is derived and compared to the experimental results. In chapter 5 the parameters that make good plasmonic materials are discussed. These parameters are used to assess the suitability of semiconductors for terahertz surface plasmon experiments. The Drude permittivity of InSb is measured here, leading to a discussion of terahertz particle plasmons in chapter 6. Finite element method modelling is used to show some merits of these over optical particle plasmons. This also includes a discussion of fabrication methods for arrays of these particles. Finally, chapter 7 is a discussion of so called spoof surface plasmons. This includes some experimental work at microwave frequencies and an in depth analysis of open ended square hole arrays supported by model matching method modelling. Perfect endoscope effects are discussed and compared to superlensing. The thesis ends with a brief conclusions chapter where some of the ideas presented are brought together.

A series of wire array implosion experiments have been carried out on MAGPIE, a 1.4MA peak current, 240ns rise-time pulsed power generator located at Imperial College London. The aim of these experiments was to investigate the azimuthal structure of ablation plasma produced during the early stages of wire array implosions. Previous work [Lebedev, S.V., et al., PoP, 2001] has demonstrated that a significant portion of the implosion time is dominated by the ablation phase, during which the dense wire cores remain at their initial radius and ablate streams of plasma towards the array axis. This ablation plasma prefills the array, and its distribution forms the initial conditions for the eventual snowplough-like implosion. In order to carry out this investigation a new two-colour end-on interferometry system was constructed. A custom software suite was developed in order to analyse the resulting data, enabling the conversion of the raw interferograms into 2D electron density maps. The experiments focused on cylindrical tungsten and aluminium arrays of between 8 and 128 wires. The structures observed in aluminium arrays were dominated by collisional features. Networks of oblique shocks were formed as the ablation streams interacted with their neighbours. Analysis of these shock structures allowed an estimate to be made for the Mach number of the ablation streams, M=7.9-8.2. In contrast, the structures observed in the tungsten experiments were characterised by smooth density profiles, consistent with relatively long ion mean free paths. The quantitative nature of the dataset makes it ideal for comparison with the results of numerical simulations. Also reported in this thesis is the development of two CW laser diagnostics, a quadrature interferometer, and a Faraday current diagnostic. The design of these diagnostics is discussed and some examples of the analysis are presented.

The work carried out in the thesis demonstrates the non linear aspects of electrostatic solitary and shock waves in multi-component plasmas, characterised by excess superthermal electrons, via the fluid theory. The electrons superthermality is modeled through the kappa type distribution function. The properties of amplitude modulated wavepackets and the occurrence of modulational instability is investigated in detail in a collisionless plasma. The plasma superthermality and the concentration of superthermal electrons may control or even suppress the modulational instability. The stability of bright and dark-type non linear structures is confirmed by numerical simulations. The nonlinear properties of electrostatic solitary waves are investigated off equilibrium via the long wavelength approximation. The nonlinearity and dispersion terms, as well the energy acquired by electrostatic solitons for different nonthermality are discussed. The soliton is found to convert into multi-solitons or loses its energy via random oscillations while crossing the interface between two plasma regions with different superthermality characteristics. The solitary wave characteristics are investigated in a magnetised plasma via non-perturbative method. An investigation of the soliton propagation characteristics and existence domain, in terms of the soliton speed (Mach number) are presented. The effect of obliquity (angle between the wave propagation vector and the magnetic field) and as well as of magnetic field effect are discussed. The basic features and the underlying physics of electrostatic solitary waves, which are relevant to space and laboratory plasmas, are briefly discussed. Finally, taking into account dissipation via the fluid kinematic viscosity, characteristics of electrostatic shocks are discussed. Small perturbation near equilibrium, i.e., off equilibrium is assumed to analyse the shock wave properties in an unmagnetised kappa distributed nonthermal plasma. The stability of the shock profile is analysed analytically and numerically. The stability conditions are discussed for the theoretical framework of kappa superthermal plasma.

This thesis investigates effervescent atomisation, a liquid fuel atomisation technique with wide industrial applications, and one which offers several important advantages over conventional atomiser types. An “inside-out” type atomiser rated at 2MW equivalent power (based on mass flow rate) was designed and tested using a state-of-the-art 2-D Phase Doppler Anemometry (PDA) system which allowed for simultaneous real-time droplet size and velocity data to be obtained. High quality data was achieved, with data rates up to 10 kHz and validation rates over 90% in 2-D PDA coincident mode in the high density sprays. Droplet diameters up to 600 μm could be measured. The parameters investigated included operating parameters (air-to-liquid by mass ratio, pressure drop across the nozzle), geometric parameters (exit orifice diameter, nozzle length-to-diameter ratio, mixing chamber diameter, mixing length and air injection geometry) and fluid viscosity. The parameter ranges investigated included 1.83-11.11% air-to-liquid by mass ratio, 4.64-7.05 barG pressure drop across the nozzle, 2-2.8mm exit orifice diameter, 60-136 mm mixing length, 20-30 mm mixing chamber diameter, 0.5-2 nozzle length-to-diameter ratio and 1-18 x10-6 m/s2 kinematic viscosity. In addition 3 air injector geometries were studied which allowed the influence of air injector hole radial symmetry and aerating hole diameter to be determined. Water and air were used as the operating fluid and assist-medium, respectively, for the operating parameter and geometric parameter tests. However, the use of water-glycerol mixtures in the fluid viscosity tests allowed the viscosity of the operating fluid to be controlled. Altering the fluid viscosity allowed the production of a range of simulated fuels (that will encompass Bio-Fuels). The effervescent atomiser designed was compared to an industrial type Y-Jet atomiser frequently used in steam-assisted boiler combustion applications. It was found that the Y-Jet atomiser performed slightly better than an effervescent atomiser without any optimisation, but that improvements in effervescent atomiser performance were possible once atomiser geometry had been fully optimised. Comparisons were also made with the droplet SMD, coefficient of discharge and spray angle predicted by correlations from the literature (obtained using earlier versions of the hardware or alternative sampling techniques). These were found to provide poor agreement with the present experimental data. Finally, global spray SMD correlations were developed; these were shown to agree well with the present experimental data.

