The study of flow and temperature fields in conducting droplets suspended in a DC/AC combination field

Electromagnetic Levitation (EML) is a valuable method for measuring the thermo-physical properties of metals - surface tensions, viscosity, thermal/electrical conductivity, specific heat, hemispherical emissivity, etc. – beyond their melting temperature. In EML, a small amount of the test specimen is melted by Joule heating in a suspended AC coil. Once in liquid state, a small perturbation causes the liquid envelope to oscillate and the frequency of oscillation is then used to compute its surface tension by the well know Rayleigh formula. Similarly, the rate at which the oscillation is dampened relates to the viscosity. To measure thermal conductivity, a sinusoidally varying laser source may be used to heat the polar axis of the droplet and the temperature response measured at the polar opposite – the resulting phase shift yields thermal conductivity. All these theoretical methods assume that convective effects due to flow within the droplet are negligible compared to conduction, and similarly that the flow conditions are laminar; a situation that can only be realised under microgravity conditions. Hence the EML experiment is the method favoured for Spacelab experiments (viz. TEMPUS). Under terrestrial conditions, the full gravity force has to be countered by a much larger induced magnetic field. The magnetic field generates strong flow within the droplet, which for droplets of practical size becomes irrotational and turbulent. At the same time the droplet oscillation envelope is no longer ellipsoidal. Both these conditions invalidate simple theoretical models and prevent widespread EML use in terrestrial laboratories. The authors have shown in earlier publications that it is possible to suppress most of the turbulent convection generated in the droplet skin layer, through use of a static magnetic field. Using a pseudo-spectral discretisation method it is possible compute very accurately the dynamic variation in the suspended fluid envelope and simultaneously compute the time-varying electromagnetic, flow and thermal fields. The use of a DC field as a dampening agent was also demonstrated in cold crucible melting, where suppression of turbulence was achieved in a much larger liquid metal volume and led to increased superheat in the melt and reduction of heat losses to the water-cooled walls. In this paper, the authors describe the pseudo-spectral technique as applied to EML to compute the combined effects of AC and DC fields, accounting for all the flow-induced forces acting on the liquid volume (Lorentz, Maragoni, surface tension, gravity) and show example simulations.

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.

Determining surface tension using DC magnetic levitation

The values of material physical properties are vital for the successful use of numerical simulations for electromagnetic processing of materials. The surface tension of materials can be determined from the experimental measurement of the surface oscillation frequency of liquid droplets. In order for this technique to be used, a positioning field is required that results in a modification to the oscillation frequency. A number of previous analytical models have been developed that mainly focus on electrically conducting droplets positioned using an A.C. electromagnetic field, but due to the turbulent flow resulting from the high electromagnetic fields required to balance gravity, reliable measurements have largely been limited to microgravity. In this work axisymmetric analytical and numerical models are developed, which allow the surface tension of a diamagnetic droplet positioned in a high DC magnetic field to be determined from the surface oscillations. In the case of D.C. levitation there is no internal electric currents with resulting Joule heating, Marangoni flow and other effects that introduce additional physics that complicates the measurement process. The analytical solution uses the linearised Navier-Stokes equations in the inviscid case. The body force from a DC field is potential, in contrast to the AC case, and it can be derived from Maxwell equations giving a solution for the magnetic field in the form of a series expansion of Legendre polynomials. The first few terms in this expansion represent a constant and gradient magnetic field valid close to the origin, which can be used to position the droplet. Initially the mathematical model is verified in microgravity conditions using a numerical model developed to solve the transient electromagnetics, fluid flow and thermodynamic equations. In the numerical model (as in experiment) the magnetic field is obtained using electrical current carrying coils, which provides the confinement force for a liquid droplet. The model incorporates free surface deformation to accurately model the oscillations that result from the interaction between the droplet and the non-uniform external magnetic field. A comparison is made between the analytical perturbation theory and the numerical pseudo spectral approximation solutions for small amplitude oscillations.

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.

White-light oscillations during a flare on II Peg

We analyse the intensity oscillations observed in the gradual phase of a white-light flare on the RS CV n binary II Peg. Fast Fourier Transform power spectra and Wavelet analysis reveal a period of 220 s. The reliability of the oscillation is tested using several criteria. Oscillating coronal loop models are used to derive physical parameters such as temperature, electron density and magnetic field strength associated with the coronal loop. The derived parameters are consistent with the near-simultaneous X-ray observations of the flare. There is no evidence for oscillations in the quiescent state of the binary.

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.

Self-focusing and envelope pulse generation in nonlinear magnetic metamaterials

The self-modulation of waves propagating in nonlinear magnetic metamaterials is investigated. Considering the propagation of a modulated amplitude magnetic field in such a medium, we show that the self-modulation of the carrier wave leads to a spontaneous energy localization via the generation of localized envelope structures (envelope solitons), whose form and properties are discussed. These results are also supported by numerical calculations.

Plasma dynamics in an inductively coupled magnetic neutral loop discharge

Magnetic neutral loop discharges (NLDs) can be operated at significantly lower pressures than conventional radio-frequency (rf) inductively coupled plasmas (ICPs). These low pressure conditions are favourable for technological applications, in particular anisotropic etching. An ICP–NLD has been designed providing excellent diagnostics access for detailed investigations of fundamental mechanisms. Spatially resolved Langmuir probe measurements have been performed in the plasma production region (NL region) as well as in the remote application region downstream from the NL region. Depending on the NL gradient two different operation modes have been observed exhibiting different opportunities for control of plasma uniformity. The efficient operation at comparatively low pressures results in ionization degrees exceeding 1%. In this regime neutral dynamics has to be considered and can influence neutral gas and process uniformity. Neutral gas depletion through elevated gas temperatures and high ionization rates have been quantified. At pressures above 0.1 Pa, gas heating is the dominant depletion mechanism. At lower pressures neutral gas is predominantly depleted through high ionization rates and rapid transport of ions by ambipolar diffusion along the magnetic field lines. Non-uniform profiles of the ionization rate can, therefore, result in localized neutral gas depletion and non-uniform processing. We have also investigated the electron dynamics within the radio-frequency cycle using phase resolved optical emission spectroscopy and Thomson scattering. In these measurements electron drift phenomena along the NL torus have been identified.

