A cryostat has been constructed for the determination of the latent heat of evaporation of liquid helium-4 from 1.8°K to the critical point. It has not yet proved possible to stabilize the cryostat behaviour sufficiently and thermal oscillations of the type reported by Taconis are strongly suspected as being the source of the difficulty. Modifications that were carried out in the design of the equipment to improve its behaviour were limited in their success so that two conclusions were reached. Firstly, a separate investigation of the problem of Taconis resonate® is necessary before reliable results can be obtained, Secondly, the existing cryostat could in the meantime readily be converted to other cryogenic uses. / Thesis / Master of Science (MS)

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Electron-ion recombination has been under study for many years, but comparisons between theory and experiment have been very difficult, especially for conditions where the ion under evaluation was a minority in concentration. This study describes a direct measurement of the recombination-rate coefficient for the recombination of minority as well as majority ions in the afterglow.

Thesis--Ph. D., University of Hong Kong, 1983.

Techn. Universiẗat, Diss., 2001--Darmstadt.

Techn. Hochsch., Diss., 2004--Aachen.

No description available.

We have performed a careful measurement of the shape of the beta spectrum of He⁶. A detailed study of the phenomenon of electron scattering in our thin lens magnetic spectrometer enabled us to interpret the spectrum shape from the end point at Wₒ = 3.50 ± .02 Mev. down to 1/14 Wₒ = 0.250 Mev. The experimental shape has been compared with the theoretically predicted shape for allowed spectra. The influence of the pseudoscalar interaction on the shape of the He⁶ spectrum has also been considered. From these measurements we have been able to set limits on the Fierz interference in the Gamow-Teller interaction as well as on the magnitude of the pseudoscalar coupling constants. These limits have been interpreted in terms of the relative magnitudes of the axial vector, pseudoscalar, and tensor coupling constants using the two component theory of the neutrino and assuming that the complete beta decay Hamiltonian proposed by Lee and Yang is or is not invariant under time reversal. We have also calculated the effect on the spectrum shape of the production of inner Bremsstrahlung in beta decay and have shown this effect to be at the limit of experimental detectability.

We focus on studying laser-induced cavitation under widely different physical conditions, from superheated jets to superfluid liquid helium. We use ultra high-speed video imaging to track cavitation bubble dynamics at frame-rates of up to 7 million frames-per-second. Cavitation is induced by focusing a 532 nm pulsed Nd-YAG laser at a spot with a minimum spot size of 150 μm and pulse duration of six ns, which forms high-pressure plasma, leading to a rapidly expanding bubble/void, which subsequently collapses. We mainly study two configurations: (1) laser-induced cavitation in liquid helium inside an optical cryostat and (2) laser-induced cavitation in HCP solid helium. Moreover, we report preliminary results of two promising studies: (3) laser-induced cavitation inside a highly turbulent flow within a square channel and (4) laser-induced break-up inside a cylindrical liquid jet, leading to its atomization. (1) Inside the liquid helium-4, we reach widely different thermodynamic conditions when adjusting the temperature between 1.4 to 5.1 degrees Kelvin. Below the lambda point at T = 2.17 K, the liquid is superfluid, with viscosity appr zero, while above this temperature, regular liquid helium approaches the critical point at ≃ 5.1 K. This greatly changes the cavitation dynamics with different amounts of vapor appearing during cavity growth and collapse, and revealed four regimes of cavitation bubble behavior. We also measure shock velocities and analyze their characteristics. (2) At pressures of roughly 25 atmospheres, superfluid helium (He-II) solidifies. With wavy time-evolving oscillations on its surface when disturbed, the interface between the solid and the superfluid exhibits fascinating behavior with wavy time-evolving oscillations on its surface when disturbed. The interface between liquid and solid can consequently behave similarly to a free surface. Here, we experimentally investigate laser-induced interfacial dynamics at temperatures between 1.2 K and 2 K and at pressures ranging from the melting pressure of approximately 25 atm to a maximum of 39 atm, which covers both the HCP and BCC structure of the solid, using ultra-high-speed imaging at frame rates up to 7 million frames per second. (3) The cavitation inside the turbulent flow; this study aims to investigate the mutual effect of the rapid straining outside the bubble on the coherent vortices within the liquid and the feedback from the modified turbulence on the shape of the vapor cavity, and to include time-resolved particle image velocimetry. (4) The cavitation inside a liquid jet helps break it up into fine spray, which is of interest for injectors in combustion engines.

No description available.