Plasma acceleration above Martian magnetic anomalies

Grande, Manuel, et al., 'Plasma acceleration above Martian magnetic anomalies', Science (2006) 311(5763) pp.980-983 RAE2008

Global oscillations in a magnetic solar model : II Oblique propagation

Pint?r, B.; Erd?lyi, R.; Goossens, M., Global oscillations in a magnetic solar model. II. Oblique propagation, Astronomy and Astrophysics, Volume 466, Issue 1, April IV 2007, pp.377-388 Pint?r, B.; Erd?lyi, R.; Goossens, M., (2007) 'Global oscillations in a magnetic solar model II. Oblique propagation', Astronomy and Astrophysics 466(1) pp.377-388 RAE2008

Evanescent field sensing: optical nanofibres, whispering-gallery-mode microspherical and microbubble resonators

In this thesis, the evanescent field sensing techniques of tapered optical nanofibres and microspherical resonators are investigated. This includes evanescent field spectroscopy of a silica nanofibre in a rubidium vapour; thermo-optical tuning of Er:Yb co-doped phosphate glass microspheres; optomechanical properties of microspherical pendulums; and the fabrication and characterisation of borosilicate microbubble resonators. Doppler-broadened and sub-Doppler absorption spectroscopic techniques are performed around the D2 transition (780.24 nm) of rubidium using the evanescent field produced at the waist of a tapered nanofibre with input probe powers as low as 55 nW. Doppler-broadened Zeeman shifts and a preliminary dichroic atomic vapour laser lock (DAVLL) line shape are also observed via the nanofibre waist with an applied magnetic field of 60 G. This device has the potential for laser frequency stabilisation while also studying the effects of atom-surface interactions. A non-invasive thermo-optical tuning technique of Er:Yb co-doped microspheres to specific arbitrary wavelengths is demonstrated particularly to 1294 nm and the 5S1/2F=3 to 5P3/2Fʹ=4 laser cooling transition of 85Rb. Reversible tuning ranges of up to 474 GHz and on resonance cavity timescales on the order of 100 s are reported. This procedure has prospective applications for sensing a variety of atomic or molecular species in a cavity quantum electrodynamics (QED) experiments. The mechanical characteristics of a silica microsphere pendulum with a relatively low spring constant of 10-4 Nm-1 are explored. A novel method of frequency sweeping the motion of the pendulum to determine its natural resonance frequencies while overriding its sensitivity to environmental noise is proposed. An estimated force of 0.25 N is required to actuate the pendulum by a displacement of (1-2) μm. It is suggested that this is of sufficient magnitude to be experienced between two evanescently coupled microspheres (photonic molecule) and enable spatial trapping of the micropendulum. Finally, single-input borosilicate microbubble resonators with diameters <100 μm are fabricated using a CO2 laser. Optical whispering gallery mode spectra are observed via evanescent coupling with a tapered fibre. A red-shift of (4-22) GHz of the resonance modes is detected when the hollow cavity was filled with nano-filtered water. A polarisation conversion effect, with an efficiency of 10%, is observed when the diameter of the coupling tapered fibre waist is varied. This effect is also achieved by simply varying the polarisation of the input light in the tapered fibre where the efficiency is optimised to 92%. Thus, the microbubble device acts as a reversible band-pass to band-stop optical filter for cavity-QED, integrated solid-state and semiconductor circuit applications.

Electromagnetic tracking and steering for catheter navigation

This thesis explores the use of electromagnetics for both steering and tracking of medical instruments in minimally invasive surgeries. The end application is virtual navigation of the lung for biopsy of early stage cancer nodules. Navigation to the peripheral regions of the lung is difficult due to physical dimensions of the bronchi and current methods have low successes rates for accurate diagnosis. Firstly, the potential use of DC magnetic fields for the actuation of catheter devices with permanently magnetised distal attachments is investigated. Catheter models formed from various materials and magnetic tip formations are used to examine the usefulness of relatively low power and compact electromagnets. The force and torque that can be exerted on a small permanent magnet is shown to be extremely limited. Hence, after this initial investigation we turn our attention to electromagnetic tracking, in the development of a novel, low-cost implementation of a GPS-like system for navigating within a patient. A planar magnetic transmitter, formed on a printed circuit board for a low-profile and low cost manufacture, is used to generate a low frequency magnetic field distribution which is detected by a small induction coil sensor. The field transmitter is controlled by a novel closed-loop system that ensures a highly stable magnetic field with reduced interference from one transmitter coil to another. Efficient demodulation schemes are presented which utilise synchronous detection of each magnetic field component experienced by the sensor. The overall tracking accuracy of the system is shown to be less than 2 mm with an orientation error less than 1°. A novel demodulation implementation using a unique undersampling approach allows the use of reduced sample rates to sample the signals of interest without loss of tracking accuracy. This is advantageous for embedded microcontroller implementations of EM tracking systems. The EM tracking system is demonstrated in the pre-clinical environment of a breathing lung phantom. The airways of the phantom are successfully navigated using the system in combination with a 3D computer model rendered from CT data. Registration is achieved using both a landmark rigid registration method and a hybrid fiducial-free approach. The design of a planar magnetic shield structure for blocking the effects of metallic distortion from below the transmitter is presented which successfully blocks the impact of large ferromagnetic objects such as operating tables. A variety of shielding material are analysed with MuMetal and ferrite both providing excellent shieling performance and an increased signal to noise ratio. Finally, the effect of conductive materials and human tissue on magnetic field measurements is presented. Error due to induced eddy currents and capacitive coupling is shown to severely affect EM tracking accuracy at higher frequencies.