The transport of heat that results from turbulence is a major factor limiting the temperature gradient, and thus the performance, of fusion devices. We use nonlinear simulations to show that a toroidal equilibrium scale sheared flow can completely suppress the turbulence across a wide range of flow gradient and temperature gradient values. We demonstrate the existence of a bifurcation across this range whereby the plasma may transition from a low flow gradient and temperature gradient state to a higher flow gradient and temperature gradient state. We show further that the maximum temperature gradient that can be reached by such a transition is limited by the existence, at high flow gradient, of subcritical turbulence driven by the parallel velocity gradient (PVG). We use linear simulations and analytic calculations to examine the properties of the transiently growing modes which give rise to this subcritical turbulence, and conclude that there may be a critical value of the ratio of the PVG to the suppressing perpendicular gradient of the velocity (in a tokamak this ratio is equal to q/ε where q is the magnetic safety factor and ε the inverse aspect ra- tio) below which the PVG is unable to drive subcritical turbulence. In light of this, we use nonlinear simulations to calculate, as a function of three parameters (the perpendicular flow shear, q/ε and the temperature gradient), the surface within that parameter space which divides the regions where turbulence can and cannot be sustained: the zero- turbulence manifold. We are unable to conclude that there is in fact a critical value of q/ε below which PVG-driven turbulence is eliminated. Nevertheless, we demonstrate that at low values of q/ε, the maximum critical temperature gradient that can be reached without generating turbulence (and thus, we infer, the maximum temperature gradient that could be reached in the transport bifurcation) is dramatically increased. Thus, we anticipate that a fusion device for which, across a significant portion of the minor radius, the magnetic shear is low, the ratio q/ε is low and the toroidal flow shear is strong, will achieve high levels of energy confinement and thus high performance.

Molecular dynamics (MD) simulations are used to compute the equilibrium dynamics of a single component fluid with Yukawa interaction potential v(r) = (Ze)^2 exp(−r/λs )/4π eps_0 r. This system, which is known as the Yukawa one-component plasma (YOCP), represents a simplified description of a non-ideal plasma consisting of ions, charge Ze, and electrons. For finite screening lengths λs, the MD results are used to investigate the domain of validity of the hydrodynamic description, i.e., the description given by the Navier-Stokes equations. The way in which this domain depends on the thermodynamic conditions of the YOCP, as well as the strength and range of the interactions, is determined. Remarkably, it is found that the domain of validity is completely determined by the range of the interactions (i.e., λs); this alone determines the maximum wave number k_max at which the hydrodynamic description is applicable. The dynamics of the YOCP at wavevectors beyond k_max are then investigated; these are shown to be in striking agreement with a simple and well known generalisation of the Navier-Stokes equations. In the extreme case of the Coulomb interaction potential (λs = ∞), the very existence of a hydrodynamic description is a known but unsolved problem [Baus & Hansen, 1980]. For this important special case, known as the one-component plasma (OCP), it is shown that the ordinary hydrodynamic description is never valid. Since the OCP is the prototypical system representing a non-ideal plasma, a number of different approaches for modelling its dynamics have been formulated previously. By computing the relevant quantities with MD, the applicability of a number of models proposed in the literature is examined for the first time.

Turbulent transport of heat and particles significantly degrades the confinement in tokamaks. Whilst the confinement improves in larger devices, these are more expensive and the economic viability of future fusion power plants depends upon understanding turbulence so that operating scenarios can be optimised. Gyrokinetic models are able to describe the plasma turbulence responsible for transport. The assumption that the equilibrium varies slowly relative to the radial width of the instability is often exploited to reduce the global gyrokinetic system to a local one. The relation between the global and local systems is a key topic in this thesis. It is shown that local solutions can only capture the true global behaviour when freedoms in the system are treated correctly. A procedure to reconstruct the global solution from the local one has been developed and successfully tested. The spontaneous transition to a regime of high confinement, observed on many tokamaks, is associated with the suppression of turbulence in a narrow region near the plasma edge, known as the pedestal, and is accompanied by edge localised instabilities (ELMs) which can eject large amounts of energy in a short time, damaging the confinement vessel. Understanding the ELM and pedestal behaviour is crucial to predict the performance of future tokamaks, as well as offering insight into techniques to reduce the threat of damage due to ELMs. The application of gyrokinetics to study microinstabilities in the edge region of MAST in the time between two ELMs is presented as part of this thesis. This work finds kinetic ballooning modes to be unstable in the pedestal whilst microtearing modes are unstable in the shallow gradient region towards the core. The transition from MTMs to KBMs at the interface between the two regions has been studied and may play an important role in the pedestal evolution.