Rotation of a nanoparticle cloud in an inductively coupled plasma induced by weak static magnetic fields

Hydrocarbon nanoparticles with diameters between 10 and 30 nanometres are created in a low pressure plasma combining capacitive and inductive power coupling. The particles are generated in the capacitive phase of the experiment and stay confined in the plasma in the inductive phase. The presence of these embedded particles induces a rotation of a particle-free region (void) around the symmetry axis of the reactor. The phenomenon is analysed using optical emission spectroscopy both line integrated and spatially resolved via an intensified charge coupled device camera. From these data, electron temperatures and densities are deduced. We find that the rotation of the void is driven by a tangential component of the ion drag force induced by an external static magnetic field. Two modes are observed: a fast rotation of the void in the direction opposite to that of the tangential component and a slow rotation in the same direction. The rotation speed decreases linearly with the size of the particles. In the fast mode the dependence on the applied magnetic field is weak and consequently the rotation speed can serve as a monitor to detect particle sizes in low temperature plasmas.

Spatial structures of plasma parameters in a magnetic neutral loop discharge

Spatial structures of plasma parameters in a radio-frequency inductively coupled magnetic neutral loop discharge are investigated under various parameter variations using spatially resolved Langmuir probe measurements. A strong coupling between the plasma production region, in the neutral loop (NL) plane, and the axially remote substrate region is observed. The two regions are connected through the separatrices and therefore, spatial profiles in the substrate region are strongly influenced by the plasma production region and the structure of the separatrices. The electron temperature in the plasma production region peaks in the centre of the NL while the maximum in electron density is shifted radially inwards due to diffusion. Details of the structures in both regions, the production region and the substrate region, are determined through the position of the NL and the gradient of the inhomogeneous magnetic field around the NL confinement region. Parameter combinations are found providing higher plasma densities and better uniformity than in common inductively coupled plasmas without applying an additional magnetic field. The uniformity can be further improved using temporal variations of the magnetic field structure.

A planar inductively coupled radio-frequency magnetic neutral loop discharge

A planar inductively coupled radio-frequency (rf) magnetic neutral loop discharge has been designed. It provides diagnostic access to both the main plasma production region as well as a remote plane for applications. Three coaxial coils are arranged to generate a specially designed inhomogeneous magnetic field structure with vanishing field along a ring in the discharge-the so-called neutral loop (NL). The plasma is generated by applying an oscillating rf electric field along the NL, induced through a four-turn, planar antenna operated at 13.56 MHz. Electron density and temperature measurements are performed under various parameter variations. Collisionless electron heating in the NL region allows plasma operation at comparatively low pressures, down to 10(-2) Pa, with a degree of ionization in the order of several per cent. Conventional plasma operation in inductive mode without applying the magnetic field is less efficient, in particular in the low pressure regime where the plasma cannot be sustained without magnetic fields.

The first polarimetric signatures of infrared jets in X-ray binaries

We present near-infrared linear spectropolarimetry of a sample of persistent X-ray binaries, Sco X-1, Cyg X-2, and GRS 1915+105. The slopes of the spectra are shallower than what is expected from a standard steady state accretion disk, and can be explained if the near-infrared flux contains a contribution from an optically thin jet. For the neutron star systems, Sco X-1 and Cyg X-2, the polarization levels at 2.4 mu m are 1.3% +/- 0.10% and 5.4% +/- 0.7%, respectively, which is greater than the polarization level at 1.65 mu m. This cannot be explained by interstellar polarization or electron scattering in the anisotropic environment of the accretion flow. We propose that the most likely explanation is that this is the polarimetric signature of synchrotron emission arising from close to the base of the jets in these systems. In the black hole system GRS 1915+105 the observed polarization, although high (5.0% +/- 1.2% at 2.4 mu m), may be consistent with interstellar polarization. For Sco X-1 the position angle of the radio jet on the sky is approximately perpendicular to the near-infrared position angle (electric vector), suggesting that the magnetic field is aligned with the jet. These observations may be a first step toward probing the ordering, alignment, and variability of the outflow magnetic field in a region closer to the central accreting object than is observed in the radio band.

Polarimetric signatures of infrared jets in X-ray binaries

We present near-infrared linear spectropolarimetry of a sample of persistent X-ray binaries, Sco X-1, Cyg X-2 and GRS 1915+105. For Sco X-1 and Cyg X-2, the polarization levels at 2.4 µm are 1.3+/-0.10% and 5.4+/-0.7%, respectively, which is greater than the polarization level at 1.65 µm. This cannot be explained by interstellar polarization or electron scattering in the anisotropic environment of the accretion flow. We propose that the most likely explanation is that this is the polarimetric signature of synchrotron emission arising from close to the base of the jet. For Sco X-1 the position angle of the radio jet on the sky is approximately perpendicular to the near-infrared position angle (electric vector), suggesting that the magnetic field is aligned with the jet. These observations may be a first step towards probing the ordering, alignment, and variability of the outflow magnetic field, in a region closer to the central accreting object than is observed in the radio band.