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Superfluid 4 He (He II) is a quantum liquid whose flow is strongly affected by quantum mechanical effects. This thesis presents experimental and numerical studies of turbulent flows in He II - quantum turbulence. Experimentally, quan- tum turbulence is investigated in thermal counterflow, pure superflow and coflow using second sound attenuation, precision local thermometry and by visualisa- tion of helium excimer molecules. The steady state and decay of the vortex line density in pure superflow and counterflow is studied and the universal quasi clas- sical decay is characterised by measurements of the effective kinematic viscosity. General dynamical behaviour is studied in detail in unsteady thermal counter- flow, with various theoretical models tested. A new model where the mean tangle curvature is dependent on the vortex line density is proposed. Temperature de- pendence and enhancement of intermittency in quasi-classical flow in the wake of a moving grid is found using visualisation. Numerically, the interaction of the tangle of quantized vortices with solid tracers is investigated, where a back reaction of the seeding particles on the tangle is identified and its relevance to visualisation experiments is discussed. Additionally, an interesting and as-yet overlooked spherical counterflow is...

Direct detection of gravitational waves is opening a new window onto our universe. Here, we study the sensitivity to continuous-wave strain fields of a kg-scale optomechanical system formed by the acoustic motion of superfluid helium-4 parametrically coupled to a superconducting microwave cavity. This narrowband detection scheme can operate at very highQ-factors, while the resonant frequency is tunable through pressurization of the helium in the 0.1-1.5 kHz range. The detector can therefore be tuned to a variety of astrophysical sources and can remain sensitive to a particular source over a long period of time. For thermal noise limited sensitivity, we find that strain fields on the order of h similar to 10(-23)/root Hz are detectable. Measuring such strains is possible by implementing state of the art microwave transducer technology. Weshow that the proposed system can compete with interferometric detectors and potentially surpass the gravitational strain limits set by them for certain pulsar sources within a few months of integration time.

The decay of quantum turbulence in superfluid (_^4)He in the zero temperature limit wasinvestigated. Quantum turbulence was created by impulsive spin-down from a specific angular velocity to rest. A measurement of the vortex line density was performed using the scattering of charged vortex rings. The vortex line density decayed as L ∝ t^(-3/2) which is the characteristic of quasiclassical turbulence forced at large length scales. The interaction of metastable spin-triplet 〖He〗_2^* molecules with quantized vortex lines in superfluid (_^4)He at temperatures below 200 mK was studied too. The molecules were generated during an injection of electrons from a sharp metal tip at high voltage. They were detected as a current into a metal collector after ionization upon colliding with the collector surface. The detected current was suppressed by even a small rotation indicating the trapping of the molecules on quantized vortices. The presence of (_^3)He impurities at the level of 0.3 ppb strongly suppressed the detected signal. The temperature dependence of the detected signal showed a sharp peak, most probably associated with the condensation of (_^3)He atoms on vortex cores. The time of flight of the molecules as a function of temperature was measured. It was observed that there are three regimes for 〖He〗_2^* molecules transportation in superfluid (_^4)He : ballistic regime for T < 100 mK, diffusive for T > 200 mK, and an intermediate regime between them. The vortex lines were created by either rotation or ion injection. The trapping diameter of the molecules on quantized vortices was found to be 96 ± 6 nm at pressure of 0.1 bar and 27 ± 5 nm at 5.0 bar. It was also demonstrated that a tangle of vortices moving in superfluid (_^4)He are capable of conveying the 〖He〗_2^* molecules through the drift region.

The Bogoliubov theory of excitations in superfluid helium is used to study collective modes at zero temperature. A repulsive delta function shell potential is used in the quasiparticle excitation energy spectrum to fit the observed elementary excitation spectrum, except in the plateau region. The linearized equation of motion method is used to obtain the secular equation for a collective mode consisting of a linear combination of one and two free quasiparticles of zero total momentum. It is shown that in this case for high-lying collective modes, vertices involving three quasiparticles cancel, and only vertices involving four quasiparticles are important. A decomposition into various angular momentum states is then made. Bound roton pairs in the angular momentum D-state observed in light-scattering experiments exist only for an attractive coupling between helium atoms in this oversimplified model. Thus, the interaction between particles can be reinterpreted as a phenomenological attractive coupling between quasiparticles, in order to explain the Raman scattering from bound roton pairs in superfluid helium.