Manipulation of magnetic anisotropy in nanostructures

Of late, the magnetic properties of micro/nano-structures have attracted intense research interest both fundamentally and technologically particularly to address the question that how the manipulation in the different layers of nanostructures, geometry of a patterned structure can affect the overall magnetic properties, while generating novel applications such as in magnetic sensors, storage devices, integrated inductive components and spintronic devices. Depending on the applications, materials with high, medium or low magnetic anisotropy and their possible manipulation are required. The most dramatic manifestation in this respect is the chance to manipulate the magnetic anisotropy over the intrinsic preferential direction of the magnetization, which can open up more functionality particularly for device applications. Types of magnetic anisotropies of different nanostructured materials and their manipulation techniques are investigated in this work. Detail experimental methods for the quantitative determination of magnetic anisotropy in nanomodulated Ni45Fe55 thin film are studied. Magnetic field induced in-plane rotations within the nanomodulated Ni45Fe55 continuous films revealed various rotational symmetries of magnetic anisotropy due to dipolar interactions showing a crossover from lower to higher fold of symmetry as a function of modulation geometry. In a second approach, the control of exchange anisotropy at ferromagnetic (FM) – aniferomagnetic (AFM) interface in multifferoic nanocomposite materials, where two different phase/types of materials were simultaneously synthesized, was investigated. The third part of this work was to study the electroplated thin films of metal alloy nanocomposite for enhanced exchange anisotropy. In this work a unique observation of an anti-clock wise as well as a clock wise hysteresis loop formation in the Ni,Fe solid solution with very low coercivity and large positive exchange anisotropy/exchange bias have been investigated. Hence, controllable positive and negative exchange anisotropy has been observed for the first time which has high potential applications such as in MRAM devices.

Rotating-frame nuclear magnetic relaxation of spins diffusing on a disordered lattice: a Monte Carlo model

The rotating-frame nuclear magnetic relaxation rate of spins diffusing on a disordered lattice has been calculated by Monte Carlo methods. The disorder includes not only variation in the distances between neighbouring spin sites but also variation in the hopping rate associated with each site. The presence of the disorder, particularly the hopping rate disorder, causes changes in the time-dependent spin correlation functions which translate into asymmetry in the characteristic peak in the temperature dependence of the dipolar relaxation rate. The results may be used to deduce the average hopping rate from the relaxation but the effect is not sufficiently marked to enable the distribution of the hopping rates to be evaluated. The distribution, which is a measure of the degree of disorder, is the more interesting feature and it has been possible to show from the calculation that measurements of the relaxation rate as a function of the strength of the radiofrequency spin-locking magnetic field can lead to an evaluation of its width. Some experimental data on an amorphous metal - hydrogen alloy are reported which demonstrate the feasibility of this novel approach to rotating-frame relaxation in disordered materials.

Magnetic modelling and magneto-hydro-dynamic simulations of an aluminium production electrolytic cell

A simulation of the motion of molten aluminium inside an electrolytic cell is presented. Since the driving term of the aluminium motion is the Lorentz (j × B) body force acting within the fluid,this problem involves the solution of the magneto-hydro-dynamic equations. Different solver modules for the magnetic field computation and for the fluid motion simulation are coupled together. The interactions of all these are presented and discussed.

Magnetic levitation fluid dynamics

This work is concerned with the accurate computation of flow in a rapidly deforming liquid metal droplet, suspended in an AC magnetic field. Intense flow motion due to the induced electromagnetic force distorts dynamically the droplet envelope, which is initially spherical. The relative positional change between the liquid metal surface and the surrounding coil means that fluid flow and magnetic field computations need to be closely coupled. A spectral technique is used to solve this problem, which is assumed axisymmetric. The computed results are compared against a physical experiment and "ideal sphere" analytic solutions. A comparison between the "magnetic pressure" approximation and the full electromagnetic force solutions, shows fundamental differences; the full electromagnetic force solution is necessary for accurate results in most practical applications of this technique. The physical reason for the fundamental discrepancy is the difference in the electromagnetic force representation: only the gradient part of the full force is accounted for in the "magnetic pressure" approximation. Figs 9, Refs 13.