The thesis investigates ion behaviour at collisionless shocks, with a focus on two areas of interest. The first area concerns the reflection of particles from collisionless shocks, a necessary mechanism for thermalization at a shock at sufficiently high Mach numbers such as ordinarily prevail at the Earth's bow shock. Previous studies have examined the trajectories of reflected ions with the assumption of a planar shock. In this study, a general framework is developed to describe the trajectory of an ion after reflection, with application to a variety of shock geometries. The conditions allowing an ion to return to the shock after reflection and to return with an increased normal velocity are studied, with three primary parameters considered: the radius of curvature, the magnetic field orientation, and the incident velocity in the shock normal direction. Each of these parameters depends on the shape of the shock and the location of incidence. Results are reported for cylindrical, spherical, and parabolic shock geometries, over ranges of shock curvatures, magnetic field orientations, and incident velocities. Second, we consider the thermalization of the ion distribution initially transmitted through the shock under low Mach number conditions, where reflection is a less significant contributor to thermalization. Previous work has considered the phase area invariant in an exactly perpendicular case. This is generalized to a quasi-perpendicular shock, and invariants of the flow are determined for a Hamiltonian formulation. The evolution of the distribution through the shock is then studied analytically and numerically. Results regarding the shape of phase shells of constant probability, the phase volume within these shells, and the temperature of the distribution are given.

Type-I ELMy H-mode is planned to be the reference inductive operational scenario for ITER. However, unmitigated type-I ELMs would cause unacceptable damage to ITER’s divertor, so a way of mitigating their effect must be found. This thesis focuses on the ergodization of the plasma edge using RMPs as a means of ELM control, and on related topics relevant to ergodic magnetic fields in MAST. Chapter 1 provides an introduction to nuclear fusion and chapter 2 provides an overview of ELMs. In chapter 3, the effect of ergodic fields on L-mode temperature profiles is calculated. It is found that no flattening of the temperature profile should be expected in MAST plasmas, which is in agreement with experimental results. In chapter 4, various metrics characterizing the degree of ergodicity in both L- and H-mode plasmas are calculated using vacuum modelling and compared with the amount of density pump-out observed experimentally in the those plasmas during the application of RMPs. The only parameter to show a correlation with the amount of density pump-out is the width of the laminar region. However, plasma response modelling provides a robust criterion for the occurrence of density pump-out that applies both to L- and H-mode plasmas. In chapter 5, the results of lower single-null H-mode ELM mitigation experiments using n=4 and n=6 RMPs are presented. ELM mitigation is achieved and refuelling of plasmas that had begun to undergo pump-out is successfully demonstrated. Both ELM frequency and density pump-out are found to increase with ELM coil current above a certain threshold. An analysis of an ergodic magnetic field formed by a current sheet on a rational surface is presented in chapter 6. Various properties of this ergodic field are calculated and compared with experimental effects observed in the plasma. Some agreement is found between the experimental data and the modelling. Finally, a summary of the thesis is given in chapter 7.

Flows driven by density differences, whether natural or induced by man, surround us on a wide range of scales. As a density difference is generated by commonly occuring temperature or salinity differences, these flows are ubiquitous. On the largest scales, they transport and mix the water in our oceans and air in our atmosphere. At an intermediate scale, frequently referred to as the mesoscale, the cold outflows which flow downwards from thunderstorms and impact the ground are a common aviation hazard. On a smaller scale, the toxins emitted by fire plumes or dense gas releases are a threat to health. This work focusses on a continuous, localised release of buoyant fluid from a horizontal source whose dimension is significantly smaller than the horizontal and vertical scales of the quiescent, uniform environment into which the flow propagates. We consider both passive releases, which are only driven by the density difference between the source fluid and the denser ambient, and forced releases, where the fluid has source momentum in addition to its buoyancy. In general, these releases give rise to turbulent plumes, a familiar example being the cloud of smoke and water vapour seen rising from a chimney stack on a cold, still morning. The first part of the research presented in this thesis focusses on the freely-propagating plume. Velocity and temperature measurements are presented which contribute considerably to the existing experimental data available in the literature. This data-set is used to validate classical plume theory and make a check of the experimental set-up so that the subsequent results can be presented with confidence. It is also possible that this dataset will be used by other researchers to validate numerical simulations of buoyant flows. The effect of varying the source balance of buoyancy and momentum upon the plume dynamics is investigated. Measurements also reveal the development region or ‘zone of flow establishment’. Frequently, plumes are restricted by some form of confinement, either vertically, horizontally or both, for example the plumes rising from the occupants of a room. Whether this restriction takes the form of a solid wall, free surface or density discontinuity, the disturbance to the flow is typically significant. The simplest confining boundary is arguably a horizontal surface located some distance H from the source of buoyant fluid. The horizontal boundary forces the vertical flow to change direction and propagate radially outwards. This type of semi-confined flow can be frequently observed in the natural world with examples including the impingement of a fire plume against a ceiling and a plume of volcanic ash with the tropopause. An investigation into this type of flow, which we refer to as the ‘impinging buoyant plume’, constitutes the second part of the research. Plume impingement has not been studied as extensively as jet impingement and several key questions remain unanswered. For example, how much energy is lost as the vertical flow is forced to turn and propagate horizontally? What effect does buoyancy have on the horizontal flow? How does the flow evolve with increasing radial distance and what is the effect of changing the source-boundary separation? These are just some of the questions addressed in this thesis guided by the novel application of highly-resolved Particle Image Velocimetry measurements to this relatively low-speed, buoyant, turbulent flow. The free and impinging plume studies both employed similar experimental techniques and analysis methods. Statistics of the steady flow were determined from a highly-resolved data-set. The third part of the research concerns a time-dependent flow and is of a more qualitative nature. The complexity of the impinging plume increases considerably when a radial confinement is added to the geometry. This restricts the radial propagation of the flow produced by the impinging plume. The plume is now effectively enclosed and buoyant fluid begins to accumulate within and thereby fill the enclosure, a configuration known as the ‘filling-box’. While previous work, which we shall go on to review in detail, has contributed analytical solutions for the density profiles in the enclosure after a certain time-scale has elapsed, in many applications, such as the spread of smoke carried by a fire plume in a room, what happens in the early moments of a confined release following impingement with the horizontal and then vertical boundaries, may be critical. This has been overlooked in earlier studies, yet is crucial as it is during these early transients that the fire is best tackled by fire-fighters. Visualisations and velocity measurements of these early filling-box transients are reported. This work provides the first detailed measurements of the velocity field induced in the filling-box by the turbulent plume during the early transients and resolves the turbulent structures that comprise the plume outflow. The experiments which investigated the impinging jet were conducted on thermal air plumes in facilities at the Laboratoire de Mécanique des fluides et de l’Acoustique (LMFA) of École Centrale de Lyon (ECL). Filling-box experiments were performed on brine plumes in fresh water in visualisation tanks in the Department of Civil & Environmental Engineering at Imperial College London (ICL). The set of experiments at ECL used a combination of Particle Image Velocimetry (PIV) and thermocouples to measure flow velocities and temperatures. At ICL, Light-Induced Fluorescence (LIF) enabled visualisation of a plane through the centre of the axisymmetric flow to complement the PIV work. These experiments enabled effective use of the equipment, techniques and expertise available at both institutions. The principal objective of this research was to use experimental measurements to answer questions of importance regarding these impinging flows which remain unresolved in the literature. Using experimental techniques unavailable to earlier researchers, the work presented herein makes a substantial contribution to the existing knowledge of these flows. Free and impinging plumes and the dynamics of the filling-box flow are studied in detail. Notably, the data gathered are of very high spatial resolution and provide a resource for those interested by not only the plume dynamics, but also radial gravity currents and the filling-box.

