Originally presented as the author's thesis--Universität Rostock, 2007. / Includes bibliographical references.

The heat capacity of 3He adsorbed on Grafoil (exfoilated graphite) preplated with four layers of 4He was measured between 1 and 50mK. The heat capacity was found to be linear up to 4OmK. At 3He surface densities below 4nm-2 two dimensional Fermi liquid behaviour was found and values of the hydrodynamic effective mass and Landau parameter Ff inferred. Subsequent steps in the heat capacity as a function of coverage were taken as evidence of independent 2D Fermi fluids. A low field DC SQUID pulsed NMR spectrometer was developed for future studies of magnetic order in 2D solid 3He films at ultralow temperatures

This thesis is a study of the heat transfer correlations between a wire and liquid helium cooled to either 1.9 or 4.3 K. The wire resembles a part of a superconducting magnet used in the Large Hadron Collider (LHC) particle accelerator currently being built at CERN. The magnets are cooled to 1.9 K and using helium as a coolant is very efficient, especially at extremely low temperatures since it then becomes a superfluid with an apparent infinite thermal conductivity. The cooling of the magnet is very important, since the superconducting wires need to be thermally stable. Thermal stability means that a superconductive magnet can remain superconducting, even if a part of the magnet becomes normal conductive due to a temperature increase. This means that if heat is generated in a wire, it must be transferred to the helium by some sort of heat transfer mechanism, or along the wire or to the neighbouring wires by conduction. Since the magnets need to be superconductive for the operation of the particle accelerator, it is crucial to keep the wires cold. Therefore, it is necessary to understand the heat transfer mechanisms from the wires to the liquid helium. The scope of this thesis was to describe the heat transfer mechanisms from a heater immersed in liquid and superfluid helium. By performing both experiments and simulations, it was possible to determine properties like heat transfer correlations, critical heat flux limits, and the differences between transient and steady-state heat flow. The measured values were in good agreement with values found in literature with a few exceptions. These differences could be due to measurement errors. A numerical program was written in Matlab and it was able to simulate the experimental temperature and heat flux response with good accuracy for a given heat generation.

The interest in the role of the solar wind termination shock (TS) and heliosheath in cosmic ray (CR) modulation studies has increased sigm6cantly as the Voyager 1 and 2 spacecraft approach the estimated position of the TS. For this work the modulation of galactic CR protons, anti-protons, electrons with a Jovian source, positrons, Helium, and anomalous protons and Helium, and the consequent charge-sign dependence, are studied with an improved and extended two-dimensional numerical CR modulation model including a TS with diffusive shock acceleration, a heliosheath and drifts. The modulation is computed using improved local interstellar spectra (LIS) for almost all the species of interest to this study and new fundamentally derived diffusion coefficients, applicable to a number of CR species during both magnetic polarity cycles of the Sun. The model also allows comparisons of modulation with and without a TS and between solar minimum and moderate maximum conditions. The modulation of protons and Helium with their respective anomalous components are also studied to establish the consequent charge-sign dependence at low energies and the influence on the computed p/p, e-/p, and e-/He. The level of modulation in the simulated heliosheath, and the importance of this modulation 'barrier' and the TS for the different species are illustrated. From the computations it is possible to estimate the ratio of modulation occurring in the heliosheath to the total modulation between the heliopause and Earth for the mentioned species. It has been found that the modulation in the heliosheath depends on the particle species, is strongly dependent on the energy of the CRs, on the polarity cycle and is enhanced by the inclusion of the TS. The computed modulation for the considered species is surprisingly different and the heliosheath is important for CR modulation, although 'barrier' modulation is more prominent for protons, anti-protons and Helium, while the heliosheath cannot really be considered a modulation 'barrier' for electrons and positrons above energies of ~150 MeV. The effects of the TS on modulation are more pronounced for polarity cycles when particles are drifting primarily in the equatorial regions of the heliosphere along the heliospheric current sheet to the Sun, e.g. the A < 0 polarity cycle for protons, positrons, and Helium, and the A > 0 polarity cycle for electrons and anti-protons. This study also shows that the proton and Helium LIS may not be known at energies <~ 200 MeV until a spacecraft actually approaches the heliopause because of the strong modulation that occurs in the heliosheath, the effect of the TS, and the presence of anomalous protons and Helium. For anti-protons, in contrast, these effects are less pronounced. For positrons, with a completely different shape LIS, the modulated spectra have very mild energy dependencies <~ 300 MeV, even at Earth, in contrast to the other species. These characteristic spectral features may be helpful to distinguish between electron and positron spectra when they are measured near and at Earth. These simulations can be of use for future missions to the outer heliosphere and beyond. / Thesis (Ph.D. (Physics))--North-West University, Potchefstroom Campus, 2004.

