We present experimental results on quantum grid turbulence produced by moving grids within superfluid 4He, both at millikelvin temperatures, with an oscillating grid, and at temperatures above 1.4 K with a linearly moving grid. Floppy devices were used at millikelvin temperatures to produce quantum turbulence. We investigated the frequency dependence of the turbulent drag on an oscillating grid. At high velocities, the turbulent drag is independent of frequency and similar to what was measured in liquid helium-4 in its normal phase. We also present measurements of the inertial drag coefficient for grid turbulence, which is significantly reduced by turbulence produced in both superfluid and normal fluid 4He. To produce (approximate) homogeneous and isotropic grid turbulence in a quantum liquid, with little to no extraneous heating in the fluid, a new linear ‘control motor‘ has been designed and tested. The motor consists of a drive coil, surrounded by three control coils. A linear current ramp is passed through the drive coil, which lifts a superconducting armature placed in the centre of the solenoid. The control coils are designed, when a steady DC current is applied to them, to have constant magnetic field derivative. The control motor performs adequately, having smooth motions with no oscillations, and with peak velocities up to approximately 30 cm/s. The velocity is not, however, very uniform during the motion of the motor. Decaying turbulence is investigated using the attenuation of second sound. We produce turbulence inside a short channel totally submersedin liquid helium-II. The turbulence is produced by a superconducting, magnetically levitated linear motor, with a grid attached to the top of the armature. The theory applied, for calculating vortex line density decay from second sound attenuation, is taken from Stalp, S. (1998) Decay of Grid Turbulence in Superfluid Helium. Ph.D. Thesis. We investigate the effects of different grid meshes on the vorticity decay curve, in particular the time at which the turbulence becomes saturated. We present comparisons of three separate meshes. We observe a shorter saturation time, and therefore a longer inertial regime with a t−3/2 dependence, for the turbulent decay produced by the smallest mesh grid. It has been suggested elsewhere that there is a t−11/10 dependence at early times in the vorticity decay curves, we observe no such dependence. Finally, we present measurements of the effective kinematic viscosity, and saturation time and their dependence on grid mesh size.

Cette thèse concerne l'étude du diagramme de phase du fer en conditions extrêmes de pression et température. La Terre possède un noyau interne solide et un noyau externe liquide, qui sont principalement composés de fer. Une détermination fiable de la température de fusion du fer à 330 GPa, pression au-delà de laquelle le noyau terrestre est solide, permet de contraindre la température du noyau, ce qui est essentiel pour comprendre la dynamique terrestre. Le diagramme de phase du fer a été étudié jusqu'à 200 GPa en cellule à enclumes de diamant chauffée par laser utilisant la diffraction par rayon X comme diagnostic de l¿apparition de la fusion. Les températures obtenues sont en accord avec celles mesurées par compression dynamique, aux incertitudes expérimentales près, et sont plus élevées que celles obtenues lors de précédentes expériences statiques utilisant un critère de fusion différent. L'appareil, les méthodes et la métrologie utilisés pour les expériences en cellule à enclume de diamant chauffée par laser sont présentées ainsi que les problèmes rencontrés dans les expériences statiques à de telles conditions extrêmes. La possibilité d'utiliser le signal de diffraction des rayons X du joint en Re à des fins d'étalonnage de la pression pour l'expérimentation dans le domaine du multi-Mbar est aussi abordée. Dans ce but, l'équation d¿état du Re a été mesurée à 144 GPa. En fin, un test préliminaire a été effectué pour vérifier la possibilité d'utiliser la spectroscopie d'absorption des rayons X en dispersion d'énergie comme une technique complémentaire à la diffraction des rayons X pour la détermination de la courbe de fusion du fer. / This thesis concerns the study of the phase diagram of iron at extreme conditions of pressure and temperature. Iron is the main constituent of the terrestrial planetary cores. In particular, the Earth has a solid inner core and a liquid outer core which are mainly composed of iron. The accurate determination of the melting temperature of iron at the inner core boundary pressure, 330 GPa, would provide an important constraint on the temperature of the core, which is essential to understand how the dynamic Earth works. The phase diagram of iron has been investigated in laser-heated diamond anvil cell experiments up to 200 GPa using synchrotron-based fast X-ray Diffraction as a primary melting diagnostic. The obtained melting temperatures agree within the experimental uncertainties with the ones obtained from shock wave experiments and are higher than those reported by previous static experiments, where a different melting criterion was used. The apparatus, methods and metrology used in the static laser heated diamond anvil cell are discussed together with the issues encountered in static experiments at such extreme conditions. The possibility of using the X-ray diffraction signal of Re gasket for pressure calibration purpose for experiment in the multi-Mbar range is also discussed. For this purpose, Re equation of state has been measured up to 144 GPa. Finally, a preliminary test has been performed to check the possibility of using energy dispersive X-ray absorption spectroscopy as a technique complementary to fast X-ray diffraction in the investigation of the melting curve of iron.