Plasma opacity calculations play an important role in solar modelling and many plasma physics and inertial confinement fusion experiments. This thesis is focussed on the fast calculation of opacity from first principles. The existing average atom (AA) opacity code IMP [1] is used alongside experimental data and detailed atomic physics to develop new models; the results show that simple models can give an excellent description of plasma spectra for a large range of conditions. The results are significant for the development of fast opacity codes which necessarily use the AA approach. The application of fast models to very large scale calculations is considered and an efficient approach to these developed; this allows the fast description of experimental data that would not have otherwise been possible [2]. Analysis of this data then allows the accuracy of the IMP model to be further discussed. The atomic model is also considered, and an improved approach implemented. These improvements makes little difference to the description of experiment provided electron exchange is included. The range of applicability of the IMP model is then extended to higher density by adding a fast description of line broadening by electrons. This gives an excellent agreement with both experiment and more advanced opacity codes. The treatment of atomic term structure can represent a significant portion of code runtime. A good compromise between detail and efficiency is the unresolved transition array (UTA) formulation; a consistent theory of UTAs is developed, and various models introduced. The accuracy of these is systematically tested. It is found that within the validity range of the UTA approach, a good description of the opacity can be gained using a simple model provided that the linewidth is correct. Various simplified calculations of this width are tested, and found to be inaccurate [3].

This thesis presents an investigation into kinematic features of fine scale turbulence in free shear flows. In particular it seeks to examine the interaction between the different length scales present in shear flow turbulence as well as the interaction between the strain-rate tensor and the rotation tensor, which are the symmetric and skew-symmetric components of the velocity gradient tensor respectively. A new multi-scale particle image velocimetry (PIV) technique is developed that is capable of resolving the flow at two different dynamic ranges, centred on inertial range scales and on dissipative range scales, simultaneously. This data is used to examine the interaction between large-scale fluctuations, of the order of the integral scale, and inertial and dissipative range fluctuations. The large-scale fluctuations are observed to have an amplitude and frequency modulation effect on the small scales, and the small scales are shown to have a slight effect on the large scales, illustrating the two way nature of the energy cascade. A mechanism whereby integral scale rollers leave behind a wake of intense small-scale fluctuations is proposed. The interaction between strain and rotation is examined with regards to the rate of enstrophy amplification (ωiSijωj). It is found that the mechanism that is responsible for the nature of enstrophy amplification is the alignment tendency between the extensive strain-rate eigenvector and the vorticity vector. This mechanism is also observed to be scale dependent for ωiSijωj > 0, but independent for ωiSijωj < 0. This is subsequently confirmed with new dual-plane stereoscopic PIV experiments performed as part of this study. Finally, computational data is used to examine the effect of experimental noise and variation of spatial resolution on the observation and understanding of this strain - rotation interaction.

The vortical disturbance environment surrounding a laminar boundary layer affects the proceedings of transition to turbulence. The exponentially growing Tollmien–Schlichting wave is observed under low free-stream turbulence levels, and is replaced by the algebraically growing streaks upon further increase in the turbulence intensity. This scenario is significantly affected by the presence of wall films which introduce additional instabilities and alter the amplification of the streaks. In this work, the effect of wall films on the linear stability of boundary layers is investigated using the Orr-Sommerfeld, Squire and the interface displacement equations. A modal analysis is conducted first, in order to identify all the unstable modes and their respective regimes of dominance. Furthermore, the physical mechanisms contributing to disturbance growth are studied using the kinetic energy equation. No unstable eigenvalues are found for wall films less viscous than the outer stream. Under such conditions, the streaks are likely to dominate; their amplification being dependent on the penetration of the free-stream vortical disturbances into the boundary layer. The ingestion of the free-stream vorticity in the mean shear is explained using the continuous spectrum of the Orr-Sommerfeld equation. A unique parameter is identified to distinguish three asymptotic regimes representing complete, partial and negligible penetration into the boundary layer, respectively. The physical mechanism is a competition between viscous diffusion and convection by the mean flow. The wall film affects the penetration into the boundary layer by modifying the wall-normal wavenumber across the interface. The penetrating free-stream disturbances efficiently generate streaks by tilting the mean vorticity. Their amplification is investigated using an initial value problem that describes the evolution of a linear perturbation. Lower viscosity wall films reduce the amplification of the streaks. However, another growth mechanism arising from the interfacial displacement dominates at long time and is enhanced for lower viscosity films.