<p>The Large Hadron Collider (LHC) is the next large particle accelerator developed at CERN, constructed to enable studies of particles. The acceleration of the particles is carried out using magnets operating at about 1.9 K, a temperature achieved by regulating flow of superfluid helium. For economical reasons, control of the helium flow is based on feedback of virtual flow meter (VFT) estimates instead of real instrumentation.</p><p>The main purpose of this work is to develop a virtual flow meter with the possibility to estimate the flow by means of two different flow estimation methods; the Samson method that has previously been tested for the LHC, and the Sereg- Schlumberger method that has never before been implemented in this environment.</p><p>The virtual flow meters are implemented on PLCs using temperature and pressure measurements as input data, and a tool for generating the virtual flow meters and connect them to the appropriate physical instrumentation has also been developed.</p><p>The flow through a valve depends, among others, on some pressure and temperature dependent physical properties that are to be estimated with high accuracy. In this project, this is done by bilinear interpolation in twodimensional tables containing physical data, an approach that turned out to be more accurate than the previously used method with polynomial interpolation.</p><p>The flow measurement methods have been compared. Since they both derive from empirical studies rather than physical relations it is quite futile to find theoretical correspondencies, but the simulations of the mass flows can be compared. For low pressures, the results are fairly equal but they differ more for higher pressures. The methods have not been validated against true flow rates since there were no real measurements available before the end of this project.</p> / <p>Le Grand Collisionneur de Hadrons (Large Hadron Collider, LHC) est le prochain grand accélérateur de particules du CERN, construit pour permettre l’étude des particules. L’accélération des particules sera réalisée en utilisant des aimants supraconducteurs qui fonctionneront à 1.9 K et la température sera régulée en contrôlant le débit d’hélium superfluide. Pour des raisons économiques, la régulation du débit d’hélium sera basée sur les réponses des estimations des débitmètres</p><p>virtuels (Virtual flow meters, VFT) au lieu d’instrumentation réelle.</p><p>Le but principal de ce projet est de développer un débitmètre virtuel qui estimera le débit avec deux méthodes différentes ; la méthode Samson qui a déjà été mise en oeuvre pour le LHC, et la méthode Sereg-Schlumberger qui n’a pas encore été implémentée dans cet environnement.</p><p>Les débitmètres virtuels seront implémentés sur des PLCs avec des mesures de température et de pression comme données d’entrée. De plus, un outil pour générer les débitmètres et les relier avec l’instrumentation physique adéquat a été développé.</p><p>Le débit à travers d’une vanne dépend entre autres des propriétés physiques qui dépendent à leur tour de la température et de la pression. Ces propriétés devront être estimées avec une grande précision. Dans ce projet, cela est fait en appliquant une interpolation bilinéaire dans des tableaux de deux dimensions. Cette méthode s’est montrée plus précise qu’avec une méthode d’interpolation polynomiale.</p><p>Les deux méthodes de mesures de débit ont été comparées. Elles dérivent toutes les deux des études empiriques et non physiques, alors les similarités théoriques sont donc peu pertinentes, mais les résultats des simulations des débits peuvent être comparés. Pour des pressions basses, les méthodes sont quasiment équivalentes, mais les différences sont plus importantes pour les pressions plus hautes. Étant donné qu’il n’y avait pas de mesures disponibles avant la fin de ce projet, les méthodes n’ont pas été validées avec des débits réels.</p>

The Large Hadron Collider (LHC) is the next large particle accelerator developed at CERN, constructed to enable studies of particles. The acceleration of the particles is carried out using magnets operating at about 1.9 K, a temperature achieved by regulating flow of superfluid helium. For economical reasons, control of the helium flow is based on feedback of virtual flow meter (VFT) estimates instead of real instrumentation. The main purpose of this work is to develop a virtual flow meter with the possibility to estimate the flow by means of two different flow estimation methods; the Samson method that has previously been tested for the LHC, and the Sereg- Schlumberger method that has never before been implemented in this environment. The virtual flow meters are implemented on PLCs using temperature and pressure measurements as input data, and a tool for generating the virtual flow meters and connect them to the appropriate physical instrumentation has also been developed. The flow through a valve depends, among others, on some pressure and temperature dependent physical properties that are to be estimated with high accuracy. In this project, this is done by bilinear interpolation in twodimensional tables containing physical data, an approach that turned out to be more accurate than the previously used method with polynomial interpolation. The flow measurement methods have been compared. Since they both derive from empirical studies rather than physical relations it is quite futile to find theoretical correspondencies, but the simulations of the mass flows can be compared. For low pressures, the results are fairly equal but they differ more for higher pressures. The methods have not been validated against true flow rates since there were no real measurements available before the end of this project. / Le Grand Collisionneur de Hadrons (Large Hadron Collider, LHC) est le prochain grand accélérateur de particules du CERN, construit pour permettre l’étude des particules. L’accélération des particules sera réalisée en utilisant des aimants supraconducteurs qui fonctionneront à 1.9 K et la température sera régulée en contrôlant le débit d’hélium superfluide. Pour des raisons économiques, la régulation du débit d’hélium sera basée sur les réponses des estimations des débitmètres virtuels (Virtual flow meters, VFT) au lieu d’instrumentation réelle. Le but principal de ce projet est de développer un débitmètre virtuel qui estimera le débit avec deux méthodes différentes ; la méthode Samson qui a déjà été mise en oeuvre pour le LHC, et la méthode Sereg-Schlumberger qui n’a pas encore été implémentée dans cet environnement. Les débitmètres virtuels seront implémentés sur des PLCs avec des mesures de température et de pression comme données d’entrée. De plus, un outil pour générer les débitmètres et les relier avec l’instrumentation physique adéquat a été développé. Le débit à travers d’une vanne dépend entre autres des propriétés physiques qui dépendent à leur tour de la température et de la pression. Ces propriétés devront être estimées avec une grande précision. Dans ce projet, cela est fait en appliquant une interpolation bilinéaire dans des tableaux de deux dimensions. Cette méthode s’est montrée plus précise qu’avec une méthode d’interpolation polynomiale. Les deux méthodes de mesures de débit ont été comparées. Elles dérivent toutes les deux des études empiriques et non physiques, alors les similarités théoriques sont donc peu pertinentes, mais les résultats des simulations des débits peuvent être comparés. Pour des pressions basses, les méthodes sont quasiment équivalentes, mais les différences sont plus importantes pour les pressions plus hautes. Étant donné qu’il n’y avait pas de mesures disponibles avant la fin de ce projet, les méthodes n’ont pas été validées avec des débits réels.