A significant factor in the overall efficiency of the balance of plant for a future magnetic fusion energy (MFE) reactor is the thermal performance of the divertor. A significant fraction of the reactor power is delivered to the divertor as plasma impurities and fusion products are deposited on its surface. For an advanced MFE device, an average divertor heat load of 10 MW/m² is expected at steady-state operating conditions. Helium cooling of the divertors is one of the most effective ways to accommodate such a heat load. Several helium-cooled divertor designs have been proposed and/or studied during the past decade including the T-Tube divertor, the helium-cooled flat plate (HCFP) divertor, the helium-cooled multi-jet (HEMJ) divertor, the helium-cooled modular divertor with integral fin array (HEMP), and the helium-cooled modular divertor with slot array (HEMS). All of these designs rely on some form of heat transfer enhancement via impinging jets or cooling fins to help improve the heat removal capability of the divertor. For all of these designs very large heat transfer coefficients on the order of 50-60 kW/m²-K have been predicted. As the conditions of a fusion reactor and associated helium flow conditions (600 °C and 10 MPa) are difficult to achieve safely in a controlled laboratory environment, the study of these divertors often relies on computer simulations and experimental modeling at non-prototypical, albeit dynamically similar, conditions. Earlier studies were based on the assumption that, for geometrically similar divertor test modules, dynamic similarity can be achieved by matching only the Reynolds number. Experiments conducted in this investigation using different coolants and test module materials have shown this assumption to be false. Modified correlations for the Nusselt number and loss coefficients for the HEMJ and HEMP-like divertor modules have been developed. These have been used to develop generalized performance curves to predict the divertor performance, i.e. the maximum allowable heat flux and corresponding pumping power fraction, at prototypical conditions. Additionally, a numerical study has been performed to optimize the fin array geometry of the HEMP-like divertor module. The generalized correlations and performance curves developed in this investigation can be incorporated into system design codes, thereby allowing system designers to optimize the divertor geometry and operating conditions.

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No description available.