The dynamics of vortex flow is studied theoretically and numerically. Starting from a local analysis, where the perturbation in the vortex flow is Fourier decomposed in both radial and azimuthal directions, a modified Chebyshev polynomial method is used to discretize the linearized governing operator. The spectrum of the operator is divided into three groups: discrete spectrum, free-stream spectrum and potential spectrum. The first can be unstable while the latter two are always stable but highly non-normal. The non-normality of the spectra is quantitatively investigated by calculating the transient growth via singular value decomposition of the operator. It is observed that there is significant transient energy growth induced by the non-normality of continuous spectra. The non-normality study is then extended to a global analysis, in which the perturbation is decomposed in the radial or azimuthal direction. The governing equations are discretized through a spectral/hp element method and the maximum energy growth is calculated via an Arnoldi method. In the azimuthally-decomposed case, the development of the optimal perturbation drives the vortex to vibrate while in the stream-wise-decomposed case, the transient effects induce a string of bubbles along the axis of the vortex. A further transient growth study is conducted in the context of a co-rotating vortex pair. It is noted that the development of optimal perturbations accelerates the vortex merging process. Finally, the transient growth study is extended to a sensitivity analysis of the vortex flow to inflow perturbations. An augmented Lagrangian function is built to optimize the inflow perturbations which maximize the energy inside the domain over a fixed time interval.

Since the Russian launch of the Zond-2 satellite in 1964 there have been over fifty years of research dedicated to the understanding of the first electric propulsion device to be flown in space, the Pulsed Plasma Thruster. The Pulsed Plasma Thruster originates from the evolution of the vacuum arc switch, but due to its microsecond operation time, the internal dynamics and nature of operation have remained unclear. The Pulsed Plasma Thruster is generally cheap to manufacture and to operate, which keeps it a popular device to research within institutes worldwide and has contributed to its longevity. As a satellite propulsion device it has unique capabilities that other propulsion systems cannot provide. The thruster operates by accelerating plasma formed in the accelerating electrodes (or nozzle) in short discrete packets of thrust or impulse. The pulsed nature of the thruster means that between pulses energy can be stored in capacitors, ready for the next discharge. The storage of energy over time means the power draw is variable and is only dependant on the frequency that the system is pulsed at. This property of the thruster makes the Pulsed Plasma Thruster extremely versatile, allowing the thruster to perform both velocity correction and control manoeuvres and attitude control manoeuvres. The Pulsed Plasma Thruster is mechanically scalable but the performance of the thruster has been shown to depend linearly on the energy storage ability of the thruster’s capacitor. The work presented here covers two areas. Firstly is the critical analysis of the physical mechanisms that occur within a Pulsed Plasma Thruster through a review of literature, experimentation and the development of a high current plasma flow model. The second area is the design, development, manufacture and evaluation of the Pulsed Plasma Thruster for use on a nanosatellite platform known as the CubeSat. Several novel observations and contributions were made during the critical analysis of the physical mechanisms of the Pulsed Plasma Thruster. The most significant was realising how the erosion of the metal electrodes affected the overall discharge process. It is postulated that the expulsion of material from emission sites (or cathode spots), the ionisation of that material and the resulting freed electrons, create a pinched plasma column between the electrodes. It is postulated that the interaction of the electrode sheath region and the intersecting plasma column cause the current flow to become limited. This was then shown to affect the efficiency with which the stored energy of the capacitor was converted to energy to accelerate the plasma. Understanding this issue is key in improving future designs of the Pulsed Plasma Thruster. The observations and conclusions made during this work were put into practice to create an eight μPPT propulsion module for a 3U CubeSat. Initial results show that a μPPT with a specific impulse of 321s, an impulse bit of 0.56μNs and a mass bit of 0.17μg has been developed. The thruster was developed for two technology demonstration CubeSats. STRaND-1 is a joint collaboration between Surrey Space Centre and Surrey Satellite Technology Limited and UKUBE-1 is a joint collaboration between Surrey Space Centre and the UK Space Agency. Both CubeSats are scheduled for launch late 2011, early 2012. The propulsion module for the STRaND-1 CubeSat will be the first to provide full axis control and the first to provide electric propulsion on this class of satellite, showing the advantages of the Pulsed Plasma Thruster for Small Satellites.

The structure and stability of colloidal monolayers depends crucially on the effective pair interaction potential between colloidal particles. In the first part of the thesis, we present two novel methods for extracting the pair potential from the two-dimensional radial distribution function of dense colloidal monolayers. The first is a so-called Predictor-Corrector routine that replaces the conventionally unknown Bridge function, with an iteratively obtained hard-disk bridge function. The second method is based on the Ornstein-Zernike relation and the HMSA closure that contains a single fitting parameter which is determined by requiring thermodynamic consistency between the virial and compressibility equations of state. The accuracy of these schemes are tested against Monte Carlo simulation data from monolayers interacting via a wide range of commonly encountered pair potentials. We also test the stability of these methods with respect to noise levels and truncation of the source data to mimic experimentally obtained structural data. Finally we apply these inversion schemes to experimental pair correlation function data obtained for charged polystyrene particles adsorbed at an oil/water interface. We find that the pair interaction potential is purely repulsive at low densities, but an attractive component develops at higher densities. The origin of this attractive component at higher densities is at present unknown.In the second part of this thesis, we study how the colloid interactions studied above influence the structure of the colloidal monolayer. Specifically inspired by recent experimental results on mixed monolayers of large and small very hydrophobic silica particles at an octane/water interface, we study theoretically the structure of two-dimensional binary mixtures of colloidal particles interacting via a dipole-dipole potential. We find that at zero temperature, a rich variety of binary crystal structures are obtained whose structure depends on the dipole moment ratio and the number fraction of small particles. At experimentally relevant finite temperatures, we find that the AB2 and AB6 binary super-lattice structures are thermodynamically stable while other binary structures e.g. AB5, which are stable at zero temperature, are thermodynamically unstable at finite temperature. Specifically, the melting temperature of the AB5 system is found to be three orders of magnitude lower than that of the AB2 and AB6 systems and at experimentally relevant temperatures, melts into a semi-disordered phase with local AB6 order.