Droplet generation with modulated AC electromagnetic field at nozzle exit

A method of droplet generation based on applying a modulated AC high frequency magnetic field in the localized region of capillary breakup is considered as ans alternative to traditional methods for high temperature liquid melt droplet serial production by pressure variation. The method is based on a pseudo-spectral approximation with a coordinate transformation adaptin to the developing free surface. The electromagnetic field is recomputed continuously with the domain shape change. Practical application cases for liquid silicon droplets of 0.5 - 2 mm diameter are considered in detail.

Time dependent electric, magnetic and hydrodynamic interaction in aluminium electrolysis cells

The waves in commercial cells for electrolytic aluminium production originate at the interface between the liquid aluminium and electrolyte, but their effect can spread into the surrounding busbar network as electric current perturbation, and the total magnetic field acquires a time dependent component. The presented model for the wave development accounts for the nonuniform electric current distribution at the cathode and the whole network of the surrounding busbars. The magnetic field is computed for the continuous current in the fluid zones, all busbars and the ferromagnetic construction elements. When the electric current and the associated magnetic field are computed according to the actual electrical circuit and updated for all times, the instability growth rate is significantly affected. The presented numerical model for the wave and electromagnetic interaction demonstrates how different physical coupling factors are affecting the wave development in the electrolysis cells. These small amplitude self-sustained interface oscillations are damped in the presence of intense turbulent viscosity created by the horizontal circulation velocity field. Additionally, the horizontal circulation vortices create a pressure gradient contributing to the deformation of the interface. Instructive examples for the 500 kA demonstration cell are presented.

Effects of magnetic fields on crystal growth

The effects of a constant uniform magnetic field on dendritic solidification were investigated using a 2-dimensional enthalpy based numerical model. The interaction between thermoelectic currents and the magnetic field generates a Lorentz force that creates a flow. This flow causes a change in the morphology of the dendrite; secondary growth is promoted on one side of the dendrite arm and the tip velocity of the primary arm is increased.

Effects of magnetic fields on crystal growth

The effects of a constant uniform magnetic field on thermoelectric currents during dendritic solidification were investigated using an enthalpy based numerical model. It was found that the resulting Lorentz force generates a complex flow influencing the solidification pattern. Experimental work of material processing under high magnetic field conditions has shown that the microstructure can be significantly altered. There is evidence that these effects can be atrtributed to the Lorentz force created through the thermoelectric magentohydrodynamic interactions.[1,2] However the mechanism of how this occurs is not very well understood. In this paper, our aim is to investigate the flow field created from the Lorentz force and how this influences the morphology of dendritic growth for both pure materials and binary alloys.

Effects of magnetic fields on crystal growth

The effects of a constant uniform magnetic field on thermoelectric currents during dendritic solidification were investigated using a two-dimensional enthalpy based numerical model. Using an approximation for three-dimensional unconstricted growth, the resulting Lorentz forces generate a circulating flow influencing the solidification pattern. Under the presence of a strong magnetic field secondary growth on the clockwise side of the primary arm of the dendrite was encouraged, whereas the anticlockwise side is suppressed due to a reduction in local free energy. The preferred direction of growth rotated in the clockwise sense under an anticlockwise flow. The tip velocity is significantly increased compared with growth in stagnant flow. This is due to a small recirculation at the tip of the dendrite; bringing in colder liquid and lowering the concentration of solute.

Ionization energies of beryllium in strong magnetic fields: a frozen core approximation

An effective frozen core approximation has been developed and applied to the calculation of energy levels and ionization energies of the beryllium atom in magnetic field strengths up to 2.35 x 10(5) T. Systematic improvement over the existing results for the beryllium ground and low-lying states has been accomplished by taking into account most of the correlation effects in the four-electron system. To our knowledge, this is the first calculation of the electronic properties of the beryllium atom in a strong magnetic field carried out using a configuration interaction approximation and thus allowing a treatment beyond that of Hartree-Fock. Differing roles played by strong magnetic fields in intrashell correlation within different states are observed. In addition, possible ways to gain further improvement in the energies of the states of interest are proposed and discussed briefly.

Dynamic, in situ optical, magnetic and magneto-optical monitoring of the growth of Co/Au and Pd/Co/Au multilayer systems

The growth of magnetron sputtered Co/Au and Pd/Co/Au superlattices on Au and Pd buffer layers, deposited onto glass substrates, has been monitored optically and magneto-optically in real time, using rotating analyser ellipsometry and Kerr polarimetry, at a wavelength of 633 nm. The magneto-optical traces, combined with ex situ and in situ hysteresis loops, provide a detailed and informative fingerprint of the optical and magnetic properties of the films as they evolve during growth. For Co/Au, oscillations in the polar magneto-optical effect developed during the deposition of An overlayers on Co and these may be attributed to quantum well states. However, the hysteresis measurements show that the magnetic field required to maintain saturation magnetization throughout the experiment was larger than available in situ, introducing a degree of confusion concerning the interpretation of the data. This problem was overcome by the incorporation of Pd layers into the Co/Au structure, thereby eliminating variation in magnetic orientation during growth of the Au layers as a contributory factor to the observations.