The interest in the role of the solar wind termination shock (TS) and heliosheath in cosmic ray (CR) modulation studies has increased sigm6cantly as the Voyager 1 and 2 spacecraft approach the estimated position of the TS. For this work the modulation of galactic CR protons, anti-protons, electrons with a Jovian source, positrons, Helium, and anomalous protons and Helium, and the consequent charge-sign dependence, are studied with an improved and extended two-dimensional numerical CR modulation model including a TS with diffusive shock acceleration, a heliosheath and drifts. The modulation is computed using improved local interstellar spectra (LIS) for almost all the species of interest to this study and new fundamentally derived diffusion coefficients, applicable to a number of CR species during both magnetic polarity cycles of the Sun. The model also allows comparisons of modulation with and without a TS and between solar minimum and moderate maximum conditions. The modulation of protons and Helium with their respective anomalous components are also studied to establish the consequent charge-sign dependence at low energies and the influence on the computed p/p, e-/p, and e-/He. The level of modulation in the simulated heliosheath, and the importance of this modulation 'barrier' and the TS for the different species are illustrated. From the computations it is possible to estimate the ratio of modulation occurring in the heliosheath to the total modulation between the heliopause and Earth for the mentioned species. It has been found that the modulation in the heliosheath depends on the particle species, is strongly dependent on the energy of the CRs, on the polarity cycle and is enhanced by the inclusion of the TS. The computed modulation for the considered species is surprisingly different and the heliosheath is important for CR modulation, although 'barrier' modulation is more prominent for protons, anti-protons and Helium, while the heliosheath cannot really be considered a modulation 'barrier' for electrons and positrons above energies of ~150 MeV. The effects of the TS on modulation are more pronounced for polarity cycles when particles are drifting primarily in the equatorial regions of the heliosphere along the heliospheric current sheet to the Sun, e.g. the A < 0 polarity cycle for protons, positrons, and Helium, and the A > 0 polarity cycle for electrons and anti-protons. This study also shows that the proton and Helium LIS may not be known at energies <~ 200 MeV until a spacecraft actually approaches the heliopause because of the strong modulation that occurs in the heliosheath, the effect of the TS, and the presence of anomalous protons and Helium. For anti-protons, in contrast, these effects are less pronounced. For positrons, with a completely different shape LIS, the modulated spectra have very mild energy dependencies <~ 300 MeV, even at Earth, in contrast to the other species. These characteristic spectral features may be helpful to distinguish between electron and positron spectra when they are measured near and at Earth. These simulations can be of use for future missions to the outer heliosphere and beyond. / Thesis (Ph.D. (Physics))--North-West University, Potchefstroom Campus, 2004.

The aim of this thesis is to analyse the cryogenic process for cooling superconducting radio frequency accelerator test cavities in the LEP accelerator at CERN. A liquefaction cryoplant is analysed, including the production of liquid helium at 4.5 K, the systems for distribution and regulation of liquid helium, and the radio frequency field used for accelerating particles. After discussing regulation problems and modifications planned for a new cavity installation in 1992, different techniques for specifying the control programs for the new installation are evaluated. Various diagramming techniques, standards and methodologies, and Computer Aided Software Engineering-tools, are compared as to their practical usefulness in this kind of process control. Finally, in accordance with anticipated requirements, possible ways of making high and low level control program specifications are suggested.

This thesis presents an implementation of path integral molecular dynamics (PIMD) for sampling equilibrium and dynamical properties within the molecular modelling toolkit (MMTK) [J. Comp. Chem. 21, 79 (2000)], an open source Python package. Rigorous simulation using this code serves to benchmark this implementation as well as the robust- ness of the path integral Langevin equation as a thermostat [J. Chem. Phys. 133, 124104 (2010)]. PIMD is used to calculate equilibrium properties for clusters of HeN-CO2 at low- temperatures, with comparison to experimental and exact results. We characterize the convergence of structural and energetic properties as a function of path-integral discretiza- tion error. The radial and angular distribution of these clusters is studied as a function of size in the absence of rotation and bosonic exchange. These distributions are subsequently used to calculate vibrational shifts of CO2. This result is compared to high-accuracy path integral Monte Carlo simulations which include rotational and exchange effects. These sim- ulations indicate that the neglect of rotational degrees of freedom leads to an unphysical localization of helium atoms and incorrect vibrational shifts when compared to experiment. Approximate real-time quantum dynamics is presented for doped helium clusters using the ring-polymer molecular dynamics (RPMD) method. The accuracy of RPMD is tested iii for low-temperature simulations and compared to exact results. Preliminary calculation of the dynamics of the helium solvated CO2 dopant with respect to the center of mass of the cluster is presented. The effect of a cartesian integrator versus a normal-mode integrator for quantum dynamics is addressed. The path integral ground-state method is applied in order to calculate T = 0 properties. A convergence study of the ground-state energy of the quantum harmonic oscillator with respect to sampling time and path discretization is shown. As a final application of this implementation, a sugar in a periodic water box is simulated at T = 300K. The calculation of rotamer populations and a dipole autocorrelation indicate negligible change with the inclusion of quantum effects. This work offers a comprehensive foundation from which to base future PIMD centered research.