This thesis presents a study of Surface Plasmon Polaritons (SPPs) in hybrid metal-dielectric waveguides. The embedding of metal in nanostructured photonic components allows for manipulating and guiding light at the subwavelength scale. Such an extreme confinement enhances the nonlinear response of the dielectric medium, which is important for applications in optical processing of information, but is paid in terms of considerable ohmic loss in the metal. It is, however, possible to embed externally pumped active inclusions in the dielectric in order to compensate for the metal loss. A novel perturbative theory for Maxwell equations is introduced and applied to various nonlinear metal-dielectric structures, deriving the propagation equation for the optical field. The nonlinear dispersion law for amplified SPPs, filamentation and dissipative plasmon-soliton formation have been studied, revealing intrinsic core and tail instabilities that prevent solitons to propagate over long distances. Stable propagation of plasmon-solitons can be achieved in insulator-metal-insulator structures with active and passive interfaces. The active SPP is coupled with the passive SPP, which absorbs the perturbations destabilising the zero background of the soliton. Theoretical modelling of optical propagation in metal-dielectric stacks predicts a modified two-band structure, allowing for gap/discrete plasmon-soliton formation. Loss and nonlinear parameters in subwavelength nanowire waveguides are evaluated and compared to the results obtained by other research groups. In all calculations, particular attention is paid in considering boundary conditions accounting for loss and nonlinear corrections, which contribute to the propagation equation with a surface term that becomes significant in the subwavelength regime.

Free energy is the criterion of stability and is essential for determining phase equilibrium properties, for example. However, calculation of free energies for complex systems, such as fluids by computer simulation, is extremely difficult. In this thesis, we show how the partition function of fluids can be calculated directly from simulations; this allows us to obtain the absolute Helmholtz free energy (F) via F- -k8TInQ. Our method radically simplifies the process of calculating absolute free energies of continuous systems. As the method has been developed in the past few months, we have not yet applied it to the study of phase equilibria. This task will be part of our future work. In the rest of the thesis, we have focused on the application of more established simulation techniques to the urgent problem of finding environmentally friendly refrigerant fluids. Methane and fluoromethanes are possible candidates. However, they are flammable. 1-1-1-2-tetrafluoroethane, on the other hand, has for a long time been used in domestic refrigeration and automobile air-conditioning systems. However, it will be banned in Europe from 2011, due to concerns about its global warming impact. Carbon dioxide has received much attention as a fluid that can be used in combination with other refrigerants to minimise flammability and toxicity, and has a very low global warming potential. Thus, it could be mixed with those refrigerants to form new environmentally friendly refrigerant mixtures. Unfortunately, little information on the thermophysical properties of these mixtures is available. We simulate the thermophysical properties of these important industrial refrigerants and their mixtures with carbon dioxide using both empirical and in-house firstprinciples potentials. Simulations also provide a microscopic-level understanding of the structure of liquids, which is not accessible via experiment. Our high-quality ab initio force fields have reproduced the thermophysical properties for carbon dioxide, methane, fluorinated methanes, and mixtures of carbon dioxide and methane and carbon dioxide and fluorinated methanes. Multi-body effects play a crucial role in determining the thermophysical properties of fluids and inclusion of a three-body effect substantially improves the prediction of the phase-coexistence properties. Our studies should be of relevance to a broad range of mixtures of fluoroalkanes and carbon dioxide. Our efforts in making the first-principle force fields for carbon dioxide and fluorinated methanes pave the way for larger fluorinated hydrocarbons to come in the future.

Recent advances in research in the areas of Plasmonics have paved the way for the development of numerous passive plasmonic components capable of field confinement and enhancement. The next step is the integration of active materials within plasmonic structures for light control. One of the main problems with plasmonic devices is a propagation loss associated with Ohmic losses in the metal. The focus of this work is directed at the interaction of surface plasmons with active materials (semi-conductor quantum dots) on the surface of nanostructured metal films, and how to use it to achieve amplification of plasmonic signals. Up to sixteen-fold enhancement of the emission is shown by the coupling of the quantum dot excitons to Bloch modes of the plasmonic crystals in the high gain regime while a more modest gain of 80% enhancement is achieved for the low gain regime. This also allows us to achieve angular control over quantum dot emission. The modulation of the gain medium provides an opportunity to control the transmission of the plasmonic crystal that is similar to the reduction of the Ohmic loss effects on the optical properties of the crystal. An increase in the transmission by about 8 times in the high-gain regime and about 7% in the low gain regime has been observed. Amplified spontaneous emission of surface plasmon polaritons propagating on the metal inetrface with the gain medium has been demonstrated with completely narrowed line- width at a low light intensity of about 5 W/cm2. The achievable gain allows one to achieve approximately a 30% increase of the propagation length of SPPs on a metal/active- polymer interface. The results from this work form a basis for further developments in active plasmonics, primarily to amplify and provide all-optical control of plasmonic signals, both of which are imperative for future plasmonic applications.

Small scale turbulence in a magnetically confined fusion plasma drives energy and particle transport which determine the confinement. The plasma in a tokamak experiment has a toroidal rotation which may be driven externally, but can also arise spontaneously from turbulent momentum transport. This thesis investigates the interaction between turbulence and rotation via nonlinear numerical simulations, which use the gyrokinetic description in the frame that corotates with the plasma. A local gyrokinetic code is extended to include both the centrifugal force, and the stabilising effect of sheared equilibrium flow. Sheared flow perpendicular to the magnetic field suppresses the turbulence, and also breaks a symmetry of the local model. The resulting asymmetry creates a turbulent residual stress which can counteract diffusive momentum transport and contribute to spontaneous rotation. The competition between symmetry breaking and turbulence suppression results in a maximum in the nondiffusive momentum flux at intermediate shearing rates. Whilst this component of the momentum transport is driven by the sheared flow, it is also found to be suppressed by the shearing more strongly than the thermal transport. The direction of the residual stress reverses for negative magnetic shear, but also persists at zero magnetic shear. The parallel component of the centrifugal force traps particles on the outboard side of the plasma, which destabilises trapped particle driven modes. The perpendicular component of the centrifugal force appears as a centrifugal drift which modifies the phase relation between density and electric field perturbations, and is stabilising for both electron and ion driven instabilities. For ion temperature gradient dominated turbulence, an increased fraction of slow trapped electrons enhances the convective particle pinch, suggesting increased density peaking for strongly rotating plasmas. Heavy impurities feel the centrifugal force more strongly, therefore the effects of rotation are significant for impurities even when the bulk ion Mach number is low. For ion driven modes, rotation results in a strong impurity convection inward, whilst a more moderate convection outward is found for electron driven modes.

The simplicity of their design and their wide range of applications make atmospheric pressure plasma sources very attractive for technological and biomedical applications as well as for lighting and pollutant abatement. Because of pd scaling atmospheric pressure plasmas are confined in small spatial volumes with dimensions of the order of a few mm. However observations show that the effect of atmospheric pressure plasma sources operating in open atmosphere may extend much further due to diffusion of plasma species. This open plasma boundary is extremely important in many applications but its complete exploitation is hampered due to lack of understanding. This is particularly true for one type of atmospheric pressure plasma source - the atmospheric pressure plasma jet (APPJ), a source that generates a transient plume of plasma in the open air. Depending on operating conditions, time-resolved imaging of the plasma jet shows that the transient plume consists of either a rapidly propagating ionization region, called plasma `blob' or `bullet', or an extended continuous discharge region that resembles a flame. The velocity of the 'plasma blob/bullet' reaches a few tens of kilometres per second and there has been much speculation about its origin. Recent theoretical and experimental studies have led to the concept of the `blob' being a positive quasi-self-sustained streamer propagating in the open atmosphere. The research presented in this thesis aims to study the behaviour of atmospheric pressure plasma jets, testing the streamer explanation of the plasma 'blob/bullet' and investigating the diffusion of plasma species into the surrounding atmosphere. This research was carried using electrical, optical and biomedical diagnostic methods to study the properties of a dielectric barrier APPJ source. In the first main study, the behaviour of the plasma plume was investigated with the aim of testing streamer theory for generation and propagation of the plasma 'blob/bullet'. A series of measurements was performed to explore the dependence of the 'blob/bullet' on the properties of the applied voltage waveform. In another set of experiments, the effect of the background pressure on jet behaviour was examined. In both cases, time-resolved iCCD images of the plasma jet were made. The results of these experiments were consistent with concept of the `blob/bullet' being a positive quasi-self-sustained streamer propagating at low applied electric field in open atmosphere. In the second main study, the interaction of an APPJ with dry plasmid DNA samples was investigated with the two aims of characterising the interaction and determining the physical extent of the plasma plume. In one part of the study, a series of measurements was performed in order to explore the amount of damage caused to plasma DNA by exposure to plasma irradiation, and to identify the species responsible for this damage. The results indicated that neutral species and radicals were mainly responsible for the damage. In another part of the study, the free plasma boundary was explored by investigating direct and non-direct plasma irradiation of DNA samples. The results showed that DNA damage occurred far from the visible plasma plume - up to 20 cm from the plasma tip in the axial direction and up to 2 cm from the plasma plume in the radial direction. Metastable molecular oxygen species were identified as the most likely cause of this distant DNA damage. It was further concluded that DNA damage is a very sensitive technique, capable of defining the region of plasma diffusion in open atmosphere.

A disruption is the sudden and uncontrolled loss of plasma confinement in a tokamak. Disruptions on the Mega Amp Spherical Tokamak (MAST) are characterised in terms of thermal quench timescales, energy balance and pre disruption energy loss. Analysis of the energy balance during disruptions on MAST has shown that approximately 10% of the stored energy is radiated during a disruption and 80% is deposited onto the divertor. The energy loss prior to the thermal quench is found to be 50% of the maximum energy in the plasma, which is half the value assumed for the ITER design. Disruptions occur when operational boundaries, in terms of current, pressure and density, are exceeded. An analysis of the operational boundaries in MAST shows that the frequency of disruptive events increases as the density is raised to 1.5 times the Greenwald density limit and that the pressure limit is consistent with empirical scalings. The current limit on MAST is triggered before the expected value of q95 is reached. Further analysis of the disrupting discharges in MAST shows that there is substantial energy loss prior to the thermal quench of up to 50%, however, disruptions at full performance are frequent. Disruption mitigation on MAST, via massive gas injection, has been performed using 0.32 bar litres (7.7x1021 particles, 10 times the plasma inventory) of a 90% helium and 10% argon mixture. The evolution of the plasma during mitigation is followed using high speed (up to 50kHz) imaging and high temporal (0.2ms) resolution Thomson scattering. High speed imaging of the plasma shows that the neutral impurities are confined to the plasma periphery. Impurity ions penetrate to the q=2 surface and mix with the bulk plasma during the thermal quench. Thomson scattering data shows significant (double the initial core density) build of density on rational surfaces, specifically q=2, prior to the thermal quench. Analysis of the power load to the divertor during mitigated disruptions shows reductions of 60% in peak power loadings compared to unmitigated. The energy balance during mitigated disruptions shows an increase in the radiated energy to 40% of the total stored energy and a decrease in the energy to the divertor of 40%. The effect of mitigation is to increase the current quench time and decrease the magnitude of halo currents by 80%

A novel experimental method using gas puffing of neutrals to stimulate charge exchange emission has been developed to measure ion temperatures and toroidal rotation velocities in the edge of MAST. This uses cold deuterium gas puffing as the source of neutrals in place of the more usual method of donation from neutral beams. A pre-existing spectroscopy system with 60 toroidal chords with spatial resolution of 1.5 mm and temporal resolution of 10 ms was adopted for this purpose. The C6+(n = 8 → 7) transition at 5290.5 A was identified as the optimal emission line, while a resonance in the cross section between the D0(n = 2) and C6+(n = 8) energy levels makes the measurement feasible. In order to assess the potential of this measurement method a model of the expected performance of the system was created. All important processes in the edge regime were identified and density profiles of electrons, carbon ions and deuterium neutrals were modelled. These were combined to produce a prediction of performance. The spectrum observed was complicated by molecular deuterium emission and a fitting routine to this complex data was developed and benchmarked. Correction factors were also identified and calculated for the emission line. System improvements were made including a new bandpass filter, gas valve and camera. Analysis of the measured intensity profiles showed a good response to gas puffing and toroidal velocity and ion temperature results are presented. A large reverse shear in the toroidal rotation is observed close to the separatrix in normal operation, but not in counter-rotation experiments. This is attributed to parallel Pfirsch-Schluter flow. The ion temperature results display a large difference between ion and electron temperature in the pedestal region in many H-mode discharges. This is linked to the collisionality of the ions such that flux surface coupling in the banana regime leads to small ion gradients.

The Reactive Atom Plasma® (RAP) process is a plasma chemical etching process. RAP was developed at RAPT Industries as a process for removing subsurface damage from silicon carbide optics. The process is being investigated at Cranfield University as a novel method for the fine surface correction of large optics, with the aim of shortening the manufacturing period of the next generation of large telescopes. RAP offers material removal rates that are up to 10 times higher than those of ion beam figuring, the current state-of-the-art technique and the convenience in that it can be operated at atmospheric pressure. Cont/d.

The role of cross field transport and volume recombination are of vital importance for a satisfactory understanding of the plasma edge in magnetically confined devices such as a Tokamak. Plasma fluctuations may travel cross field with significant velocities and play a central role in plasma transport. Cross field transport has been seen to be anomalous in most devices under a very broad range of experimental conditions. In recent years a clear indication of the relation between fluctuation, cross field particle transport and recombination has been reported.The University of Manchester Linear System (the ULS) has been used to observe the Balmer emission of the recombining plasma interacting with a dense neutral Hydrogen gas. The ULS is a device made of a cylindrical vacuum vessel 1.5 m long and 15 cm in radius. The plasma is formed in a separate chamber by a duoplasmatron source in the Demirkhanov configuration; the arc current was limited to 15 A and the potential drop was 100 V. The device is surrounded by a linear solenoid which was used to magnetize the plasma. The highest magnetic field was .1 T. Typical electron temperature in the device spans .1 to 10 eV, and the density 1. E+16 to 5. E+19.Diagnostic includes Langmuir probe and visible spectrometers. In addition, the DivCam imaging system originally designed and built to obtain 2D images of the MAST spherical Tokamak Scrape Off Layer, was used. The DivCam imaging system has enabled to obtain high resolution images of the plasma emission when interacting with the neutral gas. It appears evident that the Electron-Ion Recombination is strongly dependent upon radial transport of plasma particles: light emission attributed to EIR is only observed at a large cross field distance from the plasma source. Moreover, fast imaging of the plasma has also shown the presence of a plasma filament forming and propagating crossfield at the same region of the plasma where the EIR light is observed.To interpret the experimental observations obtained with DivCam, the OSM 1D fluid plasma solver and the EIRENE neutral Monte Carlo solver have been implemented in the linear geometry of the ULS linear system. Both the OSM and the EIRENE solvers were originally intended for tokamak and large magnetic confinement devices. Modelling of the EIR emissivity in the ULS device has demonstrated the importance of the inclusion of turbulent and blob transport in the model to obtain reasonable agreement between the observations and the theoretical predictions. The central density of the plasma filament has been estimated to be approximately .7 E+19 m-3 using EIRENE results.The emission attributed to hydrogenic ions (negative atomic H- and positive molecular ions H2+) and related to Molecular Assisted Recombinations can be estimated within EIRENE using the AMJUEL database. The database provides ion population estimations for three different collisional regimes: in the first regime a large population of vibrational excited hydrogen molecules are assumed to exist within the plasma volume; the second assumes strong Charge Exchange reactions and not vibrational excited molecule; the third assumes electron impact collisions with ground states molecule to be the only ion source. A reasonable agreement between the observations and the EIRENE prediction is only found when using the third estimation suggesting that molecular excitation and charge exchange processes are relatively unimportant under the experimental conditions considered.